Other devices

Resume : The bandgaps of GaInN alloy ranging from 0.65 to 3.43 eV are suitable for the design of high conversion efficiency multi-junction solar cells. Thus, the theoretical conversion efficiency of a four-junction solar cell designed for sunlight is as high as 62 %, which is higher than that of other materials such as Si, organic and other III-V materials. One of the most serious problems preventing the realization of high-performance nitride solar cells is the large internal electric field caused by piezoelectric and spontaneous polarizations. In this study, we fabricated the thick semi-polar (10-1-1) GaInN films on freestanding GaN substrate by MOVPE. We also fabricated the GaInN based solar cell using this semi-polar GaInN thick layer. The device consisted of a 1μm n-GaN layer, a 25 nm GaInN layer, a 100nm p-GaN layer, and a 10 nm p+-GaN contact layer. The bandgap energy determined from the PL spectrum is 2.72 eV. Almost pit free and relatively smooth surface (RMS: 0.35 nm) were obtained. The maximum EQE and absorption edge of GaInN were 17.1% at a wavelength of 395 nm and 425 nm, respectively. Therefore the internal quantum efficiency is estimated to be more than 90 %. The short circuit current density, open circuit voltage, fill factor, and conversion efficiency are 0.483 mA/cm2, 1.21V, 0.63, and 0.37%, respectively. These result showed the high potential of the semipolar (10-1-1) GaInN solar cell.

Resume : We demonstrate solar-blind ultraviolet sensing for flame detection using ultra-wide bandgap AlxGa1-xN devices. The sapphire substrate, and voltage<20V make these devices compact and low-cost, with responsivity 1.2x106A/W for the photoconductive Al0.5Ga0.5N MISFET detector, exceeding that of photomultiplier tubes ~105A/W, the gold standard in photodetection. The key is the high quality AlN-template layers grown by low-pressure metal organic chemical vapor deposition (MOCVD) on basal plane sapphire, with threading dislocation density of (1-2)×108 cm-2, with active layers were grown by pulsed MOCVD. The Al0.5Ga0.5N MISFET, with 0.2um thick selective area grown channel showed responsivity 1.2x106A/W in response to 100uW/cm2 broad area 254nm light at VGS=-2.5V, with channel electron mobility~55cm2/Vs. Action spectra showed this responsivity to fall by >200x by 290nm, the solar-cutoff. The estimated response time of ~34ms was slower than the channel transit time <1ns, indicating photoconductive gain, suitable for cameras at 24fps. MQW Al0.64Ga0.36N/Al0.34Ga0.66N pn diode detectors in short-circuit, showed solar rejection >103 from 250nm to 290nm, with peak responsivity of 0.1A/W. The photovoltaic response time of this device is <0.4us (current pre-amp bandwidth limited). Combining both approaches i.e. photoconductive and photovoltaic detection in a low-cost solar-blind platform, superior responsivity and speed are possible simultaneously.

Resume : GaN-based avalanche photodiodes (APDs) are of great interest in the ultraviolet (UV) spectral region because of their advantageous properties. Recently, separate absorption and multiplication (SAM) APD structures have been widely explored for low multiplication noise and high maximum gain. However, the realization of high-performance GaN-based UV-APDs has been hampered by high threading dislocation densities resulting from heteroepitaxial growth. In this study, the GaN p-i-p-i-n SAM APDs were designed and epitaxially grown on a c-axis n-type bulk GaN substrates with low dislocation densities by a metalorganic chemical vapor deposition (MOCVD). In addition, for a front-illuminated structure, a 5% p-AlGaN:Mg layer was introduced as a window layer instead of a p-GaN:Mg layer in order to increase responsivity in the UV spectral region. Using optimized growth conditions and a sophisticated fabrication technique, APDs with mesa areas of 30-?m-diameter showed low dark current densities at the onset point of the breakdown voltage of 74 V and no microplasmas were visually observed after multiple reverse-bias I-V scans. In addition, under illumination at ?~280 nm, the APD exhibited a maximum avalanche gain > 1x10^7 at the reverse bias of 76 V (current limited to 500A/cm2) which is higher than comparable standard p-i-n GaN APDs. The detailed growth, fabrication, and device characterization of the GaN p-i-p-i-n UV-APDs will be further discussed in the conference.

Resume : GaInN is useful as a top cell for 4-junction solar cell combination with GaInP/GaAs/Ge 3-junction solar cell. Establishment of wafer bonding technology is indispensable for realization of this device, because high quality crystals by crystal growth cannot be obtained by large lattice mismatch. In this study, we investigated the establishment of bonding technology between GaN and GaAs and its solar cell application. As the result, we confirmed the operation of 4-junction solar cell. The samples used for examination were GaInN solar cell (VOC= 2.0 V, JSH = 0.285 mA/cm2, and FF = 0.69 @A.M. 1.5G 1 sun) and GaInP/GaAs/Ge 3-junction solar cell (VOC = 2.1 V, JSH = 5.8 mA/cm2, and FF =0.75 @A.M. 1.5G 1 sun). The direct wafer bonding technology was performed by heating (~450 OC) and pressurizing (~5 MPa). By optimizing the bonding conditions, the bonding strength reached 4.5 MPa, which value is approximately 1.2 times higher than that of Au/Au junction. Next, we fabricated 4 junction solar cells by device process. The VOC, JSH, FF of this 4-junction solar cells with irradiation of A. M. 1.5G 1sun by solar simulator were 2.85 V, 0.219 mA/cm2, and 0.74, respectively. The JSH is almost comparable to theoretical value, but the VOC was about 1 V lower than the theoretical value. Because these result are thought to be affected by large resistance or Schottky junction at the junction interface, itis essential to improve the electrical properties of the bonding interface.

Resume : Sunlight-driven water splitting is a prospective means to produce clean and renewable hydrogen as a storable high-density energy carrier. Particulate photocatalyst is one of the simplest methods to produce solar hydrogen. Presently, the main concern is how to develop semiconductor materials for efficiently harnessing sunlight energy. To date, highly active water splitting photocatalysts are dominated by wide-gap oxides requiring UV light. For sufficiently harnessing sunlight energy, much narrower-gap semiconductors should be successfully applied to water splitting. Thus we have studied various non-oxides as new photocatalytic materials. Specifically, nitrides and oxynitrides have been demonstrated to be promising materials for water splitting responding to visible light. We will introduce recent developments in new semiconductors as light-harvesting materials for photocatalytic water splitting.

Resume : Solar hydrogen generation by photoelectrochemical (PEC) water splitting provides promising alternative energy resources for the conventional fossil fuel. Several semiconducting materials have been proposed, such as TiO2, ZnO, and GaN. Because the efficiency of PEC water splitting is highly dependent on visible light absorption, the ability to tune the bandgap of GaN by alloying with In makes it advantageous over other wide bandgap semiconductors. Additionally, the solar-to-hydrogen (STH) energy conversion efficiency is highly dependent on the charge carrier transport from the working electrode to the counter electrode. Providing a highly conductive path for the photogenerated charge carriers can then effectively promote better charge transport and enhanced PEC water splitting performance. Here, we report, the realization of robust PEC water splitting system using InGaN nanowires (NWs) with a tunable bandgap energy up to approximately 1.65 eV in the deep visible spectrum grown on bulk metal substrates. To optimize the growth of high quality InGaN NWs on metals, a molybdenum (Mo) substrate has been intentionally covered with titanium (Ti) buffer layer, which has been partially converted into TiN during the growth. This all-metal stack of TiN/Ti/Mo provides a unique ohmic contact, which can facilitate the transportation of charges from InGaN NWs to the counter electrode. However, the one-dimensional nanostructures are commonly characterized by a high density of surface states, which can act as surface traps for the photogenerated charge carriers and limit their transport to the counter electrode. To overcome this issue, the surface of InGaN NWs was passivated by short carbon chain 1,2 ethanedithiol (EDT) that allows the facile formation of few monolayers of sulfur (depending on the processing time), which can significantly reduce the density of surface trapping states. Furthermore, iridium metal co-catalysts were simply attached to the passivated surface of InGaN NWs by bonding with sulfur atoms to provide better charge separation. Using this semiconductor-on-metal photoanode together with the self-assembled surface treatment, the STH conversion efficiency reached 3.5% for unbiased pure water splitting (pH=7) under 1 sun illumination (AM1.5G). This is the highest STH value to date, to the best of our knowledge, for any III-nitride-based PEC water splitting single photoelectrode. Our results represent simple and reliable approaches that can be easily reproduced for scalable and cost-effective photoelectrolysis systems.

Resume : Vertically aligned 3D GaN nanopillar arrays with a sub-100 nm size, designed as conductometric NO2 gas sensors, are fabricated in this work using an improved nanosphere lithography (NSL) technique. Microline enclosures are prepared on the substrate allowing a selective deposition of polystyrene nanobeads, which are acting as a lift-off layer to define equidistant triangles of a chromium dry etching mask. Therefore, the hydrophilicity of the GaN surface has been enhanced by using a layer-by-layer self-assembly technique of PSS/PDDA/PSS-layers. This results in high surface coverage up to ~70%. Moreover, in contrast to commonly used Cl2-based GaN dry etching the nanopillars were fabricated in a hybrid top-down approach combining inductively coupled plasma reactive ion etching (ICP RIE) with SF6/H2 gases and subsequent KOH wet chemical etching, resulting in a very good control on pillar geometry. Due to the close-packing of spheres GaN nanopillars with a diameter of ~40 nm, a pitch of ~300 nm, and an aspect ratio of >10 were realized using a 500 nm nanoparticle masks. Furthermore, a detailed analytical study on the modification of the nanopillar surface condition including the surface functionalization with amine-based self-assembled monolayers (SAMs) has been performed by Auger electron spectroscopy (AES). We expect that this preparation route for 3D GaN nanopillars has a substantial impact on the development of highly selective and sensitive hybrid SAM/GaN-based gas sensors.

Resume : Anti-cancer drugs have cytotoxic effects on neoplastic cells, and the effects of various drugs on the same cell are often distinct [1]. Due to the permeability of cell membranes influenced by the opening/closing of channels in the membrane, modulation of these channels can regulate ion flux into or out of the cell thereby altering the functional state [2]. AlGaN/GaN hetereojunction field effect transistors (HFETs) are ion-impermeable and highly stable in electrolytes so they are an ideal candidate for detection of cellular activities in solutions with high ionic strengths. Here we show AlGaN/GaN HFETs for monitoring of the effects of two anti-cancer drugs, camptothecin (CPT) and doxorubicin (DOX), on different cell lines, cancerous and non-cancerous (RAJI cells and WIL2S cells respectively). The addition of CPT to a RAJI cell solution yields an appreciable increase while adding CPT to WILS2 cells showed no observable effects. The effect of DOX on both cell populations corresponds a slight decrease in the observed current signal. By looking the threshold voltage values when adding both drugs, we conclude that the current change following the addition of cell populations in gate area must result from the effect of CPT on cells. Fitting the time-dependent current responses after drug addition, we find that for the same cell line (RAJI or WIL2S), CPT treatment contains two processes with two time constants: an initial quick response/process followed by a slow one, while DOX only shows one process. The AlGaN/GaN FET biosensors shown here provide a platform for anti-cancer drug screening for cancer therapy.

Resume : Indium gallium nitride (InGaN) is known for its use in lighting applications such as LEDs or lasers. With its chemical stability it is also predestined for use in biological/medical environments. An actual scientific trend is the interest in chemical sensing applications using electrical or optical read-out. Optical sensors in particular feature the advantage of being easily implemented into wet surroundings without difficulties of e.g. electrical insulation. The photoluminescence of InGaN/GaN quantum wells (QWs) located close to the surface is sensitive to changes in the surface potential of the heterostructure. Adsorbed gas- and biomolecules alter the near-surface band bending of such structures, and hence induce a wavelength shift and change in intensity of the optical emission (Quantum Confined Stark Effect, QCSE). In order to achieve a (molecule-) selective response, we investigate methods how to functionalize the surface of our QW structures. In particular, PMMA stamps are used to apply functionalization molecules (i.e. complex silanes) to a hydrolyzed GaN surface. Specially prepared molecules added to this silanized surface are expected to selectively bond to the silanes via the lock-and-key principle. Ferritines, which are responsible for the iron transport in human blood, along with silanes and their combined sensor response are investigated in this work.

Resume : Hexagonal boron nitride (h-BN) has remarkable properties including a graphene-analogue structure, wide bandgap, and large thermal neutron capture cross-section. In particular, with a bandgap up to 6 eV, it exhibits potential for deep UV photodetectors. Most of the BN-based photodetectors reported were based on BN nanosheets or in monolayer domains with the size of micrometre scale for the usable surface area. The small size of BN films would hinder its practical application. Besides, thicker BN films would be more favoured to improve the detection yield. However, the structural properties of thick layers are unclear and their application for photodetection is still at a very early stage. In this work, we report important progress toward the technological requirements for thick BN layers and large surface area. 1~2.5 µm-thick BN layers were grown on 2-inch sapphire substrates by MOVPE. The thick layers exhibited strong band-edge absorption near 215 nm. Room temperature photoluminescence mapping showed wavelength variation of the characteristic defect band of only 0.6% and intensity variation of 13% over the whole wafer. A highly oriented 2D layered h-BN structure was formed at the film/substrate interface, which permitted an effective exfoliation of the thick BN film onto other supports. And this structure resulted in a BN membrane with metal-semiconductor-metal device prototype delaminating from the substrate. The prototype showed low dark current of 2 nA under 100 V, and 100±20% photoconductivity yield for light illumination at 205 nm. These wafer-scale MOVPE-grown thick BN layers present great potential for the development of deep UV photodetection applications, and even for flexible (opto-)electronics in the future.

Resume : Optogenetics has evolved over the past decade as one of the most innovative experimental techniques in neuroscientific research enabling the direct, cell-type specific modification of neural activity using light. It requests molecular tools which are tailored most sensitive at specific wavelengths and have to be expressed in the cell membranes using viral transfection. In addition, compact implantable technical tools have to be provided serving the light delivery and measurement of electrophysiological signals in in-vivo applications in basic neuroscientific research and ultimate in clinical practice. The paper will summarize recent efforts of our team on the integration of light-emitting diodes (LED) and laser diode chips on flexible as well as stiff, penetrating implants based on polymer and silicon substrates. The laser diode chips are combined with micromachined polymeric waveguides enabling light deliver into deeper brain structures while the optoelectronic light source is positioned above the skull. In contrast, LEDs integrated on polymer or silicon substrates are implanted directly into the brain or cochlea requesting specific efforts towards system encapsulation. The MEMS-based technological approaches used for the realization of these implantable devices rely either on commercial LED chips integrated using flip-chip bonding or the laser-lift-off of GaN-on-sapphire layers transferred onto polymer or silicon substrates using an indium-based wafer-level bonding process.

Resume : Large polarization fields are a unique property of III-Nitrides which can be used for device design. Vertical devices incorporating heterojunctions exploit the polarization sheet charges of magnitude 1013 C/cm2 or larger present at heterointerfaces. A unipolar device, the dipole induced diode, is presented for the c-plane of GaN. Rather than a wider bandgap material as the transport barrier, the results of which are highly variable due to percolative transport behavior through natural ternary alloy fluctuations, a dipole barrier is used. Heterostructures are grown with an unintentionally doped (UID) GaN layer at the negative polarization interface, which induces a depletion region in the binary GaN to act as a barrier. Such dipole induced diodes structures have been grown by Molecular Beam Epitaxy (MBE) on GaN templates with pure AlN, InGaN and bandgap matched InAlN. Diode like rectification is observed, with physically meaningful differences in temperature dependences. The JV characteristics of the AlN dipole diodes structures are relatively temperature insensitive, as the pure AlN forms an additional tunnel barrier, while the InGaN and InAlN based dipole diodes follow a thermionic emission law. Extracted barrier heights of 0.5-0.6 eV for InAlN and 0.9-1.4 eV for InGaN devices are in good agreement with 1D Schrodinger Poisson simulations. Differences in the ideality factor also reflect the distinction between transport tunneling and thermionic mechanisms.

Resume : In recent years, piezoelectric nanowires (NWs) have emerged as novel piezo-materials having the potential to increase the conversion efficiency and to extend the application of piezo-generators to supply various micro-devices without increasing their size and weight. Hence, the mechanical-electrical conversion has been demonstrated from NWs with generated maximum output voltage per single NW of about 440 mV for GaN and 1V for InN. Despite this recent progress, the conversion efficiency and capacity have to be improved for the development of efficient generators. In this work, we propose a novel design based on InGaN/GaN heterostructures integrated into the active region of the GaN NWs to tailor the piezoelectric response of the nano-system. By using an atomic force microscope equipped with a Resiscope module, allowing assessing the electrical response of individual nano-objects in representative NW ensembles, we demonstrate for the first time the piezo-conversion from InGaN/GaN NWs. In particular, we show a strong improvement of the conversion efficiency by embedding In0.4Ga0.6N insertions in GaN NWs: the piezo-generated energy is enhanced by 70 % with respect to pure GaN NWs. The impact of the growth conditions and of the GaN capping on the piezo-conversion is analyzed. These experimental results, combined with simulations performed with finite element method, open up new perspectives for the fabrication of a new class of piezo-generators.

Resume : GaN wire-based flexible piezoelectric sensors provide an original solution for mechanical sensing applications thanks to their high crystalline quality, robustness and long life-time [1]. GaN wires are grown by Metal Organic Vapor Phase Epitaxy on sapphire substrate with in-situ injection of silane [2]. This process gives N-polar wires with a hexagonal cross-section and a slight conical shape of an angle ranging between 0.3 and 2°. Their length can be tuned through growth conditions and it varies from 10 to 700 µm [3]. Wires are vertically integrated into a flexible PDMS matrix through a straightforward peeling process to make thin flexible piezoelectric sensors as already demonstrated for LEDs [4]. The PDMS mixture is first spun on the surface of the as-grown donor substrate and annealed for 30 min at 120°C. The membrane, containing the GaN wires, is then peeled and contacted with top and bottom metal electrodes to achieve a capacitive structure. This work aims to presenting the electrical performances of such devices delivering up to 2V. Variants of this sensors with different dimensions and GaN wire geometries have been first fabricated and characterized under a compression constraint through an automated mechanical bench. The output signal is studied as function of the wire length and PDMS thickness to figure out the optimal values for efficiency enhancement. The sensor sensitivity is also investigated by exploring the evolution of the device response to the variation of the applied force (0,3 ? 1N/cm²). Results show that devices with long GaN wires (>100 µm) provide higher output signal and are more sensitive to the variation of the mechanical solicitation amplitude. In addition, a small PDMS to length ratio of about 1.2 to 1.5 has to be considered to maximize the conversion efficiency and obtain a large output signal as it is confirmed by Finite element modelling calculations that will be shown. References: [1] Salomon S, et al., Nanotechnology (2014) [2] Eymery J, et al., Comptes Rendus Physique (2013) [3] Koester R, Hwang J S, Durand C, Dang D L S and Eymery J 2010 Self-assembled growth of catalyst-free GaN wires by metal-organic vapour phase epitaxy. Nanotechnology 21 015602 [4] Dai X, et al., Nano letters (2015)

Resume : Flexible electronics is a technology where a device is integrated to substrates like plastic sheets or metallic foil so it can be folded, wrapped, rolled, and twisted with negligible effect on its electronic function[1]. One interesting application of this technology is flexible gas sensors. In this work, we present wafer-scale fabrication of AlGaN/GaN HEMT sensors grown on a 2-inch sapphire wafer by metal organic vapor phase epitaxy (MOVPE) on top of an ultra-thin h-BN layer. This layer enables the separation of the HEMT from the rigid substrate and the transfer to a flexible template. While this transfer method is promising for flexible sensors, there are many processing challenges to overcome. Specifically, in order to prevent uncontrolled delamination, every step of the device fabrication process has been optimized. A wafer with more than 1000 working HEMTs has been processed with no delamination, which is the first large-scale device fabrication on h-BN. Furthermore, we have successfully verified that these HEMTs are sensitive to diesel exhaust gas, providing a feasible pathway towards flexible gas sensors. [1]Muhammad A. Alam and Satish Kumar, ?Definition of Flexible Electronics,? ed. Bharat Bhushan, Encyclopedia of Nanotechnology (Dordrecht, Springer Reference, 2014).

Resume : The noticeable progresses in strain engineering has made possible the epitaxial growth of robust GaN-on-Silicon device structures, and lead us to envisage new applications like flexible electronics based on layer transfer after removal of the easy-to-etch silicon substrate. For this purpose, high electron mobility transistor and light emitting diode heterostructures have been grown on Si(111) substrates and processed using standard techniques. After this step, the samples were bonded onto temporary sapphire carrier and the silicon substrate was thinned by mechanical lapping and finally removed Xenon Difluoride based etching. The remaining III-N films of about 2-3 µm in thickness were bonded to flexible tapes with adhesive coatings developed by 3M Company. Bending studies show that wavelength of the light emitted by blue InGaN/GaN LEDs is much less sensitive to curvature than to the injected current due to the limited thermal conductivity of the used flexible tapes (0.8-1.6 W/mK). Besides, the power density emitted by AlGaN/GaN transistors transferred to highly resistive flexible tapes with low RF propagation loss behaves the same way and saturates at 420 mW/mm at 10 GHz with a power-added efficiency of 29.6 % on a tape with a thermal conductivity of 1.6 W/mK. Thermal mapping studies confirm that the present device performance limitation is related to the thermal conductivity of the flexible tapes.

Resume : Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. After several years of research devoted to the mechanical-electrical conversion from ZnO nanowires (NWs), the attention has turned to III-nitride NWs thanks to their strong piezo-generation response. In the present work, we demonstrate the first piezo-generator integrating a vertical array of p-doped GaN NWs grown by plasma-assisted MBE. Based on a systematic multi-scale analysis going from single NW properties to macroscopic device fabrication and characterization, we establish the relationship between the GaN NW properties and the piezo-generation. The piezo-conversion of individual GaN NWs, assessed by atomic force microscopy (AFM) equipped with a Resiscope module, yields an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. Combined with the understanding of the piezo-generation mechanism in GaN NWs gained from AFM analyses, we design an efficient piezo-generator integrating NW arrays of several square millimeters in size and delivering a maximum power density of 12.7 mW/cm3 [1]. This value settles the new state of the art for III-N NW-based piezo-generators, and offers promising prospects, since the generated power density is already interesting for real world applications such as remote wireless transceivers. [1] N. Jamond, …, N. Gogneau, Nanotechnology 27, 325403 (2016) (editorial selection).

Resume : In this work, we have proposed a simple Ni pattern guided growth method to fabricate GaN nanowire array for high responsive humidity sensing. As both device electrode and growth catalyst, Ni metal can directly guide the selective-area lateral growth of GaN nanowires, which successfully bridge across a wide gap between two adjacent electrodes up to ~20 μm. The obtained Ni/GaN nanowire array/Ni sensor device exhibits a typical Schottky-contacted MSM characteristic. Its electrical transport behavior has a great dependence on the physisorption of water molecules on GaN nanowires, which is highly sensitive to monitor the relative humidity (RH) in ambient environment. Based upon utilizing energy band diagrams, the response performances of GaN nanowire humidity sensor have been investigated under different gap distance, RH and UV power conditions at room temperature. As the RH increases from 15 % to 85 %, it is clearly seen that the output current of the MSM sensor decreases gradually. And the larger the Schottky barrier at negative bias of -5 V, the higher the RH sensitivity. With the increase of UV optical power, its RH sensitivity has a large improvement from 3208 % to 10066 % at 5V and from 7818 % to 24037 % at -5 V, respectively. Interestingly, it is noted that the UV photoexcitation modulation has provided an effective way for a great enhancement of the RH sensitivity of GaN nanowire humidity sensor.

Resume : Group-III nitrides are attracted more attention to realize high-performance photodetectors (PDs) with spectral selectivity due to their bandgaps span from the deep ultraviolet (UV) to infrared region. The nitride-based UV PDs have wide applications such as chemical, biological analysis and monitoring, flame detection and optical communications. Several typical structures including p-i-n, heterostructure field-effect transistor (HFET), Schottky, and metal-semiconductor-metal-type have been used for ultraviolet detection to date. In these PDs, a two-dimensional gas (2DEG) with high electron velocity and strong confinement of electrons in the HFET-type has been considered to have a high sensitivity comparing to other types [1]. It is therefore necessary to improve the density of 2DEG in the HFET to realize high sensitivity. Generally, AlGaN/GaN (2DEG density ~ 1 × 10^13 cm^(-2)) or lattice-matched (LM) InAlN/GaN (2DEG density > 2.5 × 10^13 cm^(-2)) structure which eliminates the piezoelectric polarization is employed in the HFET. However, in contrast to the conventional GaN channel layer, AlGaN channel layer showed several metrics. Moreover, HFET-type PDs with AlGaN channel layers can detect shorter wavelengths in case to those of GaN-channel PDs whose detection wavelength is fixed at 360 nm. Recently, series of LM InAlN/AlGaN heterostructures with high 2DEG densities over 2.5 × 10^13 cm^(-2) was reported [2]. On the other hand, in the conventional metal gate HFET-type UV PDs, UV light has to irradiate the devices from the backside due to the nontransparency of the metal gate for UV light. This results in the reduction of the intensity of the detected wavelength as the thick substrate and buffer layer weak the incident UV light. Based on above analysis, we propose HFET-type UV PDs with LM InAlN/AlGaN structure and transparent ITO gate. The samples were grown on AlN/sapphire templates by metal organic chemical vapor deposition (MOCVD). The structure was initiated with a 2-μm-thick Al0.21Ga79N channel layer. Subsequently, a 1-nm-thick AlN spacer layer was deposited, followed by a 10-nm-thick In0.12Al0.88N barrier layer. Then, fabrication processes including reactive ion etching, electron beam evaporation and radio-frequency sputtering were carried out to form the HFET devices. The current-voltage (I-V) characteristics of the LM InAlN/AlGaN HFET with transparent ITO gate were conducted using a semiconductor parameter analyzer (Agilent Technologies B1505A) at room temperature. The monochromatic photoresponse characteristics of the HFET-type PDs were determined using a xenon lamp (Asahi Spectra LAX-C100) with optical filters. Drain I-V characteristics of the HFET indicate that the transparent ITO gate LM InAlN/AlGaN HFET shows typical pinched-off characteristics under both dark and UV light irradiation conditions demonstrating that the HFET can work successfully with the transparent ITO gate. The drain current Id exhibited remarkable increase under UV light irradiation. More specially, Id increases from 11 to 21 mA/mm (91% increase), 27 to 43 mA/mm (59% increase), and 45 to 66 mA/mm (47% increase) at gate-to-source voltage (Vgs) 0, 1 and 2 V respectively, under the same drain-to-source voltage (Vds) 8 V. The dark current Id of the HFET was measured to be 6.7 × 10^(-7) A/mm, which is more than one order of magnitude lower than that of the AlGaN/GaN HFET with ITO gate [3]. It is also obtained that the photocurrent to dark current ratio reached over 10^4 at Vds = 8 V and Vgs = -3 V under the UV irradiation. This presents the high sensitivity of the HFET to function as a PD. The spectral photoresponse of the LM InAlN/AlGaN HFET was obtained at Vds = 8 V and Vgs = -3 V, at which the device was pinched-off under the dark condition. The HFET-type PD achieved a high peak responsivity of 2.2 × 10^4 A/W at the cut-off wavelength around 320 nm. Moreover, a high UV (320 nm) to visible (450 nm) rejection ratio was calculated to be greater than 10^4 from the sharp cut-off. These are the best results reported so far around the cut-off wavelength of the AlGaN-based PDs. In conclusion, we fabricated high-performance UV PDs based on LM InAlN/AlGaN HFETs gated by transparent ITO films. It was demonstrated that the LM InAlN/AlGaN HFET with ITO gate can work successfully, and the drain current showed a remarkable increase under UV light irradiation, which presents high sensitivity. Moreover, the LM InAlN/AlGaN HFET exhibited much lower dark current than that of the AlGaN/GaN HFET with ITO gate. The peak responsivity and UV-to-visible rejection ratio were measured to be 2.2 × 10^4 and over 10^4, respectively, which are the state-of-the-art performances around the cut-off wavelength of the AlGaN-based PDs. The results are very promising for the development of the high-performance UV PDs. References: [1] B. S. Kang, S. J. Pearton, J. J. Chen, F. Ren, J. W. Johnson, R. J. Therrien, P. Rajagopal, J. C. Roberts, E. L. Piner, and K. J. Linthicum, Appl. Phys. Lett. 89 (2006) 122102. [2] M. Miyoshi, S. Fujita, and T. Egawa, Appl. Phys. Express 8 (2015) 021001. [3] T. Narita, A. Wakejima, and T. Egawa, Jpn. J. Appl. Phys. 52 (2013) 01AG06.

Resume : Solid-state AlGaN avalanche photodiodes(APDs) are attracting more and more attention because they have the irreplaceable advantage of detecting very weak light signals in the deep ultraviolet(UV) spectral region thanks to their wide direct band-gap and high chemical and physical stabilities. However, the demonstration of AlGaN APD devices with high gain and low noise still remains serious technical challenges due to high density of dislocations, low p-type doping efficiency resulted from large ionization energy of Mg acceptors, and low impact ionization coefficient of carriers in wide bandgap high-Al-content AlGaN alloys. In this work, A heterostructure multiplication region consisting of high/low-Al-content AlGaN layers instead of the conventional high-Al-content AlGaN homogeneous layer was proposed to increase the average hole ionization coefficient and to realize a higher gain in AlGaN avalanche photodiodes(APDs) based on separate absorption and multiplication structures. The fabricated APDs with the Al0.2Ga0.8N/Al0.45Ga0.55N heterostructure multiplication region exhibit very steep breakdown and a maximum gain of 5.5×104 at a reverse bias of 109V. Meanwhile, the large potential barrier to the conduction band formed at the Al0.2Ga0.8N/Al0.45Ga0.55N heterostructure interface can suppress the electron-initiated multiplication and hence reduce the noise of the APDs by impeding the transport of electrons. The simulation of noise confirms this effect from the heterostructure.

Resume : An array of InGaN-based flexible light-emitting diodes (FLEDs) was fabricated on a Ni-embedded electrical conducting flexible fabric with a full-scale 2-in. size. The FLED chip operation under current injection was realized using a single current probe as the negative electrode on the n-GaN surface; the conducting substrate was used as the positive electrode. The stability of the output power in the FLEDs was improved dramatically on the Ni-embedded conducting flexible fabric compared to that on the conventional polyimide flexible substrate. The former showed linear operation up to an input current 950 mA with no wavelength shift, whereas the latter exhibited rolling-over behavior after an input current of 200 mA.

Resume : We demonstrate self-powered AlGaN Metal Semiconductor Metal (MSM) solar-blind photodetectors with a record high zero-bias reponsivity of 0.34 A/W at 266 nm, which is highest amongst devices based on all material systems at zero bias. MSM devices fabricated on MOCVD-grown AlGaN (50% Al content) layers with interdigitated Ni/Au Schottky contacts exhibited a low reverse-bias leakage current (dark current) of 50 pA at 5V which was observed to be symmetric under forward and reverse bias. Photocurrent transient characteristics under a constant bias of 5V and 266 nm UV illumination showed sharp rise and fall confirming absence of persistent photoconductivity. The observed rise and fall times were less than 20 ms, limited by the instrument resolution. Spectral responsivity exhibited a pure solar-blind behavior with visible rejection ratio exceeding four orders of magnitude for front illumination. The peak responsivity was found to vary linearly with applied bias indicating the absence of an internal gain mechanism and the results were found to be repeatable and consistent across multiple devices over time. The observed zero-bias responsivity can be attributed to the presence of an internal field which causes separation of the photogenerated carriers leading to photocurrent even in the absence of an external field; origin of which will be discussed. This work is funded by Joint Advanced Technology Program (JATP), grant number JATP0152, and by the Department of Science and Technology (DST) under its Water Technology Initiative (WTI), grant number DST01519.

Resume : III-nitride deep-UV applications involve heteroepitaxial AlGaN stacks containing 50%-100% Al. Increasing bond strength with Al content, makes the diffusive mechanisms that aid defect reduction during growth more difficult. Hence, growth of “good quality” layers for UV applications involve a variety of rather complicated methods such as growth at >1300 °C, (typical GaN growth is done at temperatures <1100 °C), maskless epitaxial lateral overgrowth, pulsed atomic layer epitaxy, patterned AlN templates and miscut sapphire substrates. Compared to these methods, a systematic study showed that by judicious use of the venerable low temperature (LT) nucleation layer, AlN epilayers with crystal quality comparable to state of the art values can be grown at more reasonable temperatures of 1100 °C directly on c-plane sapphire in a single growth run. A 6X and 4X lowering in (002) (screw dislocation density) and (102) (edge dislocation density) omega scan widths to 0.04 and 0.2 degrees has been obtained. This improvement in crystalline quality is accompanied by a million-fold reduction in dark current, µA to pA, in a self-consistent set of AlGaN (50% Al) deep-UV Metal Semiconductor Metal photodetectors (MSM PD). Testing on a series of samples grown under different conditions revealed that a 30 fold reduction in screw dislocation density is primarily responsible for the dark current reduction. This work is funded by Joint Advanced Technology Program (JATP), grant number JATP0152, and by the Department of Science and Technology (DST) under its Water Technology Initiative (WTI), grant number DST01519.

Resume : We report on the gain and carrier transport mechanisms in highest responsivity AlGaN Metal Semiconductor Metal (MSM) solar-blind photodetectors on sapphire. Devices on MOCVD-grown unintentionally-doped AlGaN samples exhibited sharp absorption cut-off in the range 245-290 nm. Very high responsivity > 5 A/W at 10 V bias was measured with visible rejection ratio exceeding three orders of magnitude for front illumination. Carrier transport was studied through temperature-dependent current-voltage analysis. The studies revealed that the reverse-bias leakage current (dark current) across the Schottky contacts made to these samples was dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band edge for AlGaN with 50% Al mole fraction) at high biases. The high responsivity values and their exponential dependence on bias is attributed to an internal gain mechanism operating in these devices owing to a photo-induced lowering of the barrier at the metal-semiconductor interface. The detector performance parameters viz. dark current, photocurrent, responsivity and gain were found to depend on the crystal quality of the AlGaN absorber layer, especially the screw dislocation density. This work is funded by Joint Advanced Technology Program (JATP), grant number JATP0152, and by the Department of Science and Technology (DST) under its Water Technology Initiative (WTI), grant number DST01519.

Resume : We reported the improved performance of AlGaN/GaN heterojunction bipolar phototransistors (HPTs) with a 10-nm-thick low-doped n-type AlGaN insertion layer between emitter and base. Optical current gain was increased from 6.6×103 to 9.8×104 at 2 V bias by inserting the thin undoped layer. Spectral response measurements showed high ultraviolet to visible (350nm/400nm) rejection ratio of ~7×104 under 2 V bias, while that of the control sample without insertion layer is ~3×103. Simulation analysis reveals that the conduction band notch at the interface of base-emitter heterojunction is lowered by the insertion layer, leading to a weakened electric field and a narrowed space-charge region at the interface. This effect can reduce the recombination in the base-emitter heterojunction and contribute to the improved gain performance of the phototransistor.

Resume : In addition to light emitting devices, InGaN alloys are now attracting much attention as potential materials for solar cell devices owing to their direct and variable bandgaps ranging from 0.64 eV for InN to 3.4 eV for GaN, which allows them to cover a large region of the solar spectrum [1]. For realizing the InGaN-based solar cells, multiple quantum well (MQW) structures with a number of thin InGaN well layers have often been employed as light absorption layers [2, 3]. This is because the MQW structures can pseudomorphically realize thick InGaN layers appropriate for light absorption layers of solar cells with less difficulty in the growth technology. When it comes to the thickness balance of the well/barrier layers in MQWs, it is necessary to consider two different points: structure design and growth technology. For the structure design, the total thickness of the InGaN well layers should be enlarged to collect a large number of photons. In addition, a careful structure design is needed to maintain a high carrier collection efficiency including a low carrier recombination rate and good carrier transport properties, where the strong piezoelectric effect with the increased lattice strain should be taken into consideration. In this study, we attempted to investigate the influence of the crystal quality and the lattice strain on the InGaN/GaN MQW solar cells performance. For that purpose, we grew them by MOCVD on different substrates, which cause a difference in their lattice strain as well as the crystal quality. In this study, samples were grown on a 2-inch-diameter c-face sapphire substrate and an AlN template (1-μm-thick epitaxial AlN film on sapphire) using a horizontal MOCVD system with conventional precursors. The solar cell structure consisted of the following layers: a 3-µm-thick n-type GaN contact layer, an MQW structure with pairs of InGaN well and GaN barrier layers, and a 200-nm-thick p-type contact layer. The thickness of InGaN wells and their total thickness in MQWs were treated as experimental variables, and they were determined by using cross-sectional TEM and XRD analyses. Solar cells were fabricated using the conventional photolithographic lift-off method. The n-type contact metals were formed by the EB evaporation of Ti/Al/Ni/Au (15/60/12/60 nm), and they were subsequently annealed at 700°C in a nitrogen atmosphere for 30 s. An EB-evaporated Ni/Au (5/60 nm) finger-shaped pattern and an ALD Al2O3 (20 nm) passivation film were formed via a self-alignment photolithographic process. Here, the Al2O3 film was deposited using a Cambridge Nanotech ALD system with H2O and O3 as oxygen precursors and trimethyl aluminum as an Al precursor. The spectral response (SR) of the fabricated solar cells was evaluated by using an SR measurement system. Light absorption spectra were obtained from a combination of transmission and reflection measurements for unprocessed epiwafers. Energy conversion efficiency (ECE) and the other solar cell properties were evaluated by current–voltage (I–V) measurements under an artificial solar light illumination, with a standard AM1.5G spectrum at a 1 sun power density. To characterize the MOCVD-grown MQWs, PL measurements were carried out. The material characterization results revealed that the samples grown on AlN template showed better crystal qualities with larger in-plain compressive strains compared to the samples grown on sapphire. Typical results are follows: The X-ray rocking curves of (0002) and (10-12) reflections were approximately 200 s and 250 s for the sample grown on AlN template and approximately 300 s and 400 s for the sample grown on sapphire. The in-plain strains in the n-GaN layer were - 0.16% and - 0.24%, respectively, for the samples grown on AlN template and on sapphire. From the results of SR measurements, it was found that the samples grown on sapphire showed higher values for both the external and internal quantum efficiencies (EQE and IQE) compared to the samples grown on AlN template while those samples showed almost the same light absorption. This indicates that the carrier recombination rate in the samples grown on sapphire was lower than that in the samples grown on AlN template. As a result, a sample grown on sapphire exhibited the highest ECE of approximately 1.3 % in this study. To clarify the origin of the carrier recombination, we performed the PL measurements for those samples. The measured PL results imply that the low-EQE samples showed the extremely low PL strengths with increased PL wavelengths, which probably indicates that non-radiative recombination centers were formed in their bandgaps for those low-EQE samples. Taking into consideration the evaluation results of their crystal qualities, we concluded that the formation of the recombination centers was dependent on the large in-plain strain caused in the MOCVD-grown films rather than their crystal quality. In conclusion, in order to consider the influence of the crystal quality and the lattice strain on InGaN/GaN MQW solar cell performance, we grew the MQW structures on sapphire and AlN template by MOCVD. Material evaluation results revealed that the samples grown on AlN template have better crystal quality with larger in-plain compressive strain than the samples grown on sapphire. The SR measurement results showed that the EQE and IQE of the samples grown on sapphire were better than those of the samples grown on AlN template. The PL measurement results implied that non-radiative recombination centers caused by the lattice strain lowered the solar cell performance including IQE and EQE. This work was partially supported by the Super Cluster Program of the Japan Science and Technology Agency (JST). [1] A. G. Bhuiyan et al, IEEE J Photovoltaics 2(2012) 276. [2] R. Dahal et al, Appl Phys Lett 94(2009) 063505. [3] N. G. Young et al, Appl Phys Lett 103(2013) 173903.

Resume : III-nitride materials deposited on sapphire and silicon, are promising semi-conductorsfor applications in microwave photonics and optoelectronics fields. Unlike highly temperature sensitive optical waveguides made on more traditional materials such as Indium Phosphide (InP), Gallium Nitride (GaN) is a good candidate for optical applications in harsh environments [1-3]. Previous electrooptic investigations demonstrated an index modulation of ?n ~ 10-2 within the active layer, indicating the suitability of such structure for RF-optoelectronics [4]. We explore here the potentiality of fabricating high-speed optical modulators based on GaN material for low wavelengths range. To better meet the existing requirements for commercial products in terms of insertion losses, modulation bandwidth and driving voltage, some new configurations needs to be developed, in addition to broadband microwave characterization of GaN material by using a genetic algorithm applied to grounded coplanar waveguides [5]. We propose here to model and design GaN based electro-optic modulators taking into account both optical and microwave aspects using respectively OptiBPM and HFSS tools: the objective is to optimize the overall performances of modulators. The device fabrication is realized using template grown by MOCVD (Metalorganic Chemical Vapor Deposition). The selected design and fabrication process ensure monomode propagation in the channel waveguide. Special attention has been paid to theetching process, which is optimized for lowering the sidewall roughness and thus controlling the optical losses, as well as, the realization of n and p type contacts. The devices were characterized to evaluate the possibility of obtaining highly temperature insensitive refractive index values and low loss properties. In summary, we report a global approach for realizing III-Nitride based optical modulator devices [1] R. Hui, S. Taherion, Y. Wan: GaN-based waveguide devices for long-wavelength optical communications, Appl. Phys. Lett. 82, 1326 (2003). [2] A. Stolz, E. Cho, D. Decoster, Y. Androussi, D. Troadec, D. Pavlidis, E. Dogheche: Optical waveguide loss minimized into gallium nitride based structures grown by metal organic vapor phase epitaxy , Appl. Phys. Lett. 98, 16 (2011) 161903 [3] B. Alshehri, K. Dogheche, S. Belhasene, B. Janjua, A. Ramdane, G. Patriarche, TK. Ng, BS. Ooi, E. Dogheche: Synthesis of In0.1Ga0.9N/GaN structures grown by MOCVD and MBE for high speed optoelectronics MRS Adv. 1, 23 (2016) 1735-1742 [4] A. Stolz, SM. Ko, G. Patriarche, YH. Cho, D. Decoster, E. Dogheche: Surface plasmon modulation induced by a direct-current electric field into gallium nitride thin film grown on Si(111) substrate, Appl. Phys. Lett. 102, 2 (2013) 021905 M. Cuniot-Ponsart, I. Saraswati, M. Halbwax, YH. Cho, E. Dogheche: Electro-optic and converse-piezoelectric properties of epitaxial GaN grown on silicon by metal-organic chemical vapor deposition, Appl. Phys. Lett. 104, 10 (2014) 101908 [5] M. Hadjloum, M. El Gibari, H. W. Li, A.S. Daryoush: An ultra-wideband dielectric material characterization method using grounded coplanar waveguide and genetic algorithm optimization, Appl. Phys. Lett. 107, 15 (2015) 152908

Resume : Typically, piezoelectric nanogenerators (PNGs) demonstrate low output voltages and they drop in fractions of a second. Homojunction GaN based PNGs demonstrated higher output than Schottky junction PNGs with output voltages of 8.1 V and current density of 3 μA.cm-2. Internal carrier screening was held responsible for low output voltages and current density. Here we report a novel technique to improve the output power by controlling the electrical properties of thin films. Firstly, high resistivity of GaN thin film improved output power but GaN has a constraint to control its carrier concentration. So, a heterojunction PNG was fabricated with a p-type Cu2O thin film and unintentionally doped GaN thin film which behaved as n-type. High resistivity p-type Cu2O thinfilm was deposited by sputtering and electrical properties were optimized during growth. Internal carrier screening was suppressed due to high resistivity of p-type thin film of Cu2O. Junction leakage current was reduced drastically which increased the decay time of output voltages from fractions of a second to 1.86 seconds. Our device exhibited output potential and current density of 26 volts and 11.4 μA.cm-2 respectively under a normal compressive stress of 5 MPa. Output voltages were 3 times higher and current density was 4 times higher than homojunction GaN PNG. Our findings in this report may provide a pathway to fabricate GaN based devices with higher output power and longer decay time of output voltages such as heterojunction flexible PNGs, self-powered devices and sensors.

Resume : We have proposed a novel neutron semiconductor detector using BGaN. The large neutron capture cross sectional area of B atoms leads to a very low gamma ray sensitivity for group -III nitrides. Because of this property, only neutrons generating alpha particles by a B(n, alpha)Li reaction are detected in BGaN detectors. We have developed a BGaN neutron detector that indicates the possibility of neutron detection using a BGaN Schottky diode. However, the thickness of the sensitive layer needs to be increased. This can be achieved by fabricating sensitive layers with vertical type BGaN diodes. In this study, we fabricated vertical type BGaN diodes by metal organic vapor phase epitaxy (MOVPE) using trimethylboron (TMB) as boron sources and evaluated the radiation detection properties. The BGaN diodes were fabricated by MOVPE using ammonia (NH3), trimethylgallium (TMGa) and TMB as source gases of N, Ga and B, respectively. In order to suppress the gas phase reaction of B with nitrogen, TMB with low reactivity was used. The thickness of the radiation sensitive BGaN layer in these vertical type didoes is about 2 ?m. We evaluated the detection property of fabricated BGaN diodes by irradiating 1.47 MeV alpha particle, which is equivalent to the generated alpha particle energy by B (n, alpha) Li reaction. We determined the pulse signal by using oscilloscope and the energy spectra by using multi-channel analyzer. Comparing the detection properties of BGaN with GaN, we found that signal rise time became comparable (1 ~ 2 ?sec), and signal intensity of BGaN was small as 35 % from GaN. We consider that this decrease of signal intensity is caused by high defect density and low mobility of BGaN.

Resume : Hybrid structures incorporating both inorganic and organic conductive layers are promising for the development of inexpensive, versatile and tailored electronic and optoelectronic devices such as sensors or light emitting diodes (LEDs). The structures combine the advantages of inorganic and organic components such as a high structural stability whilst maintaining a high flexibility. One method to conformally deposit polymer layers on both planar and 3D-nanostructured substrates is oxidative chemical vapor deposition (oCVD). The oCVD process is dry and solventless as the polymer is deposited from the gas phase. Therefore, etching of fragile surfaces can be minimized. Adjusting the growth parameters highly impacts the structural and electronical properties of the resulting films. To develop a hybrid GaN/PEDOT LED, the oCVD of PEDOT on n-type GaN substrates is studied for different deposition parameters. In particular, the effect of various substrate temperatures on the resulting hybrid layer structures is investigated. The IV-characteristics of the hybrid p-n-junctions are analyzed and compared to that of structures containing an additional insulating tunnel barrier of poly(divinylbenzene) (pDVB) deposited via initiated chemical vapor deposition between the GaN and PEDOT. Initital results show pronounced diode characteristics of the hybrid devices and allow us to determine the relevant conduction mechanism at the inorganic/organic interface.

Resume : InGaN ternary alloys with their band gaps varying from 0.7 to 3.4 eV, are very promising for photodetector devices operating from UV to IR wavelength range. Being robust, environmentally friendly and compact, this expands their application from military to space, as well as for light emitting diodes (LED) and photovoltaic applications [1]. Alloys of this group have a direct band gap, attracted an intense interest in their potential applications in high power switching, high-temperature, and high-frequency device applications [2-4]. This work will provide rich informations about the photonic and electrical properties of GaN/InGaN p-i-n photodiode with an indium content equivalent to x=10%. This is correlated to the device configuration. Using Silvaco?Atlas software [5], we have designed the InGaN/Gan based p-i-n photodiode and we have studied the I-V characteristics, the spectral responsivity and the cut-off frequency as a function of InGaN thickness. The simulation and modeling reported in this work can be used to optimize the InGaN/GaN photodetectors and developing new generation of optoelectronic devices. [1] B. Alshehri, K. Dogheche, S. Belhasene, B. Janjua, A. Ramdane, G. Patriarche, TK. Ng, BS. Ooi, E. Dogheche: Synthesis of In0.1Ga0.9N/GaN structures grown by MOCVD and MBE for high speed optoelectronics MRS Adv. 1, 23 (2016) 1735-1742 [2] K. Poochindaa, TC. Chenb, TG. Stoebec, NL. Rickera: Simulation of GaN and InGaN p?i?n and n?i?n photo-devices. Journal of Crystal Growth 261 (2004) 336?340. [3] M. El Besseghi , A. Aissat, B. Alshehri , K. Dogheche , E. Dogheche , D. Decoster: Frequency response modeling and optimization of a PIN photodiode based on GaN/InGaN adapted to photodetection at a wavelength of 633 nm ,Materials Chemistry and Physics 25 (2015) 1-6. [4] YK. Su, HC. Lee, JC. Lin, KC. Huang, WJ. Lin, TC. Li, KJ. Chang: In0.11Ga0.89N-based p-i-n photodetector, P hys. Status Solidi C 6, No. S2, S811?S813 (2009). [5] ATLAS User?s Manual, Device Simulation Software, Version 5.20.2.R,SILVACO International, Santa Clara, CA, 2015.

Resume : GaN have the specific chemical stability, it is difficult to etch GaN film by general wet etching method at the room temperature. Photoenhanced chemical etching on GaN has recently identified as a method of greatly improving the chemical reactivity of GaN at the room temperature. In this study, GaN and AlGaN/GaN HEMTs was fabrication using photoenhanced chemical etching process with the etchant is KOH solution. The etching conditions can be easily controlled by the concentrations of the etchant, temperature, and lamp power. The effects of using different light sources and concentrations of KOH solution on the properties of GaN were studied for the first time. .A field emission scanning electron spectroscopy (FESEM) and an Atomic Force Microscope(AFM, Veeco) were used to characterizetheetched Surface and morphology. Observation through field emission scanning electron spectroscopy (FESEM) revealed that different types of illumination and concentrations of KOH solution resulted in different surface morphologies. The results is shown that smooth etching occurs for low solution concentration and high illumination intensity under a diffusion limited etching process. The surface roughness of 1.5nm and 50nm/min etching rate are obtained using KOH solution and Hg arc lamp

Resume : InGaN/GaN multiple quantum well (MQW) structure is a promising candidate for scintillator applications. The main requirement on scintillators is a fast, efficient, single band luminescence (PL). However, majority of our InGaN/GaN MQW samples shows a dual-wavelength emission, especially at a low excitation density. Except QW exciton luminescence (QWPL), a low-energy broad PL band (LEB) was observed. The origin of the low-energy band was studied by many authors, but it is still a matter of controversy. InGaN/GaN MQW structures on sapphire substrate with c-plane orientation were prepared by a metal–organic vapour phase epitaxy. Trimethylgallium and ammonia were used as precursors with a hydrogen carrier gas for the growth of buffer layers, triethylgallium, trimethylindium, ammonia with nitrogen carrier gas were used for the growth of MQW region including barriers. The x-ray diffraction was performed to verify the periodicity of the structure, thickness of layers and structure quality. The ratio of the intensity of QW exciton luminescence to the intensity of the LEB band IQWPL/ILEB depends strongly on the type and intensity of excitation, temperature and etc. That is why the PL studies of InGaN/GaN MQW structures with excitation above and below energy gap, high temperature-dependent PL, excitation power-dependent PL and kinetics PL measurements were done with the aims to put into context the obtained PL results and the technological parameters and discussed the LEB origin.

Resume : Avalanche photodiodes (APDs) based on III-nitrides have attracted increasing attention due to their excellent potential in offering high-sensitivity visible- or solar-blind detection. AlGaN APDs provide natural filters with tunable cutoff wavelengths, making it possible to detect very weak ultraviolet (UV) signals under strong background radiation. However, the development of AlGaN APDs suffers from some difficulties, such as high dislocation densities and low p-type doping efficiency. Another important factor that resulted in severe degradation in performance is the quick decrease of the impact ionization coefficients as Al content in AlGaN alloys increases. In this work, the conventional AlGaN homogeneous multiplication region is replaced by a high/low Al content AlGaN heterostructure layer to enhance the hole impact ionization coefficients as well as the multiplication gain. Furthermore, an intermediate n-type charge layer is inserted into the heterogeneous multiplication region to finely control the electric field distribution. Finally, an optimal SAM AlGaN APD structure with superior performance is proposed. The calculated results showed that the optimized structure exhibits an about 125% higher gain than that of the conventional APD.

Resume : Over the past decades, considerable efforts of environmental problems have been devoted to the development of photocatalytic system. Photoelectrodes have attracted a great deal of interest due to photoelectrochemical water splitting. GaN is promising candidate as a photoelectrode because of its favorable band-edge positions, low cost, controllability of properties such as structure, band gap, and catalyst characteristics, for improvement of solar energy conversion efficiency [1-5]. However, there are still has few studies on GaN photoelectrode with high conversion efficiency. Marokite (CaMn2O4) is a candidate material because it has advantages of abundant, good photoactivity [6,7]. If marokite is combined with GaN material, it could increase the solar to hydrogen efficiency by reducing oxygen evolving overpotential. So in this study, we investigated photoelectrochemical properties of CaMn2O4 cocatalyst coated on GaN photoelectrode in 1M NaOH solution. Figure 1 showed that the CaMn2O4 coated GaN has highest photocurrent density. In the chronoamperometry measurement for the stability test of the photoanode (figure 2), CaMn2O4 coated GaN has steady photocurrent densities. The results indicate that combination GaN with CaMn2O4 as co-catalyst has possibility for efficient water splitting. We will discuss more detail how this defect affects hole transfer in this conference.

Resume : Thermoelectric materials provide green energy source. Presently practical thermoelectric technology comprises mainly of Bi2Te3 and SiGe based materials. Bi2Te3 is however not only scarce and toxic, but also have a low maximum operating temperature of approximately 150°C. For SiGe, small efficiencies and narrow room for enhancement necessitate the search for a better high temperature thermoelectric material. Wide band-gap GaN and its clan of alloys are promising contenders to fill this role because they are very stable at high temperatures and non­toxic. Liu have theoretically calculated the thermoelectric parameters of GaN and AlGaN alloys1. GaN’s bulk thermal conductivity (∼ 200 Wm–1K –1 ) at room temperature is roughly two orders of magnitude higher than the SiGe2. Therefore it is critical to diminish the thermal conductivity of GaN so as to raise the ZT value to a suitable level. Two strategies have been put forward to increase the ZT value: first, decrease lattice thermal conductivity by increasing the phonon scattering, by alloying, selective implantation. Secondly, spatially isolate the donors (or acceptors) from electrons by means of modulation-doping technique and band-gap engineering. Tong et al3 estimated using the alloying strategy that the enhanced thermoelectric performances of ternary over binary alloys are largely caused by reduced lattice thermal conductivity. However, there is currently no report of using defect engineering by selective implantation to tune GaN thermoelectric properties. Proposed study aims at designing novel devices based on understanding of defects. This study also presents future scope for hybrid devices for photovoltaic and thermoelectric energy harvesting. 1. Journal of Applied Physics 2005, 97, 123705. 2. Journal of Physics and Chemistry of Solids 1977, 38, 330. 3. Analysis of thermoelectric characteristics of AlGaN and InGaN semiconductors, 2009; pp 721103-721111

Resume : Indium gallium nitride (InGaN) alloys are important materials for optoelectronic devices. Their inherent properties, such as their direct band gap, tunable from 0.65 to 3.4 eV, high absorption coefficient and radiation resistance, make InGaN compound semiconductors an excellent candidate for high efficiency photovoltaic applications. Despite the superior properties of InGaN, there are still many problems to be addressed in order to develop high-efficiency solar cells. One crucial limitation of the performance of such devices is the potential barrier in the InGaN/GaN hetero-interface caused by the electron affinity difference between InN and GaN. Another important factor influencing the photovoltaic properties of III-nitride solar cells is the existence of significant interface charges induced by spontaneous and piezoelectric polarizations. In this paper, to shed light on the effect of the aforementioned factors on the performance of InGaN-based solar cells, InGaN/GaN hetero-junctions were investigated numerically using realistic material parameters. In the simulations Poisson equation, current continuity equations, scalar wave equation and photon rate equation, were solved self-consistently, using finite-element method. Incident light transmission and absorption are treated using transfer matrix method. The employed optical dielectric functions (absorption spectra and refractive index dispersion relations) of the alloys were derived from actual experimental measurements of high quality InGaN thick films, using variable angle spectroscopic ellipsometry and Kramers-Kronig consistent oscillator models for the analysis. The photovoltaic operation of single (n-InGaN/p-GaN) and double (p-GaN/i-InGaN/n-GaN) heterojunction structures where studied by current-voltage (I-V) simulations of the devices in the dark and under AM1.5G illumination. The effects of InN mole fraction, layer thickness, strain state and doping levels on the conversion efficiency of the cells were considered. The importance of the polarization charges in reducing the barrier in the InGaN/GaN interface and in increasing the photogenerated carriers collection is revealed. In the case of single heterojunctions the efficiency monotonically increases for indium mole fractions up to 0.5 when the InGaN film is coherently strained on the GaN substrate whereas it could be maintained at high values even if the InGaN layer is partially relaxed for mole fractions up to 0.4. It is important to note that the InGaN n-doping concentration does not significantly affect the photovoltaic efficiency. Based on these results, a realistic device is proposed reaching a convention efficiency up to 14.5 %, under AM1.5G illumination, revealing the true potential of InGaN single junction solar cells when designed properly.

Resume : In recent years, GaN has been investigated as a potential material for various integrated photonic devices. In this context, the (In, Al, Ga)-N material system offers some unique properties, including a wide transparency range, relatively high second-order nonlinear coefficients, and suitability for harsh environments. A key component for the progress of integrated photonic devices (such as high speed modulators and switches) is a voltage-controlled phase modulator in an optical waveguide (WG) design. However, the realization of such a device is a challenging task. A major obstacle is the high optical loss in the device due to various factors. We studied experimentally the main contributions to optical losses in a GaN phase modulator. Our design is based on a reverse-biased p-n junction ridge WG scheme. The losses were determined mainly by using the Fabry-Perot method at a wavelength of 1064 nm, which is important for high optical power applications. We evaluated the contribution of the losses from the ridge waveguide walls? roughness, the p-doped layer absorption, and the metallic contacts. Based on these results, an optimized structure of an electro-optical GaN phase modulator is suggested. Trade-off between different design features that affect optical losses is discussed.

Resume : Two-dimensional (2D) materials open the way to new optoelectronic and photodetection devices. These materials seem to have particular interest in sensing. Indeed 2D materials, as they are monolayer thick by nature, are ultra-sensitive to their environment. Thanks to its high carrier mobility, high stability and easy fabrication technique, graphene offers an adequate platform for photodetection. However as it is an ultrathin material the light absorption and so its photoresponse is too low to be used by itself as an efficient photosensitive material. A hybridization with another adapted material is required to enhance photodetection. The wide band gap of Gallium Nitride (GaN) makes it highly efficient for UV A, B and C absorption. This make the GaN / graphene structure very promising for high gain UV photodetector. We propose to use directly grown GaN microstructures on graphene as an efficient hybrid structure for UV detection. GaN is grown by MOCVD on a graphene layer transferred on sapphire. UV photodetection is achieved by absorption of light by GaN which in turn generate charges that are acting as local gate of the graphene transistor. In this work we first intend to control and to explore the nature of the interface between graphene (2D material) and GaN (3D material) before exploring the preliminary results on UV photodetection.

Resume : Aluminum-Gallium-Nitride (AlGaN) based UV-detectors allow the detection of light in an individually tailored photon energy regime ranging from 3.4 to 6.0 eV. There is growing interest in more sensitive devices such as avalanche photodiodes (APD). AlGaN detectors with an aluminum content above 40% corresponding to a cut-off wavelength of 280 nm are intrinsically blind to solar irradiation. Additionally, in APDs multiplication gain can be achieved, increasing the efficiency of photodiodes. Here, we present AlGaN with 65% Al (sample A) and AlGaN with 60% Al (sample B) based pin-structure- avalanche photodiodes suitable for the purpose of backside and frontside illumination. The material was grown by metalorganic chemical vapour deposition (MOCVD) on 2 inch c-plane Al-polar single-side-polished sapphire substrates. In a first step, AlN was deposited using modulated ammonia flux keeping the Al precursor flow constant. Subsequently, AlGaN-70% (sample A) and AlGaN-65% (sample B) was grown serving as a filter layer for backside illumination, followed by 350 nm n-type AlGaN:Si-65% (A) and AlGaN:Si-60% (B) for n-contact formation and unintentionally doped 160 nm thick AlGaN with the same composition as active multiplication region as well as a 50 nm AlGaN:Mg-65% (A) and AlGaN:Mg-65% (B) layer. Finally, for both samples, a 50 nm GaN:Mg p-contact cap layer was deposited. Detectors of sample A show a peak responsivity of 27 mA/W at 251 nm and 35 mA/W at 249 nm, for backside and frontside illumination, respectively (see Fig. 1). Detectors on sample B reach a peak responsivity of 35 mA/W at 259 nm and 45 mA/W at 256 nm, for backside and frontside illumination, respectively, as depicted in Fig. 2. Optical transmission measurements as well as high resolution x-ray diffraction measurements have been carried out to determine the material quality. For the 00.2 reflection a full width at half maximum of the active layer of ~350 arcsec could be obtained. These photodiodes with a mesa diameter of 100 µm show avalanche gain for voltages in excess of 30 V reverse bias in linear gain mode. Highly reproducible avalanche gain below the break down threshold voltage can be observed, with a multiplication gain exceeding 2000 and 5500 for 84 V reverse bias for samples A and B, respectively. The gain is calculated as the difference between the photo and dark current at any voltage V divided by this difference at a reference voltage, thus defining unity gain. Unity gain is assumed at 30 V reverse bias, where a transition from linear to exponential increase takes place. In sample A, a dark current below 1 pA can be found for a reverse voltage up to 60 V.

Resume : With inherently low dislocation density, III-nitride nanowire-based photodetector (PD) has emerged as a promising candidate for ultraviolet (UV) light detection with high responsivity and visible light immunity. In this work, we report the first experimental demonstration of GaN one dimensional axial p-i-n PD with well-defined heterostructure and nanowire shape. The nanowire fabrication started from dry etching of GaN-on-sapphire p-i-n epitaxial thin film using SiO2 microspheres as etching masks, followed by a wet etching step using 85 ºC TMAH solution to further shrink the nanowire size. The obtained nanowire featured a cross-section diameter of 80 nm and most importantly retained the p-i-n heterostructure along the axial direction. After nanowire transfer and metal deposition, a single nanowire p-i-n photodiode with i-layer length of 500 nm exhibited good rectifying I-V characteristic with turn-on voltage of 3.1 V at 1 A/cm2 and reverse leakage current as low as 3.2 pA at -10 V. The ideality factor was extracted as n = 3.9 and the forward current could reach 1.3 µA (3.4 kA/cm2) when biased at 10 V. Under UV illumination (? = 365 nm) and reverse bias (-5 V), the nanowire presented a high responsivity of 136 mA/W, which was much higher than GaN thin film based p-i-n photodiodes (15 mA/W) due to absence of light absorption in the optical path. The GaN nanowire PD also demonstrated a good visible-blind property with the UV-visible responsivity ratio (R365nm/R450nm) reaching 10.

Resume : InGaN-based photodetectors are promising devices for light collection and monitoring of their light-emitting counterparts (LEDs and laser diodes), as well as for photovoltaics and power transfer applications. The aim of this paper is to investigate the performance under various high power density levels and reliability of InGaN photodetectors submitted to step-stress experiments in short-circuit condition and high power densities. The analyzed devices are multi-quantum well photodetectors grown on a sapphire substrate. The MQW region is composed of 25 nominally undoped In0.15Ga0.85N/GaN pairs whose thickness is, respectively, 2.2 nm and 4.8 nm, and it is grown over a 2 µm thick Si:GaN (5×10^18 cm^-3). The performance of the detectors is mildly affected by the degradation test. Under illumination at 405 nm and 5 mW/cm^2, the devices show a negligible degradation up to 44 W/cm^2, a slight decrease in photocurrent (Isc) at 111 W/cm^2, and a moderate variation in the open circuit voltage (Voc). The latter is possibly related to a change in dopant activation or in the contact quality. The combined variation of Isc and Voc causes a decrease in the peak electrical power that the device is able to supply (-11 %). The apparent charge profile suggests that stress induces the creation of deep levels inside the quantum wells, which may increase the defect-related non radiative recombination, thus reducing the photocurrent of the devices. The performance of the devices is significantly affected by the power density of the incoming light. This suggests that additional analyses need to be carried out in order to estimate the performance and reliability of InGaN photodetectors, especially when they are used in combination with concentrators.

Resume : We report on the study of a UV-switched gate-less normally-off InAlN/GaN HEMT grown by MOCVD on 8 inch silicon. After forming Ohmic contacts to the 2DEG followed by mesa isolation, gate finger of length 3 μm was recess-etched to deplete the 2DEG. The source-drain (S-D) pads were spaced 300 µm apart while the gate-recess finger (width = 350 µm) was symmetrically located between pads. The current under ambiance was 1 µA at 20 V testifying the e-mode operation. On illuminating with UV (365 nm), ON current was found to be 20 mA. After passivation of the devices with ALD Al2O3 (10 nm) followed by soft anneal, the rise & fall times were found to be 200 ms in pulsed transient measurement while maintaining an ON-to-OFF current ratio > 10^4. The On current is expected to increase significantly with a) reduction in S-D spacing b) larger device periphery c) higher power UV LED. The effect of GaN buffer layer, device design, & passivation/anneal on transient response and breakdown will be presented. For low to moderate power on-chip switching, this gateless e-mode HEMT presents a promising alternative to explore, especially by eliminating the complexities and reliability issues associated with the conventional gate. The work is funded by Ministry of Electronics and IT (MeitY) under its National Mission on Power Electronics Technology (NAMPET) program.

Resume : GaN-based photodetectors and solar cells are of great interest for applications involving detection, imaging and energy conversion in the UV - visible wavelength range. nAlxGa1-xN/GaN bipolar phototransistors and GaN/Si solar cells with their additional functionality could also be of potential interest. In this work we present the results of phototransistor and solar cell parameters simulation using the software - Sim Windows. The spectral dependence of photoresponse calculated for such phototransistors was fairly flat and showed a well defined short-wavelength threshold determined by the AlxGa1-xN emitter bandgap and the long-wavelength threshold due to the onset of absorption in the GaN base. The photosensitivity and selectivity window width can be easily regulated by varying the AlxGa1-xN composition. Decreasing the acceptor concentration in the base is favorable for increasing the photosensitivity and that increasing the applied voltage increases the photoresponse. It was detected that for medium range of powers the response jph/P is reasonably constant with power and increases considerably with decreased acceptor density. The photosensitivity near the plateau can reach values of around 800A/W. Also multicascade advanced GaN/Si solar cell with different layers width and doping concentration was investigated. It was found that the current structure has advantage than standard arsenide and Si ones. GaN/Si solar cell efficiency can achieve 42%.

Resume : In recent years, emission photodetection in ultraviolet spectral region attracted an exclusive attention. In general, the ideal photodetector should have high sensitivity, low noise, high spectral selectivity, band gap semiconductor large width and good photoresponse linearity of the radiation power. Group III nitrides are particularly suitable for the manufacture of semiconductor UV detectors, due to their large band gap width and a direct-gap. This ensures a large value of the optical absorption coefficient and inherent in these materials "insensitivity" to visible and infrared radiation. Based on the AlxGa1-xN heterostructure the devices have a wide linear dynamic range of characteristics and the ability to control the maximum quantum cutoff energy. It was developed a new phototransistor structure with disabled base on the basis of a symmetrical heterostructure nAl0,3Ga0,7N - pGaN - nAl0,3Ga0,7N. Simulation shows the possibility of creating a device with a sensitivity of 500-700 A / W on the basis of structures with electrical and structural parameters corresponding to the mass manufacturing. Phototransistor selectivity can be increased by reducing the aluminum content in the p-Al0.3Ga0.7N collector up to 20%. Sensitivity can be increased by improving MOCVD for AlGaN multilayer structures. If the sapphire substrate is replaced by a GaN substrate the lifetimes of nonradiative recombination in the phototransistor base will increase significantly.

Resume : Due to its sensitivity to surface charge and the potential to operate at high temperature, InGaN based hetero-structure sensors have attracted a lot of interest recently. The inertness and chemical stability of this semiconductor also make it an ideal candidate for bio- and chemical-sensors. The absorbates on the surface of such III-nitride hetero structures change the near-surface band bending owing to the quantum confined Stark effect (QCSE) which can be remotely evaluated by analysing the photoluminescence (PL) signal. This optical read-out not only eliminates the need for electrical contacts, which can be chemically vulnerable, but also makes fluorescent labels dispensable, which have the tendency to suffer from photo-bleaching. For this study, InGaN quantum wells on GaN buffer layers are simulated by “Nextnano” and several parameters, such as the capping layer thickness and doping concentration, are varied to optimise the sensitivity of such structures to changes in the surface potential. The optimized QW structures are then grown by metal organic vapour phase epitaxy (MOVPE) for a direct comparison. Several gases are then optically investigated by these sensors as they produce a shift in the PL emission wavelength. Such sensors for gases and biomolecules could potentially be used in the early detection of severe illnesses such as Alzheimer’s disease. Special emphasis will be on the detection of hydrogen sulphide (H2S), a gas present in human breath.

Resume : Monolithic integration of logic circuitry (line decoders) based on GaN schottky barrier metal-oxide-semiconductor field effect transistors (SB-MOSFETs) with matrix-addressable light-emitting diode (LED) arrays is proposed to reduce the number of connections for interfacing with display drivers, facilitating scalability of the emissive displays. The on-chip line decoders are constructed from monolithically-integrated transistors, which could be HEMTs [APL, 102, 192107, (2013)], vertical MOSFETs [APL, 109, 053504, (2016)] or SB-MOSFETs [EDL, 27, 81-83, (2006)]. While the first two types of devices may offer better electrical performances, epitaxial regrowth is required, significantly complicating the fabrication process and costs. On the other hand, SB-MOSFETs can be fabricated on the same LED structure without regrowth, relying on the potential barriers established by metal-semiconductor Schottky barriers. Both n-channel and p-channel transistors can be simultaneously integrated using the p-GaN and n-GaN layers respectively. The SB-MOSFETs are also normally off in nature and exhibit low leakage currents, which are important attributes for constructing logic circuits. In the present proof-of-concept design, a 4x4 micro-LED array is interfaced to two 2-to-4 line decoders constructed from on-chip SB-MOSFETs and p-GaN resistors resulting in the halving of the number of signal lines.

Resume : Nanowire (NW) heterostructures are promising candidates for the fabrication of nano-photodetectors due to their low defect density and small electrical cross-section (i.e. low capacitance), which comes without degradation of the light absorption due to antenna effects. Furthermore, the internal electric field in polar GaN/AlN NW heterostructures opens interesting possibilities for band engineering in order to modulate the spectral response and even render it bias-dependent. In this work, we present a study of MBE-grown GaN NWs containing a superlattice of 30 periods of AlN/GaN heterostructures. The heterostructure dimensions and doping profile were designed in such a way that the application of external bias leads to selective collection of photogenerated carriers from the GaN base or from the GaN/AlN superlattice. Single-NW photodetectors were fabricated on electron-transparent Si3N4 membranes. Spectral photocurrent measurements demonstrate the enhanced response in the ~330-360 nm / ~280-330 nm windows under forward/ reverse bias. The result is explained by correlation of the photocurrent measurements with scanning transmission electron microscopy observations of the same single NWs, and semi-classical simulations of the strain and band structure in one and three dimensions.

Resume : The rocketed demand to detect ultraviolet (UV) radiations have stimulated up much interest in the field of research due to its wide range of industrial, astronomical, biological and defence oriented applications.[1,2] To improve the device?s instrumentation, III-nitrides have become very promising candidates for the fabrication of UV photodetectors (PDs) due to its superior material assets such as wide-direct bandgap, inherent visible blindness so no requirement of expensive optical filters, high thermal conductivity, high electron velocity at high electric field, good radiation hardness, etc. Although, compared to UV PDs based on traditional thin-films and bulk materials, UV PDs based on low dimension semiconductor nanostructures have an advantage of higher responsivity and photoconductivity gain due to increased surface to volume ratio and reduced dimensions of the effective conductive channel.[3] Thus, they can be utilized immensely for the fabrication of reliable and versatile photodetectors. In the past few years, many attempts have been made to fabricate various structures of GaN based UV PDs such as metal-semiconductor-metal (MSM), photoconductive, p-n junction, etc. Such structure engineering offer routes to attain high detectivity and the possibility to achieve very high responsivity. In this work, we present GaN nanostructure-based technologies in MSM geometry to examine the electron transport behaviour in self-driven and biased conditions for UV photodetection devices. Markedly, utilization of nanostructures increases the light absorption efficiency which leads to a highest responsivity of 10.5 A/W at 1 V bias upon 325 nm UV illumination. The value of responsivity stated here is maximum amongst commercially available Si based UV PDs as well as so far reported studies of GaN UV PDs using Si substrates. Moreover, we elucidated the origin of self-driven PDs using a model based on band theory & demonstrated a highly responsive GaN UV PD under zero bias condition (self-driven). It was perceived that the self-driven PD exhibits dark current in the range of nA with very high detectivity of 2.4 x 1010 Jones. Noticeably, a curiously high light to dark current ratio of > 102 signifies high photodetection gain. Therefore, the highest responsivity and detectivity from devices based on GaN nanostructures may provide extensive prospects for further development of ultrafast nano-optoelectronic devices. [1] Ran Jia, Dongfang Zhao, Naikun Gao, and Duo Liua, Sci Rep. 7, (2017), 40483 [2] E Monroy, F Omnes, F Calle, Semicond. Sci. Technol. 18 (2003) R33?R51. [3] Lahnemann et. al., Nano Letters, 16, (2016), 3260

Resume : AlGaN-based back-illuminated solar blind ultraviolet metal-semiconductor-metal (MSM) flip chip photodetectors (PDs) with high quantum efficiency have been fabricated on sapphire substrate. To improve the performance of the PDs, AlGaN absorber layer was grown on high quality AlN template on the sapphire substrate by high temperature metal organic vapor phase epitaxy. To improve AlN quality, high temperature above 1300°C and low V/III ratios were applied. By inserting middle temperature intermediate AlN layers, high quality 2.5um crack free 2-inch AlN template has been achieved. The XRD rocking curve full width at half maximun (FWHM) of AlN (002) is 209 arcsec, wihile the FWHM of (102) peak is 367arcsec. A 300nm AlGaN absorber layer of Al compositon 45% was further grown on the AlN template. The photoconductive mode was demonstrated with a AlGaN MSM PDs having different Schottky contacts. The MSM AlGaN photodetectors consisted of two interdigitated contact electrodes fabricated by standard lithography and lift-off technique. On the AlGaN surface, Cr/Ni/Au and Ni/Au contact layers were deposited inter-digitally by electron-beam evaporation for an asymmetric electrode metallisation. The fingers of the interdigitated contact electrodes were 3 μm wide and 500 μm long, with a spacing of 5μm. The photoresponse cruve shows a sharp cut-off at 280 nm and peaks at 272nm. Under a reverse bias of -5V, the PD shows a low dark current density of 1.9 pA/cm2. Owing to the high quality AlN template with the back-illuminated of the device, a solar-blind AlGaN MSM photodetectors with 31% external quantum efficiency at zero-bias was achieved.

Resume : Ultraviolet (UV) radiations (the spectral interval of ? = 400?10 nm) are highly ionising that can activate many chemical processes. Consequently, the necessity to detect these radiations arise due to wide range of applications such as biological & chemical analysis (ozone, pollutants, and most organic compounds, flame detection (including missile warning or combustion monitoring), optical communications (particularly space-to-space communications at ? < 280 nm), and astronomical studies. Recently, two-dimensional (2D) materials have emerged as an alternative optoelectronic platform with new functionalities, including broadband ultrafast photodetection, saturable absorption, and on-chip electro-optic modulation. Among them, Graphene have become a promising material for broadband and ultrafast photodetection and gained substantial attention due to its distinctive properties such as high carrier mobility, low optical absorption rate of 2.3%, broad spectral absorption, short carrier lifetime, as well as mechanical flexibility.[1?3] The high carrier mobility enables ultrafast conversion of photons to electrical currents or voltages.[4] In order to develop a high performance dual band photodetector (PD) devices, various approaches have been employed to integrate 2D material with III-Nitride semiconductors.[5,6] Being a semiconductor material with wide and direct bandgap, GaN has been extensively studied for the high-power and high-temperature applications of UV PD working in harsh environmental conditions. Moreover, GaN is ideal for UV detection as it has inherent visible blindness with high responsivity and device stability. In this work, we demonstrate the dual-band (wavelength of 325 nm and 532 nm) PD based on a 2D heterostructure consisting of high-quality graphene exfoliated on GaN. A quiet fast & stable photo-detection has been observed under both the UV and visible sources. Also, the Raman spectroscopy was performed which unveils the presence of monolayer graphene with G peak to 2D peak intensity ratio about 1:3 and the absence of D peak suggests the graphene with few defects. [1] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, Science 320, 1308 (2008). [2] J.-H. Chen, C. Jang, S. Xiao, M. Ishigami, and M. S. Fuhrer, Nat. Nanotechnol. 3, 206 (2008). [3] E. H. Hwang, B. Yu-Kuang Hu, and S. D. Sarma, Phys. Rev. B 76, 115434 (2007). [4] Mueller, T., Xia, F. & Avouris, P. Graphene photodetectors for high-speed optical communications. Nature Photon. 4, 297?301 (2010). [5] Kun Xu, Chen Xu, Yiyang Xie, Jun Deng, Yanxu Zhu, Weiling Guo, Meng Xun, Kenneth B. K. Teo, Hongda Chen, and Jie Sun, IEEE Transactions on electron devices 62, 2802 (2015). [6] Yanghua Lu, Zhiqian Wu, Wenli Xu and Shisheng Lin, Nanotechnology 27, 48LT03 (2016).

Resume : The ability of detecting light in both ultraviolet and infrared band is crucial for several important applications including flame identification, surveillance, meteorology, astronomy. However, ultraviolet/infrared dual photodetector does suffer from the pains of thick and multilayer material growth, complicated device structures, and rigorous processing for device fabrication, which induces high producing cost and severely restrains its extensive applications.     Here, we report a simple and effective ultraviolet/infrared dual photodetector design using GaN and graphene ink with simple structure, convenient material growth as well as simple process for device fabrication. The GaN/graphene detector shows the responsivity of 0.19A/W and 0.23A/W in the ultraviolet (cutting off at 365nm) and infrared region (800-870nm), respectively, This results have reached the same level as high as the traditional single GaN based ultraviolet and InGaAs based infrared detectors. The photo generated electron-holes pairs by the bandgap absorption of GaN result in the ultraviolet region response, while the shift of the Fermi level and the generated local density states in graphene contribute to the photoelectric infrared response. The results present here not only make a step towards the application of ultraviolet/infrared detectors, but also offer a simple method to realize the integration of graphene with semiconductors by graphene ink. 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 : Gallium nitride (GaN) is a promising candidate for integrated photonic applications since it is transparent across most of the visible and IR spectrum. This has led to GaN being used as a chemical and biological sensing platform [1]. A grating coupler that couples an external laser source in and out of a waveguide is a fundamental building block for the development of an integrated photonics platform. The in- and out-coupling efficiencies are strongly dependant on the angle of incidence of the source, the filling factor, slope and pitch of the linear grating [2]. We present the design and fabrication of grating couplers in a GaN-on-Sapphire template for a wavelength of 635 nm and measurements of the insertion loss of the fabricated couplers into a 2D slab waveguide. FDTD modelling was used to evaluate the impact of the filling factor, etch depth and angle of light input/output on the coupling efficiencies of 400 nm pitch GaN grating couplers. An array of 40x40 ?m2 grating couplers, separated by different distances was defined by using displacement Talbot lithography in combination with direct laser lithography. Measurements of the insertion loss were made for a series of differently spaced grating couplers giving values of 3.5 dB per coupler. [1] Y. Wang et al, Appl. Phys. Express 7, 052201 (2014). [2] R. Dylewicz et al , Optical and Quantum Electronics, vol. 42, pp. 619-629, 2011.

Resume : GaN has been documented to be very stable in water solutions and compatible with in vitro studies. This offers a unique opportunity to design new biointerfaces and sensors. In addition, wide bandgap semiconductors such as gallium nitride exhibit persistent photoconductivity properties. This work incorporates this asset into the fabrication of a unique GaN based biointerface. In particular, we combine three parameters to modulate the behavior of PC12 cells on a GaN surface: surface roughness, surface functionalization, and conductivity. Templates with lithographically defined regions with controlled roughness were generated during the semiconductor growth process of a lateral polar structure. Template surface functional groups were varied using a benchtop surface functionalization procedure. In addition, the conductivity of the template was altered by exposure to UV light. We found that pattern size and roughness can be combined with surface chemistry to change the adhesion of PC12 cells when the GaN is made more conductive after UV light exposure. In addition, during neurite outgrowth, surface chemistry and initial conductivity differences can facilitate the extension to smoother areas on the GaN surface. These results can be utilized in future bioelectronics devices that take advantage of thickness dependent photoconductivity of GaN nanostructures and thin films.

Resume : This work reports on carrier transport analysis in high responsivity p-i-n deep-UV photodetectors. The epitaxial stack consisted of MOCVD grown p+GaN (20 nm) / Mg-doped Al0.45Ga0.55N (200 nm) / UID-Al0.40Ga0.60N (200 nm) / Si-doped Al0.45Ga0.55N (500nm) / UID-Al0.45Ga0.55N (500 nm)/ UID-Al0.75Ga0.25N (100 nm)/ AlN (200nm) on c-plane sapphire substrate. The devices exhibited solar-blind behavior with a visible rejection > 103 and a sharp cut-off at 286 nm with a responsivity of 0.23 A/W at a reverse bias of 10 V. The measured temperature-dependent dark current-voltage characteristics (I-V-T) in the 150-400K temperature range were observed to have an exponential dependence on temperature and bias. Modelling of the obtained I-V-T characteristics revealed the presence of a trap level with thermal activation energy of 40 meV which lowers with an increase in electric field. The leakage current could be attributed to field-enhanced thermal emission (Poole-Frenkel emission) of carriers from the trap level. This work is funded by Joint Advanced Technology Program (JATP), grant number JATPO152, and by the Department of Science and Technology (DST) under its Water Technology Initiative (WTI), grant number DSTO1519.

Resume : The development of ultraviolet (UV) photodetector (PD) can enable a wide range of applications including space communications, flame sensors, atmospheric ozone detection, bio-photonics and polarization-sensitive detection. The III-nitrides attracted much attention due to its superior material properties such as high electron mobility, high breakdown voltage, high thermal stability and high radiation resistance. One of the most challenging aspects of using a heteroepitaxial GaN (polar & semipolar) is the effect of spontaneous and piezoelectric polarization induced internal electric field on the device. However, it remains possible to fabricate a polarization free GaN UV PD by using nonpolar GaN film but the growth of high-quality nonpolar GaN leads to the formation of high dislocation densities which causes high dark currents thereby hampering the device performance. In the present study, we demonstrate fabrication of UV PD based on plasma assisted molecular beam epitaxy grown non-polar (11-20) GaN heteroepitaxial thin film over r-plane (1-102) sapphire substrate. The epitaxially grown GaN film with high crystalline quality lead to the formation of two faceted island like micro structures on the surface. The Photoluminescence spectroscopy revealed a high optical quality with a near band edge peak centered at 3.4 eV. The detector exhibited exceptionally good repeatability and stability with time under periodic illumination. The fabricated GaN UV PD exhibited a high responsivity ~ 340 mA/W at 5 V bias at room temperature which is the best performance for nonpolar GaN films grown on sapphire. A detectivity of 1.24 x 109 Jones and low noise equivalent power of 2.4 x 10-11 WHz-1/2 were also attained. The calculated rise time and decay time in the range of milliseconds were the fastest response times reported for non-polar GaN UV PD. Thus, we have demonstrated high performance non-polar GaN ultraviolet photodetector device which can serve as an excellent photoconductive material by overcoming polarization induced effects.

Resume : III-nitride direct band gap semiconductors have gained prominence in technological applications of photo-sensing from deep ultraviolet (DUV) to near infrared (NIR) spectral range. Among III- nitride wide band gap semiconductors, Indium nitride (InN) has attracted attention due to its unique potential characteristics such as high electron mobility, relatively high absorption coefficient, narrow band gap energy and peak drift velocity at room temperature, etc. Regrettably, the reports on the photo-sensing ability of the material are limited with photoresponsivity < 1 A/W. Up to now, most research efforts on the development of III-Nitride based photodetector have been focused on ultra-violet (UV) operation. Apart from UV detection, the developments in telecommunication demand for the extension of the photodetection range towards the infrared (IR) spectral region. So far the photoresponse study of InN direct band gap semiconductor is concerned, Winden et.al. reported a vertical nano-pyramid InN photodetector with spectral responsivity of 0.2 A/W.1 To the best of our knowledge, highest photoresponsivity reported on InN based photodetector is 0.44 A/W.2 Although, InN has many superior properties for photodetection, but the reported InN based photodetector devices are very few. It is because of the challenges in synthesizing high-quality InN due to its low dissociation energy and lack of appropriate substrate. Moreover, the growth of high-quality epitaxial InN on GaN, sapphire and silicon substrates lead to the generation of high dislocation density which further guide to higher leakage current at the interface. Here, we report ultrafast photoresponse from a high-quality molecular beam epitaxy grown InN film delivering the photoresponsivity of 13.5 A/W, about 30 times higher than the recently reported InN based photodetector operating in the NIR spectral range. The broadband spectral response with high photoresponsivity from visible to near infrared spectral range is experimentally demonstrated. To the best of our knowledge, this work demonstrates the faster response time (in the range of microseconds) and high photo detectivity of the device operating at room temperature. The device yields a quiet prompt saturation and decay under periodic illumination which demonstrates excellent stability and reliability of the device with switching time. The first report on the broadband spectral range of InN based photodetector open up opportunities for developing the next generation highly efficient photodetector. References: 1. A. Winden et.al. Jpn. J. Appl. Phys. 52, 08JF05, 1-4(2013). 2. L. H. Hsu et.al. Opt. Mater. Express.4, 2565-2573 (2014).