Optical devices

Resume : High-density arrays of micro-pixel gallium nitride inorganic light-emitting diodes can be successfully bonded to CMOS electronic driver arrays to provide pattern-programmability and high-speed modulation. We overview the capabilities of this technology for multi-Gb/s optical data communications utilising spatial and wavelength multiplexing and spatial modulation techniques.

Resume : GaN nanowires (NWs) with radial p-n junctions were grown by molecular beam epitaxy (MBE) using selective area growth (SAG). N-type Si:GaN NW cores were grown at sufficiently high temperatures to produce full selectivity and subsequent Mg:GaN shells were grown at comparatively lower temperatures to promote effective Mg incorporation. An N-polar selective area growth (SAG) process was utilized that is capable of producing arbitrarily large NW pitch spacing, avoiding self-shadowing effects that can interfere with the growth of a uniform shell, and allows for fabrication of single-NW or array-based LEDs in vertical geometry. Electrical contact to the NW LEDs was made by contacting the n-type core through the conductive GaN nucleation layer and to the p-type NW shell by a Ni/Au metallization layer. At room temperature, the NW LEDs produced electroluminescence (EL) with a peak emission near 435 nm. Upon cooling down to 5K, the EL at 435 nm quenches and is replaced by emission at 380 nm. Of note is that these NW LEDs are operative at 5K, suggesting high doping levels and no carrier freeze out. Presumably, the EL originates due to electron recombination in the p-type shell and the emission at 380 nm and 435 nm correspond to ultraviolet luminescence (UVL) and blue luminescence (BL) bands commonly observed in heavily doped Mg:GaN. For increasing current injection, the BL emission undergoes a significant shift to higher energy while the UVL emission is comparatively unchanged.

Resume : GaN fins with high aspect ratio are interesting alternative architectures to micro- and nanowires for 3D GaN LEDs. Here, we report on a detailed study of high aspect ratio fins, which are grown in parallel linear openings of an SiOx mask on a GaN on sapphire template. Similar structures, but with much lower aspect ratio, were named GaN nanowalls or nanosheets by other groups. These fins can outperform wires: they usually have higher active area ratios, offer higher mechanical stability and distinctly reduced edge effects. Furthermore, the characterization of internal properties by a cross-sectional view is easier to perform. On growth windows parallel to the m-direction of the GaN buffer, fins with smooth a-plane sidewalls and almost no semipolar facets at the top are grown by selective area continuous-mode MOVPE. The length of the uniform fins is limited only by the substrate size. The aspect ratio, i.e. height to width, can reach up to 14, with total heights of more than 50 µm. Here, the impact of growth parameters on the geometric structure of the fins will be discussed. Fins with smooth top facets are obtained by an improved 2-temperature-step-growth. Core-shell fin LEDs structures are demonstrated by growing InGaN QW and p-GaN shells around the fin cores. Based on the TEM analysis of different fin LEDs it is shown that the number of stacking faults and dislocations in the shell layers for the improved 2-temperature-step-grown fins can be drastically reduced.

Resume : Utilizing local strain engineering in InGaN quantum wells, we present an LED based microdisplay device with integrated collimating lenses. The device consists of red, green, and blue LED pixels, all of which are monolithically integrated on a single chip using a standard InGaN LED epitaxial structure. All LED pixels are independently addressable and integrated with parabolic collimating lenses. Intensities of individual pixels are linearly tuned via a pulse-frequency modulation scheme. The device works at room temperature without cooling and is expected to be suitable for near-to-eye display and other emerging applications requiring high-density integration of multiple color LED devices.

Resume : Even though the GaN LEDs and HEMTs have become successful products, there is still much potential behind GaN technology for further development, one of which is photonic integrated circuits (PIC), whereby different photonic components are integrated to enhance functionality of the chip, similar to the trend observed in electronic integrated circuit. Integration offers many advantages including high speed transmission, reduced power consumption, wafer-level processing and system robustness and stability. The GaN is an ideal platform for this purpose, as the building blocks are well-established.Previously, we have achieved monolithic integration of emitters (LEDs), photodetectors (PDs) and waveguides on the same InGaN-GaN LED wafer, and demonstrated visible light communication (VLC) at speeds of ~100 Mbit/s. However, such 2D planar PIC poses limitations to the scalability of the system, prompting the needs to develop 3D PICs, which require optical signals to be transmitted between different planes and regions to achieve new functionalities such as non-mechanical beam steering and photonic signal processing [Microsystems & Nanoengineering 2, 16030 (2016)]. In this work, the 3D PIC is realized by selective-area detachment of the waveguide and PDs from the sapphire substrate using a selective-area laser lift-off process.The PD is subsequently mounted onto an elevated platform; the bendable waveguide functions as an optical fiber for signal transmission between two planes.

Resume : Since the invention of blue and green light-emitting diodes (LEDs), much attention has been paid to extend emission to shorter and longer wavelengths. We have focused on Eu ions that have been widely used as an activator for red phosphor, and have succeeded in developing the world’s first red LED that operates at room temperature using Eu-doped GaN (GaN:Eu) as an active layer [1]. Light output power of the LED has been growing steadily up to sub-milli watts in these years [2]. In this talk, current status of the red LED and strategies for further enhancement of the light output power are reviewed. Previous reports on GaN:Eu indicated that the number of optically active Eu centers and the energy transfer efficiency from GaN to the Eu ions are the major factors limiting light output. It was also found that defect environments around Eu ions influence greatly the energy transfer efficiency [3]. Therefore, to improve the emission efficiency of the GaN:Eu LEDs, the exploration of growth conditions that can form selectively a defect environment with the high energy transfer efficiency is strongly required. Another limiting factor is the relatively long radiative lifetimes of the Eu ions in GaN:Eu. Modifying the spontaneous emission rate of Eu can be achieved by increasing the photonic density of states at the frequency of spontaneous emission. Using a microcavity, we have demonstrated the enhancement of the PL intensity by 21 times at room temperature [4]. These results strongly suggest that the use of a microcavity structure is also a promising method for improving the light output from the GaN:Eu red LEDs. [1] A. Nishikawa et al., Appl. Phys. Express 2, 071004 (2009), [2] Y. Fujiwara et al., Jpn. J. Appl. Phys. 53, 05FA13 (2014), [3] R. Wakamatsu et al., J. Appl. Phys. 114, 043501 (2013), [4] T. Inaba et al., AIP Advances 6, 045105 (2016).

Resume : The combination of quantum dot (QD) active layers and nanoscale cavities with high quality factors can provide a pathway to realizing compact, efficient lasers with ultra-low lasing thresholds. For the InGaN/GaN material system, this produces highly efficient blue and UV optical sources operating at room temperature. Moreover, such QD-nanocavity lasers can provide insights at the nanoscale into the dominant limitations to low threshold lasing, such as non-radiative recombination pathways, photon loss mechanisms or inefficient carrier capture. We discuss results for InGaN QD nanocavity lasers with high spontaneous emission factors, producing only 1-4 resonant modes interacting with the spectral range of the active layer. Our QD material comprises QDs with shorter wavelength emission and what we term “fragmented” quantum well regions with longer wavelength emission over a broader spectral range. The relative simplicity of the modal spectrum provides an excellent diagnostic of lasing mechanisms. With successively higher pump powers, the longer and shorter-wavelength modes undergo a dramatic change in intensity; lasing ultimately occurs at the shorter wavelength mode. Systematic removal of the cavity volume, from microdisk to ring structures, allows us to understand the effect of “parasitic” pumping of gain material on lasing thresholds. Comparing lasing results for single-layer QD active regions with triple-layer regions provides information on the importance of carrier capture.

Resume : Recently, the epitaxial growth of GaN nanowires (NWs) and multi-quantum shell (MQS) active layer has attracted much attention for new optoelectronic device applications. Such devices have several advantages such as large area of the active regions for a given device size, high crystal quality with low dislocations and no quantum confined Stark effect on the sidewall of GaN NWs. Since applying to not only LED but LD, uniformly grown NWs only at the very small area with narrow stripe width × long cavity length are required. In this paper, we report on limited area formation of GaN NWs, and experimental results of epitaxial growth. We made a guideline for structural design of GaInN/GaN MQS-LD from waveguide simulation. We clarified how to arrange NWs array, and how long stripe width is by evaluating behavior of light intensity distribution and value of confinement factor of horizontal direction. In the epitaxial growth of GaN NWs, we produced two regions; one is NWs growth area, and another is non-growth area where no openings of SiO2 mask are made. Although GaN NWs were successfully grown only at the defined stripe areas, poly-crystalline grains were deposited at the edge of the stripe areas. Modifications of the stripe pattern to eliminate such poly-crystalline grains are necessary. Therefore, we would like to discuss about the influence of stripe pattern on the reduction of poly-crystalline grain formation toward the realization of MQS-LDs.

Resume : Alloy disorder and interface roughness in low-dimensional semiconductor heterostructures inevitably degrade their optoelectronic properties. An alternative approach for confining charge carriers is the fabrication of crystal-phase quantum structures, in which confinement is achieved by alternation of the crystal structure of a given semiconductor. For instance, it has been shown that basal plane stacking faults (BSFs) in GaN nanowires act as thin quantum wells (QWs) with atomically abrupt interfaces. Here, we describe a novel crystal-phase quantum structure induced by inversion domain boundaries (IDB) and BSFs in GaN nanowires. Using time-resolved photoluminescence (TRPL), we first demonstrate that individual IDBs act as QWs. The exciton oscillator strength for these QWs is 2×1013 cm-2, a value close to that reported for nonpolar GaN/(Al,Ga)N QWs. Thus, excitons in IDBs are tightly bound. In nanowires with high densities of IDBs and BSFs, an additional line at about 3.36 eV is observed in the PL spectra at 10 K. Temperature-dependent TRPL experiments reveal that this transition originates from excitons with a one-dimensional character. We propose that the transition arises from exciton recombination at quantum wires forming at the intersection between IDBs and BSFs. These crystal-phase quantum wires exhibit exceptionally well-defined interfaces, which make them ideal systems for studying quantum effects important for a future generation of optoelectronic devices.

Resume : 3D-microrod LEDs with a core-shell geometry already attracted substantial interest. Here, we will report the electroluminescence emission (EL) characteristics of LED devices comprising an ensemble of about 10k core-shell microrods. The ensemble was fabricated by SAG MOVPE on GaN on Sapphire substrates masked by SiO2. The p-contact to these AlInGaN based blue emitting MQW LED microrods was realized using TCO layers and metal structures for current spreading while their n-contact was realized by the underlying n-GaN layer [doi:10.1117/12.2214122]. The diced chips were mounted on special flat boards dedicated for characterization of the bare LED chip and also measuring effects of encapsulation into a high refractive index material by means of a silicone lens. The EL characteristic of the device versus driving current was measured by a goniophotometer equipped with a luminance camera and a spectrometer, giving detailed insights into the spatial and angular emission pattern and spectral directionality. Along with the integral EL also the absolute luminous and radiative efficiency characteristic could be derived, resulting in an EQE maximum in the range of 10%. The discovered hexagonally shaped EL directionality is related to the 3D geometry and will be discussed with respect to encapsulation and compared with a simple optical model of the 3D-LED. Furthermore, the impact of heat sink temperature regarding efficiency and radiant intensity pattern of the prototype LED will be discussed.

Resume : We present new insight into the physics of low-current non-radiative recombination in III-Nitride quantum-well LEDs grown on bulk GaN substrates. We use an all-optical carrier lifetime measurement which enables a detailed observation of recombination dynamics. Focusing on SRH recombinations, we demonstrate yet-unreported experimental trends, including a strong influence of carrier wavefunctions. We explain these observations with a theory of tunneling-assisted recombinations. This process can impact SRH recombinations by orders of magnitude, independent of material quality, and may play a dominant role in the efficiency of III-Nitride LEDs.

Resume : The efficiency reduction of InGaN/GaN light-emitting diodes (LEDs) towards the green spectral region is usually discussed with reducing electron-hole wave function overlap due to the quantum-confined Stark effect (QCSE), or decreasing material quality in mind. Carrier localization, omnipresent in InGaN layers, has not been considered in this context until recently[1]. We show that hole localization, as it occurs in randomly alloyed InGaN layers, significantly reduces the electron-hole wave function overlap ? in addition to the QCSE, and is therefore a strong contributor to the green gap phenomenon. Using Small-Signal Time-Resolved Photoluminescence[2] to obtain differential carrier lifetimes as a function of temperature, we determine the temperature-dependence of the electron-hole wave function overlap, which is strongly affected by the localization present in blue- and green-emitting LEDs. Deeper localization in InGaN alloys with higher Indium composition leads to a delayed carrier thermalization in green-emitting LEDs[3], substantially reducing radiative recombination at operating temperatures. In contrast, Auger processes are enhanced with rising localization. We discuss the interplay between the various processes and the implications for polar device efficiency in general and the quest for a highly efficient green LEDs in particular. [1] M. Auf der Maur et al., Physical Review Letters 116, 027401 (2016) [2] F. Nippert et al., Japanese Journal of Applied Physics 55, 05FJ01 (2016) [3] F. Nippert et al., Applied Physics Letters 109, 161103 (2016)

Resume : GaN-based LEDs are very efficient sources of short wavelength visible light. We propose a new method to determine their internal quantum efficiency (IQE) from bias dependent resonant photoluminescence (PL) measurements. Considering carrier escape by tunneling out of the active region, we extrapolate the generation rate of charge carriers from photocurrent measurements under reverse bias. We then fit a recombination rate model including phase-space filling to excitation density dependent PL data obtained under forward bias to extract the IQE. The proposed method has the advantage that the IQE can be determined without assumptions on carrier generation, extraction, injection efficiency or peak IQE at low temperature. Furthermore, we show that phase-state filling strongly affects the symmetry of the efficiency curve around its maximum, which is often neglected in methods where a rate equation model is fitted to PL or electroluminescence data. The different IQE values that we obtain with conventional methods compared to ours show that a critical discussion of the assumptions made for each method is necessary. Reference: C. Mounir and U. T. Schwarz, Appl. Phys. Lett. 110, 011106 (2017)

Resume : We study the evolution of the internal quantum efficiency (IQE) with photo-excitation density of polar and semi-polar (11-22) oriented InGaN/GaN quantum well (QW) structures for blue to red light emitting designs by using time-resolved photoluminescence (TRPL) and photo-excitation power dependence measurements. At very low excitation level, the TRPL measurements show a large difference of decay times between polar and semi-polar structures due to strong differences in the values of the internal electric field inside the InGaN quantum well. The decay times of semi-polar samples were measured in the range of several nanoseconds, while the decay time of the polar samples are measured to reach up to few microseconds. This leads to a large variation of the carrier density in the active region. Long decay times help the establishment of carrier-carrier repulsions before the radiative recombination of electrons and holes occurs. This favors Auger non-radiative recombination process and hence a decrease of the IQE. We examined the impact of Auger non-radiative recombination on the evolution of the IQE via photo-excitation power dependence measurements of the PL intensity and using an ABC model. In the polar series of samples, we find that the threshold of photo-excitation density PT for the onset of reducing IQE decreases with the increase of impacting internal electric field thanks to the Quantum Confined Stark effect (QCSE). Then, the ABC model permits us to estimate the relative contributions of different recombination channels: SRH non-radiative recombination (coefficient A), radiative recombination (coefficient B), and Auger non-radiative recombination (coefficient C). We find that the intrinsic Auger effect is the dominating recombination mechanism for reducing IQE at high excitation level. In the semi-polar structure, the Auger non-radiative recombination is observed only for the red-emitting sample, using our experimental set-up. This observation was made possible because the indium composition is larger than for other semi-polar samples (i.e. 35%).

Resume : At low temperature (10 K), for low to moderate optical excitation densities, the internal quantum efficiency (IQE) of InGaN/GaN quantum wells (QWs) is in general very high (~100%). In this regime the emission is attributed to the recombination of localised electrons and holes. However, at high excitation densities (>10^12 carriers/cm^2/pulse) at low and high temperatures, the IQE drops rapidly: a phenomenon known as efficiency droop. In low temperature excitation power dependent photoluminescence (PL) measurements, we observe a saturation of the localised electron and hole emission and the emergence of an emission band, as already reported by others [Davies et al., Appl. Phys. Lett., 102, 022106 (2013); Nippert et al., J. Appl. Phys., 119, 215707 (2016)], on the high energy side of the spectrum with increasing excitation density in the droop regime. This band always occurs on the high energy side of the localised carrier emission in samples where the In fraction varied ruling out GaN defect recombination as its origin. To determine the nature of this band and its possible role in the droop process we have also performed PL time decay measurements. Notably, these exhibit plateau-like regions similar to those due to state blocking effects observed in InAs/GaAs quantum dots [Buckle et al., J. Appl. Phys., 89, 2555 (1999)]. These observations indicate that saturation of the localised state recombination pathway may play an important role in efficiency droop.

Resume : The design and optimization of III-Nitride multi-quantum-well (MQW) LEDs require a quantitative understanding of the quantum mechanics-dominated carrier flow. Typical devices can be characterized by spatial regions of extremely high carrier densities such as n-GaN/p-GaN layers and quantum wells, coupled to each other by tunneling and thermionic emission-based quantum transport. Key design challenges in LED such as improving carrier spreading among MQW and internal quantum efficiency (IQE) at high current drive, are rooted in the fundamental transport physics. Traditional modeling tools based on semiclassical physics often neglect quantum effects and realistic bandstructure details, which lead to difficulties in predicting important transport characteristics such as the turn-on voltage. This work applies a multi-scale model based on the nonequilibrium Green?s function (NEGF) formalism developed previously to study realistic III-Nitride LED devices, and reveal new insights into the device physics. The model partitions the device into different spatial regions where the high carrier domains are treated as reservoirs in local equilibrium and serve as injectors and receptors of carriers into the neighboring reservoirs. Tunneling and thermionic emission are treated in the same footing. Experimental I-V characteristics of a commercial device structure is reproduced quantitatively. Simulation also captures the Auger-dominated efficiency droop and p-side dominated light emission, which are observed in typical mid-to-high power LEDs under high drive current. Design techniques for better carrier spreading and improved IQE will be discussed, and optimized MQW designs will be proposed.

Resume : The InGaN quantum wells (QWs) grown on c-polar GaN are known to suffer from low overlap of electron and hole wave functions due to the quantum-confined Stark effect (QCSE). It strongly limits the thicknesses of the InGaN QWs feasible for application in optoelectronic devices. From the point of view of laser diodes (LDs) it would be beneficial to use wide InGaN QWs as it should enhance the optical gain if only the QCSE could be reduced. Wide QWs would strongly increase the optical confinement factor and reduce the carrier density. The reduction of the carrier density is favorable as it decreases the part of the carriers recombining through the nonradiative Auger process which is the dominant recombination path at the high injection regime in LDs. In this work we will present blue (?=450 nm) LDs grown by plasma assisted MBE with reduced QCSE which enabled us to use wide InGaN QWs. An enhancement of the optical gain is observed compared to LDs with standard QWs resulting in a lower threshold current of the devices. The experimental results will be compared with theoretical predictions. Furthermore, a comprehensive study of carrier lifetime measured by time-resolved photoluminescence in InGaN QWs with reduced QCSE will be presented. Acknowledgments: This work was supported partially by the National Science Centre (grant No. DEC-2013/11/N/ST7/02788), the Foundation for Polish Science Grant TEAM TECH/2016-2/12 and the National Centre for Research and Development Grant PBS3/A3/23/2015.

Resume : We have developed an ion beam assisted deposition (IBAD) texturing process for biaxially aligned films as substrates for GaN epitaxy. The IBAD process enables low-cost, large-area flexible metal foil substrates to be used as potential alternatives to single-crystal sapphire and silicon for GaN electronic devices. Epitaxial GaN films are grown by the MOCVD process on these engineered flexible substrates. We have achieved GaN films of several microns on polycrystalline metal foils that have in-plane and out-of-plane alignment of less than 1° FWHM and typical threading dislocation densities of 4-8 x 10^8/cm^2. We use the epitaxial GaN films on IBAD/polycrystalline metal foil as a template to deposit epitaxial multi-quantum well light emitting diode (LED) InGaN structures. From these layered structures we have successfully fabricated LED devices. These are the first LED devices fabricated directly on metal foil. We observe photoluminescence intensities from the LED structures up to 70% of those fabricated on sapphire. We will present data on performance of such devices and how these LED devices could be printed using a roll-to-roll process. This work was supported by the Department of Energy ARPA-E agency.

Resume : Flexible light emitters are a topic of intense research driven by applications such as bendable displays or lightning sources. Nowadays flexible technology is dominated by OLEDs, which however exhibit low efficiency and a limited lifetime especially for the blue spectral range. In this work, we employ InGaN/GaN core/shell nanowires (NWs) embedded in polymer membranes to demonstrate flexible LEDs. This approach allows to combine high flexibility of passive polymer films with high quantum efficiency of active nitride NWs. MOVPE grown InGaN/GaN core/shell NWs were used as active emitters. Composite polymer/NW membranes were contacted using transparent Ag nanowire networks. The assembly of free-standing membranes (active surface ~ 1cm^2) containing NWs with different bandgaps was used to demonstrate a multi-colour LED [1]. This concept therefore allows for a large design freedom and modularity since it enables combination of materials without constraints related to lattice-matching or growth conditions compatibility. Moreover, a down-converting white LED can be achieved by doping the polymer material of a single layer flexible blue LED with nanophosphors [2]. The performance and fabrication procedure of these flexible LEDs will be discussed. The concept of free-standing NW membranes can also be applied to other devices such as flexible UV photodetectors [3]. [1] DOI: 10.1021/acs.nanolett.5b02900 [2] DOI: 10.1021/acsphotonics.5b00696 [3] DOI: 10.1021/acsami.6b06414

Resume : Colour-temperature (Tc) is a crucial specification of white light-emitting diodes (WLEDs) used in a variety of smart-lighting applications. Commonly, Tc is controlled by distributing various phosphors on top of the blue or ultra violet LED chip in conventional phosphor conversion WLEDs (PC-WLEDs). Unfortunately, the high cost of phosphors, additional packaging processes required, and phosphor degradation by internal thermal damage must be resolved to obtain higher-quality PC-WLEDs. Here, we suggest a practical in-situ nanostructure engineering strategy for fabricating Tc-controlled phosphor-free white lightemitting diodes (PF-WLEDs) using metal-organic chemical vapour deposition [1-2]. The dimension controls of in-situ nanofacets on gallium nitride nanostructures, and the growth temperature of quantum wells on these materials, were key factors for Tc control. Warm, true, and cold white emissions were successfully demonstrated in this study without any external processing. [1] D. Min, D. Park, J. Jang, K. Lee. & O. Nam "Phsphor-free white-light emitters using in-situ GaN nanostructures grown by metal organic chemical vapor deposition". Sci. Rep. 5, 17372 (2015). [2] D. Min, D. Park, K. Lee. & O. Nam "Colour-crafted phosphor-free white light emitters via in-situ nanostructure engineering". Sci. Rep. to be published in March (2017).

Resume : Micro light emitting diodes (micro-LEDs) are very promising candidates for next-generation displays because they have many advantages such as high luminance, long lifetime and short response time. In spite of these many advantages, it is still challenging to realize the micro-LED displays with a high-resolution. Generally, an inorganic LED structure only has monochromatic light emission. Therefore, it requires that the fabricated red, green, and blue LEDs be laterally arranged for full color emission. However, the lateral arrangement of red, green, and blue LEDs takes three times larger area compared with a configuring having each color in same pixel position. This causes a reduction in resolution. Therefore, an innovative solution for achieving a high-resolution full color micro-LED display is needed. In this study, we fabricated a vertically-stacked passive-matrix micro-LED array structure which can independently control blue sub-pixels and green sub-pixels. The structure does not require laterally arranged subpixels. As a result, we demonstrated that the color at the same pixel position of the display can be changed from green to blue. We will report the fabrication process of the device and discuss about the optical properties in detail.

Resume : The continuous tuning of the emission wavelength of light-emitting diodes (LEDs) by external strain is of great technological significance in high quality displays and medical applications. Many researches on the wavelength tuning of LEDs have been done by controlling temperature of LEDs. However, some previous studies have shown the critical issues on optical degradation of LEDs. In this study, we demonstrate a wide range of wavelength tunability of LEDs by applying an external strain on the metal substrate without optical degradation for the first time. The external strain could reduce the QCSE induced by piezoelectric field in multiple quantum wells (MQWs) of LEDs. The key steps are wafer bonding of LEDs to the metal substrate by employing an AuSn eutectic bonding process and transfer printing by using a laser lift-off (LLO) process to separate the LED from the sapphire substrate. When the LED on metal substrate is bended by external force (convex bending process), the applied strain in the active region of LED is determined by thickness and Young's modulus of metal substrate which determine the position of neutral plane in the metal substrate. In this study, the blue LED with a 450 nm wavelength showed the 12 nm blue-shift at 3.12 x 10-3 of tensile strain and a green LED with a 530 nm wavelength showed a blue-shift of 15 nm at the same tensile strain due to the relaxation of QCSE resulting from the decrease of band bending of MQWs by the convex bending of LED on the flexible metal substrate.

Resume : GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at wavelength as short as 375 nm have attracted considerable attention for a variety of applications such as sterilization, disinfection, water and air purification, biochemistry, and solid-state lighting [1–4] . Due to the lack of mass-produced bulk GaN substrate, GaN-based UV LEDs are generally grown on foreign substrate such as sapphire by metal-organic chemical vapor deposition (MOCVD) technique. Although the light output power of GaN-based UV LEDs have been greatly improved, the performance of GaN-based UV LEDs grown on sapphire substrate is still limited by the crystalline quality of GaN epilayer because of large mismatch in lattice constant and in thermal expansion coefficient between GaN and sapphire substrate, leading to a high threading dislocations (TDs) density in GaN epilayer [5, 6]. This high TD density would generate numerous non-radiative recombination centers, thereby deteriorating the optical and electrical properties of GaN-based devices. Compared to GaN-based blue LEDs, the efficiency of GaN-based UV LEDs is more sensitive to TD-related non-radiative recombination centers because of the lack of localized states in multiple quantum well (MQW) active region [7]. Therefore, it is of particular importance to reduce TD density in GaN epilayer for realization of highly efficient UV LEDs. To overcome the large lattice mismatch between GaN and sapphire, a low temperature GaN or AlN nucleation layer (NL) is commonly introduced prior to growth of GaN epilayer at high temperature [8, 9]. More recently, high-quality GaN epilayer with reduced TD density is obtained using a sputtered AlN NL on sapphire substrate [10, 11]. The quality of GaN epilayer is sensitive to the growth of NL. A systematically comparative study of the effects of in-situ low temperature GaN/AlGaN NLs and ex-situ sputtered AlN NL on crystalline quality, optical and electrical properties of UV LED is also not well investigated yet. In addition, crystalline quality of GaN epilayer is closely correlated with the thickness of NL [12]. However, effects of sputtered AlN NL thickness on crystalline quality, optical and electrical properties of UV LED are not well understood yet. In this study, we report on the demonstration of GaN-based UV LEDs emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN NLs and ex-situ sputtered AlN NL, respectively. The growth behaviors of GaN on PSS with GaN/AlGaN/sputtered AlN NLs were comparatively investigated. The TD density in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NL is characterized by using high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), indicating that the TD density in UV LED with AlGaN NL is the largest, whereas the TD density in UV LED with sputtered AlN NL is the smallest. The light output power (LOP) of UV LED with AlGaN NL is 18.2% higher than that with GaN NL, which is attributed to a decrease in absorption of 375 nm UV light from AlGaN NL with larger bandgap. With sputtered AlN NL instead of AlGaN NL, the LOP of UV LED is further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10 nm -25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL is higher than those with thickness of 10 nm and 25 nm, revealing that the optimum thickness of the sputtered AlN NL is around 15 nm. Reference [1] A. Khan, K. Balakrishnan, T. Katona, Nat. Photonics 2 (2008) 77. [2] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song, H. Zhou, A. V. Nurmikko, Appl. Phys. Lett. 73(1998) 1688. [3] M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodriguez, S. Einfeldt, Z. Yang, N. M. Johnson, M. Weyers, Semicond. Sci. Technol. 26 (2011) 014036. [4] J. Brault, D. Rosales, B. Damilano, M. Leroux, A. Courville, M. Korytov, S. Chenot, P. Vennéguès, B. Vinter1, P. De Mierry, A. Kahouli, J. Massies, T. Bretagnon, and B. Gil, Semicond. Sci. Technol. 29, 084001 (2011). [5] S. D. Lester, F. A. Ponce, M. G. Craford, and D. A. Stei-gerwald, Appl. Phys. Lett. 66, 1249 (1995). [6] S. J. Zhou, S. Yuan, Y. C. Liu, L. Jay Guo, S. Liu, H. Ding, Appl. Surf. Sci. 355 (2015) 1013. [7] T. Wang, Y. H. Liu, Y. B. Lee, Y. Izumi, J. P. Ao, J. Bai, H. D. Li, S. Sakai, J. Cryst. Growth 235 (2002) 177. [8] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [9] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 71 (1992) 5543. [10] C. H. Chiu, Y. W. Lin, M. T. Tsai, B. C. Lin, Z. Y. Li, P. M. Tu, S. C. Huang, E. Hsu, W. Y. Uen, W. I. Lee, H. C. Kuo, J. Cryst. Growth 414 (2015) 258. [11] S. W. Chen, H. Li, T. C. Lu, AIP Advances 6 (2016) 045311. [12 J. C. Zhang, D. G. Zhao, J. F. Wang, Y. T. Wang, J. Chen, J. P. Liu, H. Yang, J. Cryst. Growth 268 (2016) 24.

Resume : Among many recent approaches to the improvement of light-emitting diode (LED) performance, the increase of radiative recombination rate by surface plasmon (SP) coupling has been actively studied for the high efficiency LEDs. SPs are the collective oscillations of free electrons in metals at the interfaces between metals and dielectrics. Specifically, the collective oscillations of electrons in noble metal nanoparticles (NPs) embedded in a dielectric matrix are localized surface plasmons (LSPs). Recently, some groups reported the physical mechanisms for the relationship between SP-coupling and quantum-confined Stark effect (QCSE). However, the direct experimental evidences, which can explain the reduction of QCSE by SP-coupling, have not been reported yet. In this study, we report the optical properties of LSP-enhanced green LEDs containing gold (Au) NPs embedded in a p-GaN layer. Excitation power-dependent photoluminescence (PL) and injection current-dependent electroluminescence (EL) measurements revealed that the blue-shift of PL and EL peaks with increasing carrier density was smaller for the LSP-enhanced LED compared with that for a conventional LED. The increased output power and decrease in blue-shift of the LED with Au NPs were attributed to the increased radiative recombination efficiency of carriers induced by the LSP-coupling process and the compensation of the polarization-induced electric fields with LSP-enhanced local fields, both of which suppressed the QCSE.

Resume : The room-temperature photoluminescence (PL) spectrum of doubly doped GaN(Mg):Eu shows a set of emission peaks due to a defect, Eu0, comprising a single Mg acceptor in close association with Eu substituting for Ga (the Eu2 defect) [1] . Low temperature PL measurements of this material exemplify hysteretic photochromic switching (HPS) between two configurations of the same Eu-Mg defect, with a hyperbolic time dependence on ‘switchdown’ from Eu0 to Eu1(Mg) [2]. At a fixed excitation wavelength of 355 nm, close to the GaN band edge at low temperature, the sample temperature and the incident light intensity tune the characteristic switching time over many orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to several hours at 50 K, 1 mW/cm2. A set of PL excitation experiments at different wavelengths above and below the band edge shows that the characteristic switching time for the Eu0 to Eu1(Mg) configurations closely mirrors the Urbach tail of the GaN absorption spectrum. We describe HPS as a nanoscale light-induced phase change around Eu-Mg defects and present evidence for stimulated migration of Mg atoms and energy transfer from donor-acceptor pairs to Eu3+ ions in GaN. [1] K.P. O’Donnell, P. R. Edwards, M. Yamaga, K. Lorenz, M.J. Kappers, M. Boćkowski, Appl. Phys. Lett. 108, 022102 (2016). [2] A.K. Singh, K.P. O’Donnell, P. R. Edwards, K. Lorenz, M.J. Kappers, M. Boćkowski, Sci. Rep. 7, 41982 (2017).

Resume : In this work, we demonstrated a submicron core-shell nanolaser array lasing under room temperature. Hexagonal nanorod lattice was fabricated on GaN-on-sapphire templates. The full hexagonal core-shell structure were achieved by re-growth of n-GaN and InGaN multiple-quantum-wells (MQWs) by metal organic chemical vapor deposition (MOCVD). The rod diagonal was estimated to be 850 nm with 900 nm translational lattice constant in a six-fold rotational symmetry. Although the width eventually shrank toward the field due to the precursor consumption during regrowth, it provided a decent out-of-plane optical confinement. The threshold pumping density was as low as 80 kW/cm2 with a quality factor as high as 1940. The peak wavelength of cathodoluminescece (CL) spectrum before and after regrowth was 364 nm and 375 nm, respectively. The CL image of full core-shell structure showed a higher intensity in the shell region, confirming that the gain medium for lasing peak around 375 nm was regrown InGaN layers. Angle-resolved PL (AR-PL) measurements were performed in two principle in-plane crystal orientations, Γ-M and Γ-K. Although the standing nanorods were aligned periodically, neither significant Fabry–Pérot fringes nor angle-dependent photonic crystal fringes were observed. This indicated the lasing modes shall be highly in-plane, and the interaction among rods happened within a short range. Three dimensional optical simulation suggested a strong candidate for the 376 nm lasing. It consists of an in-plane third-order WGM, where InGaN QWs were overlapping with the most outer ring of local intensity maxima. Power-dependent PL under different pumping densities (P) from 30 kW/cm2 to 200 kW/cm2 with a relatively large spot size (diameter~ 160 μm) were also conducted under room temperature. Two lasing peaks (λpeak=375.9 nm and 376.4 nm) firstly emerged under P= 65 kW/cm2 and the number of peaks increased as P increased. The lasing peaks can be categorized into at least three groups. The first two peaks emerged around 376 nm at low P, and vicinal peaks appeared as P increases. Under P=196 kW/cm2, five discernible peaks were observed, forming a mini-band between 373 nm to 377 nm. The peaks spacing ranged from 0.4 nm to 1 nm, which was too small to be different orders of WGM. The fine splitting was attributed to the optical coupling among high Q resonators, forming “supermodes” between or among adjacent rods. The parity of WGMs might either add up or annihilate the electric field between them, forming optical “bonds” or “nodes” between hexagons. In general, forming bonds results in wavelength redshift while forming nodes causes blueshift. The amount of wavelength shift ranges from few Å’s to one nanometer. The narrow mode spacing between lasing peaks suggested a strong coupling between adjacent whisper gallery modes (WGM), where both the GaN and InGaN could be the principle gain medium. The same power-dependent PL measurement were performed under different temperatures. As temperature decreased, the peak intensity of InGaN modes (λ~375 nm) was gradually reduced while that of GaN modes (λ~365 nm) was enhanced simultaneously. No InGaN modes lasing can be observed when T<195 K. The FWHM of GaN modes near lasing was 0.6 nm, yielding a Q about 850. Although the Pth_InGaN was significantly deteriorated, the FWHM of InGaN modes was still as low as 0.22 nm, implying Q did not change significantly with the temperature. Temperature-dependent PL measurement revealed a lowered threshold pumping density and an improved slope efficiency under room temperature than those under low temperature. The improved slope efficiency of InGaN modes also suggested that the more carriers were injected into MQW more effectively under room temperature. The improvement of injection efficiency was also attributed to the MQW’s proximity to the free surface. The high energy barrier of free surface effectively harvested the diffusing carriers from the core region and prevent them from escaping again through thermionic emission. The core-shell structure exploited the asymmetric carrier transport properties between core and shell and the vicinity of InGaN quantum wells to the free surface to enhance carrier confinement under room temperature. With wavelength-tunable active materials and a wafer-level scalable processing, patterning optically-coupled GaN-InGaN core-shell nanorods is a highly practical approach for building various on-chip optical components including emitters, modulators, coupled resonator waveguides, and optical interconnects in visible and ultraviolet spectral range.

Resume : Ultraviolet (UV) light-emitting diodes (LEDs) have attracted considerable attention because of the advantages of optional application in various fields such as biological agent identification, water or air purification, chemical sensing and excitation source for phosphors or light source for fine lithography. However, the lack of suitable transparent conductive electrode (TCE) is one of the most crucial problems for UV LEDs. The traditional indium tin oxide (ITO) suffers from low transparency in UV light wavelength region. Graphene is one of the replacements to ITO as TCE that has been investigated, but the high sheet resistance and high forward turn-on voltage limit its applications in UV LEDs. In addition, some other TCEs such as metal nanowire[1], graphene/ITO nanodot nodes[2], Ga-doped ZnO[3], Rh mesh[4] and ITO-Ag-ITO multilayer[5] have also been employed to improve external quantum efficiency of near UV LEDs. In this work, ultrathin Al-doped Ag film was used as TCE in GaN-based UV LEDs with an emission peak of 365 nm. In order to obtain the ultrathin and very smooth Ag film, a small amount of Al was co-deposited during Ag deposition to suppress the 3D island formation during the Ag film growth. The 7-nm-thick ultrathin Al-doped Ag film not only have very smooth surface morphology, but also have high thermal stability[5]. Besides, it is worth noting that the transmittance of the ultrathin Al-doped Ag film is more than 85% at 365 nm[6] compared to the traditional ITO with 75% transparency at the same light wavelength. At high injection current of 350 mA, the light output power of UV LED with ultrathin Al-doped Ag film TCE and with ITO TCE were estimated to be 41.8 mW and 36.3mW, respectively. It was indicated that the the light output power of UV LED with ultrathin Al-doped Ag film TCE was 15% higher than that with ITO TEC at the same injection current. The result indicates that ultrathin Al-doped Ag film has satisfactory properties of high transmittance in UV region for application as TCEs in UV LEDs.

Resume : The AlGaN-based high-voltage ultraviolet light-emitting diodes (HV-UVLEDs) arrays with 16 microchips connected in series were designed, fabricated, and characterized for the purpose of efficiency enhancement. Under input power of 1.2 W, the light output power of HV-UVLEDs is enhanced by 28.3% when compared to the traditional high power UV-LED (HP-UVLED).

Resume : Surface plasmons (SPs) are coherent electron oscillations that exist at a metal/dielectric interface. Specifically, the SPs in noble metal nanoparticles (NPs) embedded in a dielectric matrix are called localized surface plasmons (LSPs). When LSP is located close to the multiple quantum well (MQW) active layer of a light-emitting diode (LED) and the MQW emission wavelength matches the LSP resonance energy, the LED performance can be enhanced through the energy coupling from excitons in the active region of the LEDs to the LSP modes of metal nanostructures. This LSP coupling creates an energy transfer route that charge carriers in the active layer can take in addition to radiative and nonradiative recombination channels. The transferred energy results in the emission of photons and therefore increases the internal quantum efficiency (IQE) by suppressing nonradiative recombination channels. For the LSP enhancement to be efficient, the NP layer should be placed in close proximity to the active MQW region of the device, generally no further than ~100 nm. In this work, we proposed the fabrication of 2 types of InGaN/GaN LEDs embedded with Ag@SiO2 NPs : nanopillar LEDs and hole-patterned LEDs. In such a structure, the NPs could be placed very close to the active layer of the LED structures. Photoluminescence (PL) and electroluminescence (EL) behavior were investigated and discussed. Consequently, the PL intensities of nanopillar LEDs and hole-patterned LEDs were increased by 90% and 75% compared to reference samples without Ag@SiO2 NPs, respectively. The EL intensities of nanopillar LEDs and hole LEDs were increased by 100% and 10% compared to reference samples, respectively. The improved optical performance was ascribed to the effective energy coupling between the LSP NPs and the MQW region of the LEDs.

Resume : The recent progress in the epitaxial layer growth techniques and device fabrication processes of GaN-based LEDs is responsible for the extensive use of these devices in various applications. We developed a simplified manufacturing process for a new vertical-type LED. This vertical-type LED was fabricated the four steps namely, photolithography, consecutive epitaxial layer growth, sorting using a pocket-type shadow mask, and metallization. A thick epilayer was separated spontaneously from the existing sapphire substrate by internal stress during the cooling step in the epitaxial growth step. At the epitaxial layer growth step, the epilayers of vertical-type LED were grown by a mixed-source HVPE. Our designed HVPE combines both a source zone with a RF heating coil and a multi-graphite boat and a growth zone with three furnace. The number of well in graphite boat and quantity of the mixed-source are modulated by the thickness of the epilayers to be grown. Our sample was consecutively grown the GaN:Si, AlGaN:Si, AlGaN active layer, AlGaN:Mg, and GaN:Mg. The structural and optical properties of the vertical-type LED are also discussed. TEM results show that threading dislocation density of the epilayer was reduced with increasing thickness. Room-temperature EL peaks exhibit a blue luminescence (BL) shift from 465 nm at 30 mA to 448 nm at 130 mA. I-V characteristics of the vertical-type LED at turn-on voltage of 3.38 V and series resistance of 80 Ω. From the measurement results validating the characteristics of the emitted light, these vertical-type blue LED with the GaN provide a new concept in the field of solid-state light source. Acknowledgments This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea)

Resume : AlN material has a wide band gap, high thermal conductivity and high electrical resistance. It can be used to deep-UV detectors, LEDs, and LDs. The low-dimensional AlN nanostructures such as nanorods and nanowires were applied to the conductive fillers. AlN sphere exhibit brilliant dispersity and packing density in polymer solution, which can aid in the formation of particle networks and pathways. Recently, several reports were presented that AlN microspheres were synthesized by both sol-gel technique and gas reduction nitridation. In this study, AlN microspheres were grown by a mixed source HVPE. Our HVPE consists of the reactor combining both a source zone with a RF heating coil and a growth zone with three furnaces. The AlN microspheres are obtained by the synthesis of the source gases NH3 and the chemical compounds formed by the reactant HCl that is flowed over the mixed-source materials in a well of the graphite boat. The diameter of the AlN microsphere is hundreds of micrometers by SEM measurement. The AlN microspheres look like a spherical shape with the surface of chestnut. We confirmed that the surface is composed of AlN by using TEM and EDS. These results show that AlN microspheres were successfully synthesized by using a mixed-source HVPE method. Acknowledgments This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea)

Resume : III-nitride AlN is a promising material for high power UV LED, LD, detectors, ultra high power, and high frequency electronic devices because of its wide band gap and high thermal conductivity. For fabricating an AlN-based devices, it is important to grow a p-type AlN epilayer with high conductivity. In general, Mg is used as a dopant material for growing a p-type AlN epilayer. But, the growth of Mg-doped AlN epilayer with high conductivity is difficult because of the large activation energy of Mg and the generation of compensating centers and defects during the crystal growth. In this study, Mg-doped AlN epilayer was grown by the mixed-source HVPE. Mixed source HVPE consists of the reactor combining both a source zone with a RF heating coil and a growth zone with three furnaces for the growth of Mg-doped AlN epilayer. Al and Mg were filled in a well of graphite boat as source materials for the growth of Mg doped AlN epilayer. XPS was carried out to identify the chemical binding energy and to confirm the Mg elements in the Mg-doped AlN epilayer. XPS spectra of the Mg-doped AlN epilayer indicate the presence of Mg2 (binding energy of Mg 2p core level = 50.02 eV) in the epilayer, which can be demonstrated that Mg was doped successfully into the AlN epilayer. We fabricated the Mg-doped AlN epilayers on a sapphire substrate by using the mixed-source HVPE. By the optimizing of growth condition of Mg-doped AlN epilayer, fabrication of the UV optoelectronic devices will be expected. Acknowledgments This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea)

Resume : In this study, we report the properties of our new vertical-type AlGaN-based white-LED (WLED) without phosphor. The epilayers of WLED were grown by a mixed-source HVPE consisting of a multi-graphite boat filled with a mixed source inside the source zone, and a growth zone with three furnaces. Room-temperature electroluminescence (EL) spectrum of the WLED chip includes peaks located at approximately 2.13, 2.29, 2.57, 2.88, and 3.09 eV. The white emission of the vertical-type LED exhibits from the sum of the emitted light by the transition from one level or SD state in the continuous CB of the AlxGa1-xN alloy to the native and complex defect states. The WLED exhibits EL emission with a luminance of approximately 211 cd m-2. Photoluminescence (PL) mapping of the AlxGa1-xN active layer shows an Al composition of 0 < x < 0.015 approximately. To increase the incident current, the CIE chromaticity coordinates of the white light vary from (0.298, 0.315) near ideal white at 40 mA to green (0.313, 0.419) at 700 mA via cool blue (0.318, 0.423) at 250 mA in the white color temperature. The vertical-type WLED has various light-emitting characteristics from these transitions due to the nonuniform compound with a low composition. By optimizing the amount of source materials, the thickness of the active layer, and the number of active layers, we expect that high efficiency vertical-type WLEDs will be achieved. Acknowledgments This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No: N0001363) funded by the Ministry of TRADE, INDUSTRY & ENERGY(MOTIE, Korea)

Resume : Electrical properties, deep traps spectra and low frequency noise were studied for NUV (peak wavelength 385 nm) MQW GaN/InGaN LEDs grown by MOCVD on sapphire. DLTS spectra measured with electrical injection showed the presence of 0.56 eV and 0.8 eV electron traps and 0.75 eV hole traps. The latter were dominant in DLTS spectra measured with optical injection (ODLTS). At that the ODLTS spectra were similar for excitation only in the QW (385 nm) and the excitation of the entire active QW region (365 nm). The data strongly suggest that the observed electron and hole traps are located in the QWs of the structure. The low frequency noise as a function of the driving current If consisted of two regions with the spectral density of current noise proportional to If for low current ( dislocations-related leakage) and If^2 (G-R noise) for high current. After prolonged operation at high driving current (600 mA) the EL output of the studied LEDs decreased only slightly, at most by 15% after 1000 h exposure, but the reverse current and forward current at low voltages were gradually increasing with the testing time. Simultaneously we observed the increase in the spectral density of noise and the signal from the main electron and hole traps in DLTS/ODLTS. Similar changes in I-V characteristics, EL intensity and deep traps spectra were detected after irradiation with fluencies of 2E15-1.1E16 cm-2 of 6 MeV electrons. We discuss the similarity of this degradation after electrical stress and after irradiation in terms of native defects formation, their decoration of dislocations, and increasing the density of nonradiative recombination centers in the bulk.

Resume : Deep electron and hole traps were studied by means of admittance spectroscopy (AS) and DLTS with electrical and optical (ODLTS) injection for GaN-based MQW LEDs emitting in NUV (385 nm), blue (445 nm), and green (530 nm) spectral regions. AS spectra were dominated by freezing out of Mg acceptors at temperatures around 150K, by shallow centers in the MQW region, and, for green LEDs, by deeper MQW electron traps with activation energy 0.27 eV. DLTS spectra showed the presence of electron traps with activation energy 0.8 eV (NUV), 0.57 eV (blue) and 0.27 eV (green LEDs). In ODLTS hole traps with activation energies 0.75 eV (NUV), 0.65 eV (blue), and 0.45 eV (green LEDs) coming from the QW region were detected (the QW character was confirmed by using excitation light that generated electron-hole pairs only within the QWs). Similar hole traps could be observed in DLTS spectra of NUV and blue LEDs, but not for green LEDs. The latter observation seems to indicate the hole traps in question to be effective nonradiative recombination centers that are difficult to recharge by injection in high-indium content, high-defect-density green LEDs. The levels of the electron and hole traps were reasonably well aligned in respect to the level of vacuum. For LEDs with varying EL efficiency caused by differences in growth procedure (using or not using an overgrown nanopillar sublayer embedded with SiO2 nanoparticles), by prolonged operation at high driving currents or by electron irradiation, the changes in EL correlate with the changes in the electron and hole traps spectra. We discuss possible origin of the detected traps and considerations to be used for choosing the biasing and pulsing conditions to achieve best sensitivity.

Resume : High power 3D flip-chip light emitting diodes (LEDs) with distributed n-type via-hole-based electrodes were fabricated and investigated. In the 3D flip-chip LEDs, reflective ohmic contact to p-GaN was made by means of Ni/Ag, and contact to n-GaN layer was made by etching via holes through p-GaN and InGaN/GaN multiple quantum well (MQW) active layer. The via-hole-based n-contact electrodes structure increased the utilization ratio of active region area, and the introduction of metallization layer allowed n-contact to be arranged uniformly on the entire n-GaN surface, which exhibited a more favorable current spreading uniformity Comparison tests altering Ni metal thickness and annealing temperature were performed to optimize the reflectivity of Ni/Ag reflective layer, which enhanced the light extraction efficiency. As a result, the light output power of 3D flip-chip LEDs was apparently higher than that of conventional high power LED. The conventional lateral structure high power LED exhibited much more severe external quantum efficiency (EQE) degradation than that of the 3D flip-chip LEDs. The 3D flip-chip LEDs outperforms conventional top-emitting LEDs by generating bright-light emission with relatively lower junction temperature and thermal resistance, owing to its high light extraction efficiency, superior current spreading and heat dissipation.

Resume : Because of their advantages over conventional light sources, gallium nitride (GaN) light-emitting diodes (LED) have been widely used in many applications such as mobile, display, and lighting. However, the competition among LED companies is getting severe due to their excessive supplies of the LED products, and a need for cost-reduction has been increased at the same time. In order to reduce the cost of the LED packages and to minimize the thermal budget, the chip-on-board (COB) packages and the chip-scale package (CSP) LEDs, based on flip-chip die bonding process, have been developed. Although the flip-chip die bonding processes for the COB and CSP LED are useful to lower the thermal resistance, the precise process control for minimizing voids in solder joint between die and package is also needed for reliability. In addition, although the in-line total screening for bonding failure is very in significance for assuring reliability, there is no fast and cost-effective method in the GaN LED industry until now. A measurement method for integrated circuit and LED package structural integrity has been proposed [1]. This method is based on the thermal dynamics, in which the successive junction temperatures of the package have been monitored and analyzed to characterize the bonding quality. In order to realize the in-line total screening system for bonding failure, measurement time as well as measurement accuracy should be very important for throughput. Here, we propose a very simple method to discriminate the bonding failure in LED solder joint. The time-constant as a result of thermal dynamic analysis for the solder joint in the GaN LED packages gives the thermal specific time and we could determine two measurement points in time domain. These two measurement times are before and after thermal response time for the solder joint. In order to carry out the feasibility test for our idea, we developed three groups of the LED packages according to the solder joint quality. LEDs in A group, as a reference, had less than 10% void and had the strength of over 2 kgf in die shear test (DST). On the other hands, B and C groups showed about 50% and 90% voids and strength of over 2 kgf and less than 0.2 kgf in DST, respectively. We measured the operation voltages and the optical powers of A, B, C LED groups. Operation condition was 350 mA at 25 ℃. There was no difference between A and B groups in the operation voltage and the optical power. Although the operation voltages of C group are slightly higher than A group, the optical powers show no difference. Here, we could conclude that the classical methods such as the operation voltage and optical power of the LED packages are not appropriate for bonding quality estimation. Because of we knew that the thermal specific time for GaN LED chip and flip-chip solder joint of our LED packages is between 0.1 ms and 0.1 s, we could determine the two measurement points of 0.1 ms and 0.1s. We used the operation voltage of the LED chip as a sensor of junction temperature and the sensitivity was easily measured by changing environmental temperature. By normalizing the measured voltages to 0.1 ms, we could investigate the voltage difference of the LED packages in A, B, C LED groups. Compared to the value of A group, LED packages of B and C groups showed the higher voltage differences clearly. It means that the voids in thermal interface affected the temperature change in time domain and we could monitor it by measuring operation voltage in a specific time domain. Because our proposing method is very simple and very fast, it can be used in-line inspection step in LED package mass-production. [1] A. Hans et. al., “Transient thermal analysis as measurement method for IC package structural integrity,” vol. 24, no.6, 068105, Chin. Phys. B. 2015.

Resume : Oxide/metal/oxide (OMO) multilayers can be used as transparent conductive electrodes (TCEs) because both electrical conductivity and optical transmittance can be effectively improved using insertion metal and antireflective coating effect, respectively. However, in this multilayer scheme, the people only used narrow-bandgap oxide materials such as ITO (3.59 eV) and ZnO (3.37 eV) to reduce the sheet resistance for most cases, so both ITO/Ag/ITO and ZnO/Ag/ZnO multilayers exhibited low transmittance in the UV region. For example, C. H. Hong et al. reported an ITO/Ag/ITO TCE for UV LED, but its transmittance was as low as ~ 80% at 375nm. On the other hand, some groups reported high UV transmittance (90 % @ 385 nm) using wide-bandgap materials such as Zn0.59Mg0.21Be0.2 (4.3-5.3 eV), but this material seems not very suitable for device application due to its high series resistance. Recently, we reported a universal method to produce highly transparent and conductive electrodes using wide-bandgap materials such as AlN. In this study, we applied this method to the OMO scheme in the form of ITO/Ag/AlN/Al2O3 multilayers to enhance the UV transmittance as well as electrical conductivity by creating current path in the AlN/Al2O3 layers. As a result, our TCE scheme exhibited much higher transmittance (91.5%) and lower sheet resistance (15Ω/□) than 110 nm-thick ITO. Furthermore, 385 nm LEDs with proposed TCEs exhibited lower forward voltages (3.35 V), and higher output powers (up to 46%) at 20 mA when compared to those of the reference LED with ITO.

Resume : Recently, we have proposed a new growth scheme of using a cavity engineered sapphire substrate (CES), in which well-defined air-cavity patterns were arrayed on a sapphire. The wall-plug efficiency of InGaN/GaN blue light-emitting diodes on CES was improved up to 9% compared to that on state-of-the-art, patterned-sapphire-substrate. However, during the growth of GaN on CES, undesired GaN crystal was deposited on the top area of the cavity patterns, generating the additional threading dislocations during the coalescence of GaN layer. This is because the cavity shell including the top area was fully crystallized into single crystalline a-phase, the phase of sapphire substrate, by solid-phase epitaxy. To suppress the growth of the parasitic GaN, we investigated partially crystallized CES (PCCES) in which the cavity shell is remained in g-phase, another phase of alumina, rather than a-phase. By carefully controlling the crystallization process, we successfully obtained g-phase cavity shell while the area between the cavities was crystallized into a-phase for the GaN growth. Transmission electron microscopy analysis revealed that the parasitic GaN growth on top of the cavity patterns was successfully suppressed and the GaN layer from the planar area was coalesced on top of the pattern without generating additional dislocations. As a result, threading dislocation density in the GaN layer grown on the PCCES was reduced up to ~ 30 % compared to that on CES with a-phase cavity shell.

Resume : Tunning lasing wavelength flexibly and repeatedly in a single rod is essential to the practical applications of micro/nanorod lasers. Here, a structure that decouples the gain medium and optical cavity is proposed and implemented, where the corresponding mechanism for the laser wavelength shift is explained. Based on the above structure, a kind of wavelength-continuously-variable lasers is demonstrated on a single GaN/InGaN core-shell microrod without modifying the geometry of resonant cavity or cutting the microrod shorter. Using this method, we can control laser wavelength flexibly and repeatedly in a single facilely synthesized microrod and realize widely wavelengths variable ratio (3.6nm/μm). This new approach demonstrates great application potential in numerous fields, ranging from optical telecommunication to environmental monitoring.

Resume : One of the most important parameters in epitaxial growth is the miscut angle of the substrate. It governs kinetic processes on the growth front and influences structural, optical and electrical properties of the structures. In our previous works we have demonstrated that a controllable modification of the miscut along the substrate surface offers a valuable variety of new ideas leading to interesting applications. Using this technique, in the present work, we succeeded to fabricate InGaN/GaN light emitting diodes (LED) with a variation of wavelength originating from lateral arrangement of differently miscut substrate (miscut angle changing in range from 0.1 to 2.5 deg.). In this way, an electrically driven, monolithic, multicolor LED arrays were demonstrated. In one of them emission range was from 510 to 481 nm, what means change from green to blue color on one wafer. It demonstrates, that our technique allows to obtain locally modified parameters and precisely determine wavelength of the emitted light, suitable for practical devices. The light emission dynamics was studied with use of time–resolved photoluminescence spectroscopy. The decays time depended on the miscut angle and were only weakly dependent on temperature. At room temperature, the times were between 4 and 8 ns, for high and small angles, respectively. So long lifetimes, independent of In content, were due to low defect concentration and shown high internal efficiency of light emission in our devices.

Resume : A blue InGaN/GaN LED structure with quantum wells (QW) was grown on c-plane GaN substrate and characterized with space-, and time- resolved photoluminescence (PL). Due to special patterning, we obtained on the sample surface several regions with different vicinal angles, ranging from 0.1 to 2.3°. Since indium content in the QW (and thus the bandgap) depends on the vicinal angle, the received PL wavelength changed of 41 nm. That technology allowed investigation of several adjacent regions with different bandgaps in one sample. At low temperatures, a clear QW-related exciton peak appeared in the spectrum. Its wavelength changed according to In-content on differently oriented terraces. A significant halo (several tens of micrometers in size) was observed around the laser excitation spot (the spot size was 3 µm). The halo wavelength was the same as the parent exciton. The halo diameter was larger in regions with higher indium content, and, its size decreased under bias or when the sample was heated up. Additionally, a clear contrast in the halo’s intensity was visible on the borders between the adjacent regions of different bandgaps. These observations suggest that, the halo is an effect of lateral exciton diffusion. Low temperature stabilizes excitons and electric field induces their electric polarization, so they strongly repulse each-other by dipole-dipole interaction. Detailed results concerning dynamics and effects of external voltage will be presented.

Resume : We present a study of the internal electric fields in GaN/AlxGa1-xN (3 nm / 4 nm) multi-quantum-wells (MQWs) with x = 1, 0.5, and 0.25. Optical properties of these structures are investigated by ambient and high-pressure photoluminescence (PL) measurements. The structural characterisation is performed by XRD and TEM techniques. The optical properties are strongly affected by polarization-induced electric fields giving rise to a quantum confined Stark effect, which depends strongly on the composition of the quantum barrier. The optical emission energy redshifts by >100 meV when the Al content in the barrier increases from 25% up to 100%. Furthermore, the pressure coefficients of the PL energy are significantly reduced in comparison with bulk GaN, and they strongly depend on the composition of the barrier. The transition energies and their pressure dependence are modeled for tetragonally strained structures with the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs are also simulated using density functional theory calculations. A good agreement between these two approaches verified by experimental results indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy. The research was funded by Polish National Science Center by grant: DEC-2015/19/B/ST5/02136.

Resume : Compared to other group-III-nitride wide bandgap ternary alloys, the BAlN alloy has not been studied extensively. A detailed understanding of BAlN-Al(Ga)N heteroepitaxy is motivated by achieving solid-state ultraviolet (UV) light sources, for example, the distributed Bragg reflectors. In this study, we report on the growth, structrual and compositional analysis of multiple-stack BAlN/AlN and BAlN/AlGaN structures grown by metalorganic chemical vapor deposition (MOCVD). Scanning transmission electron microscope (STEM), geometrical phase analysis (GPA) and electron energy-loss spectroscopy (EELS) were employed to examine the strain status, the structural and compositional properties in the stacking layers. Tensile and compressive strains were also examined in the BAlN and AlGaN layers, respectively. Columnar features within BAlN layers were revealed, which are ascribed to the tensile strain, leading to a rough BAlN/Al(Ga)N interface and strong compositional inhomogeneity. V-shaped defects were formed at the BAlN/Al(Ga)N interface, based on which two distinct formation mechanisms were proposed and analyzed. We discovered that there was a significant degradation in the crystallinity as the films grew thicker. As a result, the structural integrity was disrupted, showing a tendency to form polycrystalline BAlN films. The optical reflectivity measurements have been done for both structures and will be discussed further.

Resume : Micro-light-emitting diodes (µLEDs) have received much attention for their applications as light sources of transparent display, headlamp for automobiles, head-up display for millitary and aerospace areas. However, the overall efficiency of the µLEDs is still low becasue of a trade-off between the fill factor (emission area over pixel size) and the current denstiy. Typically, the µLED has a small emission area in each pixel because it is shield by p-ype electrode for current injection. We can increase the emission area by reducing the p-type electrode area, but it decrease the current injection density of the µLED. So far, many efforts have been exerted to improve the brightness of the µLED by changing the size and shape of the pixel. However, there have been few reports on the fundamental solution to this trade-off issue.. In this study, we introduced the so-called ‘glass electrodes’ to solve this problem in µLEDs. AlN rod-shaped glass electrodes with local current path and high transmittance were applied as top electrodes of µLEDs, to improve the fill factor by enhancing the light emission of the µLED. The current path in AlN thin film is produced by electrical breakdown process. Compared to µLEDs with ITO electrodes, our µLEDs with AlN rod-shaped electrodes exhibited higher light output power density (105 % @ 300 mA) and brighter light emission per area (110 % @ 10 mA and 108 % @ 300 mA). More details including the ohmic transport mechanism for current injection and thermal stability will be presented at the conference.

Resume : Semiconductor microcavity (MC) in the strong light-mater coupling regime has generated significant impact over the past few decades. When the excitons interact with cavity photons, half-matter/half-light quasi-particles called cavity polaritons are produced with an anti-crossing behavior. The polaritons result from the coupling of cavity photons and excitons, and the dispersion curves with the lower polariton branch (LPB) and upper polariton branch (UPB) were first measured by the angle resolved photo-luminescence. Exciton polaritons have light effective mass compared to bare exciton (typically 10-4 times) and controllable energy momentum dispersion. These unique properties are good to the study of fundamental physics phenomena, such as cavity quantum electrodynamics and Bose Einstein condensates. Furthermore, exciton polaritons are promising candidates for low threshold devices including polariton light emitting diode (LED) and polariton laser diode (LD). At present, most of materials system used to study the exciton polariton is GaN bulk material due to their simple structure and better optical properties. But, it is necessary to study the exciton polariton in quantum well (QW) from the viewpoint of applications. Shorter cavity length and larger oscillator strength are two very important factors influencing the coupling between exciton and photon. Hybrid nitride microcavity has been used in the research of exciton polaritons, where the bottom distributed Bragg reflector (DBR) has been fabricated form nitride semiconductors during epitaxial growth. However, there are many difficulties relating with nitride DBR. First, due to the lattice mismatch the reflectivity cannot be comparable to the dielectric DBR. Second, many layers of nitride must be used to achieve reflectivity requirements. In this work, we used the bonding and laser lift off technologies to realized the transfer of substrate. Then, inductive coupling plasma (ICP) and chemical mechanical polish (CMP) were used to remove the high defect density nucleation layer and reduce the cavity length. All of these technologies process parameter were optimized after numerous experiments and theoretical simulation. Through these processes, we realized the strong coupling between the exciton and cavity photon in InGaN QW MC. We observed the anti-crossing behavior with the Rabi splitting value of 45meV by angle resolved photoluminescence with the detuning value of 4.7meV at room temperature. The splitting value is larger compared to similar QWs structure. The appearance of anti-crossing behavior demonstrates the strong coupling between excitons and cavity photons. The relatively large Rabi splitting indicates the high coupling efficiency.

Resume : GaN-based light-emitting diodes (LEDs) have rapidly developed owing to their potential applications to the next generation solid-state lightings. However, further improvements of the output power and the external quantum efficiency (EQE) are required for advanced technologies. In general, the output power of a LED depends on the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). The low IQE is due to the low crystal quality of the epilayer and strong polarization-induced electric fields in highly strained InGaN/GaN multiple quantum wells. Another major issue for the low EQE is a low LEE, which is mainly due to light loss by the total internal reflection and Fresnel reflection that occur at the LED surface. In this study, we demonstrate the blue LEDs with SiO2/SiNx distributed Bragg reflector (DBR) mask embedded in GaN epilayer. The process is aiming for purpose of applying the DBR mask on the sapphire for light extraction enhancement of the LED. Five pairs of circular shaped SiO2/SiNx DBR mask is successfully realized on the sapphire by deposition of multiple dielectric layers and wet etching processes. The output power of LEDs with DBR mask was increased by 33% at an injection current of 20 mA compared to that of conventional LEDs without DBR mask. The enhancement is attributed to improved IQE and LEE of an LED due to the use of a highly reflective DBR mask that reduces the defect in the epilayer and increases the reflection of light from substrate.

Resume : Deep ultraviolet light emitting diodes (DUV-LEDs) are promising light sources for great interest of applications like such as air and water purification, air purification, food processing, surface disinfection, UV curing, and medical phototherapy. However, the performances of these AlGaN-based devices are still suffering from a high defect density in the material of Al content and poor carrier injection efficiency [1, 2]. There are several methods to increase the injection efficiency of DUV-LEDs is the introduction of a p-type AlGaN electron blocking layer (EBL) on the top of the Multi-quantum wells (MQWs), such as the multi-quantum-barrier EBL (MQB-EBL) [3], the Mg-doped AlN/AlGaN superlattice [4], and an AlxGa1-xN/GaN MQB-EBL [5]. The design of the EBL is critical to prevent electron leakage from the MQWs (i.e., active region) into the p-doped layers of the DUV-LEDs and to improve efficient of the hole injection into the active region [6, 7].In this study, the investigation of different EBL structures in AlGaN-based 280 nm DUV-LEDs have been studied through a numerical simulation and the experimental. The AlN template and DUV-LEDs structures were grown on c-plane sapphire substrate at elevated temperature by Taiyo Nippon Sanso SR4000 metalorganic vapor phase epitaxy (MOVPE). The DUV-LEDs were grown by MOVPE deposition on 2.0-μm-thick AlN on c-plane sapphire substrate. The DUV-LEDs epitaxial structure consisted of an un-doped 600nm Al0.65Ga0.35N, a Si-doped 1500nm Al0.55Ga0.45N, 6 periods of Al0.42Ga0.58N/Al0.52Ga0.48N MQWs, Mg-doped 32nm p-EBL with different structures for LED-I to LED-V samples, a Mg-doped 100nm p-Al0.1Ga0.9N and a Mg-doped GaN contact layer. The DUV-LEDs devices were made by flip-chip processes. The spectral light output power of the DUV-LEDs package obtained by using integrating sphere measurement and measured from the backside of LED chips at the current injection of 35 A/cm2. The simulation and experimental results show that the adoption of EBLs is critical to improve the device performance. In comparison with a conventional structure using EBL with constant AlxGa1-xN composition (x = 0.7) and the introduction structure using EBL combine with constant AlxGa1-xN composition (x = 0.7) and an Al-content grading AlxGa1-xN (x from 0.7 to 0.2) possesses numerous advantages such as lower hole trapping, lower hole injection barrier, lower electron leakage from active region to p-layer, and higher internal quantum efficiency, which can improve efficiency of hole injection into the Multi-quantum wells, therefore, the output power of UV-LEDs can be enhanced 6 times ~ 10.0 mW. [1] M. Shatalov, W. Sun, R. Jain, A. Lunev, X. Hu, A. Dobrinsky,Y. Bilenko, J. Yang, G. A. Garrett, L. E. Rodak, M. Wraback,M. Shur, and R. Gaska, Semicond. Sci. Technol. 29, 084007(2014). [2] M. S. Shur, IEEE Trans. Electron Devices 57, 12 (2010). [3] H. Hirayama, Y. Tusuke, T. Maeda, and N. Kamata, Appl. Phys. Express3, 031002 (2010). [4] T. M. Al tahtamouni, J. Y. Lin, and H. X. Jiang, AIP Advances 4, 047122 (2014). [5] T.-H. Wang, J.-L.Xu, X.-D.Wang, Physica E 47, 51 (2013). [6] T. Kolbe, T. Sembdner, A. Knauer, V. Küller, H. Rodriguez, S. Einfeldt, P. Vogt, M. Weyers, and M. Kneissl, Phys. Status Solidi C 7, 2196 (2010). [7] T. Kolbe, F. Mehnke, M. Guttmann, C. Kuhn, J. Rass, T. Wernicke, and M. Kneissl, Appl. Phys. Lett. 103, 031109 (2013).

Resume : Currently, there is a big interest to development of efficient InGaN-based and AlGaInP-based micro-LEDs for displays of mobile phones and, in a distant prospect, for Li-Fi communication. In contrast to large-size high-power devices, micro-LEDs operate typically at elevated current densities and temperatures. Therefore, such mechanisms as thermal efficiency droop, non-thermal droop originated from Auger recombination, and surface recombination become the major factors limiting performance of the micro-LEDs. This paper reports on the simulation study aimed at better understanding of general trends in evolution of the main LED characteristics and efficiency with reduction of the LED size. We have considered a blue LED structure with five-period 2.7 nm InGaN / 8 nm GaN MQW active region sandwiched between a thick n-GaN and 180 nm p-GaN contact layers. A 20 nm p-AlGaN was placed between the last MQW barrier and p-GaN contact layer to suppress the electron leakage at high current densities. The undoped InGaN quantum wells were assumed to be separated with n-type GaN barriers to improve uniformity of carrier injection in the wells. Round-shaped thin-film flip-chip LED dice with various outer diameters ranged from 16 μm to 300 μm have been studied by simulations. All the dice had a single column n-electrode formed to the n-GaN contact layer via cylindrical aperture etched through the p-layers and active region of the LED structure. The aperture diameter of 10 μm was not changed while scaling the die outer size. The bottom surface of the n-GaN contact layer was assumed to be textured after removing the substrate used for heterostructure growth, providing enhanced light extraction from the LED dice. Actually, such an assumed fabrication technology was similar to that used at Osram OS for UX:3 chip processing with the difference that single column n-electrodes were exploited in the LED dice examined in this study. Simulations of the LED operation was performed with the SimuLEDTM commercial package by STR, considering self-consistently the current spreading, heat transfer, and light extraction from the die. The heat sink from the LEDs was accounted for by the heat transfer coefficient of 1×10^5 W/K·m2 which was the same for all the dice. Surface recombination was considered to occur at the inner (next to the column n-electrode) and outer edges of the active region. The surface recombination velocity of 10^3 cm/s and ambipolar carrier diffusivity in the InGaN quantum wells of 2 cm2/s were used in the simulations. Ray tracing of the emitted photons has predicted the light extraction efficiency (LEE) to the top hemisphere of ~86% for the immersion medium with the refractive index of 1.5. Being practically independent of the die dimensions and operating current density, this obtained LEE corresponds well to that calculated for large-size UX:3 chip, demonstrating applicability of the above fabrication technology to micro-LEDs. Contribution of surface recombination to the total carrier losses is found to increase substantially towards smaller sizes of the LED dice. On the other hand, surface recombination becomes suppressed considerably at high densities of current flowing through the LEDs. The reason for such a behavior is the carrier life time shortening remarkably with the carrier concentration (current density) in the active region and reducing the carrier diffusion length, which lowers the carrier losses for surface recombination. However, LED efficiency at high current suffers from the droop originated from Auger recombination. Competition of two non-radiative channels, surface and Auger recombination, results in the peak LED efficiency shifted to higher current densities in small-size LEDs. This simulated trend is in qualitative agreement with available observations. One more factor affecting the small-size LED performance is current crowding. The current spreading is found to become more uniform towards smaller die sizes, except for the “dead” area formed by surface recombination at the edges of the active region. Localization of current at the electrode edges typical for large-size devices results in enhanced Auger recombination losses, lowering the high-current emission efficiency compared to small-size LEDs. As a result, the efficiency corresponding to a certain a priori chosen current density can be maximized by a proper choice of the LED die dimension. The size-dependent current crowding is predicted to affect substantially the current-voltage characteristics of LEDs via scaling the diode series resistance with the die dimensions and its dependence on temperature. In turn, this effect is found to influence the LED electric efficiency, which can be characterized by the efficiency deficit, i.e. the difference between the external (EQE) and wall-plug (WPE) efficiencies. Simulations predict the efficiency deficit to decrease remarkably with the die dimensions, so that micro-LEDs possess minimal electrical losses. In addition, the onset of substantial rise in the efficiency deficit becomes shifted to higher current densities in small-size devices. The onset is found to correlate with the transition from injection-controlled to series-resistance controlled part of the current-voltage characteristics. In summary, general trends in the behavior of current-voltage and light-current characteristics and both EQE and WPE have been analyzed by simulations for scaling the lateral dimensions of round-shaped LED dice from 300 μm down to 16 μm. The revealed trends allow clear interpretation in terms of size-dependent interplay of thermal effects, current crowding, and surface and Auger recombination, eventually limiting the LED efficiency. In particular, surface recombination is found to be the major factor making unfavorable the low-current operation of micro-LEDs. The interplay of the above mechanisms is predicted to result in non-monotonous, i.e. that providing a maximum, efficiency dependence on the die dimensions in a certain range of the operating current densities. At the current densities well exceeding 100 A/cm2, small-size (16-30 μm) LEDs provide both EQE and WPE greater than that typical for large-size LEDs. The results obtained in this study may serve as a guideline for optimization of the micro-LED chip design, as well as for finding their optimal operation condition, i.e. the most suitable range of operating current densities.

Resume : Yellow emitting YAG:Ce3+ phosphor has been widely used in conjunction with blue light emitting diodes (LEDs) to fabricate solid-state white LEDs. However, its low light conversion efficiency, low color rendering index, and weak emission in red are considered as chronic problem. Here, we fabricated a large-area (30x30 cm2) polymer composite film embedded with colloidal semiconductor quantum dots (QDs) and presented high-quality white light generation by placing these composites on a blue LED platform to substitute phosphors. Semiconductor QDs show high quantum yield and present wide bandgap tenability. However, QDs are vulnerable to oxygen and water. When embedded in polymers, QDs can be stabilized and utilized effectively in various applications. In this work, polymethyl methacrylate (PMMA) was used as the polymer matrix in which CdSe-based QDs were embedded. PMMA was chosen because it readily peeled off after coating on a glass (30 x 30 cm2). QDs (0.07g) in toluene (10mL) that were mixed with PMMA solvent (40mL), and the QD-PMMA (PMMA to QDs in toluene ratio of 1:8) composites were spin-coated. The film presented high visible transparency and uniformity in thickness and color. Yellow emitting QD films together with blue LEDs generated large-area uniform white light. Green and red emitting bilayer of QD-polymer film was also fabricated in order to avoid optical loss through Forster resonance energy transfer. Single and bilayer scheme were characterized and compared in terms of color uniformity, color rendering index and conversion efficiency.

Resume : The performance of InGaN/GaN multiple quantum well (MQW) based visible-light Schottky photodiodes (PD) were improved by optimizing the source flow of TEGa during InGaN QW growth. The samples with five pairs InGaN/GaN MQWs were grown on sapphire substrates by metal organic chemical vapor deposition. Among the fabricated Schottky-barrier PDs, it was found that the smaller the TEGa flow, the lower the reverse-bias leakage is. The photocurrent can also be enhanced by depositing the InGaN QWs using lower TEGa flow. High responsivity of 1.94 A/W was obtained at 470 nm and -3 V bias in the PDs grown with optimized TEGa flow. Analysis results show that the lower TEGa flow used for depositing InGaN may lead to superior crystalline quality with improved InGaN/GaN interface, and less structural defects related non-radiative recombination centers formed in the MQWs.

Resume : Nitride-based ultraviolet light-emitting diodes (UV-LEDs) has received considerable attention for various applications such as sterilization, biochemistry, water purification, and solid-state lighting. GaN-based UV-LEDs is generally grown on sapphire substrate. The difference in lattice constant and in thermal expansion coefficient between GaN and sapphire substrate causes a high density of threading dislocations (TDs) between 108 and 1010 cm-2. The TDs act as nonradiative centers and hence limit the internal quantum efficiency (IQE) of LEDs. For blue LEDs, the InGaN localized states can provide a safe passage for radiative recombination, resulting in a defect-insensitive emission probability. However, the multiple quantum well (MQW) active region of UV-LED usually comprises lower indium composition and thus fewer localized states for charge carriers to by-pass the dislocations, which makes UV-LEDs more sensitive to TD than blue LEDs. Therefore, it is critical to reduce TD density for realization of highly efficient UV-LEDs. Recently, it has been reported that high-quality GaN film can be obtained using a sputtered AlN nucleation layer on PSS. It is well known the size of PSS plays an important role in improving the efficiency of LEDs. However, effects of pattern size on crystalline quality, optical and electrical properties of UV-LED grown on PSS with sputtered AlN nucleation layer are not well understood yet. In this study, we use a combination of a cone-shaped PSS together with ex-situ sputtered AlN nucleation layer to reduce TD density. The influence of pattern size on the crystalline quality, optical and electrical properties of UV-LEDs was investigated in detail. We demonstrate nitride-based UV-LEDs emitting at 375 nm grown on patterned sapphire substrate (PSS) with ex-situ sputtered AlN nucleation layer. Effects of pattern size on crystalline quality of GaN film was investigated by in-situ reflectance monitoring, high-resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was observed from in-situ reflectance measurement that transition from three-dimensional (3D) island to two-dimensional (2D) coalescence can be prolonged with a larger PSS size, leading to reduced full-width at half-maximum (FWHM) of symmetric GaN (002) and asymmetric GaN (102) rocking curves. From cross-sectional TEM images, it was found that threading dislocation (TD) density of UV LEDs with sputtered AlN nucleation layer decreases with increasing PSS size and fill factor. The light output power of UV LED grown on large size of PSS with a fill factor of 0.71 is 131.8% higher than that of UV LED grown on small size of PSS with a fill factor of 0.4. The improvement is mainly attributed to reduced TD density in InGaN/AlInGaN active region.

Resume : Encapsulation materials free from deep ultraviolet (DUV) photolysis are desired to increase the light extraction efficiency of DUV light emitting diodes (LED). By comparing perfluoro-4-vinyloxy-1-butene polymer (BVE) with various terminal ends, a CF3 terminal was determined to be robust against DUV rays. [K. Yamada, et. al., Appl. Phys. Express 8, 012101 (2015); S. Nagai, et al., Jpn. J. Appl. Phys. 55, 082101 (2016)]. In this work, the five-membered ring structures in (1) perfluoro-2, 2-dimethyl-1, 3-dioxole co-polymerized with tetra-fluoroethylene (PDD-co-TFE), terminated with -CF3 or PDD, and (2) CF3-terminated BVE were compared. The rings of PDD and BVE have ‘-O-C-O-‘　and ‘-C-O-C-‘ in their main chains, respectively. An AlGaN-based DUV-LED (262 nm) encapsulated by PDD-co-TFE exhibited damage to the electrodes after operating at 350 mA for 832 h, and leakage between p- and n-electrodes occurred. The IR absorption spectrum and temperature-programmed-desorption mass spectrum (TPD-MS) showed that the PDD ring was decomposed by DUV-induced photolysis, and the adsorbed N2 and O2 detected by TPD-MS indicate the creation of molecular size voids related to PDD photolysis. Photodissociation was considered to start at the ‘-O-C-O-‘ structure in the ring. In contrast, the LED encapsulated with BVE with -CF3 ends exhibited no damage for more than 6000 h in BVE, nor any damage to the electrodes, showing the sufficient robustness against DUV for use as an encapsulation resin.

Resume : In alloy semiconductors, localization of excitons due to alloy disorder plays an important role in the near-band-edge optical properties. We have so far studied the effects of localization on the excitonic optical properties of AlGaN ternary alloy semiconductors. We have earlier reported that the binding energy of biexcitons was enhanced by biexciton localization due to alloy disorder. In this work, we evaluated the Stokes shifts of excitons and biexcitons in an Al_{0.61}Ga_{0.39}N epitaxial layer as a function of temperature to understand in detail the mechanism of exciton and biexciton localization. We performed photoluminescence (PL) and PL excitation (PLE) spectroscopy at temperatures ranging from 5 to 500 K. Exciton and biexciton luminescence spectra were observed up to 500 K, which enabled us to obtain the PLE spectra of excitons and biexcitons. We observed exciton resonance in both the exciton and biexciton PLE spectra up to 500 K. Furthermore, the biexciton two-photon resonance was clearly observed in the biexciton PLE spectra up to 500 K. Based on these observations, we evaluated the Stokes shifts of excitons and biexcitons. The Stokes shifts of both excitons and biexcitons decreased with increasing temperature up to 300 K and became independent of temperature above 300 K. These observations could not be explained only by the localization due to alloy disorder, which suggests at another mechanism of exciton and biexciton localization in AlGaN ternary alloys.

Resume : Precise evaluation of the internal quantum efficiency (IQE) of nitride-based light-emitting devices is essential for the improvement of their luminescence efficiency. We have so far studied the IQE of InGaN-based light-emitting diodes (LEDs) as functions of the excitation power density and temperature. We revealed that the IQE increased and then decreased with increasing excitation power density, and explained that the initial increase was caused by the saturation of nonradiative recombination centers (NRCs) with increasing carrier density. In this work, we evaluated the IQE of InGaN-based near-UV LEDs with different threading dislocation densities (TDDs) as functions of the excitation power density and temperature. We analyzed the initial increase in the IQE to evaluate the effects of the saturation of NRCs on the IQE. Analysis was performed using a rate equation model based on the radiative and nonradiative recombination of excitons, in which the saturation of NRCs was taken into consideration. The initial increase in the IQE could be described well by the model. The analysis showed a clear correlation between the increase rate of the IQE and the TDD: the increase rate decreased with decreasing TDD. This correlation unambiguously indicated that the threading dislocations acted as saturable NRCs. The analysis of IQE indicated whether the IQE had reached 100% at a low temperature.

Resume : Investigation on the surface plasmon (SP) coupling effect on nitride emitting device has drawn great attention. It has been argued that through SP-coupling effect, the spontaneous recombination rate in active region is dramatically enhanced and leads to significant improvement of internal quantum efficiency (IQE) when nano-size plasmon metals were placed within nanometer range distance to active region of LEDs [1-4]. However it is difficult to separate then identify plasmon coupling effect from other effect such as light extraction, current spreading, epi quality, etc. The author provides a unique method to investigate the QW-SP coupling behavior as a function of distance between plasmon metal and QWs in LED epi. A wide shallow microgroove with extreme small curvature was etched on the p-GaN layer of the LED epi. KOH solvent was used to modify the surface roughness down to 7nm. 20nm of Ag layer was evaporated to cover the trench, and generating pseudo-nano Ag fluctuation. A spatial resolved micro-photoluminescence was measured by scanning 2.5µm-diameter laser spot across etching area with step size of 1µm. Power density dependent PL measurement was used to compare the droop effect at different depth of the groove. Temperature variable PL was measured to study the relationship between IQW and plasmon coupling effect. Figure 1 shows schematic diagram, top-view SEM image, and corresponding depth profile of the etched microgroove. The dimension of the fabricated arc-shaped microgroove is 100µm in width and 200nm, corresponding to an extreme aspect-ratio of 1:500. Such aspect-ratio was intended that the different of PL induced by surface curvature was minimized. Figure 2(a) shows the integrated PL intensity of Ag deposited groove as a function of horizontal scanning location. Significant PL enhancement was observed when the distance between Ag and the QWs (defined as D) was reduced to less than 35nm, and saturation was reached below 10nm with maximum enhancement measured at the groove bottom. Figure 2(b) shows the plot of integrated PL intensity as a function of D, indicating a trend almost following exp(-Im(ksp)x. Large enhancement of PL intensity was contributed to surface plasmon coupling effect. Figure 2(c) shows the normalized PL intensity as a function of scanning position at different low temperature (300k~10k). It is found that by lowering temperature, the PL difference between with and without Ag (thus the PL enhancement) seems to be less dependent on D. Figure 2(d) shows the IQE, calculated by dividing the PL intensity at RT to that at 10K, as a function of scanning position. The similarity between the above RT PL enhancement curve and the IQE curve indicates that it is possible that the same mechanism induces the improvement of efficiency either by lowering temperature or plasmon coupling effect. Optical pumped power-dependent IQE measurement was done at different depth of the Ag deposited groove: the bottom, middle and outside respectively. They show very different behavior among these locations, providing another tuning factor used to build a relationship between efficiency droop and surface plasmon coupling effect.

Resume : The conventional white LED is packaged by coating GaN-based LEDs with phosphor powder embedded in organic resins. It suffers from blue light reflected back to the chip at the GaN/air interface due to total internal reflection, as well as yellow light traveling backwards due to photon scattering happens at the boundary of the phosphor particle and the silicone matrix. Recently, Y3Al5O12:Ce3+(YAG:Ce) single crystal[1] or polycrystalline ceramic phosphor platelets (CPPs) [2]as an alternative photon converter has been applied due to its properties of reduced photon scattering, high transparency, good mechanical performance and excellent chemical stabilities. However it still suffers from bad heat dissipation, refractive index contrast: 2.5(GaN)-1.0(air)-1.8(YAG crystal), due to the air gap between the platelets and the LED chip. To improve heat dissipation and light extraction efficiency, an unique compact material structure of white LED directly grown on YAG:Ce single crystal is proposed in this study. The commercially available (002)-oriented YAG:Ce single crystal substrates with thickness of 400μm, 14mm×14mm of size, and Ce3+ ions doping concentration of 0.3 at% were used as substrate during the LED epi growth and then as the photon converter material afterwards. Firstly, AlN buffer layers were deposited on YAG:Ce substrate at 300℃-600℃ by RF reactive magnetron sputtering method with power of 300W- 600W in Ar-50 vol%N2 mixture at total pressure of 0.2Pa. Then a 2μm thick un-doped GaN film followed by a 2.5μm thick Si-doped GaN were grown using Metal Organic Vapor Phase Epitaxy (MOVPE). After that 10 pairs of GaInN(7nm)/GaN(3nm) 450nm-emission multiple quantum wells were grown, above which regular AlGaN electron blocking layer (EBL) and a Mg-doped p-GaN layer with a thickness of 200nm were finally grown. Figure 1(a) shows the XRD pattern and AFM image of sputtered AlN buffer layer on YAG:Ce substrate before LED epi growth, indicating (002) orientation and surface quality of Ra=0.257nm. Figure 1(b) shows the SEM image of YAG-LED after GaN growth, presenting a relative smooth p-GaN surface morphology. The cross section SEM image shows abrupt epi layer boundary. Electroluminescence characteristics were measured from 2 mA to 20 mA using quick-test configuration (shown in Figure 2(a)). A relative stable blue to yellow ratio was achieved above 50mA. Figure 2(b) shows the I-V characteristics of the YAG-LED. It shows a turn-on voltage around 3V. The voltage at higher current is quite large, which could be lower after further material optimization and chip fabrication. Compared to conventional coated and remote phosphor, the proposed configuration provided better light conversion as well as thermal dissipation, eventually higher efficiency. REFERENCES: [1] S. Wang, Y. Li, L. Feng, L. Zhang, Y. Zhang, X. Su, W. Ding, and F. Yun, Optics Express, 24, 15, 17522 (2016) [2] N. Wei, T. Lu, F. Li, W. Zhang, B. Ma, Z. Lu, and J. Qi, Appl. Phys. Lett.101, 061902 (2012).

Resume : White light emitting diodes (LEDs) will ultimately replace incandescent bulbs and fluorescent tubes for a host of outdoor and indoor lighting applications due to the advantages of low power consumption and long lifetime. Current white LEDs are fabricated mainly based on the “blue LED + yellow phosphor” approach. However, phosphor-converted white LEDs have distinct disadvantages, such as non-radiative internal losses, optical loss due to light backscattering, heating-related effects and the long-term reliability of phosphors. Monolithic phosphor-free white LEDs which are exclusively grown on c-plane substrates have been reported in recent years. Basically, such white LEDs require at least a mixture of either blue/green/red (RGB) emissions or blue/yellow emissions, where in both cases InGaN based long wavelength emission, either green or yellow, is required. However, it is well-known that the optical efficiency of InGaN on c-plane substrates decreases with increasing indium content as a result of polarisation induced quantum confined Stark effect (QCSE). This becomes more severe when the emission moves toward longer wavelength such as green or yellow spectral region. Furthermore, another issue is due to the great challenge in incorporating indium into GaN on c-plane substrates. These two fundamental issues lead to the well-known “green/yellow” gap. Consequently, it is extremely difficult to achieve white LEDs with high efficiency and high color rendering index. This obstacle is expected to be overcome by growing InGaN on semi-polar (11-22) GaN,[1] where the polarization effect can be significantly reduced, leading to an enhancement in quantum efficiency. Furthermore, the (11-22) semi-polar surface significantly favours indium incorporation into GaN, which is crucial for growth of longer wavelength InGaN/GaN quantum wells (QWs) with high indium content required for white LEDs.[1] Furthermore, an increasing demand on the application of Li-Fi requires a white LED with an ultra-fast response. Current LEDs on c-plane substrates suffer from a long carrier recombination lifetime as a result of QCSE, typically on a scale of a few to 10 nanoseconds for blue emission and ~100 nanosecond for green emission. Phosphors generally exhibit even longer response time, typically on a microsecond scale. In contrast, semi-polar InGaN QWs exhibit a much shorter carrier recombination lifetime, typically 100s ps for blue. Therefore, it is also essential to develop semi-polar or non-polar based phosphor-free LEDs for Li-Fi applications.[1] The great challenge is due to the crystal quality of semi-polar GaN on sapphire. Recently, our group has demonstrated semi-polar (11-22) InGaN LEDs with a wide spectral region of up to amber on our overgrown semi-polar GaN templates with significantly improved crystal quality,[2] demonstrating a number of major advantages of semi-polar LED compared with currently c-plane LEDs in terms of reducing efficiency droop and resolving the "green and yellow" gap issues. In this work, two different kinds of device structures, labelled as sample A and sample B, have been designed based on a simulation (called APSY, provided by Cross-light Ltd.), and then have been grown on our high quality semi-polar overgrown GaN for white LEDs. In both cases, there are two different kinds of InGaN/GaN QWs, namely, single quantum well (SQW) for blue emission and one pair of InGaN multiple quantum wells (MQWs) for yellow emission. However, for sample A, the MQWs were grown firstly, followed by the SQW, while for sample B the SQW was grown firstly and then the MQWs. In both cases, the growth conditions remained identical. Photoluminescence measurements show similar spectra in both cases, namely, two strong emission peaks, one in the blue region and another in the yellow region. However, electroluminescence (EL) measurements shows different behaviours. Sample A shows two EL peaks at 445nm and 540 nm, respectively, where a photo image captured shows a white emission. However, only one EL peak at 548 nm appears for sample B even when the injection current increases up to 300 mA. The EL measurements have agreed well with our simulation, demonstrating that both the hole transport and the electron overflow play important roles on emission mechanism. Comparison between EL and PL along with the simulation results provide a useful information for further improving our structure design. Further work is in progress to enhance chromaticity coordinates and correlated colour temperature. [1] T. Wang, Topical review, Semicon. Sci. Technol. 31, 093003 (2016) [2] J. Bai, B. Xu, F. G. Guzman, K. Xing, Y. Gong, Y. Hou, and T. Wang, Appl. Phys. Lett. 107, 261103 (2015)

Resume : We report on the study of the key features of the potential profile in InGaN/GaN multiple quantum wells (MQWs) emitting in a wide spectral range using spatially-resolved photoluminescence (PL) spectroscopy. The structures emitting from 450 to 600 nm were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate. The spatial PL intensity distributions were similar in all the samples under study. Meanwhile, the spatial variation of the PL band peak position as well as band width increased as the emission wavelength redshifted. The large band width values of the single-spot PL spectra indicated the presence of the double-scaled potential profile. These observations show that the red shift of the PL band is caused mainly by introduction of a broad spectrum of localized states. In blue-emitting MQWs, the emission properties are governed by exciton localization at small-scale potential fluctuations, while large-scale fluctuations facilitate accumulation of excitons in the regions of the order of 250 nm in diameter. In amber-emitting MQWs, the spectral range of the localized states becomes governed by large-scale potential fluctuations, while the increased density of NRCs in the areas with higher In content results in lower emission efficiency.

Resume : Improving the quality of InGaN-based emitters requires persistent development of sophisticated growth methods. Pulsed delivery of metal precursors in epitaxial deposition is one of currently most promising approaches to grow high-quality multiple quantum wells (MQWs). We report on the study of optical and structural changes in cyan InGaN/GaN MQWs grown using pulsed metalorganic chemical vapor deposition (MOCVD). During the growth of InGaN MQWs, ammonia flux was constant, and metalorganic (MO) precursors were delivered in 20 s pulses with the pause between the pulses varied in the range from 0 to 15 s. The surface morphology measurements using atomic force microscopy (AFM) revealed that all the samples have very smooth surfaces. However, the increasing pause between MO pulses resulted in a higher density of V-pits. Photoluminescence (PL) measurements showed that the PL band slightly blueshifted as the pause between MO pulses was increased. The measurements of an S-shaped temperature dependence of the PL peak position enabled us to characterize the band potential fluctuations. The localizing potential transformed as the pause between MO pulses was increased exposing different origin of the localized states. The spatially-resolved PL measurements revealed the increasing PL intensity inhomogeneity correlated with the transformation of localizing potential. The optimization of timing results in increasing emission efficiency of cyan InGaN/GaN MQWs.

Resume : Blue emission is the most significant part of light spectrum needed for white light generation. The difficulties of achieving blue light are originating from the request of more energetic photons coming from special materials with wide/direct bandgap and high crystal quality. Therefore, intense research activity is devoted to the generation of blue light emission by searching for new phosphors. Nitride phosphors exhibit an excellent luminescence properties and thermal stabilities to be used in light emitting diodes and flat panel displays. Among them, aluminum nitride (AlN) shows a large bandgap of 6 eV and very high thermal conductivity that nominate it as a good candidate for high temperature working devices. Although, many rare earth ions-doped AlN have been reported, there are very few works on Ce-doped AlN at the core of this study. In addition, the UV-blue emission of Ce is an interesting feature from application point of view for white light generation. Seeking for realization the blue emission with AlN, cerium-doped aluminum nitride (Ce-AlN) thin films were prepared at room temperature using radio frequency (RF) reactive sputtering. X-ray diffraction and high resolution transmission electron microscopy (HRTEM) revealed a well crystalline textured microstructure with single < 002> out-of-plane orientation. Strong blue emission from the prepared samples was detected when excited by 325 nm laser. Electron energy loss spectroscopy (EELS) has been used to reveal the dominant oxidation state of Ce atoms, which undergoes a change from of Ce(IV) to Ce(III) ions after annealing. The chemical composition was analyzed by simulation of Rutherford backscattering spectrometry (RBS) and compared to HRTEM images. A clear correlation between microstructure, composition and sample photoluminescence (PL) was established. It was found that surface oxidation during post-deposition annealing plays an important role in the PL response of the samples. Moreover in some samples, a low amount of oxygen flow rate has been intentionally introduced during the preparation in order to test the influence of oxygen in the PL behavior. The role of oxygen in the excitation mechanism was explained from the photoluminescence excitation measurements. The strong blue emission in this new (oxy)-nitride material holds great potentials for solid state lighting applications due to its thermal and chemical stability as well as the luminescence efficiency. Moreover, the comprehensive approach conducted within this study could serve as a guideline for better understanding and the design of the luminescence behavior in rare earth-doped (oxy)-nitride thin films.

Resume : Micro light emitting diodes are the attractive devices for many applications including high-speed telecommunication, interconnecting and possibly also single-photon emission. While the strategies based on bottom-up approach including self organized nano-columns are frequently applied and considered, they are challenging and expensive especially processing-wise. In this work we applied the opposite - top-down strategy, in order to fully profit from mature and well developed planar MOVPE technology. By means of application of combination of well established hard masking technique with photo-lithography and nickel nano-dots formation we were able to obtain mask which allowed to fabricate columnar structures with diameter of around 140nm and height of around 1µm. The advantage of these technique is that it does not require electron beam lithography. The process itself can be divided into four stages. First stage require deposition of ~300nm thick SiO2 and 8-15nm thick Nickel films on surface of a sample. Second stage is photo-lithography of dots which are be used to copy them by dry etching of nickel film. Third stage is annealing the sample in 850oC for one minute and formation of sub-micron crystalline nickel dots. This stage conclude the fabrication of hard mask. Last stage is dry etching of underlying SiO2 film which is used as mask for dry etching our epi-structure. The size of the final column can be controlled mainly by nickel film thickness and size of pre-annealing photo-lithography. It can be fined tuned by optimization of dry etching process and temperature and time of annealing. We believe that application this approach for production of µLEDs will be more viable for future mass production, at lower cost since the process needs very typical processing steps which are already available in most labs nowadays. Also avoiding slow electron beam lithography whole process is much faster especially for big samples with many µLEDs structures. This work was supported by Polish National Science Center within the research project OPUS no. UMO-2015/17/B/ST7/04091

Resume : GaN based light emitting diodes (LEDs) can cover the entire range of visible light spectral regime, thereby showing great potential in solid lighting systems and color displays. Conventional vertical epitaxial techniques that employ standard sapphire substrates to achieve Ⅲ-Ⅴ nitrides planar films usually exhibits high threading dislocation density (108-1010 cm-2) and V defects. Additionally, the presence of large spontaneous polarization and piezoelectric fields induced by the residual strain in the MQWs limits internal quantum efficiency due to the quantum confined stark effect (QCSE).In order to overcome this problem, alternative approach has been employed by achieving semi-polar or nonpolar GaN facets. Selective area epitaxy (SAE) is a promising technique in creating three dimensional (3D) GaN structures with semi-polar GaN facets, such as micro-stripes and micro pyramids. However, SAG technique typically requires complicated processes involving lithographic patterning and subsequent dry etching to fabricate the patterned substrate.Simple techniques thus are highly desired to meet the growing requirement of LED industry. In this study, we present a simple, flexible and low cost method for 3D patterned sapphire substrate fabrication by means of a laser drilling process. High quality single pyramid array grew on this patterned substrate by selective area epitaxy. The morphology of the growth LED was analyzed by scanning electron microscopy (SEM). The material properties were examined by x-ray diffraction (XRD), Raman spectroscopy, room temperature and low temperature PL. Several kinds of holes dimension have been tried. The dimension of hole influences the epitaxy process and finally the material quality. Perfect separated pyramid array with high material quality can be achieved by adjusting the laser drilling parameters.Material quality can be improved by increasing the depth of holes, since the threading dislocations (TDs) can be effectively annihilated by crystal nucleus coalescence in deep hole. However, it can’t be grown into pyramid structure if the aspect ratio of depth and hole length is higher than 1/2. Additionally, the aspect ratio of pitch and diameter of hole should be larger than 3 and smaller than 9 in order to get separate and uniform pyramid structure. The internal quantum efficiency was estimated to be improved by a factor of 3 for pyramid structure compared with planar LEDs. Totally separated pyramid arrays is benefit for the flexible LED fabrication. Laser lift-off (LLO) process has been employed to transfer the pyramid array to flexible substrate. Separate pyramid arrays achieved by laser drilling process can prevent the fracture appearance during LLO process and the bending process. The peak wavelength of the device test by EL is 430nm which is consistent with the PL results.

Resume : White light-emitting diodes (WLEDs) technology is reaching the maturity phase necessary to replace the traditional technology of incandescent lamps because of their many advantages, including their long lifetime, easy fabrication, and because they are environmentally friendly and low cost. Especially, cerium doped YAG (yttrium aluminium garnet) phosphor (YAG:Ce3+) emitting yellow light, combining with a blue LED chip, has been a commonly used way to achieve WLEDs in commercial. However, the two main drawback of YAG:Ce3+ based WLEDs are its low conversion efficiency and bad thermal stability. To realize a white light emitting, a larger amount of phosphor is needed to increase the yellow light, yet increases the device cost and decreases the efficiency of WLEDs due to stronger re-absorption of light and more substantial back scattering of the emission. Additionally, the YAG:Ce3+phosphor is usually mixed into the encapsulation material like silicone orepoxy-based, which is not good conductivity for heat, thereby the accumulation of heat will decrease the lifetime of phosphor. So simple and efficiency techniques are highly required to improve the performance of YAG:Ce3+based WLEDs. In this study, we utilize the fluorescence enhancement of local surface plasmon resonance (LSPR) of noble metal, presenting some simple ways to coat the Au nanoparticles (NPs) onto the surface of YAG:Ce3+ particles. The Au NPs were synthesized by hydrothermal method and the geometrical characteristics like shape and size were analyzed by scanning electron microscopy (SEM). The Au NPs optical property was examined by Uv-vis spectrophotometer. The YAG:Ce3+phosphor properties were examined by photoluminescence (PL), electroluminescence (EL) andtimeresolved photoluminescence (TRPL). Since the shape and size of Au NPs greatly determine its LSPR property, different synthesis conditionshave been tried to match the emitting spectra of YAG:Ce3+ phosphor. To coat the Au NPs onto the phosphor,different ways have been tried, and a relatively easy-control way is utilizing binding molecule to attract the Au NPs to the phosphor surface. In the comparative analysis, the YAG:Ce3+coated with Au NPs has shorter lifetime than pure YAG:Ce3+and higherPL intensity. However, too much larger or smaller amount of Au NPs coated on the phosphor cannot improve the phosphor property due to the absorption and quenching effect. Totally we synthesize suitable Au NPs and achieve an easy and manageable way, which employs a third part material as a binder, to coat the Au NPs onto the surface of YAG:Ce3+ phosphor. By changing the amount coated on the phosphor, a relative good fluorescence enhancement is measured, which is promising in WLEDs device with low cost, high power and well optical performance.

Resume : III-Nitrides semiconductor materials with bandgap changing from 0.7 eV to 6 eV has shown a major attention for the prospect development of optoelectronic devices [1]. The purpose of this work is to study, design and develop a PIN photodiode based on InxGa1-xN and GaN materials deposited by MOCVD [2]. Structural, microstructural and optical analyses are achieved using XRD, TEM, PL, AFM and SEM [3]. The design of the PIN structure is governed by the limitation of the active surface in order to limit the global capacitance. Static and dynamic characterizations have been realized to qualify the photodiode response [4]. For the fabricated photodiodes, photocurrent value has reached a maximum of 1 mA with laser power of 75 mW [5]. Investigation on size-dependent capacitance for photodiodes has been carried out for different size of photodiodes. Among the different noise components, the most important for the photodiode is the shot noise which is due to the velocity distribution of the carriers and varies according to the photodiode current. The measurement of the noise permits to achieve the cut-off frequency of the photodiode at -3dB. This method shows a significant way to achieve the dynamic response of fabricated photodiodes. Photodiodes have shown cut-off frequency of 220 MHz using Noise Setup [6]. References [1] H. Morkoç, Nitride Semieonduetors and Devices. Springer-Verlag Berlin Heidelberg, 1999. [2] C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra, ?High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap,? Appl. Phys. Lett., vol. 93, no. 14, pp. 14?17, 2008. [3] A. Gokarna, A. Gauthier-Brun, W. Liu, Y. Androussi, E. Dumont, E. Dogheche, J. H. Teng, S. J. Chua, and D. Decoster, ?Optical and microstructural properties versus indium content in InxGa1?xN films grown by metal organic chemical vapor deposition,? Appl. Phys. Lett., vol. 96, no. 19, p. 191909, 2010. [4] J. C. Carrano, T. Li, D. L. Brown, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C. Campbell, ?High-speed pin ultraviolet photodetectors fabricated on GaN,? Electron. Lett., vol. 34, no. 18, pp. 1779?1781, 1998. [5] M. Wraback, H. Shen, J. C. Carrano, C. J. Collins, J. C. Campbell, R. D. Dupuis, M. J. Schurman, and I. T. Ferguson, ?Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN,? Appl. Phys. Lett., vol. 79, no. 9, pp. 1303?1305, 2001. [6] J.-P. GOUY, ?Etude Comparative de la Photodiode PIN de la Photodiode a Avalanche et du Photoconducteur Sur Materiaux III-V,? Ph.D Thesis, 1989.

Resume : For GaN-based light emitters both, the radiative and non-radiative recombination processes are important to achieve high-efficiency structures. Especially non-polar structures, like m-plane GaInN/GaN quantum wells (QWs), are promising to increase the radiative recombination due to the absence of internal polarization fields that lower the efficiencies in c-plane structures.At the same time the reduction of defects is important to inhibit non-radiative recombination. Even for thin c-plane QWs, where the polarization field is negligible, our time-resolved photoluminescence measurements show minimum radiative lifetimes around 2ns, but values between 1ns and 200ps for m-plane QWs. The large difference of up to one order of magnitude can partly be explained by a higher exciton binding energy on non-polar QWs. As confirmed by simulations, the main contribution stems from a modified valence band structure caused by the anisotropic strain in non-polar QWs, which decreases the effective hole masses. By introducing a metamorphic AlInN buffer layer we aim to reduce the strain state of the QW, which results in larger radiative lifetimes. This indicates effective hole masses comparable to c-plane structures and makes the impact of strain on the valence band structure evident. In the same way, a high strain energy density causes the formation of defects that can act as non-radiative recombination centers. Due to strain reduction by a buffer layer the non-radiative lifetimes can be improved as well.

Resume : Sapphire is an attractive material as underlying substrates for III-nitride semiconductors due to its optical and electrical property. It is known that the polarity of nitride semiconductor could control with an interlayer of nitrided sapphire. In this study, we have experimented with local nitridation of sapphire with the use of femtosecond laser. A laser oscillator with a wavelength of 1045 nm and a pulse width of 450 fs was used. The laser beam was focused by a collective lens. The irradiation was performed in the vacuum chamber with flowing 50 sccm nitrogen gases. Above the threshold laser power of ablation, periodic structures were formed at the surface of sapphire substrates. The periodic pitch was approximately 250 nm, which corresponded to λ/4. The periodic structures with smaller pitch than the incident laser wavelength have attracted much interest as femtosecond-laser-induced periodic structures. At around the threshold laser power of ablation, high count value of nitrogen ion (CN-) were detected at the irradiated region compared with the non-irradiated area by TOF-SIMS. The result suggested sapphire was nitrided by laser irradiation in N2 ambient. This technique permits local nitridation with easier process and higher-spatial control. The effect on the polarity of nitride semiconductor is verifying.

Resume : Micro-LED (μ-LED) arrays have attracted great attention because of their wide applications to self-emissive micro-displays, multisite photo-stimulation of nerve cells, and biological neural networks [1–3]. Recently, individually addressable μ-LED arrays have been implemented to achieve a high resolution adaptive driving beam(ADB)/Advanced Forward lighting System(AFS) for headlamp of vehicles, which have 64 μ-LED pixels per 1 mm2, and a pixel size of 115 μm x 115 μm with a pixel pitch of 125 μm in both directions [4]. In order to realize high-brightness InGaN based individually addressable μ-LED arrays for the high resolution ADB/AFS application, the light output power as well as thermal dissipation of the μ-LED arrays should be further improved. However, there are little report on the improvement of output power of the μ-LED arrays for the high resolution ADB/AFS application. In this study, we have focused on the effect of reflectivity of p-type electrodes and coverage of p-type electrode to p-GaN on MESA area in μ-LED arrays on light output power of the μ-LED arrays. For this purpose, we have fabricated InGaN based μ-LED arrays with low reflectivity and small coverage of p-type electrode to p-GaN on MESA area (uncontrolled sample), and with high reflectivity and large coverage of p-type electrode (controlled sample). Both the InGaN based μ-LED arrays consisted of 256 μ-LED pixels per 2 mm x 2 mm (a pixel size of 115 μm x 115 μm with a pixel pitch of 125, 135, or 145 μm in both directions). Both devices showed individual control of μ-LED pixels, and yielded similar eletrical properties. On ther order hand, light ouput power of the controlled sample was greatly improved by 4.2 times at injection current of 100 mA compared to that of the uncontrolled sample. This result shows that the reflectance and coverage to p-GaN on MESA area of p-type reflective electrodes are important factors to improve the light output power of the μ-LED arrays for the high resolution ADB/AFS application. In addition, the variation of dark spaces in several emission images (total 9 pixels) were confirmed as a function of distance between pixels for controlled samples at an injection current of 20 mA. This result showed that the dark space almost disappeared at 10 μm of pixels distance. Finally, we analyzed thermal dissipation with current injection of the controlled and uncontrolled μ-LED array samples. [1] H. X. Jiang, S. X. Jin, J. Li, J. Shakya, J. Y. Lin, III-Nitride blue microdisplays, Appl. Phys. Lett. 78 (2001) 1303–1312. [2] V. Poher, N. Grossman, G. T. Kennedy, K. Nikolic, H. X. Zhang, Z. Gong, E. M. Drakakis, E. Gu, M. D. Dawson, P. D. Dawson, P. M. W. French, P. Degenaar, M. A. A. Neil, Micro-LED array: a tool for two-dimensional neuron stimulation, J. Phys. D: Appl. Phys. 41 (2008) 094014. [3] N. Grossman, V. Poher, M. S. Grubb, G. T. Kennedy, K. Nikolic, B. M. Govern, R. B. Palimini, Z. Gong, E. M. Drakakis, M. A. A. Neil, M. D. Dawson, J. Burrone, P. Degenaar, Multi-site optical excitation using ChR2 and micro-LED array, J. Neural Eng. 7 (2010) 016004. [4] S. Grötsch, M. Brink, R. Fiederling, T. Liebetrau, I. Möllers, J. Moisel, H. Oppermann, and A. Pfeuffer, "μAFS High Resolution ADB/AFS Solution," SAE Technical Paper 2016-01-1410, (2016), doi:10.4271/2016-01-1410

Resume : The InGaN/GaN superlattices (SLs) and multiple quantum wells (MQWs) are widely used as active regions or buffer layers in laser diodes, ligth-emitting diodes, etc. As a rule, the electrical and optical characteristics of this devices are investigated in detail. However, the characteristics of InGaN/GaN SLs and MQWs are not studied intensively. Recently the influence of well thickness on carrier transport in unipolar InGaN/GaN MQW structures was studied [1]. The authors showed that low-temperature transport of electrons cannot be explained by conventional quasi-equilibrium drift-diffusion model. In this regard it is interesting to obtain new experimental data on electrical and optical characteristics of InGaN/GaN SLs and MQWs. In this work the influence of GaN barrier thickness on photoluminescence and transmittance spectra is studied, as well as current-voltage (I-V) and capacitance-voltage characteristics of unipolar structures. Three unipolar structures grown using a MOCVD method on sapphire substrate (0001) were used for experiment. The active n-region included ten InGaN/GaN quantum wells with 2 nm thickness. All structures differ by GaN barrier thickness (3, 6 and 12 nm). The thickness of top GaN layer for all structures remains constant (20 nm). As a top contact we used Ni (Schottky barrier). Preliminary we studied dependence of barrier height on temperature using n-GaN/Ni structure. As an ohmic contact for buffer n-GaN layer we used fusing In contact of large area. All measurements were conducted in the temperature range T = 10-300 K. According to measurement results the short wavelength shift of photoluminescence peak with the barrier thickness decrease is observed. The results for optical measurements agrees with other works and can be explained by Quantum-Confined Stark Effect [2]. Using analytical approximation we estimated the built-in electric field in QWs (~0.1-1 MV/cm). The measurements of I-V curves showed increase of current limitation with barrier thickness increase. This limitation is more pronounced at reverse bias for Ni/GaN contact. The analysis of I-V curves showed that the transport mechanism of carriers are different in each sample. The sample with 6 nm GaN barrier thickness has a complex shape of I-V curve, containing regions of negative differential resistance for both bias polarities. The analysis of experimental results considers resonant tunneling under condition of inhomogeneous electric field distribution in SLs [3], and hooping conductivity at high current density for MQWs [4]. [1] D.A. Browne, B. Mazumber, Y.-R. Wu, and J.S. Speck., J. Appl. Phys., Vol. 117, 185703, 2015. [2] I. S. Romanov, I.A. Prudaev, V.V. Kopyev, Jpn. J. Appl. Phys., Vol. 55, 05FJ15, 2016. [3] A. Wacker, Physics Reports, Vol. 357, p 1, 2002. [4] I. Prudaev, O. Tolbanov, S. Khludkov, Phys. Status Solidi A, Vol. 212, p 930, 2015.

Resume : Deep-ultraviolet emitting structures based on 1.5nm Al0.65Ga0.35N/3 nm Al0.8Ga0.2N multiple quantum wells, embedded in compositionally graded AlxGa1-xN films in the form of graded-index separate-confinement heterostructure, were grown by plasma-assisted molecular beam epitaxy on 6H-SiC substrates. The graded AlGaN layers blocked threading defects(TDs) in the vicinity of the AlN/AlGaN heterointerface, resulting in a reduction of the TDs in the active layer. The intensity of the TM polarized amplified spontaneous emission (ASE) spectra with peak emission at about 270 nm was found to increase linearly with the number of quantum wells. Furthermore, while the peak intensity for devices with a single quantum well varies linearly with the pump fluence, it varies superlinearly for larger number of quantum wells, suggesting light amplification by stimulated emission. These results are consistent with numerical simulations, which indicate that the confinement of the optical mode in this device structure increases with the number of quantum wells and simultaneously the threshold under optical pumping decreases with increasing of the number of quantum wells. Finally, the band structure simulations indicate that the device is in the form of a p-n junction due to polarization-induced p- and n-doping in the compositionally graded AlxGa1-xN films on either side of the multiple quantum wells, and thus, paving the way for the development of an electrically pumped deep-ultraviolet laser.

Resume : We demonstrate a highly reliable transparent conductive electrode (TCE) that integrates silver nanowires (AgNWs) and high-quality graphene as a protecting layer. Graphene with minimized defects and large graphene domain was successfully obtained by decoupling nucleation step from growth through a facile two-step growth approach.1,2 Ultraviolet light emitting diodes (UV-LEDs) were fabricated using AgNWs or hybrid electrodes where AgNWs were combined with two-step grown graphene (A-2GE) or conventional one-step grown graphene (A-1GE). Then, the device performance and reliability of the UV-LEDs with three different electrodes were compared. The light output power of the LED with AgNWs is poor, and its light output power is not observed when the injection current exceeds 40 mA because the device failed by larger voltage drop caused by AgNWs agglomeration and junction breakdown under high power operation. When operated at high injection current levels, the devices inevitably generate extreme amounts of heat, obstructing the reliability and performance. However, the UV-LEDs with A-1GE and A-2GE show stable operation up to an injection current of 100 mA. This result strongly implies that both devices with graphene have thermal stability during long-time operation. One can notice that the difference in light output power between the devices with A-1GE and A-2GE progressively increases as the current increases, indicating that the A-2GE as the TCE offers device reliability better than that of the device made use of A-1GE. From our observation, the A-2GE would be a reliable TCE for high power UV-LEDs.

Resume : III-nitride nanophotonics on silicon is a booming field, both in the near infrared and in the UV-VIS spectral range. The integration of active microresonators can lead to integrated circuits with a small footprint. So far, single microdisk laser devices have been demonstrated, mostly under optical pumping. Combining microdisk lasers under electrical injection with passive devices represents the next challenge in realizing a viable III-nitride nanophotonic platform on silicon. We have developed the fabrication of microdisks to operate under electrical injection and to achieve lasing with whispering gallery modes. This work builds upon our recent demonstration of room-temperature optically pumped microdisk lasing emission between 275 nm to 470 nm with quality factors greater than 1000. In the case of electrically injected microdisks a circular p-contact is fabricated on top of the disk and connected to a larger pad via a metal microbridge to allow for electrical probing. We have successfully fabricated mechanically stable microbridge contacts using a two-step optical lithography process. We will also show a new strategy to collect the microdisk emission with side waveguides and distribute the light in the layer plane. The challenge consists in the very narrow gap between disk and waveguide of only around 100 nm, which is necessary for efficient coupling in the blue spectral range.

Resume : Semi-polar GaN, particularly (11-22) GaN, has emerged to be a promising candidate for fabricating emitters in the long wavelength region, e.g. green and yellow, which is critical for solid state lighting, visible light communications and opto-genetics. InGaN/GaN based quantum wells grown along the (11-22) direction demonstrate a significant reduction in piezoelectric polarization fields compared with those on c-plane polar GaN, thus effectively increasing internal quantum efficiency (IQE). Furthermore, the (11-22) plane exhibits a lower indium chemical potential than either non-polar or polar surfaces, thus effectively enhancing indium incorporation into GaN which is crucial for achieving long-wavelength emission where high indium content is required. However, the crystal quality of semi-polar (11-22) GaN grown on sapphire is far from satisfactory. In order to address these material challenges, our group has developed a cost-effective approach of overgrowth on regularly-arrayed micro-rod templates, achieving a step-change in the crystal quality of semi-polar (11-22) GaN on sapphire.1 Based on such overgrown semi-polar (11-22) GaN, we have demonstrated high-performance semi-polar InGaN LEDs with an emission wavelength of up to the amber spectral region.1 Our very recent study indicates that the micro-rod diameter plays a vital role in effectively reducing both the dislocation density and the basal staking fault (BSF) density of our overgrown (11-22) GaN, showing a distribution of BSF-free regions (3.0 µm width) periodically spaced by BSF-containing regions (1.3 µm width) along the [-1-123] direction.2,3 It is well-known that dislocations generally act as non-radiative recombination centre, while the influence of BSF on optical performance of InGaN/GaN quantum wells is unknown. In this report, we have developed a novel nano-patterning approach, which allows us to fabricate a number of individual areas which are smaller than the areas of either any BSF free regions or any BSF-containing region. Based on this approach, we have carried out a systematic study of the optical properties of the semi-polar (11-22) InGaN single quantum well (SQW) structures on the overgrown GaN in order to study the influence of BSF on optical properties of the semi-polar InGaN SQW. A series of semi-polar (11-22) InGaN/GaN SQW LED structures with an emission wavelength from blue to yellow have been grown on our GaN.1 As shown in Figure 1a, a number of nano-windows are fabricated by randomly spin-coating nanosphere silica particles with a diameter of 500 nm on the surface of each sample, followed by depositing a nickel layer. After that, the nanosphere particles are removed, leaving a number of sparely distributed nano-windows with a diameter of 500 nm. As presented in Figure 1b, both the nano-windows and the spacing of the nano-windows are randomly distributed. The separations of these nano-windows are large enough, allowing us to perform optical measurements on the SQW within such a nano-window area by using our micro-photoluminescence system equipment equipped with a time-correlated single photon counting (TCSPC) system, where a 375 nm pulsed diode laser with a pulse width of 50 picoseconds (ps) is used as an excitation source. Our optical study is based on a large number of measurements on these individual nano-windows, where some nano-windows are on the BSF free regions, and some on BSF-containing regions. The results measured at room temperature do not show significant difference, meaning that unlike dislocation which generally acts as non-radiative recombination centre BSF might not play an important role in the optical performance of semi-polar InGaN SQW. Further investigation to understand the influence of BSF on exciton dynamics at a low temperature is on-going, demonstrating a completely different role to dislocations. References: 1 J Bai, B Xu, FG Guzman, K Xing, Y Gong, Y Hou and T Wang, Appl. Phys. Lett. 107, 261103 (2015). 2 Y Zhang, J. Bai, Y Hou, X Yu, Y. P. Gong, R M Smith, T Wang, Appl. Phys. Lett. 109, 241906 (2016) 3 Y Zhang, J Bai, Y Hou, R M Smith, X Yu, Y Gong and T Wang, AIP Advances 6, 025201(2016)

Resume : The presence of high built-in piezoelectric fields into polar GaN effects on optical properties of nitride based structures. These electric fields arise due to mismatch of the crystal cells of adjacent heterostructure layers. There are several papers concerning the studies of InGaN/GaN quantum wells (QWs) with different applied bias voltage [1]. In present report the internal electric fields in active region of LEDs structures based on multiple InGaN/GaN QWs were investigated by electroreflectance (ER) and photocurrent (PC) spectroscopy. ER and PC spectra were obtained at room temperature in the range of 2.2-3.5 eV with different AC and DC bias using lock-in technique. The sample under investigation is LED heterostructure grown along [0001] direction on sapphire substrate by MOCVD technology and mounted by the p-side to the heat sink. We observed the changes of the ER line shape in the range of 2.3-3.0 eV and stable line at 3.2 eV while DC bias of pn-junction varies from +0.5 V to -4.5 V. This modification is connected with redistribution of ER sources between different QWs due to the different strength of the internal electric field in the active area. Thus, the detail comparison of ER and PC spectra from active LED regions shows high influence of internal electric fields on recombination probability in different QWs. 1. W.G. Scheibenzuber, U.T. Schwarz, L. Sulmoni, J.-F. Carlin, A Castiglia, N Grandjean. Appl. Phys. Lett. 97(18), 181103 (2010)

Resume : Electron blocking layer is considered an important part of nitride light emitters such as LEDs and LDs. It’s main role is to limit the electron overflow of the active region and their non-radiative recombination in the p-side of the device structure. Although it serves its purpose in LEDs, EBLs role in LDs still remains controversial, as this layer also limits unintentionally the hole injection due to insufficient doping and polarization effects. To shed more light on this issue we fabricated structures with different types of EBL: conventional step-like EBL, very low EBL, GaN/AlGaN superlattice EBL, and aperiodic superlattice EBL. The performance of all these structures was modelled using SiLENSe software. Experimental results show, that under normal conditions (not too high temperature) the construction of EBL has almost negligible impact on device parameters like threshold current and efficiency. However, the differences become visible when it comes to operating voltage and thermal stability expressed in terms of T0 parameter. Additionally, the modelling of these devices shows pronounced changes in the spatial distribution of the non-radiative recombination. We believe (which is supported by our preliminary measurements) that these changes may influence degradation processes.

Resume : Three dimensionally (3D) faceted InGaN quantum wells (QWs) are attractive for phosphor-free white light-emitting diodes. However, In-rich InGaN QWs emitting the longest wavelength are usually fabricated on the (0001) polar facet, where the optical transition probability is low due to the strain-induced piezoelectric field. To circumvent this issue, we fabricate 3D QWs on {11-22} semipolar GaN substrates by selective area metalorganic vapor phase epitaxy. Firstly, various stripe mask patterns were formed along either the [-1-123] or [-1100] direction on the (11-22) and (-1-12-2) surfaces. We found that polar-plane-free 3D GaN structures composed of the (-1-12-2) and (-110-1) semipolar planes, and (-1100) nonpolar plane could be fabricated with a particular mask pattern on the (-1-12-2) plane. Then, InGaN/GaN three-periods QWs were fabricated subsequent to the 3D GaN growth. Spatially resolved cathodoluminescence spectroscopy revealed polychromatic emissions at ~390, ~440, and ~480 nm from the (-1100), (-1-12-2), and (-110-1) facets, respectively. Local structure analyses showed that the facet-dependent indium composition chiefly determines the polychromatic emission property. In addition, time-resolved photoluminescence clarified that the radiative recombination lifetime for the long wavelength emission of the obtained 3D QWs without the (0001) plane is much shorter than that of the conventional 3D QWs with the (0001) plane.

Resume : InGaN/GaN multiple quantum well (MQW) solar cells were fabricated and investigated with 3.5 nm In0.25Ga0.75N quantum well and three different barrier thicknesses: 6.5, 14 and 25 nm. Results showed that the barrier thickness greatly influenced on the device property. The short circuit current density (Jsc) and external quantum efficiency (EQE) were increased when the barrier thickness was reduced, because the reduction of barrier thickness would weaken piezoelectric polarization and enhance tunneling transportation of photo-generated carriers. The cells showed a wide spectrum response of a wavelength of 470 nm, which is still not as long as expected. In addition, a stair-like profile was observed in J-V curves, and a low value appeared for both fill factor and Jsc. Therefore, a deep investigation on the above abnormal phenomena are necessary. Samples were grown on c-plane sapphire substrates using Thomas Swan low-pressure metalorganic chemical vapor deposition system. On a 2-μm-thick Si-doped n-GaN layer, the light absorption region of 10 periods of In GaN (3nm)/GaN (6.5/14/25nm) MQWs were grown. A 70 nm Mg-doped p-GaN layer was finally deposited for the window layer. In content in InGaN layer was estimated to be 0.25 by x-ray diffractometer (XRD) using a Panalytical X'pert PRO instrument. The mesa size dimensions of a solar cell were 1×1 mm2. The semitransparent current spreading layers of Ni/Au (5/5 nm) were deposited on p-GaN by evaporation. Cr/Au were used for n and p pads. The J-V characteristics were measured using an AM 1.5 (1 sun) solar simulator light source and Keithley 2410 source meter. Light was dispersed through an Acton monochrometer with a 2400-g/mm UV-optimized grating to measure the external quantum efficiency (EQE). The incident light power was calibrated by a reference silicon photodetector. For the three samples, the curves of high-resolution XRD all show many well-defined high-order satellite diffraction peaks, indicating a good layer periodicity for the entire MQWs and abrupt interfaces between wells and barriers. With reducing barrier thickness, the weaken piezoelectric polarization and the enhanced tunneling transportation result in the increase of Jsc and EQE. It should be noted that the J-V curves exhibit different degrees of staircase characteristics and accompanied by low fill factor and EQE values, which may reflect certain correlation with the polarization and barrier thickness, that need a further investigation. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 61404059), the project for distinguished youth scientific researchers in the Universities of Fujian.

Resume : In recent years, the application fields of UV-LED based on III-nitride materialskeep expanding and these create a huge potential market. The AlGaN-based UV-LED devices (265nm-400nm) show a great prospectingin the structural analysis of biological and chemical agents, biological agents, biological and chemical weapons detection and warning, water and air purification, disinfection and sterilization, printing, packaging, advertising, building materials, decoration, electronics, household appliances, optical fiber, automobiles and other military and civilian fields. At present, the UV-LED attracts more and more attention of the major companies (Philips, LG, etc.) and research institutions (University of South Carolina, RIKEN, University of Texas-Austin, University of California-Santa Barbara, etc.)all over the world. However, different from the blue LED, many urgent practicalproblems restrict the performances of UV-LED currently, such as the huge device heat, low light extraction efficiency and poor reliability, resulting in poor light efficiency, short life.Besides the improvement of chip fabrication technique, the packaging technology also has an important impact on the characteristics of UV-LED devices. In this report, we studied the growth parameters and doping process ofhigh Al-content AlGaN films by metal-organic chemical vapor deposition(MOCVD). The structures of multi quantum wells were designed and deposited on high-quality AlGaN layers. Thenwe fabricatedthe UV-LED flip-chip structures, and carried out the researches on UV-LED device packaging design and flip-chip packaging technology at the low thermal resistance.This study laid a foundation for high-power, high-density UV-LED devices in the practical applications. REFERENCES: [1] S. Wang, W. Tian, F. Wu,J. Zhang,J. N. Dai*, Z. H. Wu, Y. Y. Fang, Y. Tian, and C. Q. Chen, Efficient optical coupling in AlGaN/GaN quantum well infrared photodetector via quasi-one-dimensional gold grating, OPTICS EXPRESS, 23, 7, 8740 (2015) [2]Renli Liang, Jun Zhang , Shuai Wang , Taotao Ding, Jiangnan Dai ,Changqing Chen ,Experimental Study on the Effects of Eutectic Voids on the Thermal Performance Within Flip-Chip Ultraviolet Light-Emitting Diodes, IEEE Transactions on Components, Packaging and Manufacturing Technology 6(10):1-5 (2016)

Resume : Localization of carriers, especially that of holes, plays an important role in carrier dynamics in InGaN quantum structures; localization effects become incrementally important as indium content increases. In this presentation, we analyze the impact of carrier localization on recombination and diffusion. We compare three LED structures, grown on c-sapphire and emitting at 440 nm (blue), 500 nm (cyan), and 530 nm (green). For studies, we employ a number of optical techniques: time-resolved and time-integrated photoluminescence (PL), differential transmission spectroscopy, and light-induced transient gratings. The LED structures were excited resonantly with 250 fs laser pulses. We observe an untypical redshift of PL spectra of 30 meV in the cyan and 20 meV in the green structures that increases with excitation and time (up to 40 ns). We explain it by density-dependent and diffusion-driven carrier redistribution between the shallower and deeper localized states. Quantum efficiency (calibrated with an integrating sphere) is approximately similar for green and cyan structures, while there is a considerable difference in PL decay times, 12 ? 15 ns for cyan vs 20 ? 30 ns for green. We demonstrate the correlation between the increase in carrier diffusivity and the onset of efficiency droop, which shows that carrier transport to remote defect states plays an important role in density-dependent non-radiative losses.

Resume : AlGaN-based deep UV LEDs are expected as an alternative light source for mercury lamp. One of the challenges for achieving their high efficiency is to reduce the resistance of p-type contact [1]. The Schottky barrier for a hole at the anode contact can be reduced by using a material with a large work function for the electrode. Since the work function of carbon nanotubes (CNTs) easily changes by doping because of their low density of state, a relatively large work function can be obtained [2], and hence good contact for p-GaN and p-AlGaN can be expected. In this study, electrostatic doping [3] was performed on the CNT electrode using ionic liquid to improve the hole injection efficiency of the DUV LED. The CNT thin-film electrode was formed on p-GaN contact layer by the dry transfer method [2]. An ionic liquid (DEME-TFSI) was dropped onto the CNT electrode. A probe was inserted into the ionic liquid and a gate voltage was applied to dope carriers in the CNTs. With increasing gate voltage, the LED current increased. The operating voltage was reduced by about 0.5 V for a constant LED current. This result shows the possibility of reduction of power consumption in DUV LEDs by field effect doping to CNT electrode. [1] J. Chen and W. D. Brewer, Adv. Electron. Mater. 1, 1500113 (2015) [2] A. Kaskela, et al., Nano Lett. 10, 4349 (2010) [3] H. Shimotani, et al., Adv. Funct. Mater. 24, 3305 (2014)

Resume : Improvement of the external quantum efficiency (EQE) of blue/green GaN-based laser diodes has been required for realizing a blighter larger laser display. The EQE, however, decreases with increasing In contents of the GaInN/GaN active layer because of the quantum-confined Stark effects (QCSE) and lattice defects. Recently, an active layer grown on a non-polar GaN side-wall of nanowire is expected to be one of the possible candidates to overcome above problems. In fact, the optical pumping stimulated emissions from this structure was reported [1]. For fabricating the laser structure, characterizations of the actual local structure of active layer are required. In this study, we have successfully analyzed the local structure of GaInN/GaN multi-quantum wells (MQWs) grown on a single GaN {1-100} side-wall of nanowire by using the x-ray micro beam from the 8 GeV synchrotron radiation facility (SPring-8). GaInN/GaN MQWs samples were grown on a GaN {1-100} side-wall of nanowire by MOCVD. The diameter of the nanowire including the MQWs was 400 nm. By irradiating the x-ray microbeam with a size of 0.27 μm×0.23 μm on a side-wall of nanowire, reciprocal space maps for 2-200 reflection of GaN and GaInN/GaN MQWs were obtained. From this result, the period of MQWs was estimated to be 13.6 nm (designed period:12 nm). The In contents of MQWs and the effect of QCSE also will be discussed in the presentation. [1] S. Ishizawa et al., Appl. Phys. Exp. 4 (2011) 055001.

Resume : Recently, we have demonstrated that an array of microstructured hollow cavities leads to a significant enhancement in diffraction strength, thus giving rise to large light extraction efficiency. Adjusting the morphologies of hollow cavities can potentially further enhance their ability related to the diffraction of light. Here, we quantitatively demonstrate that increasing the volume of hollow cavities yields intensified diffraction strength particularly for incident light beyond a critical angle, suggesting a rational strategy for greater light extraction. For this experiment, we prepared GaN-grown sapphire substrates containing hollow cavities with progressively increasing volume, and measured their diffraction strength by using a homebuilt wavelength-resolved diffraction measurement setup. According to the measured result, as the volume of the hollow cavities embedded in the GaN medium increases, various orders of diffraction modes appear strongly over the visible spectrum (Wavelength 400 – 700 nm). To support the intensified diffraction due to the increase of the hollow cavities, their angular transmittance was calculated by conducting the full-vectorial numerical simulation. In accordance with the measured result, large hollow cavities show significantly enhanced transmittance for light that otherwise would be trapped by total internal reflections. In conclusion, we have demonstrated that rationally designed hollow cavities serve as strong-diffraction photonic elements, enabling to develop high-efficiency InGaN/GaN-based light-emitting diodes. Moreover, these high-index-contrast gratings can be utilized for other optical applications including organic light-emitting diodes and even solar cells.

Resume : Group-III materials are the basis of solid-state lighting (SSL) because their emission can cover the whole visible spectrum. However, the planar nitride thin film used for commercial light-emitting diodes (LEDs) is typically grown on sapphire substrates, which results in high threading dislocation densities due to their thermal- and lattice-mismatch. Moreover, the degraded crystal quality and increased piezoelectric field lead to decreasing quantum efficiency as the indium composition higher than 25%, forming the “green gap” for LEDs. The roll-over of quantum efficiency under high injection current, called “efficiency droop”, also becomes pronounced with increasing indium composition. In the past, GaN and SiC bulk substrates have been used to address these issues at the expense of high material costs. Recently, Group III-nitride vertically aligned nanowires and devices grown on silicon have been investigated, due to their numerous advantages over planar epitaxial LEDs, such as a reduced polarization field, wavelength tunability by increasing indium composition, and enhanced light extraction. However, nanowire LEDs on Si suffer from the formation of amorphous SiN layer and significant light absorption from Si. Moreover, the high density of surface states limited the quantum efficiency and output power of nanowire devices. The carrier and phonon confinement in these nanowires also led to reduced heat dissipation and high junction temperature, especially for the high-power application. It is noted that commercial high-power planar LEDs have to be transferred to a metal heat sink via laser liftoff or wafer bonding. In this paper, we will report our results on high-performance (Al, In)GaN quantum-disk-in-nanowires LEDs emitting at ultraviolet (UV)-to-near infrared wavelength range grown on molybdenum (Mo) substrates using molecular beam epitaxy (MBE), which demonstrated the potential of nanowires emitters for various applications. The nanowires were prepared using Veeco Gen930 plasma-assisted MBE. A layer of 500 nm Ti was deposited on the Mo substrates before loaded into the chambers. The Qdisks-in-nanowire LEDs structures on polycrystalline Mo substrate have n-type GaN grown at 660 °C, 8 stacks of InGaN-disks/GaN-barrier grown at different conditions. The p-GaN was grown at a substrate temperature of 570 °C. The NW-LEDs were fabricated following planarization, etching, and Ni/ITO p-contact deposition process. For UVLEDs, 15 (Al)GaN-disks, separated by AlGaN-barriers, and Ni/Au for p-contact have been used to avoid the absorption of UV in ITO layer. We achieved, for the first time, the direct growth high-quality nanowires on titanium (Ti) coated bulk Mo substrates. Our extensive studies on the growth mechanisms confirmed the epitaxial relationship between the cubic titanium nitride (TiN) transition layer and the hexagonal GaN nanowires. Thus the platform simultaneously implements buffer layer, n-metal contact, reflector, and heat-sink, thus greatly simplifies the fabrication process for high-power emitters. The InGaN/GaN Qdisks-in-nanowire LEDs on Mo exhibited a low turn-on voltage of ∼2V without efficiency droop when operating at an injection current density of 1.6 kA/cm2 and ∼5 V. The light output power is higher than reported LEDs emitting at long wavelength, thus is promising for high-power device operation. The low self-heating of the LEDs was revealed by our current-dependent Raman measurements. The reliable operation of our nanowire-LEDs (NW-LEDs) was evident in the constant-current soft burn-in test. By tuning the indium composition, we achieved blue, green, and yellow LEDs on Mo. When using different Qdisk and barrier structure, we also achieved LED emission in the UV range. In conclusion, we reported the first nanowires growth process on metal substrates. The Qdisks-in-nanowires grown on Mo exhibited high crystal quality, and the fabricated devices show full-color emission from UV to near infrared.

Resume : (Al,Ga)N based light emitting diodes (LEDs) are seen as the next technology for ultra-violet (UV) sources. However, low efficiency values are frequently observed for the standard technology based on sapphire substrates. A main reason is the high dislocation densities (DD) in the LED active region, which leads to low internal quantum efficiencies (IQE). An efficient way to drastically limit the DD influence on the IQE of UV LEDs is to use quantum dots (QDs), which strongly localize the excitons and favor electron-hole radiative recombinations. Using molecular beam epitaxy in a RIBER reactor, polar and semi polar AlyGa1-yN QDs / AlxGa1-xN active regions have been designed by adjusting Al concentrations with x > y to insure a compressive strain of the QD layers. N-type AlxGa1-xN layers with electron doping above 10^18 cm^-3 have been obtained for x as high as 75%, and QDs with Al concentrations up to 20% and densities above 5.10^11 cm-2 have been grown. In addition, electron blocking layers on the p-type side have been adjusted to insure an efficient carrier injection in the QD active region. Electroluminescence (EL) and current-voltage characteristics from different polar and semipolar QD LEDs have been obtained. An EL emission from the UVA to the UVB region has been observed, depending on the QD design: specifically, an emission covering the UVA region between 325 and 400 nm is obtained for GaN QD-based LEDs whereas AlyGa1-yN QD-based LEDs emit at shorter wavelengths, down to 300 nm, as a consequence of the QD larger bandgap energy and the decrease of the quantum confined Stark effect. These results pave the way for the fabrication of efficient LEDs in the UV range. This work is supported by the ANR Project “NANOGANUV”.

Resume : Highly polarized ultraviolet (UV) emission from AlGaN-based UV emitters has the unique application. For example, the irradiation by a linearly polarized UV light can induce optical anisotropy in photoreactive polymers. This property is particularly suitable for the alignment of liquid crystals without using the rubbing and patternable processes. To obtain the polarized UV emission, it is necessary to fabricate the metal or dielectric subwavelength grating (SWG). However, it is still difficult to realize the large-scaled stable metal gratings under 150 nm period when using the e-beam lithography or imprint lithography. In this study, we fabricated the 200 nm, 100 nm, and 50 nm period aluminum (Al)-based SWGs by using solvent-assisted nanotransfer printing (S-nTP) process for highly polarized UV light with 365 nm wavelength. To overcome the problems of resolution limit and low transfer yield of conventional nanotransfer printing (nTP), the S-nTP was employed and it could produce the extremely fine (down to sub-10 nm) nanostructures with an excellent transfer yield of 100%. The transverse magnetic (TM) mode UV intensity was 4 times greater than the transverse electric (TE) mode UV light and a high polarization ratio (PR) of 0.65 was achieved. These results indicate that the S-nTP is an excellent method to fabricate the Al SWG with good uniformity and surface coverage and the high polarization ratio of UV emission.

Resume : Lateral current transport in group-III nitride-based devices is hampered by very low hole mobility in p-doped layers. Current spreading layers such as transparent conducting oxides deposited on the epitaxial layer structures after growth are widely applied to achieve larger contact areas. Spreading resistance and transparency of such current spreading layers are limiting factors for device performance of light emitters. Tunnel junction structures have been proposed as an attractive alternative to improve lateral current spreading across the contact region of p-type GaN layers. However, the effectiveness of the tunnel junction is limited due to the achievable maximum donor and acceptor concentrations as well as the activation of hydrogen passivated Mg acceptors buried below an n-type GaN layer. In this study, we have grown heavily doped GaN:Mg/GaN:Ge tunnel junctions on top of conventional LED structures using exclusively metal-organic vapor phase epitaxy (MOVPE). In particular, the activation process of the Mg-doped GaN layer is critical for device performance. We compare post-growth thermal annealing schemes applied to LED mesa structures with thermal annealing of the p-type GaN:Mg layer during growth with regard to the efficiency of the activation. Furthermore, we investigate the potential of MOVPE regrowth of the GaN:Ge layer on top of a post-growth activated GaN:Mg layer.

Resume : Minimizing the contact resistances on both p- and n-layers in deep UV light emitting diodes (LEDs) and lasers is essential for improving the efficiency of these devices suffering from the high operating voltages and resistive heating. Ohmic contacts to n-AlxGa1-xN are especially challenging for high Al mole fractions mainly due to the low electron affinity. Standard metallization schemes such as Ti/Al/Ti/Au can form ohmic n-contacts only up to about 40% Al mole fraction. In this study, we investigated vanadium-based contacts to n-Al0.8Ga0.2N layers in a four-metal electrode V/Al/Ni/Au configuration. Si-doped Al0.8Ga0.2N layers were grown on low defect density AlN by MOVPE with a specific sheet resistivity of 0.02 Ωcm. After metal deposition, the samples were annealed in a modular process rapid thermal annealer under various annealing conditions. Contact characteristics were measured using the linear transfer length model at room temperature. The V/Al/Ni/Au contacts on n-Al0.8Ga0.2N annealed at 800°C under N2 atmosphere exhibit ohmic characteristics with specific contact resistivities as low as 8x10-5 Ωcm2 at a current density of 0.1 kAcm-2. In contrast, Ti/Al/Ti/Au electrodes form rectifying Schottky contacts. We finally demonstrate UVC LEDs emitting at 265 nm using V-based n-contacts and compare their performance to standard Ti/Al/Ti/Au electrodes. A significant reduction of the operating voltage was observed with Vop 11 V at a current of 50 mA and with output powers of 2 mW.

Resume : GaN/AlN quantum discs exhibit unique electrical and optical properties [1, 2]. To investigate their specific features, nanoscale characterization techniques are required. In this work we probe the electronic structure of single GaN nanowires (NWs) with GaN/AlN multiple quantum discs (MQDs) by means of electron beam induced current (EBIC) microscopy and cathodoluminesce (CL) mapping. The wires consist of three segments: an n-GaN cap, an active region with 20 periods of narrow (0.6 nm) GaN discs / 6 nm Al barriers and a n-GaN /AlN core/shell base (in certain wires the shell is composed of GaN/AlN multilayers). SEM and TEM images were used to assess the thicknesses and to build a band profile simulation. EBIC mappings performed at different applied biases demonstrate the presence of a built-in field in the MQDs region, in agreement with simulations [2]. EBIC mapping enables analyses of the field extension and the minority carrier diffusion lengths. We demonstrate that the presence of the radial shell around the n-GaN base of the NWs impacts the EBIC signal-to-noise ratio, series resistance [1, 2], but also CL emission, which exhibits a strain-induced shift of the GaN emission [3]. Finally, we evidence a high-energy CL emission peaks from the GaN/AlN radial heterostructures at 15 K [4, 5]. [1] DOI: 10.1021/acs.nanolett.6b00806 [2] DOI: 10.1021/nl1010977 [3] DOI: 10.1088/0957-4484/23/32/325701 [4] DOI: 10.1103/PhysRevB.93.205410 [5] DOI: 10.1088/0957-4484/23/45/455205

Resume : III-N nanowires (NWs) have recently often been employed as basis for light-emitting diodes (LEDs). At the free NW sidewalls, strain induced by lattice mismatch can elastically relax, thus enabling axial (In,Ga)N/GaN heterostructures with high quality at large In content. This relaxation leads to a three-dimensional strain variation in the active region of NW-LEDs. Here, we study the consequences for the electroluminescence (EL). We measure a double peak structure, and the relative intensity of the two peaks exhibits a peculiar evolution with injected current. We calculate strain, electric field, and charge carrier distributions for both planar and NW-LEDs and simulate their EL spectra. On this basis, we identify as the physical origin of the double peak structure a stronger quantum-confined Stark effect in the first and last quantum well (QW) of the heterostructure with four QWs. This phenomenon results from the nitrogen crystal polarity of the investigated NW-LED and is observed as well in the simulated EL spectra of planar N-polar but not of planar Ga-polar LEDs. The simulations show also that the peculiar evolution of the relative EL peak intensities with injected current, observed only in case of the NW-LED, is caused by the strain relaxation characteristic for NWs. Therefore, we directly identify consequences of the NW morphology on the emission properties of LEDs. Furthermore, we provide important insights on the recombination mechanisms of N-polar LEDs in general.

Resume : The InGaN/GaN quantum well (QW) structures are widely used for LEDs and lasers since early 1990s. More recently application of nitride heterostructures, as fast scintillators for detectors of ionizing radiation, have emerged. The advantages of nitride heterostructures compared to other scintillation materials are high radiation resistance, high exciton binding energy and consequently short excitonic decay time as well as high luminescence efficiency. A demand for very fast, efficient scintillators with decay time of few nanoseconds was created in the last years due to necessity of fast scanning electron microscopes for inspection machines in electronic industry. Structures designed for scintillating applications require large QW number covering particle penetration depth. In this work we compare luminescence results obtained on InGaN/GaN multiple QW structures with different number of QWs. The aim was to increase the intensity of fast excitonic QW emission and decrease of the luminescence of QW defect band, which has slower luminescence response and is undesired for fast scintillator applications. We show that increasing the number of InGaN QWs in combination with decreased QW growth rate is an efficient method to reach this aim. Photoluminescence and cathodoluminescence results will be compared.

Resume : We developed watt-class green and blue laser diodes which were fabricated on free standing semipolar {20-21} GaN and conventional c-plane GaN substrates, respectively. Although several research groups have recently developed green laser diodes on semipolar GaN substrates, which have weaker piezoelectric fields and higher indium homogeneity in InGaN active regions compared to c-plane GaN, watt-level output power remains to be achieved. By utilizing the {20-21} plane, we have successfully fabricated the first watt-class green lasers at 530 nm, and achieved maximum output powers in excess of 2 W, which to the best of our knowledge is the highest reported value for GaN-based green laser diodes. A wall plug efficiency of 17% was realized at a current of 1.2 A under continuous-wave operation, which corresponds to an optical output of approximately 1 W, and is also the highest reported value to date. In addition, high-power and high-efficiency blue laser diodes at 462 nm were successfully fabricated on conventional c-plane GaN substrates. The output power and wall plug efficiency were 5.0 W and 36%, respectively, at a current of 3.0 A under continuous-wave operation. These laser diodes are promising light sources for future laser display applications meeting the ITU-R Recommendation BT.2020.

Resume : Visible InGaN-laser diodes have seen tremendous performance development in the recent years. In this talk an overview of current technical status and recent research and development-results of InGaN-laser diodes at Osram Opto semiconductors will be given: Both – efficiency and available output power - have increased to unprecedented values. Driven by this the lasers are used in an increasing number of exciting applications. It ranges from low power mW single mode lasers in blue, cyan and green wavelength range to high power blue laser diodes in the watt-output power class. Furthermore, R&D devices with roll-over beyond 100W out of one package will be demonstrated by integration of power lasers in a multi-chip package. Accompanying, the established applications of visible InGaN lasers and also upcoming possibilities and applications will be discussed. Typical applications range from industry, illumination and visualization to lighting by converted blue laser light.

Resume : Considerable attention has been focused on InGaN green LDs during the past few years due to the demand for green laser diodes in pico-projectors and laser displays. Since the first breakthrough of InGaN-based green LDs was achieved by Osram Corp. in 2009, green InGaN LDs with emission wavelength above 500 nm grown on c-plane, (11-22) plane, and (20-21) plane have been realized. However, the threshold current density of InGaN-based green LDs is still high. The reasons for high threshold current density for green LDs mainly include increased defects and polarization field in InGaN QWs as nearly two-fold higher indium (In) composition is required in green LDs compared to blue LDs. In this article, by observing the morphology evolution of green InGaN/GaN QWs and studying the catholuminescence property, we investigate indium-segregation-related defects that are formed at green InGaN/GaN QW interfaces. We found In cluster defects exist not only at dislocation regions but also at dislocation-free regions. Meanwhile, we also propose the approach and suggest the mechanism to remove them for green InGaN/GaN QW grown on both GaN templates and free-standing GaN substrates. By engineering the interface of green InGaN/GaN QWs, we have achieved green LD with low threshold current density of 1.85 kA cm-2. The output power of the green LD is 58 mW at a current density of 6 kA cm-2 under continuous-wave operation at room temperature. We performed optical characterization of InGaN/GaN QW active regions of green laser diodes (LDs) with different threshold current density by temperature dependent photoluminescence (PL). The internal quantum efficiency (IQE) was evaluated to be 39% and 59% for green LDs with threshold current density of 8.50 kA cm-2 and 1.85 kA cm-2, respectively. Additional non-radiative recombination centers with activation energy of 10 meV were found in the sample with lower IQE, which is attributed to defects located at the interface of InGaN/GaN QWs.

Resume : In last few years, there has been strong development of plastic optical fibers (PMMA POF) and high speed visible light communication. With the growing demand for such specialized telecommunication systems, the need of developing basic components of visible light optoelectronic systems - such as optical amplifiers ? appeared. Moreover, optical amplifiers - if placed after laser diode source ? can work as an optical booster or ? if placed in front of detector ? as an optical signal enhancer. This work shows the optoelectrical properties of (Al,In)GaN optical amplifiers emitting at 430 nm and 450 nm. All samples have graded index separate confinement heterostructure (GRIN SCH) grown by MOVPE technique on bulk GaN substrates. We investigated properties of semiconductor optical amplifiers made by different type of devices like AR/AR coated broad area lasers, AR/AR coated tapered waveguide lasers and uncoated superluminescent diodes. The measured optical gain in CW operation in room temperature was over 20 dB for 1 mW of input optical power and over 10 dB for 10 mW of input power. We also investigated the temperature dependence of the amplifiers. The results shows that every type of the investigated devices can work as an optical amplifier, however the superluminescent diodes shows the best performance among others.

Resume : Today most of the InGaN-based laser diodes (LDs) are homo-epitaxially grown on 2-inch expensive free-standing GaN substrates, making the cost of LDs orders of magnitude higher than that of light-emitting diodes (LEDs) grown on hetero-epitaxial substrates. Si substrates have a few major advantages in wafer size, material cost, and uniformity, as well as its depreciated automation processing line. Direct growth of III-Nitride semiconductors on large diameter Si is a potential game-changing technology to GaN-based LD industry. However, hetero-epitaxial growth of GaN on Si encounters a large mismatch in both lattice constant and coefficient of thermal expansion, often resulting in a high density of defects and even micro-crack networks. By carefully engineering Al-composition step-graded AlN/AlGaN multilayer buffer between Si and GaN, we have not only successfully eliminated the crack formation, but also effectively reduced the dislocation density. Upon the high-quality GaN-on-Si platform, we have realized continuous-wave electrically injected blue/violet InGaN-based LDs at room temperature, which could also be a useful alternative on-chip light source for monolithic-integrated Si photonics. This work was financially supported by the National Key R&D Program of China (Grant No. 2016YFB0400104), the National Natural Science Foundation of China (Grant Nos. 61534007, 61404156, 61522407, and 61604168), the Key Frontier Scientific Research Program of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-JSC014), the Natural Science Foundation of Jiangsu Province (Grant No. BK20160401), and the China Postdoctoral Science Foundation (Grant No. 2016M591944).

Resume : We propose a strategy for increasing the overlap integral of electron and hole wavefunctions in c-plane high AlN mole fraction (xAl) AlxGa1-xN multiple quantum wells (MQWs) operating at around 260 nm. By applying out-of-plane one-dimensional quadratic modulations in composition (QCMs), xAl(z), in both the wells and barriers along the c-axis (z-direction), local bandgap energies, Eg(z), and concentrations of immobile residual charges originating from the polarization discontinuity, C(z), are simultaneously controlled throughout the MQW structure. We have examined several schemes of xAl(z), and eventually achieved the optimized band profiles: the overlap integral value is substantially increased to 93% when the well width (Lw) is smaller than 4.0 nm. In addition, the integral value greater than 80% is predicted even in thicker wells (Lw=10 nm). These predictions guarantee that our optimized QCM scheme has sufficient tolerance in latent errors in the well width and xAl(z). We note that such high overlapping values are also realized for the MQWs of wide range of emission wavelengths. The results presented here imply that the one-dimensional QCM is an effective method for improving the internal quantum efficiency of the MQW emission in polar c-plane AlxGa1-xN regardless of the well thicknesses. This finding is quite a useful information for designing laser structures, as the active layer of such large volume is advantageous to obtain a high optical confinement factor.

Resume : An uneven multiple quantum well (MQW) grown on an n-AlGaN template with high-density macrosteps exhbited the high external quantum efficiencies (EQEs) of 6.1 and 6.0% at 283 and 298 nm, respectively, without the use of a p-reflective electrode, encapsulation, and so forth. The n-AlGaN cladding layer with high-density macrosteps was grown on an AlN template on (0001) sapphire with a miscut of 1.0°relative to the m-plane. The n-AlGaN layer was estimated to have a threading dislocation density (TDD) of 5E8/cm² from (0002) and (10-10) omega-scan X-ray rocking curves (XRCs) with good reproducibility. Compared with n-AlGaN on sapphire with a 0.3° miscut, that on sapphire with a 1.0° miscut has lower TDD.[PCT/JP2011/068115 (2011)] The high EQE is considered to be due to the high internal quantum efficiency (IQE). The IQEs is considered to be greater than 60% which was conservatively estimated by using improbable light extraction efficiency of 10% and current injection efficiency of 100%. The TDD of n-AlGaN indicated carrier localization enhanced by macrosteps. Compared with the MQWs grown on 0.3° miscut sapphire, the electroluminescence (EL) outputs are 20% greater than those for 265 and 285 nm AlGaN-based DUV-LEDs and EL spectral width of DUV-LEDs grown on 1.0° miscut sapphire is broader, indicating the compositional modulation induced by macrosteps.[PCT/JP2015/060588 (2015)] The uneven MQW exhibited no adverse effects on the lifetime, showing the robustness of the AlGaN.

Resume : We report on successful electrical excitation of non-polar a-plane InGaN quantum dots (QDs). These were grown employing MOVPE with a method similar to the Stranski-Krastanov mode. The QDs lie within an electrically contacted p-i-n diode. As previously shown with photoluminescence studies, these QDs are sources of highly linearly polarised single photons. Here, we present a study of the polarisation properties of the electroluminescence of these QDs. A very high degree of linear polarisation is measured, deterministically linked to a special crystal direction. Furthermore, we study the second order correlation function of a QD under DC excitation and find evidence for single-photon emission with a g(0)=0.18, corrected for the instrument response function. This is unprecedentedly low for electrically driven nitride QDs. Finally, we probe the temperature dependence up to 80K of the same spectral line, underlining its assignment to a QD. Our approach to obtain electroluminescence differs from most previous studies on InGaN QDs by using planar epitaxial growth as opposed to either growing QDs as restricted quantum wells in nano-pillars or etching QDs out of quantum wells. In contrast to the latter, our method allows for future development towards structures such as photonic crystal cavities. Thus, our work presented here is a major step towards the development of a highly polarised, efficient, electrically pumped single-photon source.

Resume : The Auger process is widely cited as the limiting mechanism in the performance of InGaN LEDs, that is, it is responsible for droop. An alternative often-cited third-order process is electron overflow and escape of electrons from the active region of the LED heterojunction. Here we present experimental evidence to suggest that third-order processes, whether Auger or electron overflow, are not significant in AlGaN MQW?s at high Al content (i.e., >60%), under electron-beam excitation at room temperature. Such optical sources that emit in the deep UV have applications that include chemical/biochemical detection, water purification, surface decontamination, medical diagnostics and treatment, non-line-of-sight communication, and materials curing. A custom bench-top electron-beam-pumping system was designed and built to pump AlGaN devices with high-energy electrons (10-30 keV) at incident power densities exceeding 1 MW/cm2, with the system design extendable to compact versions. The AlGaN heterostructures were specifically designed and tailored to match the deposited energy distribution of the incident electrons; the heterostructure also included doped n-type conducting layers and metallization for electrical discharge but did not contain an electron-blocking layer. Using the focused e-beam pumping system we have demonstrated cathodoluminescence from these heterostructure with a record-high pulsed peak optical output power of 230 mW of spontaneous emission at ? = 246 nm, at room temperature. The dependence of the absolute output power was measured over a range of excitation power densities from < 0.1 kW/cm2 to 1 MW/cm2, with carrier density of ~1e20/cm3. A double-logarithmic plot of optical output power vs. excitation power density clearly reveals a slope of unity over the entire range. The results indicate that the carrier recombination process is dominated by radiative recombination at all measured levels of beam current. (Early designs of the AlGaN heterostructure did display a departure from unity slope at high excitation power densities toward a 2/3 slope.) Thus, our optimized devices show no evidence of significant contribution from non-radiative recombination at the lowest power densities or from Auger recombination or carrier overflow transport processes at the highest power densities. In summary, our findings suggest that third-order carrier recombination processes, either Auger or carrier overflow, are not significant in AlGaN at high Al content (i.e., >60%), at room temperature.

Resume : A better understanding of the fundamental materials properties of AlGaN is essential to optimize growth, simulation, and design of mid-IV optoelectronic devices. Here, the strength of polarization fields in AlGaN is discussed using power dependent photoluminescence measurements and investigations of the influence of MQW well width on the emission wavelength. AlGaN/AlGaN MQW structures with emission at 240-280 nm grown on c-plane (0001) sapphire and AlN substrates were investigated. First, an analytical model for carrier screening was established, which is based on perturbation theory. It predicts that photo-generated free carriers in the MQWs will screen the internal electric field and reduce the QCSE. Additionally, it showed a shift of the peak energy with different photo-generated carrier concentration. Experimentally, photoluminescence (PL) was used to characterize the peak energy shift with excitation power. As predicted, a shift of the peak position to higher energies with increasing excitation power was observed. The theoretical fitting curve showed good consistency with the experimental data over carrier densities spanning four orders of magnitude. The estimated internal electric field, based on this method, is around 1.1MV/cm-3. This is comparable to estimations and calculations for the emission peak shift of MQWs with different quantum well widths.

Resume : A vertical-cavity surface-emitting laser (VCSEL), invented by Iga in 1977, is one of the semiconductor-based laser diodes, emitting a laser beam in a direction vertical to the semiconductor wafers. GaAs-based VCSELs emitting red or infrared light have been already commercialized and utilized in various products, such as laser mice, laser printers, and high-speed data transmission systems. Now the VCSELs are expected to cover not only red but also blue and green regions, the three primary colors of light, targeting much broader ranges of applications, like retinal scanning displays, adaptive headlights, and high-speed visible light communication systems. Recently, intensive efforts have been made to develop GaInN VCSELs emitting violet or blue light, showing a considerable amount of improvements. In this talk, we present current status and prospects of the GaInN VCSELs, including our results. First, the developments of lattice-matched AlInN/GaN distributed Bragg reflectors (DBR), which is one of the most important structures in the VCSELs, are described. We have successfully obtained high quality AlInN layers grown with high growth rate. Based on the AlInN growth, we also developed Si-doped conducting AlInN/GaN DBRs. Next, we describe our GaInN VCSEL characteristics. A 3mW light output power at room temperature and an operation up to 90 degree C have been demonstrated. Finally newly developed key structures for GaN-based VCSELs, such as buried tunnel junctions for current confinement and convex structures for lateral optical confinement, will be described.

Resume : We are reporting an optically pumped vertical cavity surface emitter laser (VCSEL) in deep UV region. The device structure was a hybrid design with a 2lambda cavity sandwiched by 31-pairs AlGaN/AlN DBR and 10-pairs HfO2/SiO2 DBR. The VCSEL cavity consisted of an active region with 5 pairs of 2nm Al0.65Ga0.35N quantum well and 5nm Al0.80Ga0.20N barrier on top of Al0.80Ga0.20N spacer layer. The emission wavelength of the active region was set at 247 nm to match the reflectivity peak of the AlGaN/AlN DBR. Both of the semiconductor based DBR and cavity were epitaxially grown on 1.5 um thick AlN/sapphire templates via metalorganic chemical vapor deposition while the dielectric DBR was formed by e-beam evaporation. The reflectivity values at 247nm of the AlGaN/AlN and HfO2/SiO2 DBRs were 94 and 98% respectively. Under pulse excitation of 193nm ArF excimer laser at room through the dielectric DBR, we observed stimulated emission peak at 245.7 nm with the threshold excitation power density of ~350 kW/cm^2. Details of VCSEL cavity and DBR growth and their characteristics will be presented.

Resume : Semiconductor lasers have been attracting much attention since their birth in 1962. Compared with solid-state and gas lasers, semiconductor lasers are featured with small volume, low operation current and voltage, low cost etc. Determined by the band gaps of semiconductor materials, the semiconductor lasers can cover a wide spectral range from violet to infrared and some of them have been well studied and/or successfully commercialized. However, semiconductor lasers with wavelengths in the green region, typically 500 ~ 600 nm, are still undeveloped, which is called the ‘green gap’. Two-dimensional InGaN quantum-wells (QWs) are commonly used as the active region of violet, blue and green semiconductor lasers. To get green emission lasers, higher In content in the InGaN QW layer is necessary, which causes more defects and larger strain-induced built-in electric field. Defects capture carriers and built-in electric field causes Quantum Confined Stark Effect (QCSE). These reduce the emission efficiency of the green InGaN QW greatly and are the main reasons of the “green gap”. Quantum dot (QD) is an ideal candidate to solve these problems. Strain is released during the self-assembled growth procedure and the deep localization state in QDs impedes carriers from being captured by defects. QDs are zero-dimensional structures in which electrons and holes are strictly confined in a small volume, which is particularly important to get high emission efficiency and low threshold current. Therefore, QDs have great potential in fabricating green lasers with excellent performance. In this work, by combination of InGaN QDs and vertical-cavity structures, we demonstrate room temperature, continuous wave and low threshold green lasing from InGaN-QD based VCSELs for the first time. The lasing wavelength extends from 491.8 nm (blue-green) to 565.7 nm (yellow-green), covering most of the ‘green gap’. For different VCSELs, the same QD active region was used with the wavelengths controlled by adjusting the cavity length. The emission wavelength, threshold current and CW lasing are record results for GaN-based green VCSELs. In addition, the devices are bonded on a copper plate to improve thermal dissipation and the thermal resistance of the VCSEL is as low as 915 KW-1. The ability of controlling the emission wavelength is believed to be useful in full-color small-size laser displays and projectors.

Resume : We demonstrate the laser operation of an optically pumped GaN-based vertical cavity surface emitting laser (GaN-VCSEL) with a 50-µm (281λ) cavity containing a curved mirror that is monolithically formed on a GaN wafer. The output wavelength of the device is 434 nm. The threshold excitation power for a 349-nm injection beam (spot size: 200 µm, pulse width: <5 nm, frequency: 1kHz) is 0.3 mW (0.96 mJ/cm2), which is half that for the GaN-VCSEL with a 4.5-µm (25λ) cavity having planar mirrors [T. Hamaguchi et al., Physica Status Solidi (a), 213 (5), 1170–1176 (2016).] and is comparable to values reported for 8.5λ cavity with planar mirrors [W. J. Liu t. al. Scientific Reports, 5, Article No.9600 (2015)]. The proposed structure allows for GaN-based VCSELs to be constructed with long cavities, which greatly simplifies the fabrication process. A curved mirror not only compensates diffraction loss but also laterally confines the optical modes [H. Kogelnik et al., Applied Optics, 5, 10, 1550 (1966)], which theoritically allow for aperture miniaturization to the sub-micron scale and for the threshold current to be reduced to sub-mA. Because this idea is applicable for semi-polar GaN substrates, we believe these results to be a milestone for GaN-based green VCSELs that, until now, have been difficult to grow on c-plane GaN wafers.

Resume : RT-CW operations of GaN-based VCSELs have been reported [1-3] but showing higher threshold currents and/or lower slope efficiencies compared to those of GaAs-based VCSELs. Hashemi et al. pointed out that such inferior characteristics could be due to no lateral optical confinement in a GaN-based VCSEL, and proposed a convex structure for better lateral optical confinement [4]. In this study, we investigated GaN-based VCSELs with the convex structures for lateral confinement. Our VCSEL structure was basically the same as that in ref. 3. Then, an additional dielectric mesa was inserted just on the ITO contact to form the convex structure. The convex structure resulted in longer resonant wavelength, corresponding to a larger refractive index. A current confinement structure was still formed by the combination of an ITO contact and a SiO2 aperture with an 8 µm diameter. I-L-V characteristics and lasing spectra were measured under a RT-CW condition. A threshold current was 2.1 mA, which was one third of a typical value in a standard VCSEL without convex structure. We then found that clear multiple peaks were observed in the lasing spectra. In addition, a daisy-like NFP was also observed in the convex VCSELs while a single peak pattern was observed in a standard VCSEL. These results suggests that our convex VCSELs should have lateral optical confinement. [1] D. Kasahara et al, Appl. Phys. Express, 4, 072103 (2011). [2] G. Cosendey et al, Appl. Phys. Lett., 101, 151113 (2012). [3] K. Matsui et al, Jpn. J. Appl. Phys., 55, 05FJ08 (2016). [4] E. Hashemi et al, Jpn. J. Appl. Phys., 52, 08JG04 (2013).

Resume : Monolithically integrated high-reflectivity (HR) mirrors for III-N vertical cavity surface emitting lasers are a major focus of study in optoelectronics in the visible spectral region of 420-440nm. Implementations of such HR mirrors include dielectric distributed Bragg reflectors (DDBRs) (e.g. SiO2/HfO2), III-N-based DBRs, and nano-porous GaN/GaN DBRs. There is a desire to minimize thickness in these stacks, a particularly difficult feat for near-ultraviolet 300-370nm DBRs. A simulated “airgap DBR” comprised of 5 pairs of (Al)GaN and free space would produce a reflectivity of 99.5% at 370nm and stopband of 46 nm. In this work, we report a complementary conductivity-selective wet-etch method that etches either highly or lightly-doped (Al)GaN n-type layers with alternating binary doped profiles to achieve a versatile air-gap/semiconductor DBR. In particular, an H3PO4–based electrochemical (EC) etch selectively etches highly-doped n-type AlGaN (n > 1e18 cm-3) over unintentionally doped (UID) (Al)GaN. An KOH-based electrode-less photoelectric chemical (ELPEC) etch optimally etches UID (Al)GaN and preserves n-type (Al)GaN. These etches were tested on MOCVD-grown structures with up to 10 pairs of n+-(Al)GaN and UID (Al)GaN, with AlN mole fraction up to 0.05. The etch selectivity of >300 was found, and air/n-AlGaN and air/UID-AlGaN DBR microstructures were fabricated. Detailed analysis of the selectivity and related properties of the wet etches will be discussed in the conference.

Resume : Bottom-up grown InGaN-based nanocolumn photonic crystals (NC-PCs) exhibit excellent emission properties and the light diffraction at -point band edges contributes to lasing of NCs based on two-dimensional distributed feedback (2D-DFB) scheme. We demonstrated blue to green optically pumped lasing of triangular-lattice NC array. Increased NC period (L) and diameter (D) result in the red-shift of the band edge wavelength; for a 550 nm lasing, D =210 nm and L=275 nm. However, the increased NC diameter tends to undermine the nano-crystalline effect, such as dislocation-filtering and strain relaxation. To overcome the situation, in this study, we present a novel arrangement of nanocolumns, i.e., cluster array of NC-PCs. In this scheme, neighboring 3 to 7 thin NCs are clustered, which clustering unit is arranged in triangular lattice. Each nanocolumn keeps a small size, but a proper period (L’) for the arrangement of clustering unit provides a longer wavelength. The cluster array of NC-PCs with 3-, 4-, and 7-cluster were prepared on GaN templates by Ti-mask selective area growth of RF-MBE. The novel NC-PCs exhibited sharp photoluminescence emission peaks corresponding to the light diffraction at the photonic band edge. The angle resolution PL measurement was performed to provide experimentally the photonic band diagram. For a 4-cluster NC-PC with an extremely small D of 100 nm and L’=300 nm, the lasing at 546 nm was observed with the threshold excitation density of 0.35 MW/cm2.

Resume : Exciton cavity polaritons (polaritons) are mixed light?matter quasi-particles generated by strong coupling between cavity photons and excitons. This hybrid nature of polaritons pave the way for development of new concept for nonlinear photonic devices such as polariton lasing, switching, and transistor. Especially, group III nitride-based microcavities make up an excellent system for both electrically and optically driven polariton devices at room temperature. However, nitride materials have an intrinsic polarization along c-axis induced by asymmetric wurtzite crystal structure. In addition, multiple-quantum wells (MQWs) grown along this polar direction severely suffer from quantum-confined Stark effect, weakening the oscillator strength. Here, we first demonstrated a novel structure for polariton system resulting from strong coupling between whispering gallery mode (WGM) photons and two-dimensional excitons using a core?shell GaN/InGaN MQWs hexagonal wire. High quality and nonpolar InGaN MQWs were conformally formed on a GaN core nanowire. The excitons in the MQWs are spatially well matched with photons of WGMs inside the wire. Both high oscillator strength of nonpolar MQWs and spatial overlap with WGMs lead to giant large Rabi splitting energy of ?180 meV. This structure offers a robust polariton system with a small footprint, which can be utilized for various interesting applications.

Resume : Optical strong light−matter coupling describes the phenomenon of energy oscillating between an optical cavity and a two-level system. This periodic energy exchange can be observed in the frequency domain of various observables such transmittance or photocurrent as a splitting of the single cavity resonance into two polariton branches. We present the design, realization, and characterization of optical and electrical strong light−matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials. Using bound-to-bound transition in 40 periods quantum cascade detector as a two-level system and metamaterial nano-cavities with deep sub-wavelength mode-volumes (Veff=2x10-3 (λ/n)3), we demonstrate normal-incidence room temperature strong coupling with Rabi splitting on the order of 15% of the fundamental bare cavity frequency at short wave infrared spectral region of 1.8 micron. As the bare metamaterial resonance, frequency is varied across the intersubband resonance, a clear anti-crossing behavior is observed in the frequency domain both in transmission and photocurrent measurements. By changing the metamaterial array period the photocurrent response changes from light-matter interaction that occurs at the single resonator level to collective resonance phenomenon that enhances the photo-signal responsivity from 2 mA/W to 15 mA/W and open the way to normal incidence two dimensional multi-element planar tunable infra-red detector.

Resume : In order to obtain polariton condensation at 300K, materials with large oscillator strengths and large exciton binding energies must be employed as active regions. In this communication, epitaxial GaN cavities grown by MBE on patterned silicon substrates [1] will be used to illustrate some of the particularities of polariton condensation in large Rabi splitting microcavities (80meV). These Rabi values, combined with large photon lifetimes leading to Qs larger than 2000, enable to achieve polariton condensation from low to room-temperature in a large detuning range between the exciton resonance and the cavity photon mode [2]. In determining the condensation phase diagram of this microcavity it has been found that, contrary to a common assumption in wide-bandgap microcavities, the strong exciton-LO phonon interaction does not necessarily lead to a reduction of the polariton condensation threshold. The conditions for this to hold will be discussed. Furthermore, thanks to a homogenous photonic potential, with variations of the bare cavity mode smaller than 4% of the Rabi at a micrometer scale, we have been able to study experimentally how does the condensation threshold varies with the excitation spot size. It will be finally shown that the optimum spot size is determined by the typical spatial scale associated to the photonic inhomogeneous broadening. [1] J. Zuniga-Perez et al., Appl. Phys. Lett. 104 (2014) 241113. [2] O. Jamadi et al., Phys. Rev. B 93 (2016) 115205

Resume : Microlasers covering the UV to blue spectral range are important building blocks for biochemical detection applications. This work presents a series of microdisk lasers realized within the same GaN-on-Si photonic platform scheme, and operating at room temperature under pulsed optical pumping over a broad spectral range extending from 275 nm to 470 nm. The III-nitride microdisks embed either binary GaN/AlN multiple quantum wells (MQWs) for UV operation, or ternary InGaN/GaN MQWs for violet and blue operation. The fabrication process is based on e-beam lithography and the release of the nitride membrane after selective under-etching of the silicon substrate, and has led to high quality factor resonators, microdisks as well as photonic crystal cavities, within the same platform. A detailed comparison of these microlasers leads to the conclusion that for our Q>1000 microdisk resonators, the pumping threshold is determined by the type of optical excitation, resonant with the barrier or with the MQW excited states, and the emission efficiency of the MQWs [Sellés et al., APL 109, 231101 (2016) and Sci. Rep. 6, 21650 (2016)].

Resume : Dilute antimonide III-nitride semiconductors have emerged as a new generation of compound semiconductors. By introducing a very small amount (1-5%) of antimony (Sb) into the host lattice, the bandgap of GaN can be reduced to as low as 2 eV. First principle studies reveal that the reduction of GaN energy bandgap with Sb incorporation is primarily due to upward shift of the valence band-edge, which is in direct contrast to downward shift of the conduction band-edge by alloying with In. Therefore, precise tuning of the band-edges can be obtained, and the energy bandgap can be drastically reduced to deep visible and near-infrared spectral range with simultaneous incorporation of In and Sb in GaN, while maintaining a relatively small lattice mismatch to the underlying GaN template/substrate. Herein, we have successfully synthesized InGaSbN nanowire heterostructures and demonstrated that the emission wavelengths can be readily varied from the blue, green to red spectral range with small amount of Sb incorporation in InGaN. Subsequently, we have demonstrated the first dilute-Sb quaternary III-nitride nanowire functional LEDs where Sb incorporation eventually leads to red light emission from InGaSbN, compared to green light emission from InGaN under similar growth condition. This work has shown the extraordinary potential of dilute Sb III-nitrides to emerge as a new quantum material platform for the development of high efficiency optoelectronic, photonic and electrochemical devices.

Resume : The current-voltage (I-V) curve of the pn-junction device contains the most important electrical characteristics and is typically described by the Shockley equation. The Shockley equation has been first developed for homo-junction devices. However, in InGaN-based hetero-structure devices, not only the diffusion current but various current transport and recombination mechanisms play an important role. Hence, it is insufficient to use the simple Shockley equation to explain the I-V characteristics of the InGaN-based LEDs employing the MQWs as an active layer. In the LED devices, the injected current can be separated into two components: the radiative (IR) and the nonradiative current (INR). It is essential to understand the characteristics of each current component in LED devices to find a way to represent the I-V curve more accurately. While there have been frequent analyses of the I-V curve by the Shockley equation, the data of IR and INR vs. V still requires a deeper understanding. In this presentation, we show the characteristics of IR and INR as a function of V in the LED devices. We carefully analyze each current component as a function of V. Based on these analyses, we give an explanation on the characteristics of IR and INR as a function of V including the physical mechanisms of carrier dynamics. Lastly, we propose the modified Shockley equation for modern InGaN-based LEDs.

Resume : Micron and nanoscale structures made from III-N materials have many promising applications in photonics. This paper reports on the optical properties of core-shell microtubes achieved by a hybrid top-down/bottom-up approach. GaN microtube array was created from a 2µm thick layer by combining Displacement Talbot Lithography, to define the ring pattern, and Inductive Coupled Plasma dry etching. GaN faceting and InGaN shell regrowth was subsequently performed by Metal organic vapor phase epitaxy. Characterisation was performed by room temperature Cathodoluminescence hyperspectral imaging, SE imaging and Energy Dispersive X-ray (EDX) measurements. The InxGa1-xN shell grown on the outer m-plane facets was observed to emit in a broad band composed of multiple peaks, with the center emission energy of the band varying between 2.51 eV and 2.4 eV, from top to bottom. InxGa1-xN shell was also observed on the inner facets of the tubes with an emission energy varying between 2.4 eV and 2.3 eV, from top to bottom. The lower inner emission was found to be associated with the formation of a-plane facets while EDX measurements suggest that the energy variation across the length of the microtube is related to an inhomogeneous Indium incorporation. The high filling factor and dimensions of the array imply that this phenomena results from a combination of surface diffusion and gas-phase diffusion, pointing out the growth challenges on the outer and inner walls of hollow cylinders of GaN.

Resume : AlGaN based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due to the low refractive index contrast and high strain between AlGaN alloys (tensile strain of ~2.47% for GaN on AlN). We propose exploiting near-bandedge excitonic resonances in the real part of AlGaN?s dielectric function to create a large refractive index contrast (7-11%) between AlxGa1-xN and AlN. AlGaN?s refractive index increases sharply near the bandgap before band-to-band absorption becomes dominant. Using numerical simulation, we also investigate the trade-off between increased refractive index contrast and increased absorption losses as the photon energy approaches the bandgap, along with its impact on peak reflectivity and stopband width. Using this technique, we have grown tri-layer and bi-layer UV-C DBRs by MOCVD with reflectivities as high as 97% at 226 and 247 nm. The formation of a tri-layer DBR may have been due to the composition pulling effect. A tri-layer DBR has a lower peak reflectivity and narrower stopband than an equivalent bi-layer DBR. The sensitivity of the DBRs? peak reflectivity, center wavelength and stopband width to fluctuations in the period thickness and composition have also been studied by numerical simulation. At such short wavelengths, the center wavelength is very sensitive to variations in the layer thicknesses, and a variation of even two monolayers (~0.5 nm) per period leads to a shift in the center wavelength by ~1.5 nm.

Resume : In the past two decades, significant progress has been made in AlGaN quantum well ultraviolet (UV) light emitting diodes (LEDs). Their performance in the UV-C band, however, is much inferior to that of GaN-based blue/green/near-UV LEDs. They are generally characterized by low external quantum efficiencies (EQEs) and large device operation voltages, which is largely attributed to the commonly measured large dislocation and defect densities, poor hole transport, and low light extraction efficiency. Ternary AlGaN nanowire UV LEDs have also been investigated but they can only produce very low output power (below 1 uW). In this context, we have investigated the molecular beam epitaxial growth and characterization of AlGaN tunnel junction (TJ) UV LEDs on Si substrate. The LED structure consists of n+-GaN/Al/p+-AlGaN TJ, AlGaN double-heterojunction, and n-/p-GaN contact layers. It is found that with the use of TJ, the device resistance is reduced by one order of magnitude, and light output power is increased by two orders of magnitude, compared to reference AlGaN nanowire UV LEDs without TJ. For devices operating at 242 nm, a maximum output power of 0.37 mW is measured, with a peak EQE of 0.012%. To the best of our knowledge, this is the shortest wavelength ternary AlGaN nanowire LEDs. These performance characteristics are also significantly improved compared to the previously reported AlGaN nanowire UV LEDs, and are close to AlGaN quantum well UV LEDs operating around this wavelength. AlGaN nanowire TJ UV LEDs operating at 275 nm will also be discussed. At this wavelength, a maximum output power of 9 mW and a peak EQE of 0.4% are measured. A detailed characterization of Al TJ and further improved device performance will be presented.

Resume : Despite all the advances in nanowire growth and fabrication, the basic physics of these structures is not very well understood. To be able to exploit a nanowire to the best of its capabilities, it is important to thoroughly understand the behavior of the device. We have presented an investigation of size dependent carrier and photon dynamics in InGaN/GaN nanowire array of 20 nm and 50 nm in size with respect to the well. The nanowires were prepared by a combination of dry and wet etching. These nanowires manifested their higher efficiencies through very bright photoluminescence. Nanowires support higher extraction efficiency coming from large surface-to-volume ratio and relatively fewer defects [1]. We observed the shift in PL peak to the higher energy side and narrower FWHM with increase in confinement resulting from 1D density of states indicating their suitability for the LEDs and lasers. The time resolved absorption of these samples suggested that the nanowires facilitate faster recombination owing to increased electron-hole wavefunction overlap originating from 2D confinement and reduced QCSE [2]. We verified the improved performance by performing systematic experiments, which is further verified theoretically. These properties make them suitable for the effective implementation of nanowire based solid state lighting [3]. References- 1. Nano Lett., 2016, 16 (2), pp 973–980. 2. PHYSICAL REVIEW B 93, 205410 (2016). 3. ACS Photonics, 2016, 3 (12), pp 2237–2242.

Resume : Nitride-based III-V semiconductors have been successfully used in the fabrication of blue and green light-emitting (LEDs) and laser diodes (LDs) for a long time already, leading to a whole revolution in lighting technology, yet their full potential has not been completely unchained. Ultraviolet emitters are still a great challenge that is being researched worldwide. Problems begin as the wavelength is reduced just below 400nm, even when the usual and well-tested InGaN/GaN multiple-quantum-well structure is used. LEDs demonstrate low quantum efficiencies, often believed to be caused by insufficient carrier confinement. But LDs have a different problem, as their otherwise very satisfying operating parameters are penalized only by the short lifetime, which prevents practical applications and commercialization for most manufacturers. The reason for the early failure of 360-400nm LDs has not been clarified yet. We are advancing hypotheses and looking for possible tweaks in our epitaxial process to counter them. In particular, we are investigating crystal impurities of different kinds, their diffusion during device operation, and how the presence of hot electrons and intense ultraviolet radiation can affect the whole picture (for example, we developed epitaxial structures with superlattices designed to affect the probability of hot electrons). In this poster, we would like to share with you our results so far.

Resume : Reducing the threading dislocation is one of the key priorities in advancing the commercial viability of high-efficiency, deep-UV light emitting diode (LED) (operating at 250?280 nm) used for sterilization. We report on the high optical and structural quality of AlGaN/AlGaN based deep UV multiple quantum wells (MQWs) emitting at ~ 270 nm. The structure was investigated optically, using photoluminescence (PL) and time resolved photoluminescence (TRPL) spectroscopy. An ultrafast Ti:Sapphire laser accompanied by a harmonic generator of the third order (266 nm) was used as the excitation source, while a CCD camera attached to a streak camera was used to detect the signals from the sample. The LED structure was grown by metal-organic chemical vapor deposition. Initially, an AlN regrowth layer was deposited over the AlN substrate. This layer was followed by AlGaN superlattices and an Si-AlGaN layer, respectively. The active layer, grown over the Si-AlGaN layer, comprised of 3nm Al0.4Ga0.6N/ 12 nm Al0.5Ga0.5N MQWs. Finally, an mg-AlGaN electron blocking layer was grown to cap the MQWs. Using Shockley-Read-Hall (SRH) method, we demonstrate that the MQW sample has a high internal quantum efficiency (~ 95%). We attribute this result to the significantly diminished density of dislocations in the MQWs, owing to the close lattice match between the substrate and the MQW layers. The power and temperature dependence of the carrier dynamics were investigated using TRPL and PL excitation measurements. TRPL measurements show a single exponential PL lifetime of ~ 1.5 ns, with radiative recombination processes dominating the recombination rate at all examined power and temperature settings. Planar and cross-sectional transmission electron microscopy (TEM) and scanning TEM characterizations, were performed to systematically investigate the nature of dislocations and other structural defects.

Resume : The development of semiconductor light sources operating in the deep-ultraviolet (DUV) region, such as UV light-emitting diodes (LEDs) and laser diodes (LDs), has been attracting considerable attention because of their wide variety of potential applications such as air and water purification, surface disinfection, UV curing, and medical phototherapy. However, compared with InGaN-based visible LDs or LEDs, the development of DUV light sources based on AlGaN material system is still a challenge. High-quality AlN templates are promising candidates for the growth of high-Al-content AlGaN-based high-power DUV light sources. However, very high threading dislocation densities (TDDs) are usually observed in AlN templates grown on commonly used substrates, such as sapphire, owing to the large lattice mismatch between the films and substrates. In this work, we report on the characterization of high quality AlN epi-layers grown directly on high-density cylindrical patterned sapphire substrates (CPSS) using hydride vapor phase epitaxy (HVPE) for the first time, and evaluate these templates by AlGaN-based multiple-quantum wells (MQWs) on these AlN templates. The pattern’s diameter, height and a center-to-center distance of the CPSS in this experiment are 1.5, 0.5 and 2.0 µm, respectively. The AlN epi-layers were grown on CPSS by HVPE at a relatively low temperature (1140℃) and polished by chemical mechanical polishing (CMP) for improving the surface morphology. The full-width at half maximum (FWHM) of the X-ray (0002) and (10-12) ω–scan rocking curves of the AlN epi-layer after CMP were 267 and 388 arcsec, respectively. The surface roughness on the AlN epi-layers after CMP was approximately 0.2 nm, as measured by atomic force microscopy (AFM). To evaluate the effect of AlN-on-CPSS template, we measured the photoluminescence (PL) of the AlGaN MQWs simultaneously grown on AlN-on-CPSS and AlN-on-plane sapphire templates. The PL peak wavelength for the AlGaN MQWs grown on AlN-on-CPSS and the AlGaN MQWs grown on AlN-on-plane sapphire templates are 285 and 283 nm, respectively. The peak intensity of PL spectrum for the AlGaN-based MQWs grown on AlN-on-CPSS template was enhanced by 150% compared to that of the AlGaN-based MQWs grown on AlN-on-plane sapphire. These results show that the AlN-on-CPSS template can significantly improve the performance of AlGaN-based DUV light source.

Resume : Manipulating stimulated emission light in nanophotonic devices on scales smaller than their emission wavelengths to meet the requirements for optoelectronic integrations is a challenging but important step. Surface plasmon polaritons (SPPs) are one of the most promising candidates for sub-wavelength optical confinement. In this report, based on the principle of surface plasmon amplification by the stimulated emission of radiation (SPASER), we fabricated a designed III-Nitride-based plasmonic nanolaser with hybrid metal-ultrathin-oxide-semiconductor (MUTOS) structures. Using geometrically elliptical nanostructures fabricated by nanoimprint lithography, elliptical nanolasers able to demonstrate single-mode and multi-mode lasing with an optical pumping power density as low as 0.3 kW/cm2 at room temperature and a quality Q~factor of up to 123 at a wavelength of ~ 490 nm were achieved. The ultra-low lasing threshold is attributed to the SPP coupling-induced strong electric field confinement in the elliptical MUTOS structures. In accordance with the theoretical and experimental results, we found that the size and shape of the nanorods (NRs) are key for manipulating the hybridization of the plasmonic and photonic lasing modes in the SPASER. This finding provides innovative insight that will contribute to realizing a new generation of optoelectronic and information devices.

Resume : AlGaN related deep-ultraviolet light emitting diodes (deep-UV LEDs) are in strong demand for various applications including sterilization, water purification, and decomposition of chemical materials. In addition, deep-UV light of which wavelength is shorter than 280 nm is useful for virus elimination, since the light with this wavelength can break the molecular bonding in virus. However, deep-UV LEDs still have lower efficiency than required for these applications, mainly due to its relatively lower material qualities of layers with high Al contents. Therefore, detailed understanding of defects in AlGaN active region is very important. In this study, we observed anomalous photocurrent reversal in AlGaN-based deep UV LEDs as the wavelength of illuminating light varied and investigated the relationship between this phenomena and defect formation at the interfaces in the active layers. The wavelength dependent photocurrent measurements were performed in a series of 265nm deep-UV LEDs on zero-bias condition. A sharp near band edge (~265 nm) absorption was observed in addition to a broad (350~700 nm) peaks in photocurrent spectroscopy, while the current direction of these two peaks was opposite. In addition, the current direction of the longer wavelength absorption peak was reversed when a certain forward bias was applied. This forward bias induced current reversal also showed very slow recovery time after the applied bias is removed. This result can be consistently explained by existence of hole trap at AlGaN/GaN interface and those related band bending caused by piezo- and spontaneous polarization field. Temperature dependent changes of this photocurrent reversal were also measured in order to confirm our interpretation. The possible origin of these interface defects is also be discussed.

Resume : Effective acceptor doping of wide-band-gap semiconductors is still an outstanding problem. Beryllium has been suggested as a shallow acceptor in GaN, but despite sporadic announcements, Be-induced p-type doping has never been practically realized. Be-doped GaN possesses two luminescence bands; one at 3.38 eV and a second near 2.2 eV at an energy close to that of the parasitic yellow luminescence often found in undoped GaN crystals. We have performed high hydrostatic pressure studies of bulk, Be-doped gallium nitride crystals using the diamond anvil cell technique. We observed a splitting of the yellow luminescence line under hydrostatic pressure into two components, one which is strongly dependent on applied pressure and another whose pressure dependence is more modest. Together with hybrid functional calculations, we attribute the strongly-varying component to the beryllium-oxygen complex. The second component of the yellow luminescence possesses very similar pressure behavior to the yellow luminescence observed in undoped samples grown by the same method, behavior which we find consistent with the CN acceptor. At higher pressure, we observe the vanishing of yellow luminescence and a rapid increase in luminescence intensity of the UV line. We explain this as the pressure-induced transformation of the Be-O complex from a highly localized state with large lattice relaxation to a delocalized state with limited lattice relaxation.

Resume : AlGaN-based ultraviolet light-emitting diodes (UV LEDs) have recently attracted great attentions due to their promising potential in numerous applications. Along with the fast growth of UV LED market, reduction of the manufacturing cost of UV LEDs is getting more and more important. Epitaxial integration of GaN on large size Si substrates has been widely deemed as one of the most effective approaches for a significant cost reduction of GaN-based LEDs. In this work, we report high-power AlGaN-based 385 nm ultraviolet light-emitting diodes (UV LEDs) grown on Si(111) substrates. High-quality crack-free UV LED epitaxial structures on Si(111) were realized by using AlN/AlGaN multi-layer buffer, with X-ray rocking curve (XRC) full-widths at half maximum (FWHMs) around (0002) and (10-12) of 299 and 320 arcsec, respectively. Vertical thin-film (TF) UV LED chips were fabricated employing silver based reflective ohmic electrodes. At a forward injection current of 500 mA (1000 mA), a typical TF-UV LED exhibited a light output power (LOP) of 960 mW (1766 mW), corresponding to an external quantum efficiency (EQE) of 59.7% (55.0%), and a wall plug efficiency (WPE) of 52.9% (44.0%). The peak EQE of the UV LED reached 61.9% at 150 mA, and the efficiency droop at 1000 mA with respect to the peak EQE was 6.9%. The high power efficiency of UV LEDs grown on Si(111) could be attributed to the high crystalline quality and the enhanced light extraction of the TF-UV LED structure.

Resume : Al-rich AlGaN and AlN are ideal choices for achieving solid-state deep ultraviolet (DUV) light sources. To date, however, the realization of high efficiency DUV optoelectronic devices has been limited by the presence of dislocations and the inefficient p-type conduction in conventional planar AlGaN structures. In this work, AlN/h-BN nanowire heterostructures were grown on a silicon substrate by plasma-assisted molecular beam epitaxy under nitrogen rich conditions. Following the growth of AlN nanowires, a h-BN thin layer was subsequently deposited using an in situ electron beam source. The AlN/h-BN nanowire heterostructures exhibit stronger photoluminescence emission at 210 nm compared to the AlN nanowires without the h-BN capping layer, attributed to the reduced surface recombination. We have further demonstrated, for the first time, a functional AlN/h-BN nanowire light emitting diode that operates at 210 nm. The underlying AlN segment is doped n-type using Si. By exploiting the acceptor-like boron vacancy formation, we have discovered that h-BN can function as a highly conductive, DUV-transparent p-type electrode. As a consequence, the AlN/h-BN device shows excellent current-voltage characteristics, with a turn-on voltage < 6 V. Moreover, the electroluminescence intensity of such an AlN/h-BN nanowire LED device is more than one order of magnitude stronger than AlN p-i-n LEDs without a BN layer, which is attributed to the enhanced hole injection and reduced electron overflow.

Resume : Radio-frequency sputtering is a powerful and low-cost technique for the deposition of low-temperature large-area AlInN for application in photovoltaics. We report the effect of introducing an AlN buffer layer (d = 0, 4, 8 and 15 nm) on the electrical properties of a 80-nm thick n-Al0.38In0.62N (absorption band edge ~625 nm) on p-Si(111) heterojunction, to improve the carrier extraction. Mesas were defined by sputtering with an Ar plasma, while ohmic contacts were formed by Ti/Al/Ni/Au (n) and Ti/Al (p) nitrogen annealed 5 min at 550ºC. For devices without buffer, the peak external quantum efficiency (EQE) is mitigated (27% at 500 nm) due to a reduction of the internal electric field at the AlInN/Si interface. Best results are achieved for d = 4 nm, which leads to an EQE of 44% at 560 nm thanks to the contribution from the AlInN added to the one of Si. For thicker buffers the electric field at the heterointerface increases at the price of reducing collected carriers (i.e. the EQE drops) due to inefficient transport through the AlN barrier. Illuminated current-voltage curves (1 sun AM1.5G) point to conversion efficiencies of 0.68, 1.52, 0.41 and 0.005% for increasingly AlN thickness, showing a constant current for negative bias with a kink at ~-0.6 V. By reverse biasing the structures with d = 0 and 4 nm, we achieve a broad and flat EQE (62% at 700 nm) for -0.6 V, in accordance to J-V curves. Results are compared to theoretical simulations to identify the responsible mechanisms.

Resume : A pencil-like morphology of homoepitaxially grown GaN nanowires is exploited for the fabrication of thin conformal intra-wire InGaN nanoshells, which host quantum dots in nonpolar, semipolar and polar crystal regions. All three quantum dot types exhibit single photon emission (g(2)(0) < 0.5) with narrow emission linewidths (< 2 meV) and high degrees of linear optical polarization (>70%). The host crystal region strongly affects both single photon wavelength (covering 370 – 520 nm spectral range) and emission lifetime, reaching sub-nanosecond time scales for the semi- and nonpolar quantum dots (< 500 ps). A detailed insight into optical properties of InGaN nanoshells, performed by low-temperature TEM-CL, reveals strong variations of emission wavelength and emission intensity on few-nm spatial scale, throughout the nanoshell structure. The observed variations are attributed to random In fluctuations which induce localization sites in the varying InGaN potential landscape. These localization sites trap 0D excitons, acting thus as single photon sources, with linear optical polarization, varying emission wavelengths and lifetimes. The hereby reported pencil-like InGaN nanoshell is the first single nanostructure able to host all three types of single photon sources and thus a promising building block for tunable quantum light devices integrated into future photonic circuits.

Resume : By using the Mg pre-flow technique for the growth of the AlGaN electron blocking layer, the Mg-doping concentration in this layer can be significantly increased. In this situation, the increased hole concentration near the boundary between the AlGaN layer and the top quantum-well barrier can reduce the potential barrier of the AlGaN layer for hole tunneling and increase the potential barrier for blocking electrons. Therefore, by increasing the Mg doping concentration in the AlGaN layer, carrier injection efficiency and hence LED emission efficiency can be enhanced. The Mg pre-flow technique means the growth pause of AlGaN or GaN for a duration of 5-10 min, in which Mg supply into the MOCVD growth chamber is continued. Based on this technique, LED output intensity can be increased by up to 10 times with the turn-on voltage reduced by 0.3-0.4 V although the device differential resistance is slightly increased. Simulations are undertaken based on the LED structure obtained from the observation of the transmission electron microscopy and the Mg concentration distribution obtained from the observation of the secondary ion mass spectroscopy. From simulations, it is found that the potential barrier of the AlGaN layer is indeed decreased for holes and increased for electrons. The electron and hole densities trapped by the quantum wells are indeed significantly increased. When the Mg concentration is increased in the p-GaN layer, instead of the p-AlGaN layer, the effect is reversed.

Resume : Si photonics compatible with large-scale low-cost fabrication technology is deemed as a promising path to realize optical interconnections and optoelectronic integration. However, Si with an indirect band structure cannot emit light efficiently. III-nitride semiconductors with a direct band structure have been widely used for efficient light emitting diodes and laser diodes with a huge commercial success. Microcavity lasers featured with low-loss and high-quality whispering gallery modes hold a great potential for ultralow-threshold lasing. Therefore, GaN-based microcavity lasers directly grown on Si may serve as an alternative on-chip light source for Si photonics application. High quality GaN film grown on Si is the key material foundation for the realization of microcavity lasers. However, direct growth of GaN on Si encounters a large lattice mismatch and a huge misfit in coefficient of thermal expansion (CTE), and often presents a high density of threading dislocations (TDs) and crack networks. A properly grown AlN/AlGaN multi-layer buffer between GaN and Si can not only build up compressive strain to compensate the tensile stress due to the CTE mismatch during cool down, but also induce inclination and annihilation of TDs at the interfaces. Upon the high-quality GaN-on-Si platform, we have grown InGaN/GaN multiple quantum well laser structure. During the fabrication process of microcavity lasers, a two-step etching method was developed to define the microdisk while removing the etching damage of the sidewalls, which significantly suppressed non-radiative recombination due to the etching damage and reduced optical loss. Finally, electrically pumped GaN-based microcavity lasers directly grown on Si are realized.

Resume : Current crowding and photon absorption by p-type electrode are important device parameters that reduce the external quantum efficiency (EQE) of lateral-geometry LEDs. P-type electrode must have low contact resistance and so metal-based electrodes are mainly employed, by which some photons generated from active region were absorbed. Thus, in this study, to further increase the EQE by improving light extraction efficiency (LEE), a patterned SiO2 current blocking layer (CBL), also acting as light scattering center, was investigated. It is shown that the CBL effectively prevents photon from being absorbed by the topmost p-pad. To obtain patterned SiO2 CBL, nanoimprint lithography was used to form nanorods (a diameter of 260 nm, a height of 180 nm, and a period of 600 nm) on the flat SiO2 layer (a thickness of 60 nm). LEDs (chip size: 300 ? 800 ?m2) fabricated using ITO-only, ITO/conventional SiO2 CBL, and ITO/patterned SiO2 CBL exhibited forward-bias voltages of 3.01, 3.10, and 3.10 V at 20 mA, respectively. The LEDs with patterned SiO2 CBL yielded 39.6 and 11.9 % higher light output powers, respectively, at 20 mA than LEDs with the ITO-only and conventional CBL. On the basis of the emission images obtained from the LEDs with ITO-only, conventional CBL, and patterned CBL, and FDTF simulations (LEE), the enhanced light output power was explained in terms of improvement of light extraction and current spreading effects.

Resume : In this study, a novel packaging structure was proposed to reduce the heat accumulation of flip-chip ultraviolet light-emitting diodes (UV-LEDs) and provided a new idea for enhancing the stability of devices by introducing graphene oxide (GO) silicone embedding the structure gap in surface mounted devices (SMD). The GO silicone showed an excellent performance in the thermal conductivity, imperviousness and antioxidant capacity. Experimental results proved that the GO silicone embedding structure proposed in this paper could reduce the thermal resistance of die bonding layer by more than 15%. The junction temperature was reduced by 1.3? at the current of 500 mA and the light output power was enhanced. What?s more, it was found that the lifetime of UV-LED with proposed structure could be obviously improved by %. Besides the low junction temperature, it was attributed to the additional imperviousness and antioxidant capacity resulted from the graphene oxide introduced in the proposed package structure. This novel approach provides a simple and effective strategy for improving UV-LED performance.

Resume : Selective etching of a sacrificial layer is a crucial step for the fabrication of vertical cavity surface emitting lasers (VCSELs) with double dielectric distributed Bragg reflectors, top-emitting resonant-cavity LEDs as well as structures that require suspended membranes. Appropriate sacrificial layers should enable the growth of high-quality layers on top and provide a removal with high etch selectivity to surrounding layers. We will show that doping-selective electrochemical etching of highly n-doped GaN layers fulfills these criteria and yields smooth surfaces for high performance devices. Crack-free GaN nanomembranes with an area up to 100 µm x 100 µm and thickness of 300 nm were fabricated. The electrochemical etching was carried out in 0.3 M nitric acid using a three-electrode setup and an applied bias voltage of 30 V vs. a Ag/AgCl reference electrode. The lateral etch rate of around 50 µm/min allows a complete lift-off of our nanomembranes within few minutes. The root-mean-square surface roughness of the etched surfaces was measured using atomic force microscopy (AFM) over an area of 2 µm x 2 µm. Without any post treatment, both the Ga- and the N-polar surface of the etched undercut shows a roughness below 3 nm which enables the integration of the nanomembranes in optical devices. The fast and smooth etching in combination with the possibility to lift-off hundreds of mesas simultaneously over a large area provides a key technology for future device processing.

Resume : A cobalt-doped ZnO (Co-doped ZnO, CZO) dilute magnetic film deposited by pulsed laser deposition was employed as an external electron retarding layer (n-electrodes) for both lateral- and vertical-type green InGaN light emitters (wavelength: ~ 520 nm). The CZO films were grown using a 5 at. % cobalt-doped ZnO target in Ar at a substrate temperature ranged from 100 to 700 C. The laser energy of 400-600 mJ was also modified to obtain the optimum properties of CZO films. When a laser energy of 600 mJ was used, the better electrical and magnetic characteristics were achieved in the 400 C-grown film. This film was suitable for the fabrication of lateral light emitter. The output powers (@350 mA) of lateral light emitters without and with the CZO electrode were 87.6 and 103.1 mW, respectively. Besides, to prepare the vertical light emitter with the low-temperature device process, the 100C-grown CZO film deposited using the laser energy of 550 mJ was chosen as the n-electrode. The output powers (@1000 mA) of vertical light emitters without and with the 100C-grown n-electrode were 254.8 and 283.6 mW, respectively. Through the deposition of CZO electrode, the electron mobility of emitter can be reduced obviously. Thus, the excessively large mobility difference between electron and hole carriers occurred in the conventional light emitter can be reduced efficiently, decreasing the non-radiative recombination rate and improving its light output power. Details of the enhanced performance of InGaN green emitters using CZO as an external electron retarding layer will be discussed.

Resume : Magnesium is usually used for p-type doping of GaN however it suffers from some limitations due to large acceptor ionization energy at room temperature. [1]. Further problem is ubiquitous hydrogen that further reduces hole densities. Therefore larger concentrations of Mg are required to achieve significant hole concentration sufficient for device operation. The fundamental mechanism for p-type conduction in GaN is the post growth high temperature annealing which activates Mg by breaking the Mg-H complex and donate a hole [2]. Here, we demonstrate improved p-type doping by ionizing Mg dopants which are otherwise inactive due to formation of Mg-H complexes using Phosphorus as an additional dopant. This may be due to polarization induced electric field caused by the addition of phosphorus which creates a non–uniform doping profile across the semiconductor. In addition, phosphorus may be helping to reduce the Mg-H complex creating suitable by products. The improved efficiency is verified through Hall and PL measurements. To demonstrate the impact of non-uniform doping across the semiconductor we have doped MOCVD grown p-GaN with phosphorus spin-on-dopants to create a non-uniform doping profile thereby inducing a built-in electric field. This built-in electric field may be the reason for further ionization of Mg dopants which are otherwise masked by H atoms by the formation of Mg-H complexes.

Resume : Pulsed-MOCVD growth of InGaN quantum wells was shown to be an effective way of enhancing quantum efficiency of light emitting diode (LED) structures. Interruptions in the flow of metalorganic precursors allow the deposited atoms to migrate across the surface, improving their distribution and diminishing the formation of structural defects. In this work, we study the impact of the interruption time between the flows of metalorganic precursors on optical properties. We employ photoluminescence and differential transmission spectroscopy techniques, were resonant excitation of InGaN quantum wells (grown on c-sapphire) is performed with 250 fs laser pulses. We show that there is an increase in carrier lifetimes in the samples with longer interruptions of metalorganic precursor flow. Also, the longer carrier lifetime correlates with the higher quantum efficiency of the active LED layer. This can be explained by a more homogeneous distribution of indium atoms on the growth surface, leading to a better quality quantum wells with less defects. On the other side, PL spectra shift to the blue side in the samples with longer interruptions, most likely due to partial evaporation of indium atoms.

Resume : Formation of low-density GaN quantum dots (QDs) in AlGaN and AlN matrices is the key point for fabrication of single electron transistors, single and entangled photons sources operating at evaluated temperature. The GaN QDs formation on the (0001)AlN surface is usually implemented by using the Stranski-Krastanov growth mode, or by the droplet epitaxy technique. Normally, the density of QDs is quite high as 10^10cm^-2. In the present work we propose an original method for formation of low-density GaN QDs using decomposition of the GaN wetting layer on the (0001)AlN surface, which does not involve the induction of strain or the crystallization of the Ga droplets. The QDs formation has been in situ investigated by RHEED technique, and ex situ by AFM, SEM with EDS, TEM and micro-PL methods. At first 2D GaN was grown: we deposited 3-4 Ga monolayers onto the (0001)AlN surface at different temperatures (540-750°C), then the surface was treated by the ammonia flux (10 sccm). Then the obtained GaN layer was decomposed at vacuum and the substrate temperature rapidly increased up to 900°C. TEM images reveals the single GaN QDs with height being 3nm and diameter being 10nm. Three-dimensional RHEED pattern confirmed the QDs formation. Exciton and biexciton peaks from single GaN QD was observed in micro-PL spectra. From the number of single emission lines which are detected in the illuminated area of the sample corresponding to the laser spot size the density of QDs can be deduced to ~10^8cm^-2.

Resume : AlGaInN-based white LED can achieve lifetimes of up to 100000 h depending on junction temperature and current density. However state of the art silicone encapsulated LEDs with powder-based phosphors reveal a strong dependence of field lifetimes on environmental conditions such as humidity, corrosive gas and air pollutant exposure. This work studies the effects of critical air pollutants in different concentrations on the light output power of various white LED types over time. A special focus is put on the effects of the pollutant hydrogen sulfide. Furthermore the observed degradation mechanisms on the semiconductor chip, the contact layers, the phosphor and the package components are analyzed. Such degradation is highly dependent on inertness and the gas permeability of the encapsulation and package materials. The gas permeability of glasses and ceramics is several orders of magnitude lower than for silicones and epoxy. At the same time, glasses and ceramics are inert to many chemical substances. Consequently, we propose ceramic phosphors and/or hermetically sealed LED-packages as a possible solution for LED applications in harsh environments.

Resume : GaN-based LEDs on Si substrate has made great progress over the last decade. However, there are still some problems which hamper the improvement of LEDs on Si. Among these, the optical absorpsion substrate is a main problem. To reduce light absorption of substrate, one way is inserting a distributed Bragg reflector between the active layer of LEDs and substrate for reflecting the light emitted downward [1]. Another way is to remove the silicon substrate and transfer the LEDs structure to a new carrier. Zhang et al.[2] utilized metal-to-metal bonding technique combine with the selective lift-off process to transfer the LEDs structure from Si onto copper carrier. Xiong et al. [3] used the same technique of metal-to-metal bonding to transfer LEDs from the native Si substrate to a new Si carrier. Chen et al. [4], Luo et al. [5] and Wong et al. [6] transferred the LED structures from Si substrate onto copper carrier by using electroplating technique. For the LED transferring technology, HNA (HF:HNO3:CH3COOH=1:1:1) solution was often used to remove silicon substrate. In this wet chemical etching process, how to protect the electrodes deposited on chip and the new carrier is a main problem. In this work, a crack-free InGaN multiple quantum wells LEDs structure was transferred from 2-inch Si substrate onto a thin epoxy resin carrier. Due to the good mechanical and anticorrosive properties of the epoxy resin layer, it can protect the electrodes deposited on chip easily during Si etching in HNA solution and can served as a carrier after substrate remove. Combining with the metal deposition after silicon remove to serve as a metal reflector, electrical and thermal conductive layer, the performances of thin-film LEDs transferred to epoxy resin carrier show a significant improvement. References [1] Y. Yang, Y. Lin, P. Xiang, M. Liu, W. Chen, X. Han, G. Hu, G. Hu, W. Zang, X. Lin, Z. Wu, Y. Liu, and B. Zhang, Appl. Phys. Express, vol. 7, 042102, 2014. [2] B. Zhang, T. Egawa, H. Ishikawa, Y. Liu, and T. Jimbo, Appl. Phys. Lett. vol. 86, 071113, 2005. [3] C. Xiong, F. Jiang, W. Fang, L. Wang, C. Mo, and H. Liu, J. Luminesc., vol.122–123, pp. 185–187, 2007. [4] T. Chen, Y. Wang, P. Xiang, R. Luo, M. Liu, W. Yang, Y. Ren, Z. He, Y. Yang, W. Chen, X. Zhang, Z. Wu, Y. Liu, and B. Zhang, Appl. Phys. Lett., vol. 100, pp. 241112- 241116, 2012. [5] R. Luo, W. Rao, T. Chen, P. Xiang, M. Liu, W. Yang, Y. Wang, Y. Yang, Z.Wu, Y. Liu, H. Jiang, and B. Zhang, Jpn. J. Appl. Phys., vol. 51, 012101, 2012. [6] K.M. Wong, X. Zou, P. Chen, and K. M. Lau, IEEE Electron Device Lett., vol. 31, pp. 132–134, 2010.

Resume : The light-emitting diode (LED) lights offer higher energy conversion efficiencies and longer light times. As an LED chip generates heat during operation, it causes a reduced life-time of LED chip and degeneration of the light emission stability. An LED module also offers a limited heat dissipation path. As a result, for reducing the operating temperature, LED lights need a heat sink which is used for a general cooling system because the heat is generated in an LED chip. To enhance heat dissipation, a fin-type has been commonly used to passively cool LED modules with natural convection for an extended surface area of the heat sink. However, a fin-type is poor cooling performance through horizontal direction due to the low heat transfer rate. Therefore, it needs to enhance the lateral spreader through a horizontal surface of the heat sink. The graphene oxide (GO) is promising candidates for thermal management because of its higher thermal conductivities. However, the thermal properties of GO are restricted by its high anisotropy, leads that the out-of-plane of a thermal property is two order lower than the value of in-plane direction. To overcome this issue, the three-dimensional (3D) structure of GO with metal-based materials suggests as ideal materials for thermal transport. In this presentation, the 3D structures by use of the GO and Copper nanoparticles (Cu NPs) coated on the heat sink for improvement of heat dispersion will be discussed. The 3D structure of GO mixed Cu NPs is spray-coated on heat sink. The thermal resistance of heat sink is characterized by Thermal transient tester (T3ster), and the temperature LED package on heat sink is observed by FLIR camera. The thermal resistance of our proposed structure is lower 46 % than that of Al heat sink. It means that the combination of GO and Cu NPs enables improvement of the heat dissipation property. Therefore, the proposed structures are possible effective thermal management for long operating lifetime in the high performance of LEDs. The physic behind the heat transport and lifetime in our proposed structures will be discussed in detail.

Resume : Recently, Nitride based Deep UVLED becomes a very important topic. It can reduce its size from traditional UV lamp and has a potential to reach very high efficiency with the improvement of growth technology. Currently, the major limitation of Deep UV LED is due to the high activation energy of p-AlGaN layer, contact issues, absorption issues, stability issues, etc... Many studies have shown that the hole injection is the bottleneck, where electron and holes are imbalanced so that the efficiency would be very bad. Due to the limitation of hole activation, many devices are still using p-GaN layer as the hole injection layer, where the strong light absorption further limits the performance. To solve this problem, the p-type Supper Lattice (SL) was proposed to substitute p-GaN. Our 1D simulation show that under certain configuration of QW (AlxGa1-xN) /QB (AlyGa1-yN), it is possible to enhance the hole density and becomes transparent for UVC wavelength. However, to understand if it really helps for hole injection, we will need to apply the quantum transport model for super lattice structures. With the newly developed localized landscape model[1], it is possible for us to investigate the hole injection in SL structures directly. Therefore, we applied 2D Poisson, drift-diffusion solver with localized landscape model to understand the carrier transport, IQE and light extraction enhancement. We found that SL structure will only works in certain configuration (very thin SL layer for p-layer). With the flip chip design, a large voltage drop might be also from n-AlGaN layer. Therefore, an overall optimization is needed and we will present our optimized result in this talk. [1] D. Arnold, G. David, D. Jerison, S. Mayboroda, and M. Filoche. "Effective confining potential of quantum states in disordered media." Physical review letters 116.5 (2016): 056602.

Resume : Spectral blue-shifting caused by nanostructuring of LEDs has been exploited for changing the color of light emission for applications like monolithic broadband emission [ACS Photonics, 3, 1294 (2016)]. However, such spectral manipulation is limited to blue-shifting, as nanostructures are known to give rise to strain relaxation of the MQWs that reduces the Quantum Confined Stark Effect (QCSE). Here for the first time we demonstrate a novel nanostructure design that induces strain in the MQWs using a top-down fabrication approach. We discover that if the etch is terminated just before the MQWs are removed, within a range of ~30 nm, no strain relaxation would occur but instead a spectral red-shift is observed, as estimated by molecular dynamics simulations and k.p calculations. Our preliminary experimental study shows up to 10nm of red-shift in PL by fabricating the nanostructure on c-plane InGaN/GaN LED structure on sapphire with an emission wavelength > 560nm. The extent of spectral red-shift depends heavily on the aspect ratio of the nanostructure. As such, the nanostructures are fabricated on the n-face of laser-lifted off blue LED sample having a weaker QCSE due to lower In content; a PL red-shift from 459 to 468nm is observed. Such phenomenon can be exploited for broadband-emission LEDs, as well as for realizing green light emission without increasing In content, overcoming the green gap issue. A systematic study of the effect and their applications will be presented.

Resume : AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) have attracted much attention due to their applications such as water purification and sterilization. However, the external quantum efficiency (EQE) of DUV LEDs is still much lower than that of InGaN-based visible LEDs, limiting the expansion of the applications and the market. The low EQE of DUV LEDs is attributed to poor crystal quality of the AlGaN layer, leading to low internal quantum efficiency (IQE), and low light extraction efficiency (LEE). We have investigated AlGaN-based DUV LEDs with periodic air-voids-incorporated nanoscale patterns enabled by nanosphere lithography and epitaxial lateral overgrowth (ELO). Nanoscale ELO on the nano-patterned substrate (NPS) improved the crystal quality of overgrown layers at relatively low growth temperature of 1050 oC and at small coalescence thickness less than 2 um. Full-width at half-maximum values of x-ray rocking curves for (002) and (102) planes of AlN were found to decrease from 235 arcsec to 186 arcsec and from 457 arcsec to 432 arcsec, respectively. In addition, the periodically embedded air-void nanostructure enhanced the LEE of DUV LEDs by breaking the total internal reflection. The EQE of DUV LEDs on the NPS was enhanced by 122% at current density of 22 mA/cm2 compared to the reference DUV LEDs. We believe that the significant enhancement of EQE was attributed to simultaneous improvements in the crystal quality of AlGaN epitaxial layer, i.e., IQE and LEE.

Resume : AlGaN-based UV/DUV lasers have a great potential in the fields of laser processing especially surface processing, medical, biotechnology and so on, but it is difficult to obtain high hole concentration p-AlGaN with high AlN molar fraction at RT. One promising technique is the use of electron beam excitation. If the AlGaN-based lasers by electron beam excitation are obtained, UV laser tube like mercury lamp and X-ray tube will be realized. In this report, we investigated the AlGaN-based MQW laser by electron beam pumping. We also discuss that what is the point of laser oscillation by electron beam excitation. From Monte Carlo simulation results, we found that the active layer is excited most efficiently at an acceleration voltage of approximately 15 kV. Moreover, it is best to design the position of the active layer at ~200 nm from the surface. According to these results, a 500-nm-thick Al0.15Ga0.85N cladding layer, a 100-nm-thick Al0.05Ga0.95N guide layer, GaN (3.5nm)/Al0.05Ga0.95N (12nm) 10QW active layer, a 100-nm-thick Al0.05Ga0.95N, and a 100-nm-thick Al0.15Ga0.85N cladding layer were stacked in that order. The cavity length of the laser was ~50 ?m. From luminescence intensity-excitation power density characteristics, spectrum, and polarization characteristics, it was concluded that this GaN/AlGaN sample reached laser oscillation. The threshold power density, laser oscillation mode, and wavelength were ~211 kW/cm2, TE-polarization, and 352.8 nm, respectively.

Resume : Micro-LED arrays have attracted much attention due to high pixel density, high light intensity and flexibility. For the transfer of micro LEDs to another substrate, complicated processes such as laser dicing and chemical etching of sacrificial layers are required. In this study, we have demonstrated the growth of micro chip-sized LED arrays and its direct transfer to a Si substrate, which is a simple and cost-effective process. Separate GaN layers of various LED chip sizes (14 um x 14 um, 50 um x 50 um and 100 um x 100 um) were successfully grown on a stripe-patterned alumina membrane by metalorganic vapor phase deposition. For the alumina membrane structure, first, photoresist(PR) pattern of a stripe with the width and the spacing of 2 um was formed on a sapphire substrate. The stripe length and the number of arrays determine the size of separate GaN layers. Then, an amorphous Al2O3 layer with a thickness of 120 nm was deposited by atomic layer deposition(ALD). The substrate was annealed at 1100 oC for 2 hours in air to remove PR, leaving stripe cavities, and to crystallize the ALD alumina into alpha-phase for the GaN growth. The wafer including GaN layers on the stripe pattern was flipped over on a Si substrate and the separate layers were transferred on it through Au-Si eutectic bonding. And then, a mechanical force was applied gently to break the thin membrane structure and to separate the sapphire substrate. In addition, we found that the threading dislocation density of GaN layer on the alumina membrane was 1.2x10^8 cm^-2, which is 40% lower than that of GaN grown on the planar sapphire substrate. In the presentation, more details of the results including the growth behavior of GaN on the stripe pattern and potential application to LED display will be discussed.

Resume : We present a unique approach to analyze the free-carrier concentration within Si-doped GaN cores of a microrod LED and the related impact on the luminescence properties of the coaxial shell layers by nanocathodoluminescence (CL) and µ-Raman spectroscopy. The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subesquently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. On top of the Si-doped core an InGaN single quantum well (SQW) was deposited epitaxially followed by an intrinsic GaN layer and a Mg-doped p-GaN shell. Aspect ratios of up to 40 have been realised. CL and Raman measurements reveal a high free-carrier concentration of 7 x 10^19 cm-3 in the bottom part and a decreasing doping level towards the tip of the microrod. Structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense CL originating from the InGaN SQW on the non-polar side walls, which ehibits a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium content. The QW emission is strongly reduced in the vicinity of the defects generated in the shell. Moreover, a locally confined red shifted emission appears at the intersection of defects and SQW, which most probably indicates indium clustering.

Resume : For the realization of high efficiency optoelectronic devices, GaN nanowires (NWs) constitute a promising alternative to GaN planar layers. Different applications can be foreseen, in particular their use as optical microcavities ([1]) for the development of low-dimensional and possibly very low-threshold lasers ([2]). Indeed, lasing effect in single GaN NWs under optical pumping and at room temperature have already been reported for both Fabry-Pérot ([3]) and whispering gallery modes ([4]). However, for practical applications lasing under electrical injection is indispensable but has not been reported so far. In this work, we will report lasing effect observed under optical excitation in p-i-n core-shell GaN NWs, which constitutes a promising step in the realization of an electrically injected GaN NWs laser. The core-shell NWs are grown by selective-area MOCVD on Ga-polar GaN template. First, we will focus on the growth conditions used in this work. Then, lasing effect will be presented and the nature of both the gain and the cavity modes involved will be discussed. [1] Coulon, Pierre-Marie, et al. Optics express 20.17 (2012): 18707-18716. [2] Trichet, Aurélien, et al. New Journal of Physics 14.7 (2012): 073004. [3] Johnson, Justin C., et al. Nature materials 1.2 (2002): 106-110. [4] Tessarek, Christian, et al. ACS Photonics 1.10 (2014): 990-997.

Resume : Polarized emission characteristics of GaN-based devices has attracted great interest due to a potential application to backlight for liquid crystal display. However, c-plane GaN on c-plane sapphire, a typical growth scheme for the fabrication of GaN-based devices, exhibits no anisotropic polarized emission due to isotropic strain. In this study, we demonstrated polarized photoluminescence of c-plane GaN on c-plane sapphire by using a stripe-shaped cavity-embedded sapphire substrate (SCES), which is expected to generate anisotropic strain in the GaN on the top. A stripe-shaped photoresist (PR) pattern was defined along the [1-100] direction of Al2O3. An 120 nm-thick amorphous alumina was deposited on the PR-patterned substrate by atomic layer deposition. Then, the substrate was annealed at 1100 oC for 2 hours in an air ambient furnace to remove PR, resulting in stripe-shaped cavities on the surface, and to crystallize the amorphous alumina membrane into alpha-Al2O3. A 2 μm-thick GaN was grown on the substrate at 1040 oC by metalorganic chemical vapor deposition. X-ray reciprocal space mapping revealed that the GaN layer on SCES was under anisotropic strain of -0.184% and -0.026% along the direction perpendicular and parallel to the stripe pattern, respectively. The PL peak positions measured in the direction parallel and perpendicular to the stripe pattern, were 3.438 eV and 3.446 eV, respectively, suggesting splitting of the top-most valance band is dependent on the anisotropic strain. The intensity ratio of the peaks was calculated to be 4.3, indicating the strong polarized emission behavior of the GaN layer. In the presentation, mechanism for the anisotropic strain of GaN and the change of the band structure will be discussed.

Resume : Spectral diffusion is a linewidth limiting phenomenon of an emitter due to interactions with charge fluctuations in its environment. The effect is prominent in III-nitride quantum dots (QDs) and is of crucial importance as it leads to linewidths that can reach up to several meV. Moreover, due to its rapid nature, the effect can be too fast to be resolved using typical spectroscopy (and results in a significant Gaussian component of the lineshape). Here, we use an alternate method to reveal the time-scale of the diffusion in GaN fluctuation QDs via careful measurements of the intensity autocorrelation. GaN fluctuation QDs exhibit several advantageous properties such as comparatively stable and pure single photon emission, making them an important platform for the fundamental study of III-nitride nanostructures. Autocorrelation measurement of the whole emission peak from an isolated QD reveals the typical antibunching behavior for a single quantum emitter, with g(2)(0)<0.5. However, when selecting only one half of the peak for autocorrelation, an additional bunching with a characteristic time of ~22ns appears due to the spectral diffusion of the peak into and out of the selected spectral window. This measurement provides us with direct information on the temporal scale of the spectral diffusion process for the first time in these QDs, which will have important implications for the generation of indistinguishable photons for advanced applications.

Resume : III-nitrides based devices relies on heteroepitaxy, therefore nitrides possess high density of crystalline defects. Characterization which provides important parameters for devices operation is on demand. Stimulated emission (SE) threshold is one of used parameters to determine quality of epitaxial GaN layers and LED structures. Photoluminescence (PL) properties of a dense electron-hole plasma were studied in a set of MOCVD grown GaN epilayers with thickness varying from 2 up to 25 um. Using picosecond interband carrier photoexcitation, a threshold of SE was measured by picosecond PL and light-induced transient gratings techniques. Both techniques revealed an unexpected decrease of the SE threshold for the thinner and thus more defective epilayers, having higher nonradiative recombination rates. A model of thickness-dependent evolution of SE in GaN layers is presented. The model includes calculations of emitted light mode profile, its overlap with in-depth profile of photoexcited carriers and finally modeling of carriers and photons time dependent density dynamics. The modeling shows that increase of layer thickness allows optical mode to spread in depth of sample, thus reducing the overlap of optical mode and gain profile. As consequence threshold of SE is increased while sample thickness increases. We conclude that SE threshold measured at picosecond excitation represents optical properties of sample, but not so much dependendent on carrier lifetime or defect density.

Resume : III-nitride light emitters play an essential role for solid state lighting due to its high spectral power features energy-efficient[1,2]. Owing to the significant advantages of reduced piezoelectric polarization field, efficient droop elimination, tunable energy bandgap, and light extraction enhancement, the nanowires (NW) emitters are preferable over planar based LEDs [1-3]. To planarize and passivate NW, several filler materials has been used such as parylene, SiNx, BCB and polymide for LED and solar cells [3]. These filler materials serve to overcome the band bending effect in achieving controllable surface for GaN-based devices [4, 5]. Few groups reported that polar and nonpolar surfaces of GaN layers suffer from band bending effect due to the surface Fermi level pinning [4, 6]. This results in negative charges accumulations at the surface leading to a drop in the light efficiency [7]. Accordingly, several factors contribute to this problem such as dangling bonds, defects, surface states, and adsorbates [7]. Specifically for NW emitters, parylene has been shown to improve 25% of photoluminescence (PL) spectra of NW emitters [8]. In this paper, we demonstrate the NW-LED passivation using BCB to enhance its optical and electrical properties. We compare the enhancement of PL spectra from the BCB as filler material against parylene. We further deposited dielectric distributed Bragg reflector (DBR) on these filler materials for the purpose of facet coating on the actual device to enhance the electrical-optical properties of the NW emitters. Figure 1 shows PL spectra t at room-temperature for BCB and parylene layers deposited on orange NW emitter sample. A 1µm thickness of parylene was deposited using a thermal evaporator system and an enhancement factor of about 35% in light intensity at 363 nm peak wavelength was observed. In contrast the BCB exhibits more than 2 magnitudes higher order of PL enhancement (around 80%). The 1 µm BCB is deposited by spin-coating followed by thermal curing on the NW sample. It has been shown that BCB is excellent for uniform passivation and enhanced the transmission of NW solar cells [9]. Figure 2 shows high reflectivity spectra was achieved by depositing dielectric DBR on the filler materials of BCB and parylene, respectively. The facet coating on actual NW emitters is ongoing and the results will be communicated in near future.

Resume : III-Nitride nanowires (NW) have tremendous potential to substitute two dimensional structures in the field of solid state lighting because of its superior structural properties. The dislocations present in the planer structures are originated at the interface due to the lattice mismatch with the substrate. However, in nanowires, these dislocations don’t seem to propagate along the c axis, rather have been found to follow an inclined path towards side wall. The present study of GaN(~2 nm)/AlN(~10 nm) nanowire heterostructures grown on GaN NWs by PA-MBE with 25 periods demonstrates the strong presence of polarization induced quantum confinement stark effect(QCSE), which causes a change of quantum well shape from rectangular to sawtooth, resulting an enormous red shift of band edge emission. We have measured the photoluminescence (PL) spectra of GaN/AlN nanowire heterostructures at low temperature (10K) and have compared with the GaN nanowires without heterostructure grown by PA-MBE on Si(111) and Sapphire substrates. Low temperature PL studies have shown the red shifted broad band emission at 2.95 eV which emphasizes the presence of large polarization field whereas the pure GaN NWs ensembles shows sharp transition at 3.45 eV. We have also simulated these heterostructures with different GaN well thickness by keeping AlN thickness constant. We will show that by reducing GaN layer thickness below 1 nm, one can significantly minimize the QCSE. Further work on the dependence of NW diameters on QCSE and Si doping is on progress and will be reported later.

Resume : The insertion of an indium containing layer before the growth of the InGaN/GaN quantum well (QW) active region is known to increase the internal quantum efficiency (IQE) of light-emitting diodes. This buried layer can be a superlattice (SL), which consists of multiple InGaN/GaN QWs. To explain the IQE increase, several mechanisms have been proposed in the literature. Li et al. [APL 102, 041115 (2013)] suggest that SLs help decreasing the threading dislocation density via a bending mechanism. Another possibility [Takahashi et al., JJAP 39, L569 (2000)] is that the In present in the SL seeds the formation of V-pits, thereby preventing carriers from reaching non-radiative (NR) recombination centers associated to dislocations. Armstrong et al. [JAP 117, 134501 (2015)] suggest that SLs improve the QW efficiency by burying impurities and associated NR defects. In this work, we study single QWs, which emit at 405 or 450 nm. We investigate the impact of the substrate (freestanding (FS) GaN or sapphire) and the In composition of the SL. A difference in the photoluminescence (PL) decay time at 300 K is observed for samples grown on FS GaN with (890 ps) and without (27 ps) SL, confirming the role of NR recombination centers associated to point defects. We carried out a comprehensive study on the reduction of NR recombination in QWs induced by the buried SL, by performing PL, time-resolved PL, cathodoluminescence, atomic force microscopy, and secondary ions mass spectroscopy.

Resume : III-nitride-semiconductor position-controlled nanostructures embedded in thin films are highly desired for integrated planar lightwave circuits relying on a photonic crystal and/or a waveguide architecture. Such nanostructures in nanowires or pyramids have already been reported. However, to date there is no existing report about position-controlled nanostructures embedded in III-nitride thin films. In this work, we demonstrate the realization of position-controlled GaN nanostructures embedded in AlN films. Those position-controlled GaN nanostructures were fabricated by a three-step procedure. Firstly, an AlN film was grown on Si(111) substrate using ammonia-source molecular beam epitaxy. Secondly, small dips were formed on the AlN film using electron beam lithography and inductively coupled plasma etching techniques. Finally, a GaN/AlN single quantum well (SQW) was regrown by metal organic vapor phase epitaxy. Beyond a SQW related emission peak centered around 3.2 eV characterized with a linewidth of about 540 meV measured on samples with a 2-nm-thick GaN SQW, low temperature cathodoluminescence (CL) measurements revealed sharp lines ranging from 3.7 to 4.0 eV with a linewidth of 62 ± 16 meV located at the sidewalls of the dips. We will discuss in a systematic fashion the structural and optical properties of those nanostructures by combining the data obtained via atomic force microscopy, X-ray diffraction, low-temperature CL and micro-photoluminescence spectroscopy.

Resume : Having the ability to emit a wide range of wavelengths in the visible spectrum with high light conversion efficiency, GaN-based light-emitting diodes (LEDs) have progressively become a viable light source for illumination and sensing applications. In this work, a vertical 3D GaN-based nano-optoelectronic platform is developed for non-destructive in situ diagnostics of electrochemically active bacteria, which are utilized as biocatalysts in microbial fuel cells (MFCs). To build this system, we employ GaN LEDs as a light source and a photodiode as a detector of the transmitted light, respectively. From the optical in situ measurements, insights of the biochemical and physical processes inside the biofilms can be obtained, including their thickness profiles. As direct coupling of the biofilm to the sensor platform will minimize the environmental interferences, the biocompatibility of a GaN working electrode as a surface for biofilm cultivation, which is at the same time used as an LED, has therefore been investigated. Additionally, to enhance biofilm growth and current density of the MFCs, vertical 3D GaN nanostructures with different sizes are employed (i.e. nanopillar and nanofin arrays). Depending on their geometry, the bacteria (Geobacter sulfurreducens)/biofilm can then grow either in the spaces between the GaN micro/nanostructure sidewalls or directly on their top parts. As an outlook, an optoelectronic lab-on-chip will be integrated with a microbioreactor.

Resume : Considerable attention has been focused on InGaN green laser diodes (LDs) in the past few years due to the demand for green LDs in pico-projectors and laser displays. The epitaxial growth of InGaN quantum well (QW) active region with high internal quantum efficiency (IQE) and high differential gain is a key point to realize LDs with good performances. Step-flow growth can result into smooth morphology and thus sharp QW interface. However, InGaN QW emitting in the green spectrum range usually shows two-dimensional (2D) island morphology at temperature lower than 700 oC. In this article, we studied the reasons for 2D island morphology of InGaN QW, and suggested that it is due to small diffusion length of adatoms at low growth temperature. Therefore, reducing atomic terrace width to reduce the distance that adatoms needed to diffuse prior to incorporation was utilized in order to obtain step-flow growth. Then the structural and optical characteristics of green InGaN/GaN MQWs with different growth mode were also studied. Green InGaN/GaN MQWs with step-flow morphology was showed to have a better well/barrier interface than that with 2D island morphology. The IQE of green InGaN MQWs with step-flow morphology is estimated to be 30%, while that of green InGaN/GaN MQWs with 2D island morphology is estimated to be 15%, which results from eliminating of a kind of non-radiative recombination centers with an activation energy of 18.8 meV.

Resume : Thanks to the good crystalline quality of InGaN/GaN nanowires (NWs), these nano-objects exhibit intense optical emission in the spectral domain corresponding to the so-called green gap, where 2D films have a limited radiative efficiency. In this work we analyse the optical properties of thick In-rich InGaN/GaN NW heterostructures using Photoluminescence (PL) and Cathodoluminescence (CL). Self-assembled GaN NWs embedding single ~50 nm InGaN insertions with 40 (±5) % In content were synthetized by Plasma Assisted MBE. The In content dispersion from wire to wire was verified by SEM-EDX statistics and the crystal quality was assessed by high-resolution HAADF STEM and EDX measurements. Resonant and non-resonant PL on NW ensembles as a function of temperature and of excitation power were used to probe the NW optical quality and in particular to assess the quenching of InGaN emission with temperature. A broad emission originating from InGaN insertions centred at 2.3 eV was observed by PL. Single NW analyses by CL evidenced emission peaks in the 2.1- 2.5 eV range, yielding information on the wire-to-wire compositional variations. The optical IQE was analyzed for NW ensembles showing a dramatic dependency on the growth conditions affecting the crystal quality. An IQE increase from 6% to 44% was observed for optimized capping of InGaN insertions with GaN. These results are a step forward towards the realization of high-efficiency InGaN/GaN optical nano-devices in the green spectral domain.

Resume : With increasing N concentration in GaP, wave functions of discrete N-N pairs overlap to form an intermediate band (IB) of GaP1-xNx at a suitable energy position for the IB-type solar cell. It is crucial for realizing high efficiency IB-type solar cells to characterize defect states which act as nonradiative recombination (NRR) levels and to find a way to minimize them during growth. We studied the IB photoluminescence (PL) of a GaP1-xNx (x=0.0075) grown by MOCVD on a GaP substrate under the CW above-gap excitation (AGE, hνA=2.38eV) and an intermittent below-gap excitation (BGE) light with varying photon energy hνB. The intensity change of the IB-PL due to the BGE implies the detection of NRR levels which are activated by the BGE energy. In case of 0.947eV BGE energy at 5K, the intensity of the IB-PL (2.10eV) showed 5% increase with increasing the BGE density up to 2W/cm2. The increase was 8% for lower energy emission (1.71eV). We obtained the hνB dependence of the intensity change in the PL with hνB=0.800, 0.925, 0.947 and 1.165eV, respectively. The contribution of 0.947eV BGE was higher than that of 0.925eV for 2.10eV emission, while the relation was inverse for the 1.71eV emission when compared at the same BGE photon number density. The PL intensity of another discrete peak (2.21eV) decreased distinctly by adding the BGE of 0.925 and 1.165eV. Together with their AGE density and temperature dependence, these results are interpreted to the energy distribution of NRR levels via the IB. It gives us a direct clue for understanding radiative and NRR processes inherent to the IB in GaP1-xNx.

Resume : The III-Nitride based lateral light emitting diode (LED) exhibits severe current crowding, as well as efficiency droop, mainly due to the lateral geometry. Therefore, there is a significant interest in improving the efficiency of III-Nitride based vertical LED. In this work, we demonstrate that an affordable, high thermal and electrical conductive and transparent (-201) β-Ga2O3 substrate is an excellent candidate for fabrication of high quality bright vertical LEDs, as it improves the heat management and simplifies device fabrication. We investigate optical and electrical properties of high-quality InGaN/GaN multi-quantum well (MQW) vertical LED grown by metal organic chemical vapor deposition on (-201) β-Ga2O3 substrate, using GaN buffer layer. We assess MQW samples with different InN content (x = 0.05-0.15) at growth temperature of 725-820 K. The width of the well (3 nm) and the barrier (6 nm) are estimated by secondary ion mass spectrometry. Room temperature (RT) photoluminescence (PL) spectra (excited by 325 and 400 nm) of In0.15Ga0.85N vertical LED showed high-intensity emission (~458 nm) with a weak defect band. We demonstrate a superior internal quantum efficiency of ~73%, compared to that reported in literature. Time-resolved spectroscopy analysis using Ti:sapphire femtosecond laser (frequency doubled, λ = 400 nm) and Hamamatsu streak camera revealed a biexponential decay with a decay time of 3 ns and 24 ns, dominated by radiative recombination at RT. Electroluminescence was investigated at different forward current densities. Compared to the lateral LED, our vertical LED grown on β-Ga2O3 exhibits excellent current-voltage characteristic with low resistance at forward bias and a lower leakage current at reverse bias.

Resume : Surface Plasmon (SP) is a promising way to improve the performance of optoelectronic devices. In our previous work, we realized high performance GaN detectors by fabricating Ag nanoparticles on the surface of GaN epilayer, the responsivity of which increased more than 30 times. Localized Surface Plasmon (LSP) effect was regarded as the reason to this enhancement, which has been further elaborated in our previous report. However, the direct evidence for these hypothesis is still lack and the physical origin of SP enhancing the performance of semiconductor photoelectric devices remains to be clarified. Hence a feasible method is urgent needed to explain the SP enhancement.     To reveal the physical origin of this enhancement, a means of Kelvin Probe Force Microscopy (KPFM) was adopted and  SP induced surface potential reduction around Ag nanoparticles on GaN epilayer was observed. Under ultraviolet illumination, the localized field enhancement by SP forces the photogenerated electrons drifting close to the Ag nanoparticles, leading to a reduction of surface potential around Ag nanopaticles on GaN epilayer. For the isolated Ag nanoparticle with the diameter around 200nm, the distribution of SP localized field located within 60nm from the boundary of Ag nanoparticles. For the dimer Ag nanoparticles, the localized field enhancement between the two nanoparticles was the strongest. The results presented here provide direct experimental proofs for the existence of localized field enhancement by SP, which will help us to further understand and improve the performance of SP-based optoelectronic devices in the future.    This work was supported by the National Natural Science Foundation of China (grant nos 61322406, 61274038 and 61574142), the national key research and development program of China (2016YFB0400900), the Jilin Provincial Science & Technology Department (grant nos 20140520116JH and 20150519001JH), the CAS Interdisciplinary Innovation Team, the Youth Innovation Promotion Association of CAS (grant no. 2015171)

Resume : GaN based material is the most likely candidate for phosphor-free, single chip, white light emitting diodes (LEDs) because it can cover the whole range of visible wavelength. Therefore, in order to achieve phosphor-free broadband LEDs, two-dimensional film and three-dimensional (3D) GaN structures have been studied using a metal-organic chemical vapor deposition (MOCVD). It has been proven that multi quantum wells on various facets of a 3D structure emit different color that originated from complex of (i) different indium composition due to diffusion length of group-III precursors under growth step; (ii) different quantum confinement effect because of various growth rate of facets; and (iii) quantum-confined Stark effect. Therefore, we can achieve broadband emission from the 3D structure. Here, we present fabrication and characterization of 3D LED structures using selective area growth technique via MOCVD and UV lithography.[1] LEDs with these 3D structures consistently emit the same white light regardless of the variation of injection current. Since we could change the emission color with a mask pattern or growth parameters, color temperature and color rendering index can be adjusted. Moreover, it is cost effective compared to using phosphor or multiple substrate and adoptable to mass production. This 3D structure is one of possible candidate for a future micro-LED display.

Resume : The development of deep ultraviolet emitting devices based on AlN has been very limited due to the lack of lattice-matched substrate for AlN growth and the low hole concentration and low conductivity of p-AlN as a result of inefficient p-type doping. In this regard, low-dimensional nanostructures have proved to significantly enhance Mg-dopant incorporation and p-type conduction. We have studied, for the first time, the molecular beam epitaxy and properties of Mg-doped AlN nanowalls and have demonstrated high-efficiency, large-area AlN nanowall LEDs at 210 nm. Mg-doped AlN nanowalls were grown on GaN-on-sapphire templates pre-patterned with GaN nanowalls. The photoluminescence at room-temperature unprecedentedly exhibited a strong emission peak at 210 nm due to free exciton emission and another at 235 nm due to Mg-acceptor related transition, which suggests excellent material quality. The internal quantum efficiency could also reach ~95% under relatively high excitation power. The effects of nanowall width on the strain and optical emission were also revealed. Narrower nanowalls exhibited stronger optical emission and enhanced Mg-dopant incorporation due to the reduced formation energy for Mg-dopant as a result of smaller strain. The current density of our AlN LEDs could therefore reach 141 A/cm2 which previous AlN diodes have never achieved. The smaller strain was also manifested by the blueshift of the emission of AlN nanowall LEDs with decreasing nanowall width.

Resume : The indoor lighting source, which is a most application of white light emitting diodes (WLEDs), desires warm white light (correlated color temperature (CCT) < 4800 K) with good color rendition (color rendering index (CRI) >75). However, in order to achieve warm white light from commercial WLEDs, developing a phosphor emitting red color component was required instead of yellow phosphor. The phosphor-free WLEDs based on GaN three dimensional (3D) structures could be considered as important building blocks for realizing multi-color emission from various crystal planes. Although phosphor-free WLEDs based on 3D structure have been reported, it still remains challenging problems related to low luminescence efficiency and a lack of red spectral component. Here, we propose InGaN/GaN based dodecagonal ring structure (DRS) emitting warm white light with high efficiency. The DRS was fabricated by selective area growth technique and wet etching method. The wet etching induces strain relaxation of 3D structure with a minimization of built-in field, and forms regions for high indium contents on semi-polar facet in GaN DRS. The InGaN/GaN DRS emit ultra-broadband visible light with low CCT (4,500 K) and high CRI (Ra=80) and show high internal quantum efficiency with fast radiative recombination times over a broad spectral range owing to polarization field reduction. The InGaN/GaN based DRS become a platform toward highly efficient warm white light emitting source without the use of phosphors.

Resume : Light emitting diodes (LEDs) with short emission wavelength in the range of 200-320nm have been demonstrated to be promising for applications in environmental purification. Because AlGaN has direct band gaps between 3.4 and 6.2eV, Al-rich AlGaN alloys are prime candidate for realization of UVC LEDs. However, AlGaN epilayers grown on sapphire substrates contain a high density of threading dislocation of the order of 1010cm-2. The reduction of high threading dislocation density is indispensable to obtain higher efficiency and output power of device and reliability[1]. Many studies have been devoted to improve crystal quality of AlxGa1-xN epilayers such as low temperature buffer layer, two-step growth, gradual variation in V/III ratio and epitaxial lateral overgrowth using patterned sapphire substrates (PSS). However, the increasing concentration and injection efficiency of hole is as important as reducing defect density. In spite of significant absorption of UVC light from p-AlGaN with low Al composition, p-GaN is widely used as p-type cladding layer due to low hole concentration and lateral spreading of high Al composition p-AlGaN. In this paper, we report on growth of high efficiency AlGaN based UVC LEDs using high temperature metal organic chemical vapor deposition (HT-MOCVD). The structures of grown UVC LEDs are same but p-electron blocking layers (EBLs). To enhance doping efficiency and injection rate of hole into multiple quantum wells (MQWs), graded AlxGa1-xN/AlyGa1-yN superlattice structures (SLs) were inserted as EBLs. According to the results from electroluminescence(EL), the emission intensity of wavelength about 275nm from UVC LED with graded EBL increased about factor of 2 compared to the single p-AlGaN EBL inserted sample[2]. In order to obtain further increased light output power from the UVC LEDs, we applied both SLs as conventional multiple quantum barriers (MQBs) and combinative structure of graded p-AlxGa1-xN/AlyGa1-yN SLs (varying x) as EBLs. We expect SLs (both MQBs and polarization doped SLs) will intensify the lateral spreading of holes with higher electron blocking efficiency. The concentration of hole can be increased by grading of Al composition in the SLs due to generation of extra holes by polarization doping effect. Numerical calculation and electrical characterization are being conducted. Detailed will be shown in the conference. [1] Jinwan Kim, Jaedo Pyeon, Minhwan Jeon, and Okhyun Nam. Jpn. J. Appl. Phys. 54, 081001 (2015) [2] Comparison of UVC LEDs with polarization doped and conventional EBLs by electroluminescence measurement. (not published)

Resume : Exciton cavity polaritons (polaritons) are bosonic quasi-particles offering special semiconductor systems for exploration of interaction. The temperature limit of polaritons for wide bandgap semiconductor such as GaN is expected to be above room temperature (RT) due to high oscillator strength. However, to our knowledge, the condensate of one-dimensional polariton of GaN has never been reported due to qualities of cavity and active medium. In this work, using a hexagonal GaN microwire (MW) grown by metalorganic chemical vapor deposition with selective area method, we firstly demonstrate the one-dimensional polariton condensate in nitride semiconductor system at RT. Different from two-dimensional cavity of GaN, the one-dimensional condensate show unique physical phenomena, which is the propagation of condensate over microscopic length from the excitation area. The main difference between two-dimensional and one-dimensional polariton is absence of strain-induced potential fluctuation for as-grown high quality MW due to the dislocation bending. The physical origin of the propagation is from the positive local potential induced by exciton reservoir from non-resonant pumping within small size. The single MW structure with the control of this local excitation provide all-purpose method for manipulation of the propagation of condensate at RT.

Resume : Core-shell III-nitride structures offer great promise for UV light emitters due to their lower defect density, reduced quantum confined Stark effect, high-quality lateral non-polar growth and improved extraction efficiency, all known problems of current planar UV devices. This paper reports on the fabrication and characterization of AlN/AlGaN core-shell nanorods emitters by a hybrid top-down/bottom-up approach. Displacement Talbot lithography has been used to fabricate a 1.5 µm pitch hexagonal array of 200 nm thick Ni metal dots via lift-off to act as a hard etch mask. An AlN nanorod scaffold was then created by inductive coupled plasma dry etching of an AlN template with a Cl2/Ar-based chemistry. The impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve straight nanorod arrays with high aspect ratios will be presented. Details of AlN facet recovery will be described by means of KOH based etching, Metal Organic Vapor Phase Epitaxy (MOVPE) regrowth and high temperature annealings, systematically monitored by scanning electron microscopy. Finally, AlxGa1-xN shells regrowth by MOVPE with various compositions will be presented along with optical characterisation via hyperspectral cathodoluminescence imaging to assess their emission wavelength range and spatial homogeneity. The reported hybrid top-down/bottom-up approach is promising and opens new perspectives for UV LEDs based on core-shell nanostructure.

Resume : GaN on Si opens up several new possibilities. Si offers low cost substrates and CMOS compatibility for optoelectronic devices. Patterning of the Si substrate enables access to different crystal orientations for the GaN growth and also control of the defect density. Accessing the semipolar crystal orientations offers the benefits of a reduced quantum-confined Start effect and higher incorporation of InN to achieve longer wavelength emitters. (10-11)-oriented GaN has been overgrown on Si substrates using MOCVD which have been patterned into stripes in which a multiple quantum well (MQW) structure has been grown emitting around 470 nm. Cathodoluminescence (CL) hyperspectral imaging at room temperature revealed several features. Black spots associated with non-radiative recombination at threading dislocations (TDs) appear in the GaN CL intensity image. The MQW CL intensity image showed dark lines and mode structures overlaying the MQW emission peak. Electron channelling contrast imaging confirmed the presence of TDs at the dark spots. The dark lines can be associated with misfit dislocations induced during the semipolar growth. Finite difference time domain simulations of the mode structure revealed that the peak spacing originates from Fabry-Perot type resonances from the Si sidewalls. The source was a dipole at the position where the two GaN growth fronts from the opposite sidewalls of the Si meet. These structures show promise for higher wavelength emitters and laser diodes.

Resume : Design and growth of mid-UV laser diodes presents some challenges that are different to those in devices emitting in the VIS and IR range. This includes the absorbing p-GaN contact layer, low modal gain due to low waveguiding efficiency, and low p-conductivity for the cladding and waveguide layers. Here, theoretical and experimental work, targeted to address these challenges is presented. First, we present a theoretical framework that shows pathways to optimize carrier injection into the laser diode by optimizing the p-injection and p-cladding layers. Thereby, care is taken to reduces losses and optimize gain in the devices. Next, electrical data from diodes following these design rules is shown and compared to the predicted results. It is found that the simulation data can be used as a valuable guide for the growth of UV lasers. Further increase of current density is achieved by optimizing the MQW and EBL design. This includes optimization of the Al content in the barriers, quantum well thickness, and number of quantum wells. Comparing e.g. the laser threshold of AlGaN/AlN and AlGaN/AlGaN MQWs to the carrier injection efficiency it becomes apparent that only a tradeoff between the electrical and optical properties will lead to electrically injected UV lasers. Finally, based on the simulation data and experimental data a set of design rules will be presented that can be used to guide UV laser design in the future.

Resume : The optical polarization switching in multiple quantum wells (MQWs) on a partially relaxed (20-21) AlxGa1-xN-on-GaN is investigated via photoluminescence (PL) and k·p modeling. The strain in QWs is varied by AlxGa1-xN as its degree of relaxation (DOR) along [10-1-4] changes with XAl, allowing the shear component to change independent of other variables such as the In composition dependent biaxial strain and inhomogeneity in QWs. In0.03Ga0.97N/Al0.12Ga0.88N MQWs (x3) were grown on a series of 40-nm-thick partially relaxed AlxGa1-xN (0.1

Resume : III-nitride based heteroepitaxially grown quantum well layers contain several orders of magnitude higher number of threading dislocations (TD) compared to their III-arsenide counterpart. Thus, non-radiative recombination at TDs is expected to reduce the efficiency of light emitting devices. Nonetheless, highly efficient blue LEDs based on InGaN/GaN quantum wells have been realized and commercialized despite the high TD density. In contrast to the remarkable technical breakthroughs, the physics of the most crucial component – the active medium – remains not fully understood and is still debated. Using cathodoluminescence spectroscopy performed in a scanning transmission electron microscope (STEM-CL), we analyze the optical properties of an InGaN single quantum well (SQW) with nanometer scale resolution close to individual threading dislocations. Embedded into GaN barriers the InGaN SQW was grown by metal-organic vapor phase epitaxy on a GaN/sapphire template. Cross-sectional STEM images prove the comparably high quality of the InGaN/GaN SQW in general. In dislocation free domains, the SQW layer thickness amounts to 1.2 nm. The spatially averaged spectrum exhibits emission contributions from the SQW at 395 nm wavelength. Most strikingly, the SQW emission is blue-shifted up to 70 meV in vicinity to TDs with concurrently reduced intensity. The nature of the blue shift will be discussed in terms of local variations of strain, composition and thickness introduced by TDs.

Resume : The semiconductor AlN has been shown to be highly radiative with recombination lifetimes as short as 10 ps at low temperature. For ternary AlGaN c-plane quantum wells (QWs), alloy fluctuations and built in fields that reduce wave function overlap can lead to longer than expected lifetimes. Thin AlGaN QWs designed to increase wave function overlap show a PL lifetime with two distinct fast and slow components at low temperature that merge into one at room temperature. The QWs studied ranged from 0.6 to 2 nm and were grown pseudomorphic to high quality bulk c-plane AlN. It is postulated that the initial fast component reflects the highly radiative nature of AlN containing material while the longer component is due to localization phenomenon, including well width fluctuations. We compare our experimental results to a theoretical model incorporating the effects of excitons and Coulomb correlated electron-hole pairs, that utilizes a non-equilibrium Green’s function formalism with self-energies evaluated in the self-consistent T-matrix approximation. The model gave recombination rates that increased over two orders of magnitude, consistent with the observed decrease in the fast initial lifetime by nearly two orders of magnitude from 10 to 0.16 ns for the 2 to 0.6 nm wells. Results imply that narrower QWs with high exciton binding energy enable a radiative lifetime comparable to those measured in bulk AlGaN of similar Al content.

Resume : Nowadays, GaN-based light-emitting diodes (LEDs) have the potentials in general lighting and visible light communication (VLC) simultaneously. However, its optical modulation bandwidth (BW) is still low, which limits the application in VLC. Localized surface plasmon (LSP) coupling with quantum wells (QWs) has attracted quite a lot of researchers’ interest. Since the density of states (DOS) of SP mode can be very high, the LSP-QW coupling rate will be very fast, which can increase the spontaneous emission rate (SER) [1]. Akselrod et al thought that SER enhancement could exceed even 1000 times [2]. But there are seldom reports on BW improvements of LED by LSP. In this work, we have fabricated LSP-QW coupled samples based on the photonic crystal (PhC) structure on the green LED wafer, as shown in Fig. 1(a). The green LED wafer used in this work was grown by metalorganic chemical vapor deposition (MOCVD) on the double polished c-plane sapphire substrates with five quantum wells (InGaN (2.5 nm) / GaN (12.5 nm)) and 160 nm thick p-GaN layer. First, the 150 nm thick SiO2 was deposited on the surface of p-GaN. The PhC patterns were obtained by nano-imprinting using Eitre 3 instrument with a standard PhC stamp whose period is 545 nm. The patterned LEDs were etched by induced coupled plasma (ICP) etcher with the pore depth 150 nm, which is 10 nm away from QWs. Then the 20 nm thick Ag film is deposited on the patterned surface. After thermally annealing at 600°C for 10 minutes in N2 ambient, Ag nanoparticles (NPs) are formed in the PhC holes, followed by the lift-off to remove the Ag NPs on the mask. Time resolved photoluminescence (TRPL) measurement is an effective way to prove LSP-QW coupling effect, as shown in Fig. 1(b). The decay curves are fitted using double exponential function. The fast decay time are obtained as 0.28, 0.77 and 0.95 ns for PhC-Ag LED, PhC LED, bare LED respectively. The enhancement by 3.8 times of PhC-Ag LED compared to PhC LED or bare LED are achieved, which indicates that LSP has a strong coupling with QWs. PL intensity of PhC-Ag LED decreases by 1.5 times compared to PhC sample, which can be attributed to the dissipation of metal. However, cathodoluminesece (CL) intensity of PhC-Ag LED increases by a factor of 2 compared to PhC sample. To get more details, CL line scan with the beam passing through the center of Ag NP and PhC hole respectively was performed. As is shown in Fig. 2, CL intensity profile with the beam passing through Ag NP and PhC hole shapes like an “M” and a bell, showing that the LSP-QW coupling was enhanced while the electron beam hits Ag NPs. To get deeper understanding of these results, three dimensional finite difference time domain (3D-FDTD) simulations are performed by Lumerical FDTD Solutions. The Purcell factor (Fp) is calculated, which is defined as the radiation power of a dipole near Ag NPs to the ratio of the classical radiation power of a dipole in a bulk dielectric material. The Purcell factor is directly related to the local density of states (LDOS) calculated from the imaginary part of the dyadic Green’s Function in the direction of the dipole moment at the dipole position [3]. Calculated Purcell factor can be as high as 27, which indicates that a significant reduction of the spontaneous emission lifetime could be achieved. Moreover, the BW improvements are observed in the LSP-QW coupling LED. The quench mechanism of the recombination rate in electroluminescence is studied too.

Resume : Highly efficient nitride-based light emitting diodes (LEDs) are extremely needed for solid state lighting application. Recently the patterned sapphire substrate (PSS), which are orientated with the epitaxially lateral overgrowth (ELOG) technique has been widely investigated to improve the crystalline quality and light extraction of LEDs [1]. The different pattern shapes of the PSS were fabricated by combining conventional photolithography and dry or wet etching technique, and could be used to effectively scatter or redirect the guided-light inside an LED chip to find escape cones. Besides it could improve the light extraction of the LEDs to shrink the pattern size. For the same area of the sapphire substrate, the smaller pattern size of the PSS can increase the number of the patterns, and then increase the opportunity of light scattering [2]. In this work, the cone-shape NPSS was fabricated using two-inch c-plane sapphire substrates by nano-imprint lithography (NIL) and inductively coupled plasma (ICP) dry etching. The pitch and the size of the hexagonal arrangement pattern were 550 and 420 nm, respectively. The height of the pattern was 245 nm. After the patterning process, the growth of GaN film was carried out by using low pressure metal organic chemical vapor deposition (MOCVD). First, a low-temperature (545 °C) GaN buffer layer was grown for 90 s. Afterwards, the chamber temperature was raised to 1045 °C for the recrystallization of the GaN buffer layers, followed by the two-step high temperature growth. The first step denoted as three dimensional (3D) growth was carried out with 3 sccm silane flow rate in 1025 °C and the second one denoted as two dimensional (2D) growth was the recovery stage in 1085 °C. Then cyan LEDs (emission peak at 490 nm) were grown on the GaN templates. The GaN epilayers grown on the planar sapphire substrates and NPSS were characterized by X-ray diffraction (XRD). The full width at half maximums (FWHMs) of the (002) and (102) reflections in the XRD rocking curves were 275 and 340 arcsec for the sample on NPSS, while those for planar samples are 280 and 364 arcsec. It is also worth mentioning that the silane contributes to the 3D growth as the antisurfactant [3] which can delay the nucleation island coalescence to reduce the threading dislocation density (TDD) or inhibit the TD propagation upward. In addition, the cyan LEDs grown on the GaN templates based on both NPSS and planar sapphire substrates were tested by room temperature photoluminescence (PL) and electroluminescence (EL) measurements. The PL and EL intensity of the LEDs grown on NPSS are enhanced as 3 times and 61% compared with those grown on planar sapphire substrates. The enhancement of the PL and EL intensity can be attributed to the increase of the light extraction efficiency of the LEDs. In summary, we have performed MOCVD growths of GaN epilayers both on NPSS and planar sapphire substrates. The improvement of the crystalline quality can be found in the XRD result, and it is also due to the the silane addition which causes the effective 3D growth. The cyan LEDs on the two different kinds of sapphire substrates are fabricated. It is indicated that the PL and EL intensity are enhanced as 3 times and 61% for LEDs on NPSS compared with those on planar sapphire substrates. Acknowledgements This work was supported by National Key Basic Research Program of China under Grants Nos. 2011CB301905, 2013CB328705, and National Natural Science Foundation of China under Grants Nos. 61334009, 60976009, U0834001. References [1] Y. J. Park, H. Y. Kim and J. H. Ryu, et al., Effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes, Opt. Express, 19, 2029-2036 (2011). [2] C.C. Wang, H. Ku and C.C. Liu, et al., Enhancement of the light output performance for GaN-based light-emitting diodes by bottom pillar structure, Appl. Phys. Lett. 91 121109 (2007). [3] K. Pakula and R. Bozek, et al., Reduction of dislocation density in heteroepitaxial GaN: role of SiH4 treatment, J. Cryst. Growth 267, 1–7 (2004).

Resume : Nitride LEDs development presents significant scientific and societal issues. The aim is to get low-cost, high efficiency LEDs with accurate colour-rending (typically the Colour Rending Index has to be higher than 90). Type-II InGaN-ZnGeN2 quantum wells (QWs) were proposed for the improvement of efficiency in active regions and realizing then devices operating in a large wavelength range from UV to IR. Due to their large band gap (from 0.8 to 6.2 eV), III-N materials, as GaN and alloys, are still used for LEDs development. Nevertheless, they present several huge limitations mainly due to the evolution of InGaN properties for higher Indium concentrations. Strain and polarization effects affect then the LED quality through the reduction of the spontaneous emission. New high-performance devices require the development of new materials and the introduction of ZnGeN2 layers could be an alternative solution. ZnGeN2 derives from the III-nitride elements by replacing the III-group alternatively by a group II (Zn) and a group IV (Ge). Both the energy band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The crystallographic organisations are similar and the recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II InGaN-ZnGeN2 heterostructure. Inserting a ZnGeN2 layer in a conventional type-I InGaN QW structure yields significant modifications . The strong confinement of holes in the ZnGeN2 layer allows the use of a lower In-content InGaN QW with uniform In content. In the type-II InGaN-ZnGeN2 QW designs, a thin AlGaN layer was used as a barrier for better carrier confinement. The type-II InGaN-ZnGeN2 QWs lead to a significant enhancement of the electron-hole wave function overlap as compared to those of the conventional QWs. We describe here the results of simulation of the proposed type-II QWs. We focus on the designs of type-II InGaN-ZnGeN2 QW structures for blue and green-emitting LED applications. Different design protocols are compared by changing the width and the position of the ZnGeN2 layer. The self-consistent 6-band k?p method is used to perform the band structure calculations, which take into account the effect of strain, the valence band mixing, and the spontaneous and piezoelectric polarisations.

Resume : Micron Light emitting diode (LED) shows extraordinary performance under high injection level (~kA/cm2). Uniform current spreading, good thermal dissipation, the influence of strain relaxation and many other reasons attribute to the excellent properties of micron LEDs (μLEDs). The spontaneous polarizations as well as the strain induced piezoelectric will affect the optical and electrical properties of GaN based micron LEDs to a certain extent. The built-in piezoelectric field induced by compressive stress will lead to band bending in the area of InGaN/GaN multi-quantum well (MQW). The phenomenon weakens the restriction of quantum wells on carriers and reduces radiation recombination probability at the same time. However, there are few reports on polarizations and their influence on the capability of GaN based micro-light emitting diodes with different diameters. In this work, Kelvin probe force microscopy (KPFM) has been used to characterize strain relaxation distribution in InGaN/GaN MQW micro-pillars with diameters ranging from 10 to 300 μm. The results of KPFM can be confirmed by the data getting from Micro-photoluminescence (µPL). KPFM is used to measure the contact potential difference, VCPD, between a metal tip and the semiconductor surface directly, which corresponds to the difference in their work functions. Ling et al.calculated band diagram of c-plane LED under forward bias operation and observed a severe situation of band bending caused by polarization charges. In the MQW active region, the conduction band close to n-GaN is higher than that close to p-GaN, which can be detected sensitively by KPFM. In our study, The surface potential of μLEDs with different diameters are measured. The dependencies of VCPD on relative position to the mesa are shown . The relative position is defined as the ratio of the distance between the edge and test point to the radius of μLED pillar. Strain relaxation will relieve the band bending induced by polarization charges, which lead to the decrease of the contact potential difference. The distribution of strain relaxation can be showed directly and the strain-relaxation-effected area expand gradually when the size of μLEDs becomes smaller. In our study, The surface potential of μLEDs with different diameters are measured. The dependencies of VCPD on relative position to the mesa are shown. The relative position is defined as the ratio of the distance between the edge and test point to the radius of μLED pillar. Strain relaxation will relieve the band bending induced by polarization charges, which lead to the decrease of the contact potential difference. The distribution of strain relaxation can be showed directly and the strain-relaxation-effected area expand gradually when the size of μLEDs becomes smaller. The results getting from the µPL spectra of μLEDs with different diameters is in agreement with that of KPFM. It is suggested that the PL intensity increase is caused by strain relaxation and its decrease is due to etching damage.The ratio of unaffected area is estimated through the dependencies of integral intensity and peak wavelength on relative position to the mesa. In summary, Kelvin probe force microscopy has been successfully applied to measure the strain relaxation distribution of micron LEDs. It can be infer that smaller size μLEDs can effectively reduce the disadvantages induced by polarization field, thus leading to excellent performance of μLEDs.

Resume : Nowadays, GaN-based light-emitting diodes (LEDs) have the potentials in general lighting and visible light communication (VLC) simultaneously. However, their optical modulation bandwidth (BW) are still low, which limits the application in VLC. Localized surface plasmon (LSP) coupling with quantum wells (QWs) has attracted quite a lot of researchers’ interest. Since the density of states (DOS) of SP mode can be very high, the LSP-QW coupling rate will be very fast, which can increase the spontaneous emission rate (SER) [1]. Akselrod et al thought that SER enhancement could exceed even 1000 times [2]. But there are seldom reports on BW improvements of LED by LSP. In this work, we have fabricated LSP-QW coupled samples based on the photonic crystal (PhC) structure on the green LED wafer, as shown in Fig. 1(a). The green LED wafer used in this work was grown by metalorganic chemical vapor deposition (MOCVD) on the double polished c-plane sapphire substrates with five quantum wells (InGaN (2.5 nm) / GaN (12.5 nm)) and 160 nm thick p-GaN layer. First, the 150 nm thick SiO2 was deposited on the surface of p-GaN. The PhC patterns were obtained by nano-imprinting using Eitre 3 instrument with a standard PhC stamp whose period is 545 nm. The patterned LEDs were etched by induced coupled plasma (ICP) etcher with the pore depth 150 nm, which is 10 nm away from QWs. Then the 20 nm thick Ag film is deposited on the patterned surface. After thermally annealing at 600°C for 10 minutes in N2 ambient, Ag nanoparticles (NPs) are formed in the PhC holes, followed by the lift-off to remove the Ag NPs on the mask. Time resolved photoluminescence (TRPL) measurement is an effective way to prove LSP-QW coupling effect, as shown in Fig. 1(b). The decay curves are fitted using double exponential function. The fast decay time are obtained as 0.28, 0.77 and 0.95ns for PhC-Ag LED, PhC LED, bare LED respectively. The enhancement by 3.8 times of PhC-Ag LED compared to PhC LED or bare LED are achieved, which indicates that LSP has a strong coupling with QWs. PL intensity of PhC-Ag LED decreases by 1.5 times compared to PhC sample, which can be attributed to the dissipation of metal. However, cathodoluminesece (CL) intensity of PhC-Ag LED increases by a factor of 2 compared to PhC sample. To get more details, CL line scan with the beam passing through the center of Ag NP and PhC hole respectively was performed. As is shown in Fig. 2, CL intensity profile with the beam passing through Ag NP and PhC hole shapes like an “M” and a bell, showing that the LSP-QW coupling was enhanced while the electron beam hits Ag NPs. To get deeper understanding of these results, three dimensional finite difference time domain (3D-FDTD) simulations are performed by Lumerical FDTD Solutions. The Purcell factor (Fp) is calculated, which is defined as the ratio of the radiation power of a dipole near Ag NPs to the classical radiation power of a dipole in a bulk dielectric material. The Purcell factor is directly related to the local density of states (LDOS) calculated from the imaginary part of the dyadic Green’s Function in the direction of the dipole moment at the dipole position [3]. Calculated Purcell factor can be as high as 27, which indicates that a significant reduction of the spontaneous emission lifetime could be achieved. Moreover, the BW improvements are observed in the LSP-QW coupling LED. The quench mechanism of the recombination rate in electroluminescence is studied too.

Resume : The InGaN/GaN laser diodes (LDs) have attracted much attention due to their wide applications in laser display and laser lighting. Temperature-dependent photoluminescence (TDPL) is an important characterization method used to investigate the luminous efficiency and internal quantum efficiency. In this study, the temperature and power-dependent photoluminescence in InGaN/GaN blue LDs is investigated. From the temperature-dependent PL spectra, we observed conventional “S-shaped” behavior of the peak wavelength, both “U-shaped” and “W-shaped” behavior of the linewidth. From the temperature and power-dependent photoluminescence results, the shoulders in the “W-shaped” linewidth disappear as the power density increases. We attributed the “W-shaped” behavior of the linewidth in our blue LDs to the saturable absorption centers in the quantum wells.

Resume : In this work we report the growth of a tunnel junction (TJ) n++/p++ GaN on top of a regular GaN p-n junction using ammonia-assisted molecular beam epitaxy. A reference GaN p-n junction was also grown. The goal is to replace the highly resistive and partially transparent p-type contact by a low resistivity n-type contact. The surface morphology was studied by scanning electron microscopy and atomic force microscopy. Smoother surfaces are obtained for the samples with TJ. The samples were clean-room-processed using standard photolithography and reactive ion etching steps. The main difference between both samples concerns the top surface metallization. For the standard LED, the mesas are covered with a semi-transparent thin NiAu layer while for the TJ-based LED, only a TiAlNiAu small pad is deposited. We measured the electrical and optical characteristics of these LEDs at room temperature under CW conditions. The salient features are that the TJ-based device showed homogeneous emission (without semi-transparent metallic layer), and an EL peak at 364nm (corresponding to UV-A emission). The total differential resistivity (including n-type contacts, p-n junction and tunnel junction specific resistivities) of the LED with TJ is 10-2 Ω.cm2 measured at a current density of 100 A/cm2. This work is partially funded by GANEX (ANR-11-LABX-0014) and by the CEA Grenoble.

Resume : Photonic integrated circuits are key elements for signal processing and development of quantum information devices. The III-N materials present a series of key advantages for the development of photonic circuits from the near-infrared to the visible and ultra-violet. The III-nitride materials have a very large transparency window and their crystal symmetry allows second-order nonlinear processes.In this talk, we will present recent progress achieved with III-nitride photonic circuits grown on silicon(111). A silicon platform is a key advantage for integration on large surfaces at low cost using the CMOS fabrication environment. We have developed two-dimensional photonic circuits integrating photonic crystals, microdisks and suspended waveguides as well as couplers. Quality factors up to 80000 have been achieved in the near-infrared for microdisks (34000 for photonic crystals). Second-order and third-order optical nonlinearities have been demonstrated at room temperature under cw excitation. We will show that third-harmonic generation can be used for near-infrared mode imaging with sub-wavelength spatial resolution and pattern analysis equivalent to the one obtained with near-field scanning probe microscopies. Phase-matched second harmonic generation has been demonstrated in microdisks by varying the disk diameter by steps of 8 nm and achieving double resonance for the pump and harmonic with whispering gallery modes satisfying the phase matching condition.

Resume : Exploring the limit of low threshold lasing has been a central goal in the development of nanocavity lasers. GaN-based microdisk cavities provide advantages of high quality factors and small modal volumes, and have already demonstrated low threshold lasing. We have previously observed lasing in one-micron diameter microdisks containing either 3 layers of InGaN quantum dots (QDs) or quantum wells (QWs) with thresholds as low as 0.28 mJ/cm2 and 0.18 mJ/cm2, respectively. To further lower the lasing thresholds, we formed “micro-ring” laser structures with different geometries. The micro-rings are 1 micron in outer diameter, and have inner holes with diameters of 200 nm, 400 nm and 500 nm, respectively. Although we indeed observed threshold reduction with decreased micro-ring volume in cavities containing 3 layers of QDs, we also saw a concomitant decrease in slope efficiency of the lasing curves. We believe that leakage of carriers to the surface plays a strong role in this; yet it is remarkable that the lasing thresholds continue to diminish as the surface effects increase. To better understand the role surfaces play in lasing, we systematically compare micro-rings with active layers comprising either QDs or QWs. Changing the micro-ring geometry alters the total surface area of the laser that is subject to non-radiative recombination, while changing the active layer composition allows us to further understand the relative role of the surface in determining laser performance.

Resume : Surface phonon polariton resonator has been discussed on SiC nanostructures, where it is claimed that the polariton systems have a higher quality factor than plasmon systems using metallic nanostructures. We have studied semiconductor/metal composites for applications to absorber or emitter in terahertz region at room temperature, and have observed intense absorption of s-polarized light resonant to LO phonon energy due to electric dipolemoments induced by vibrating polarization charges at interfaces between GaN and Ti-stripe plates. In this study, we show the mechanism of electric-dipole absorption and interface polariton generation in the field dominated by the optically induced dipoles, particularly in AlN/Al stripe structure, where AlN produces the largest polarization charges at the heterointerfaces with metals or other dielectric materials among III-nitride materials. In this structure, s-polarized light was strongly absorbed at the LO resonant energy of 911 cm^-1 when the electric field was perpendicular to the Al stripe lines. Besides the surface polariton modes of AlN, the AlN/Al-interface modes were detected using p-polarized incidence light. The dependence of reflectance spectrum on incidence angle revealed that these mode energies agree with the theoretical values based on the permittivity function dominated by the optically induced electric dipoles. Thus, metal/III-nitride composites can supply various functions of IR absorption, emission, and propagation.

Resume : Three-dimensional (3D) GaN-based structures allow for the integration of InGaN quantum wells (QWs) on their non-polar sidewalls while keeping the material free of extended defects. These core-shell heterostructures are promising approaches for the next generation LED devices. A significant advantage is the large active area on the 3D structures in relation to the surface area of the substrate. In order to optimize the internal quantum efficiency (IQE) of the 3D heterostructures, a detailed knowledge of the radiative and non-radiative recombination channels and their rates is required. We compare the recombination and relaxation dynamics of excited electron-hole-pairs in InGaN QWs of 3D microrod and fin heterostructures (microwalls). Both were grown by continuous selective area metal organic vapour phase epitaxy (MOVPE) on SiOx masked GaN templates. The dynamics are investigated by time-resolved photoluminescence measurements for different temperatures and excitation wavelengths, with appropriate photon energies above the GaN bandgap and resonant to the InGaN, respectively. The InGaN luminescence of the 3D structures show decay times in the range of 200 ps. In contrast, for QWs grown on c-plane GaN structures, which are strongly influenced by the quantum confined Stark effect, decay times of a few ns are determined. Additionally, we analyse the homogeneity of the indium in the QWs along the height of the structures and the length of the fins of several mm.

Resume : Efficient tunnel junctions (TJs), which provide a means of carrier conversion between p-type and n-type material in semiconductor devices, could be particularly advantageous for the III-Nitrides where the poor conductivity of p-GaN significantly impacts the design and efficiency of LEDs and lasers. In this paper, we will describe device results following the development of a hybrid growth approach that involves growing the active region and and top p-GaN layers of the device by the standard (MOCVD) growth technique, and subsequent regrowth of the n-side of the TJ by ammonia assisted molecular beam epitaxy. This hybrid TJ approach has been successfully demonstrated on a number of different types of GaN based light emitters, including edge emitting lasers, and vertical cavity surface emitting lasers (VCSELs) where major efficiency improvements were demonstrated for a VCSEL with GaN TJ intracavity contact as compared to a reference VCSEL with transparent conducting oxide intracavity contact. In this paper we will discuss the implications of the TJ on increasing the design space for laser diodes and potential for improved efficiency.

Resume : To date, AlGaN quantum well LEDs in the UV-C band exhibit very low efficiency. In addition, there have been no demonstration of electrically pumped quantum well lasers in the UV-B and UV-C bands. In this talk, I will present an alternative solution for addressing the critical challenges of deep UV photonics. With the use of plasma-assisted molecular beam epitaxy, we have demonstrated that nearly defect-free AlGaN nanowires can exhibit high efficiency, tunable emission from 207 to 340 nm. More importantly, we have discovered that Mg-dopant incorporation is significantly enhanced in nanowire structures without the formation of extensive defects. The resulting Mg-impurity band leads to efficient hole hopping conduction. At room-temperature, we have measured free hole concentration up to 1018 cm-3 for Mg-doped AlN nanowires. We have further conceived the concept of Al tunnel junction and demonstrated AlGaN nanowire tunnel junction deep UV LEDs on Si substrate. These devices exhibit an output power > 8 mW, the highest value ever reported of deep UV LEDs on Si. Moreover, with the incorporation self-organized quantum dots in nearly defect-free AlGaN nanowires, we have demonstrated electrically pumped semiconductor lasers operating in the UV-B and UV-C bands. For the devices operating at 239 nm, the threshold current is only ~ 0.3 mA at room temperature. The achievement of electrically pumped high power AlGaN nanowire lasers is currently in progress and will be reported.

Resume : We report on high efficiency tunnel-injected ultraviolet light emitting diodes (UV LEDs) emitting at 287 nm. Using a polarization engineered Al0.65Ga0.35N/In0.2Ga0.8N tunnel junction structure, we achieved on-wafer peak external quantum efficiency of 2.8%. III-Nitride ultraviolet light emitting diodes (UV LEDs) are promising in various applications including sterilization, water purification and medical sensing. However, both the light extraction efficiency and electrical efficiency face fundamental challenges for the conventional UV LED structures. This stems from the poor p-type conductivity and high p-type contact resistance. Recently, we demonstrated a tunnel-injected UV LED structure to address both the absorption and electrical loss issues. Using this tunnel-injected UV LED structure, we have achieved efficient UV light emission at 325 - 257 nm. In this work, we show that efficient deep UV LEDs can be achieved through non-equilibrium tunneling hole injection. The tunnel-injected UV LEDs were grown by plasma assisted molecular beam epitaxy on metal polar Al0.72Ga0.28N template with a threading dislocation density of 3×109 cm-2. The UV LEDs contain three periods of 2 nm Al0.5Ga0.5N quantum wells, 50 nm compositionally graded p-AlGaN layer with Al mole fraction grading from 95% to 65%, thin tunnel junction layer with 4 nm In0.2Ga0.8N, 200 nm n Al0.65Ga0.35N, and a 40 nm graded n AlGaN top contact layer with Al composition grading down from 75% to 15%. The reverse graded contact layer enables flat conduction band between the metal contact and high composition AlGaN for the injected electrons. Alloyed V/Al/Ti/Au metal stack, and non-alloyed Al/Ni/Au/Ni metal stack were used for bottom and top contact, respectively. The top contact was designed to have partial metal coverage on the mesa area. A low power ICP-RIE etch was used to remove the down-graded n AlGaN top contact layer within the device mesa region that has no metal contact to minimize internal light absorption. The contacts were characterized by transfer length measurement methods. The specific contact resistances for the annealed bottom contact and the non-alloyed top contact are 1.2×10-5 Ω cm2 and 2.2×10-6 Ω cm2, respectively. This demonstrates the benefit of using a down-graded AlGaN contact layer for contacts to high Al content n-AlGaN layers. The electrical characteristics of 30 × 30 um2 devices shows a high voltage drop of 10.5 V at 20 A/cm2, which is mainly attributed to the extra voltage drop across the TJ layer. This could originate from the poor effective p-type doping due to the observed high density background compensation charge (> 1×1019 cm-3). As a result, there is extended depletion in the p-AlGaN layer, and great reduction in the tunneling probability. Therefore, optimizing the tunnel junction design is expected to reduce the operation voltage. A low differential resistance of 1.7 ×10-3 Ω cm2 was extracted at 1 kA/cm2. This is more than one order lower as compared to the conventional UV LED devices. Electroluminescence measurements confirm non-equilibrium hole injection into the active regions. Single peak emission was obtained at a wavelength of 287 nm, with peak on-wafer EQE (WPE) value of 2.8% (1.1%). Both efficiency values are comparable to state-of-the-art reports at similar emission wavelength. A high power density of 54.4 W/cm2 was obtained at 1 kA/cm2. The microscope image indicates uniform light emission from the whole device area even though the metal contact covers small part of the mesa area through a reverse graded contact layer. This demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and provides a structural design towards high power UV emitters. We acknowledge funding from the NSF (ECCS-1408416 and PFI AIR-TT 1640700). Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. References: [1] Y. Zhang, S. Krishnamoorthy, J. M. Johnson, F. Akyol, A. Allerman, M. W. Moseley, A. Armstrong, J. Hwang and S. Rajan, Appl. Phys. Lett. 106, 141103 (2015).

Resume : We examined InGaN/GaN double quantum wells (QWs) with In-concentration of 17% and width of QWs equal to 2.5 nm. The barrier thickness of 1-2 monolayers was applied. Samples were grown by PA-MBE on bulk GaN substrate. Time-integrated and time-resolved photoluminescence as well as variation of PL energy, EPL, with a distance from laser exciting site were measured for temperatures between 4K and 100K. A drastic blue-shift of EPL, from about 2.30 eV (540 nm) to 2.6 eV (460 nm) with a power of exciting laser accompanied by the corresponding decrease of PL decay time (in the range of tens of microseconds) were observed. It confirms large Quantum Confined Stark Effect caused by built-in electric field in these polar structures and thus strongly indirect character of excitons in the examined coupled QWs. Excitons diffuse along the QW ? plane on distances of few tens of micrometers. We grew also the same QW-structures with the tunnel junction which allowed to study an influence of externally applied electric field on the indirect (dipolar) exciton in the studied coupled QWs. We observed a shift of EPL to higher (lower) values with voltage applied in reverse (forward) direction. This behavior is accompanied by a significant decrease (increase) of the PL decay time (between 2 and 20 microseconds). We have estimated the effective electric field acting on the InGaN/GaN QWs with tunnel junction. This work was supported by National Science Centre Poland grants nr 2013/11/B/ST3/04263 and 2015/17/B/ST7/04091.

Resume : Recently, there has been increasing attention given to the interband tunnel junctions (TJ) for effective carrier conversion between n-type and p-type material region in nitride LEDs and LDs. It was shown that TJ resistance for wide bandgap semiconductors can be effectively suppressed by engineering of the piezoelectric fields in the region of the TJ. The low resistance TJs were realized using GaN/InGaN/GaN heterostructures. For TJ nitride devices the MBE technology seems to be more efficient than MOVPE, due to the fact that diffusion of hydrogen is suppressed in n-type layers. Therefore the activation of MOVPE grown p-type regions (which are deeply buried below n-type layers) by post growth annealing is problematic. In this work we demonstrate TJ LDs operating at wavelengths 450-480 nm grown entirely by plasma assisted MBE. The diodes consist of standard LD structures capped with heavily doped InGaN QW TJ. We estimated specific resistance of our TJ to be lower than 5e-5 Ohm*cm2 at current density 10kA/cm2. We will discuss the influence of the TJ growth parameters like InGaN well width, doping profiles for performance of TJ LDs. Our demonstration opens possibility for new architecture of edge emitting LDs and monolithic VCSELs, where high absorption of the p-type DBR is challenging issue. Acknowledgements: This work was supported by the Polish National Centre for Research and Development Grant PBS3/A3/23/2015 and Foundation for Polish Science Grant TEAM TECH/2016-2/12

Resume : In this paper, I will provide an overview of the optical characteristics of III-nitride quantum wells (QWs) under high injection. In the first part, I will present the investigation of the Mott transition occurring in single high-quality GaN/AlGaN QWs. I will explain how we determine the carrier density at which the Mott transition occurs in these III-nitride heterostructures [1]. I will also provide clear experimental evidence on the peculiar stability of biexcitons at those carrier densities [2]. This is of crucial importance to establish a comprehensive theory to better understand the physics of III-nitride lasers. In the second part, I will present similar studies on InGaN/GaN QWs. I will show that these experiments are perfectly suited to investigate the non-radiative recombination under high injection. Such studies provide a deeper insight on the physical phenomena responsible for the efficiency droop in III-nitride LEDs [3,4]. [1] G. Rossbach et al., Phys. Rev. B 90, 201308 (2014). [2] M. Shahmohammadi et al., Nat. Commun, 5, 5251(2014). [3] M. Shahmohammadi et al., under review. [4] W. Liu et al., Phys. Rev B, accepted (2016).

Resume : One of the most significant limitations for the quantum efficiency of group III-nitride based light emitters is the spatial electron-hole separation due to the quantum-confined Stark effect (QCSE). To overcome this problem, Hönig et al. [1] proposed a novel concept, the Internal-Field-Guarded-Active-Region Design (IFGARD), which suppresses the QCSE for wurtzite crystals in the [0001] direction. Here, we show how encapsulating the active region by additional guard layers results in a strong reduction of the built-in electric field in c-plane wurtzite nanostructures. Even more importantly, we demonstrate the first experimental evidence for the successful realization of an IFGARD structure based on GaN/AlN heterostructures embedded in GaN nanowires. By means of power-dependent and time-resolved µ-photoluminescence (µ-PL) we experimentally proof the validity of the unconventional IFGARD structure. We managed to tune the emission of 4-nm-thick GaN nano-discs up to 3.32 eV at low excitation powers, which is just 150 meV below the bulk GaN bandgap. Our results demonstrate an almost complete elimination of the QCSE in comparison to conventional structures which show approximately 1 eV red-shifted emission. The suppression of the QCSE results in a significant increase of the radiative exciton decay rates by orders of magnitude and demonstrates the potential of IFGARD structures for future light sources based on polar heterostructures.

Resume : Semipolar growth of III-nitrides provides a route to reducing the inbuilt electric fields present in polar directions, decreasing device efficiency. However semipolar growth introduces a new range of defects, including arrowhead features referred to as chevrons. These form due to irregularities in coalescence and propagate to the surface of the material. We report on their impact on the optical properties and conductivity of devices. Cathodoluminescence (CL) hyperspectral imaging and AFM measurements were used to study the impact of chevrons on the luminescence of two (11-22)-oriented InGaN/GaN MQW structures grown on different overgrowth templates. The first, a blue-emitter, showed a 200meV red-shift in emission energy along the chevron arms, and a 50meV redshift in tail-like structures protruding from the tips. These shifts are likely due to changes in InN incorporation in the new facets introduced by the chevrons. There was also a slight blue-shift within the chevron, which we attribute to changing thickness of the quantum wells. The second sample was a fully contacted amber LED and showed a similar blue shift. Simultaneous CL and Electron Beam Induced Current mapping showed low emission intensity and minimal current flow along the chevron arms, indicating that non-radiative recombination dominates in that region. The complementary microscopy techniques are shown to provide important details of the properties of the chevrons.

Resume : GaN/(Al,Ga)N is the materials system of choice for deep ultraviolet emission. However, planar GaN/(Al,Ga)N heterostructures suffer from high densities of threading dislocations and cracks. These problems are avoided in GaN/(Al,Ga)N nanowires (NWs) that spontaneously form in molecular beam epitaxy on, e. g., Si. NWs also offer a superior light extraction, and the strain relief at the NW sidewalls leads to reduced polarization fields as compared to planar layers. Here, we study three GaN/(Al,Ga)N NW ensembles with a nominal Al content of 0.52 and a single embedded GaN quantum disk (QD) of 1.2, 2.3 and 4.3 nm thickness. Photoluminescence (PL) spectroscopy reveals that the QD transition energy depends only weakly on its thickness and is always higher than the band-gap energy of GaN, in contrast to results of Schrödinger-Poisson simulations that predict a significant quantum-confined Stark effect. Using time-resolved PL experiments, the oscillator strength of the QD transition is extracted from the temperature dependence of the radiative lifetime. The corresponding value is close to that observed in nonpolar quantum wells, reflecting that built-in electrostatic fields are virtually absent. The probable origin of this phenomenon is an axial gradient in the Al content, which is detected by energy dispersive x-ray spectroscopy. This unintended gradient induces polarization doping, resulting in a high electron density in the QD that screens the built-in electrostatic fields.

Resume : Semipolar InGaN/GaN quantum-well (QW) structures with the (20-2-1) and (20-21) surface orientations have attracted a lot of interest in recent years for applications in efficient light emitting diodes. Studies revealed unexpected differences in the energy splitting of QW emission energy as well as the optical polarization degree between the two orientations. Here, we present electro-reflectance spectroscopy results of both types of QWs embedded in p-i-n diodes simultaneously grown by metal-organic vapour phase epitaxy. By sophisticated analysis of the signals, we are able to extract absorption related transition energies assigned to different valence sub-bands as a function of externally applied voltage. Different orientations of the electric field vector of the probe light allow conclusions about the relative oscillator strengths of the different transitions especially in comparison to elaborate kp perturbation theoretical model calculations. The field-dependent energy red-shift yields very similar effective hole masses of both participating valence sub-bands and defines an upper limit of the built-in polarization field of the two types of QWs. The external voltage dependent photo- and electro-luminescence properties are used for additional insight into polarization degree and transition energy dependence.

Resume : In this contribution we demonstrate how to experimentally determine the anisotropic deformation potential D5 for GaN and InN from photoluminescence spectroscopy (PL) applied to samples of different strain states and indium compositions. Via polarization resolved PL at low and at room temperature we are able to determine the valence band splitting caused by the anisotropic strain in m-plane structures. The strain of multi quantum well (MQW) structures can be controlled by varying the buffer layers and thus we are able to investigate the polarization and valence band splitting over a wide range of anisotropic strain states and MQW indium compositions. The control of strain is realized by an AlInN layer by varying the composition and thickness and thus the degree of relaxation. For thin QWs the in-plane lattice constant is given by the AlInN layer. This gives us the possibility to investigate the influence of different anisotropic strain at various MQW indium contents on the splitting of the valence bands. The strain controlled structures show a strongly reduced optical polarization for high indium containing MQWs, even at low temperatures. In addition to the unusual optical polarization our analysis indicates a sign switch of the D5 deformation potential for InN, while the D5 for GaN is in good agreement with literature. Both features suggest a possible polarization switching in m-plane GaInN/GaN quantum well structures for either high indium contents or high anisotropic strain.

Resume : Nitride based laser diodes utilize as an active medium extremely strained InGaN quantum wells. As the nitride materials are piezoelectric in their nature, this strain is reflected in a strong piezoelectric field of the magnitude even higher than 1 MV/cm. This tilts the energy bands and shifts the emission spectrum position through the Quantum Confined Stark Effect (QCSE). As the laser diode is operated at elevated currents, the built-in electric field is reduced due to the screening by injected carriers. This leads to the increase of emission energy (blue-shift). It has been a subject of many discussions whether the field is preserved at lasing, or is it completely screened. Moreover, the magnitude of the piezoelectric field depends on the device emission wavelength or in the other words on the In composition of the quantum wells. In the present work we compare the InGaN laser diodes of different QWs compositions with superluminescent diodes built on the same structures. We compare the blue shift of the emission wavelength caused mostly by the screening of the piezoelectric field. The superluminescent diodes allow us to study the emission spectrum at higher carrier densities than for laser diodes. In laser structures, we clearly see the saturation of the blue-shift caused by the Fermi level stabilization at threshold current. While, on the other hand, we see the continuous shift of the emission wavelength in case of superluminescent diodes. This suggests, that the piezoelectric fields are not fully screened at threshold current. We also see that for UV laser diodes the emission line shift is much smaller than for blue wavelength devices. This implies practically complete screening of the electric field for UV laser while lasing of blue laser diodes occurs at high electric field conditions.

Resume : Laser Light sources, the future of video projection. Even if high brightness laser projection is stil in its early stages, the latest improvements in GaN laser should trigger an acceleration of laser projection market adoption in digital cinema and other large venue projection applications. Laser baser projection provides multiple advantages : scalable light power, native primary colors, longer life cycles. BBright develops high power laser light sources and packaging technologies for red, green, and blue color. This presentation will cover the following items: - General review on video projection devices, how it works, and what are the main markets - Laser light sources in video projection, pro and cons - Other applications of laser light sources (medical, illumination)