Group III-Nitrides: fundamental research, optoelectronic devices and sensors

Resume : The history of LEDs has been evolved for more than 50 years. Even though the nitride LEDs are somewhat younger, this material system enabled the lighting revolution that we are currently experiencing. The foundation has been the tremendous improvements in brightness level and efficiency that have been the in the focus of research for a long time. Material properties required design and process improvements to overcome limitations. Nowadays, it is recognized that LED offer a big variety of opportunities beyond efficient lighting. There are many applications that could only be addressed by the unique characteristics of semiconductor LED light sources. Thus, it is much more than cost cutting and efficiency triggering further research on group III nitride optoelectronics. A brief history of group III nitride based LED developments along with an introduction to OSRAM Opto Semiconductors’ numerous activities in research and development of next generation LEDs / Lasers and future trends for lighting applications will be given. Examples of technology and products for various application fields will illustrate the claimed technology leadership that is based on the close interplay of material development, thorough understanding of scientific background and of technological solutions yielding.

Resume : UV AlGaN-based semiconductor laser diodes have wide applications such as bio-/chemical photonics, material processing, and high-density data storage. Nevertheless, the full potential of these devices has been limited by low-efficiency current injection due to the difficulty of producing low resistivity p-type high Al-content AlGaN alloys. An alternative approach to address this problem is the use of the electron beam excitation. In this presentation, we shows the electron beam excited laser in the UV region, using a nitride semiconductor device with a GaN/AlGaN MQW active layer. We also discuss that what is the point of laser oscillation by electron beam excitation. After growth of a 500-nm-thick GaN layer on GaN substrate, a 500-nm-thick Al0.15Ga0.85N cladding layer, a 100-nm-thick Al0.07Ga0.93N guide layer, GaN (3nm)/Al0.07Ga0.93N (12nm) 10QW active layer, a 100-nm-thick Al0.07Ga0.93N, and a 100-nm-thick Al0.15Ga0.85N cladding layer were stacked in that order. The cavity length of the laser was ~50 μm. Despite measuring the same sample, we found clearly difference in the threshold power density required for laser oscillation. Also, the threshold excitation power density was minimized at 20 kV. This result almost matched with the simulation result. The threshold power density and wavelength were ~ 160 kW/cm2 and 352.8 nm, respectively.

Resume : The new method is presented, designed for exact determination of the recombination mode of optically excited matter, measured by standard time resolved photoluminescence (TRPL) instruments. The considered processes include non-radiative Shockley-Read-Hall (SRH) monomolecular, standard bi-molecular, and Auger tri-molecular recombination. In addition the optical emission is analyzed taking into account the emission due to both monomolecular and bi-molecular process. The method allows us to determine the dominant optical emission mode and also the values of ABC parameters in fundamental time evolution equation of optically excited samples measured by TRPL [1,2]. It is shown that the method could be applied to any quantum structures and other systems investigated optically. Moreover, it is also shown that the method could be successfully applied for analysis of the processes in the nitride polar (with polarization induced electric field) and the nitride nonpolar (i.e. without polarization induced electric field) multiquatum well (MQW) systems. The analysis of temporal evolution of the emission allows to determine the recombination modes from the structure, allowing to identify basic factors affecting optical emission from nitride quantum structures. [1] R.N. Hall, Phys. Rev. 83, 228 (1951). [2] W. Shockley, W. T. Read, Phys. Rev. 87, 835 (1952).

Resume : Ga1-xInxN have attracted great interest and is intensively studied as materials for bright light emitters that cover continuously from the ultraviolet to near-infrared region by proper alloying. The band gap of the InxGa1−xN alloy system now spans the entire optical window from the near infrared to the ultraviolet. Difficulties in growing high-quality crystals are caused of the extremely high equilibrium vapor pressure of nitrogen. In present study, we report ab initio quasiparticle electronic structure calculations of InxGa1−xN, 0 ≤ x ≤1. Calculation was performed within the framework of density functional theory using the PBE functional for exchange and correlation. Beyond that, quasiparticle effects were taken into account by a subsequent GW correction of the Kohn-Sham eigenvalues within the framework of many body perturbation theory. Very good agreement between experimental and theoretical data was found. For pure GaN and InN GW calculations gives the band gap values equal to 3.2 and 0.65 eV while corresponding experimental data are 3.4 and 0.7 eV. For single wurtzite unit cell of Ga0.5In0.5N the calculated band gap is found to be equal to 1.0 eV while for model supercell with the same stoichiometry but some disorder this value is 2.0 eV that indicates strong effect of ordering on the alloy’s band gap. The almost linear dependence of the band gap value as a function of x was obtained.

Resume : The combination of luminescence spectroscopy - in particular at liquid He temperatures - with the high spatial resolution of a scanning transmission electron microscopy (STEM), as realized by the technique of low temperature scanning transmission electron microscopy cathodoluminescence microscopy (STEM-CL), provides a unique, extremely powerful tool for the optical nano-characterization. Examples will be given: We analyze the optical and structural properties of a modulation doped GaN:Ge DBR with an InGaN multiple quantum well on top. The structure was grown by metal organic vapor phase epitaxy on a sapphire substrate with a GaN buffer layer and consists of 100 pairs of GaN/GaN:Ge bilayers. Cross-sectional STEM-CL exhibits an modulation of monochromatic intensity in the DBR layers at low temperature: In the GaN:Ge DBR layer the excitonic emission (D0,X) at 356 nm is significantly reduced compared to the undoped layer. On the high energy side an electron hole band recombination (e,h) at 347 nm is apparent indicating the exceeding of the Mott transition in those GaN:Ge layers. In spectrally resolved STEM-CL-linescans the antiphase modulation is evidend. Further examples on the nano-characterization of nano-wires, core-shell heterostructures, and GaN quantum dots will be given.

Resume : This work is concentrated on studies of the band gaps, Eg, and related photoluminescence, PL, in polar InGaN/GaN structures, which due to narrow QWs and/or barriers exhibit an electron and a hole wave function overlap (WFO) between adjacent layers. The resulting effects consist in coupling of double QWs and formation of superlattices, SLs. Depending on the strength of built-in electric field the effective Eg changes drastically and excitons formed in such systems have intra-well or inter-well indirect character. Firstly, we present examination of double InGaN QWs with different GaN barrier width d . An effective WFO for an electron and a hole from adjacent QWs leading to inter-well character of excitons occurs for d ≤1 nm. Switching of the excitons between intra-well and inter-well ones as a function of exciting laser power is demonstrated. Then we examine In(Ga)N/GaN SLs, based on the structures described above. For sufficiently thin barriers such SLs can be formed. Finally, short period SLs In(Ga)N/GaN with QW and QB thickness varying from 1 to 15 atomic layers are studied with the purpose of determining their band gaps. Contributions of the built-in electric field and the WF hybridization to evolution of Eg is analyzed. Tuning of Eg in much wider range than for InxGa1-xN alloys, including closing of the band gap, is demonstrated. Financial support by the Polish National Science Center, grant DEC 2013/11/B/ST3/04263 is acknowledged.

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

Resume : BGaN is a promising material to achieve lattice-matching with SiC or AlN if sufficient boron concentration in alloy is achieved. On the other hand, incorporation of an increasing amount of boron is accompanied by a substantial degradation of material quality, which manifests through the defect-related luminescence bands. In this study, we have studied the influence of boron incorporation on yellow luminescence (YL) band seen in photoluminescence from BGaN alloys. The BGaN thin films were grown using metalorganic chemical vapour deposition (MOCVD) on sapphire substrates. Both GaN and AlN layers have been used as buffer layers for the growth of BGaN films. The YL band has been found to red shift as the boron concentration increases at a faster rate than the BGaN bandgap. A shoulder on the lower energies side of the YL band starts developing at the expense of a gradual decrease in intensity of the YL band. As a consequence, the colour of the YL band changes from yellow to amber with increasing boron content. 6-MeV electron irradiation followed by rapid thermal annealing reduces the intensity of the YL (amber) band, especially its low energies side. The mechanisms for such transformation are discussed.

Resume : Point defects or unintentional impurities may act as radiative or nonradiative recombination centers. Theoretical advances now enable us to calculate the electronic and optical properties as well as radiative and nonradiative carrier capture coefficients with unprecedented accuracy. These capabilities have allowed us to evaluate Shockley-Read-Hall coefficients and identify specific defects that play a key role in limiting the efficiency of nitride devices. Bare gallium vacancies are unlikely to occur, but their complexes with hydrogen or oxygen acts as compensating and recombination centers [1]. Calcium has also been identified as a detrimental impurity [2]. Based on simple rules, one expects defect-assisted recombination to become less important in wider-gap materials; but a number of examples show a different trend. We provide an explanation by noting the crucial role of excited states in recombination processes [3]. Iron is an important example [4]. Work performed in collaboration with A. Alkauskas, C. Dreyer, G. Kresse, A. Janotti, J. Lyons, S. Marcinkevicius, J. Speck, J.-X. Shen, and D. Wickramaratne, and supported by DOE and NSF. [1] C. E. Dreyer et al., Appl. Phys. Lett. 108, 141101 (2016). [2] J.-X Shen et al., Appl. Phys. Express 10, 021001 (2017). [3] A. Alkauskas et al., Phys. Rev. B 93, 201304 (2016). [4] D. Wickramaratne et al., Appl. Phys. Lett. 109, 162107 (2016).

Resume : In bulk AlN crystals as well as in epitaxial layers frequently broad defect-related photoluminescence (PL) bands are found in the spectral range from 1.4 – 4.5 eV. Using different spectroscopic methods like cw and time resolved PL at varying sample temperature, transmission, PL-excitation spectroscopy etc., we find clear indications of the involvement of (relatively shallow) DX states and effective-mass-like donor states related to oxygen and silicon. The capture of free electrons into these states and their thermalization properties are analysed, and their relation to Hall and optically detected EPR data is discussed. The deep acceptors acting as final states in the donor-acceptor pair-type PL transitions are most likely related to Al vacancies and their complexes with oxygen. For Be doped GaN bulk crystals unintentionally co-doped with oxygen, a broad PL band coinciding with the yellow PL at around 2.15 eV is found. This PL band has even at room temperature an exceptionally long lifetime of ~ 1 sec, which by thermal activation with an apparent energy of ~ 0.26 eV becomes much shorter for temperatures above 300 K. Gated PL spectra again indicate a donor-acceptor pair transition, with both DX-like and EMT-like donor states involved, and a Be related acceptor.

Resume : We have studied hole traps in p-type GaN grown by MOCVD using a pn junction structure by DLTS. The diode structure consists of 100-nm p(++) ([Mg]=8e19 cm-3)/700-nm p([Mg]=2e17 cm-3)/200-nm n(+)([Si]=1e19 cm-3) on n-type GaN substrate, which enables us to detect traps in p-type GaN due to the depletion region toward the p-side under negative bias condition. The Ohmic contacts were formed by Ni/Au on the top of p(++)-GaN and by Ti/Al on the back side of n(+)-GaN, respectively. No variation of the junction capacitance was observed in the frequency range up to 1 MHz, indicating negligible series resistance in the diode structure used. This enables to perform capacitance DLTS with the measurement frequency of 1 MHz. The CV measurements revealed the almost flat profile with the concentration of 2e17 /cm3 in the depth range from 170 to 280 nm which corresponded well to the ionized acceptor concentration in p-type GaN. This confirms that the depletion layer extends to the p-type region in the pn junction. DLTS measurements were performed in the temperature range from 200 to 550 K. The low measurement temperature is restricted due to the freeze-out of Mg acceptors. Four hole traps are detected with the energy levels of Ev+0.46, 0.88, 1.0 and 1.3 eV whose trap concentrations are estimated to be 1.6e15, 2.4e16, 3.7e15 and 5.3e15 /cm3, respectively. The energy level of Ev+0.88 eV is close to that of the hole trap labeled H1 observed in n-type GaN, which has been ascribed to carbon-related or Ga-vacancy-related defects [1,2]. The energy level of Ev+0.46 eV is close to the theoretically predicted energy level of nitrogen vacancy [3]. The results obtained by the admittance spectroscopy and current DLTS will be also presented. This research is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through its “Program for research and development of next-generation semiconductor to realize energy-saving society.” [1] Y. Tokuda, CS MANTECH, 19 (2014). [2] Y. Tokuda, ECS Transactions, 75, 39 (2016). [3] Q. M. Yan, A. Janotti, M. Scheffler, and C. G. Van de Walle, Appl. Phys. Lett. 100, 142110 (2012).

Resume : The recent discovery and understanding of inexpensive routes to grow high quality doped and un-doped gallium nitride (GaN) thick films by Hydride Chemical Vapour Deposition (HCVD) could lead to the fabrication of high efficiency low-cost optoelectronic devices, such as ultra-bright light emitting diodes and high-efficiency solar cells. HCVD technique to grow un-doped high-quality GaN thick films on different substrates, fused silica, silicon and Au/SiO2 had been described in previous works. In this research doped GaN thick films on Au/SiO2 using different dopants such as Be, Mg and Zn is presented. Ga-Be, Ga-Mg and Ga-Zn diluted alloys and ammonia as nitriding agent were used as row materials in a 3-zone horizontal quartz tube reactor to grow p-type GaN 40-μm/h films. NH4Cl is evaporated in the entrance of the reactor, argon is the carrier gas that takes the vaporized NH4Cl to middle of the reactor where it discomposes in NH3 and HCl, this reacts with the metallic alloy to yield volatile chlorides that are carried to the substrate and react with ammonia at high temperatures to grow doped GaN on the surface of the substrate. Doped and un-doped GaN synthesized by this method grow in hexagonal columns in the c-direction independently of the substrate. The best substrate to grow GaN columns is Au/Silica. Zn dopant improves luminescence of GaN better than Mg. It is possible to grow free standing un-doped, Mg- and Zn-doped GaN thick films with a columnar structure by this HCVD technique. The GaN thick films grown in this work showed the typical near band-edge emission at around 3.5 eV, and three extra-peaks around 3.3, 2.9 and 2.6 eV that are probably donor acceptor pares (DAP) with different donor levels. These GaN TFs are highly luminescent with efficiency comparable to those previously seen in thin films grown by more expensive techniques such as MOCVD and MBE, which would suggest that the material is a good candidate to be used in the fabrication of optoelectronic devices.

Resume : Defect incorporation in Al/GaN is dependent on the defect formation energy and hence on associated chemical potentials and the Fermi level. For example, the formation energy of CN in Al/GaN varies as chemical potential difference (µN- µC) and -EF (Fermi level). Here, we demonstrate a systematic point defect control by employing the defect formation energy as tool by (a) chemical potential control and (b) Fermi level control. Chemical potential control (µN and µC) with a case study of C in MOCVD GaN is reported. We derive a relationship between growth parameters, metal supersaturation (i.e. input and equilibrium partial pressures) and chemical potentials of III/N and impurity atoms demonstrating successful quantitative predictions of C incorporation as a function of growth conditions in GaN. Hence growth environment necessary for minimal C incorporation within any specified constraints may be determined and C is shown to be controlled from >1019cm-3 to ~1015 cm-3. Fermi level control based point defect reduction is demonstrated by modifying the Fermi level describing the probability of the defect level being occupied/unoccupied i.e. defect quasi Fermi level (DQFL). The DQFL is modified by introducing excess minority carriers (by above bandgap illumination). A predictable (and significant) reduction in compensating point defects (CN, H, VN) in (Si, Mg) doped Al/GaN measured by electrical measurements, photoluminescence and secondary ion mass spectroscopy (SIMS) provides experimental corroboration. Further, experiments with varying steady state minority carrier densities at constant illumination prove the role of minority carriers and DQFL in defect reduction over other influences of illumination that are kept constant.

Resume : Acceptor impurities remain some of the most intriguing species in III-nitride semiconductors. For instance, potential carbon contamination during growth motivates investigations of C acceptors [1], which can act as a carrier trap and source of luminescence. The search for dopants more efficient than Mg has spurred interest in group-II acceptors such as Be [2]. And although isolated native acceptor species are not thought to be abundant in as-grown GaN [3], complexes between gallium vacancies and donor impurities are likely to be present. In this talk I will review recent theoretical efforts to characterize such acceptor species in III-nitride semiconductors. Density functional theory is employed to predict the formation energies, acceptor ionization energies, and optical transition levels. Hole localization is found to be a key feature of these acceptors, and hybrid density functional theory is applied so that these physics are captured. We also directly compare our calculations to recent pressure-dependent photoluminescence studies [3]. 1. J. L. Lyons, K. Krishnaswamy, L. Gordon, A. Janotti, and C. G. Van de Walle, IEEE EDL 37, 154 (2016). 2. H. Teisseyre, J. L. Lyons, A. Kaminska, D. Jankowski, D. Jarosz, M. Boćkowski, A. Suchocki and C. G. Van de Walle, J. Phys. D: Appl. Phys. 50, 22LT03 (2017). 3. J. L. Lyons and C. G. Van de Walle, NPJ Comp. Mat. 3, 1 (2017). This work was conducted in collaboration with H. Teisseyre, A. Janotti, A. Alkauskas, and C. G. Van de Walle

Resume : By growing an alternating p-GaN/u-GaN layered structure with molecular beam epitaxy, we can obtain extremely low effective resistivity based on Hall measurement. With 4 and 2.5 nm in thickness for the p-GaN and u-GaN layers, respectively, a resistivity level as low as 0.038 Ohm-cm can be achieved. Such a low resistivity level is obtained by combining the high hole mobility in the u-GaN layers and the high hole concentration in the p-GaN layers. The holes can effectively diffuse into the u-GaN layers from the neighboring p-GaN layers for high-mobility migration. However, the theory behind the Hall measurement is based on the assumption of a uniform p-type layer. In such a layered structure, the Hall measurement result relies on the depth of current penetration. In other words, the relatively larger measured resistivity of a layered structure of larger layer thicknesses can be due to two possible causes: high effective resistivity and shallow current penetration in the Hall measurement. To understand the real cause, we grow samples of two thick layers with varied upper-layer thickness and low resistivity in the lower layer. By comparing the Hall measurement results of those samples, we can figure out the current penetration depth. This study can shed some light on the mechanism of Hall measurement when it is applied to an nm-scale layered doped structure. Based on the result of the current penetration depth, we can more accurately evaluate its effective resistivity.

Resume : Functional Mn-Mgk cation complexes in GaN co-doped with Mn and Mg are expected to extend the functionality of light-emitting III-nitride systems to the near-infrared (NIR) range [1-3]. We report on the excitation-dependent resonance Raman analysis and photoluminescence measurements, performed in order to gain insight into the nature of impurity-induced Raman features in (Ga,Mn)N:Mg with IR-emitting cation complexes. We have found that in contrast to the case of (Ga,Mn)N, the resonance enhancement of Mn-induced modes in Mg co-doped samples is not observed at an excitation of 2.41 eV, but shifts down to 1.58 eV, an effect explained by a resonance process involving the excitation of holes from ionized Mn4+ donor states to the valence band of GaN. Selective excitation within the resonance Raman conditions allows to resolve the structure of the Mn-induced phonon band into two distinct components, whose relative intensity varies with the Mg/Mn ratio, and thus is related to the concentration of cation complexes. Moreover, from the relative intensity of the 2LO and 1LO Raman resonances at inter-band excitation energy, we estimate the Huang-Rhys parameter, and consequently the strength of the electron-phonon interaction, which is found to increase linearly with the Mg/Mn. This work was supported by NATO SfP Grant 984735 and by the Austrian Science Fundation - FWF P24471 and P26830. 1. G. Capuzzo et. al., Sci. Rep. 7, 42697 (2017). 2. T. Devillers et al. Sci Rep. 2, 722 (2012). 3. T. Devillers et al. Appl. Phys. Lett. 103, 211909 (2013).

Resume : III-Nitride quantum dots with their large band offsets and wide range of bandgaps are promising nanostructures for room temperature quantum information technologies, since excitons can be sustained at higher temperature. Growth of high-quality GaN/AlN quantum dots with Stanski-Krastnanov growth mode was developed in 2002 by Metal Organic Chemical Vapour Deposition (MOCVD) [1]. The quantum dots exhibited a large biexciton binding-energy and a strong phonon interaction, realizing a single photon emission at 200 K in 2006 [2]. However, the magnitude of the binding energy of biexciton was not large enough to realize single photon emission at room temperature. Recently, we have acheived the single photon emission at or above room temperature via a position controlled GaN/AlGaN nanowire quantum dot. The structure was grown by selective area MOCVD and consists of GaN/Al0.8Ga0.2N core-shell type nanowires with a GaN quantum dot inclusion near the tips. The quantum dots themselves typically have a lateral dimension of ∼10 nm and a vertical dimension of ∼1 nm[3]. A large binding energy of biexciton (> 60meV) realized in the quantum dot enabled single photon emissions at room temperature in 2014 and at 350 K (77℃) in 2016, respectively[4,5]. We also succeeded in the formation of interface-fluctuation GaN quantum dots in a thin GaN/AlxGa1-xN (x < 0.25) quantum well grown on a sapphire (0001) substrate. The high temperature growth process led to a striking suppression of spectral diffusion and narrow emission linewidths as low as ~90 micro-eV. A high purity single photon emission from this interface fluctuation quantum dot was achieved and resulted in a measured g(2)(0) value smaller than 0.1 at 10 K. This value is the lowest ever reported for a III-nitride quantum dots, showing the remarkable nature of these quantum dots[6]. References [1] M Miyamura, K Tachibana, and Y Arakawa, Appl. Phys. Lett. 80, 3937 (2002). [2] S. Kako, C. Santori, K. Hoshino, S. Gotzinger, Y. Yamamoto, and Y. Arakawa, Nat. Mater. 5, 887 (2006). [3] K. Choi, M. Arita, S. Kako, Y. Arakawa et al., J. Cryst. Growth 370 328 (2013). [4] M. Holmes, K Choi, S. Kako, M. Arita, and Y. Arakawa, NanoLett., 14 982 (2014). [5] M. Holmes, K Choi, S. Kako, M. Arita, and Y. Arakawa, ACS Photonics 3 543 (2016). [6] M. Arita, K. Choi, S. Kako, and Y. Arakawa, NanoLett., in press (2017).

Resume : Correlating two or more microscopy and spectroscopy techniques on the same nanoscale object may yield a relevant amount of information, which is difficult to achieve by other means. In this contribution, we present several results of correlated studies of micro-photoluminescence (μ-PL), high-resolution scanning transmission electron microscopy (HR-STEM) and laser-assisted atom probe tomography (APT) on single nano-objects containing quantum confined systems based on III-N materials. This approach can be applied to the study of heterostructure interface definition, presence of extended defects such as stacking faults or dislocations, carrier localization and optical emission in quantum confined systems [1-4]. Furthermore, the use of complementary techniques may be extremely helpful for a correct interpretation of atom probe results and for understanding the limitations of the applied techniques [3,5]. Finally, we will show the first results of PL spectroscopy in situ in an atom probe. [1] L. Rigutti et al., Nano letters (2014), 14, 107–114. [2] L. Mancini et al. Appl. Phys. Lett. (2016), 108, 042102 [3] L. Rigutti et al. J. Appl. Phys. (2016) 119, 105704. [4] L. Mancini, Ph.D. Thesis (2016) and L. Mancini et al., submitted (2017). [5] L. Mancini et al. J. Phys. Chem. C (2014) 118, 24136-24151.

Resume : Group III-nitride semiconductors based on AlxGa1-xN alloys span a wide range of bandgap energies that could enable the realization of color tunable laser diodes. In particular, Favennec et al. demonstrated that materials with wider bandgaps present lower thermal quenching of rare earth (RE) luminescence, which drives our motivation to explore the case of AlGaN with high AlN molar fraction. Furthermore, in order to achieve laser diodes at the nanometer scale, or nano-emitters, AlxGa1-xN nanowires (NWs) are explored. In this work, AlxGa1-xN NWs grown by molecular beam epitaxy on Si (111) substrate, implanted with europium (Eu) ions at the same fluence of 1×1014 Eu/cm2, are studied. The as-implanted samples containing Al, were further submitted to rapid thermal annealing (RTA) treatments in nitrogen for 30 seconds, at 1000°C and 1200°C, while the GaN samples were only annealed at 1000°C. Raman spectra demonstrate that the Eu implantation did not significantly affect the AlN NWs. Furthermore, for all the AlxGa1-xN NW samples, a crystal recovery was obtained after RTA annealing. Photoluminescence (PL) measurements revealed that Eu3 luminescence was observed in all samples with the most intense emission assigned to the 5D0 - 7F2 transition in the red spectral region, indicating that such implantation and annealing conditions successfully activated the Eu ions. The same transition is found to shift by 2 nm towards longer wavelengths (from 622 to 624 nm) by increasing AlN molar fraction (from x=0 to x=1), in good agreement with results obtained for AlxGa1-xN layers. After annealing the AlxGa1-xN NWs at 1200°C, the Eu relative emission intensity at 622 nm (with respect to the yellow band intensity), was enhanced by a factor of two for each sample.

Resume : AlxGa1-xN based ultraviolet (UV) LEDs attract a lot of attention as an alternative to mercury vapor lamps that suffer from environmental problems and technical limitations. However the low structural quality of AlxGa1-xN materials leads to a drop in the UV-LED efficiencies. Our approach to enhance the radiative efficiency in AlxGa1-xN LEDs is to confine the carriers in 3 dimensions in quantum dots (QDs) to minimize the probability of non-radiative recombinations with dislocations. In this work, we investigate the properties of AlyGa1-yN QDs (0 ≤ y ≤ 0.4) for high efficient UV emitters. The samples were grown on AlxGa1-xN (0001) with 0.5 ≤ x ≤ 0.8. The influence of the epitaxial strain and of the QD composition on the formation of high QD densities (up to 10 12 cm-2) are shown. In addition, the fabrication of ultra-small QDs (with height < 2 nm), enables to minimize the influence of the internal electric field on the QD emission energy. Time resolved photoluminescence (PL) combined with temperature (T) dependent PL measurements enabled us to determine the QD internal quantum efficiency (IQE) at low T, showing IQE values between 30% and 50% in the 270 nm – 370 nm range. In addition, the PL yield decreases only by 25% from low T to room T. A very strong reduction of the decay times (from tens of µs to a few ns) was also observed for higher Al content and smaller size QDs, due to the strong increase of the electron–hole wavefunction overlap. These properties can be related to two contributions: the QD size and/or the internal electric field reduction, which will be discussed. Finally, the design of AlyGa1-yN QD based LEDs was investigated and UV LEDs emitting in the 300-360 nm range were fabricated.

Resume : Non-polar (11-20) a-plane InGaN quantum dots (QDs) are strong candidates for polarised single-photon light sources due to their large and tuneable band offsets, strong quantum confinement, and the lack of internal fields in the (11-20) direction. The resultant strong exciton oscillator strength, reduced inhomogeneous broadening and phonon scattering, and short emission lifetimes allow these polarised sources to achieve higher operation temperatures and much faster repetition rates. With these QDs embedded in nanopillars for higher extraction efficiency, or in planar LED structures for electrical pumping, we report the observation of optically-pumped ultrahigh repetition-rate, strongly polarised single photon emission with a predefined and deterministic polarization axis up to 220 K, an operation temperature easily accessible by commercial Peltier coolers. Furthermore, we report single photon electroluminescence up to 130K in our device structures. The simple planar growth method and inclusion of p and n doping also allow them to be easily made into electrically pumped devices. Hanbury Brown and Twiss (HBT), time-resolved, and polarization-resolved micro-photoluminescence (µPL) experiments were performed at various temperatures on these QDs. Sharp emission features from QDs were recorded up to 250 K in the optically pumped samples. Linearly polarised single photon emission was observed from 4.7 to 220 K. A g(2)(0) of 0.37, lifetime of 521 ps, and a degree of linear polarization (DOLP) of 0.83 were obtained at 4.7 K. All three of these characteristics remained almost insensitive to increasing temperature. At 220 K, the g(2)(0), lifetime, and DOLP became 0.47, 341 ps, and 0.80 respectively. Due to the limited emission intensity, g(2)(0) and DOLP were not obtainable at 250 K. The axis of polarization is consistently along the (1-100) m-direction. Based on the behavior of the redshift and inhomogeneous linewidth broadening, we have demonstrated temperature-insensitive quantum confinement, and a much-reduced Huang-Rhys factor and acoustic phonon coupling strength, comparing to their polar (0001) QD counterparts. These latest results demonstrate the potential of non-polar a-plane InGaN QDs as strong candidates for future on-chip Peltier-cooled polarized single photon emitters. For the electrically pumped source single photon emission was achieved under excitation with a constant current giving a second-order correlation value of 0.18.

Resume : In the active layer of the blue light-emitting diodes (LEDs) used in the solid-state lighting, quantum wells of essentially immiscible InGaN alloys are exclusively used. For realizing compact and low power consumption light sources for high color rendering index LED lighting and for sterilization and disinfection for human lives, near- to deep-ultraviolet LEDs are indispensable and are under development using AlGaN alloys. Whereas, since the growth of high-quality crystal is herculean, equally immiscible AlInN alloys have never been used as light-emitting media, although their bandgap energies cover from UV-C to near IR wavelength regions. The authors have recently demonstrated planar vacuum fluorescent display devices using m-plane AlInN epitaxial nanostructures grown by metalorganic vapor phase epitaxy, which were composed of a dense array of approximately 50-nm-thick nanoboards. As they emitted polarized UV-C, blue, and green light despite the presence of high-concentration nonradiative recombination centers, the emission mechanisms and their microscopic origins are worth investigating. In this presentation, two distinct length-scale compositional modulations in the m-plane AlInN epilayers will be shown to discuss their origins. A few-nm-scale periodical compositional undulation along < 20-21>-axis was found in the pseudomorphic region thinner than 50 to 60 nm from the AlInN/GaN interface, and more macroscopic compositional pulling effect was seen in the nanoboard regions.

Resume : The research on GaN quantum structures grown on nonpolar and semipolar directions suppresses internal electric field, which increases oscillator strength of the confined exciton. However, the growth along these directions results in much-enhanced densities of extended defects, like basal (BSF) or prismatic stacking faults (PSF). Most typically they take a form of a few atomic monolayer thick, cubic-like inclusions in a wurtzite host. Our polarization and time resolved micro-photoluminescence measurements (spatial resolution <0.5 μm) conducted on GaN layers grown on semipolar (11-22) direction reveals a presence of unexpectedly sharp (FWHM down to 0.2 meV), discrete lines in the 3.20 – 3.45 eV spectral range. Planar density of these lines determined from optical measurements are of the order of 1010/cm2. The  PL dynamics in single ns temporal range excludes that donor-acceptor pairs are responsible for the observed emission. The individual lines show a high linear polarization degree, with three dominating polarization directions in the lines statistics. They clearly correspond to directions of three types of defects visible in HR-TEM images taken in plan-view near [11-22] zone axis. The defects are high density of 5-20 nm long segments of the BSF (perpendicular to c axis) and two groups of partial dislocations associated with BSF. Our results provide the first indication for the optical determination the type of the extended defects in GaN semipolar structures.

Resume : Contactless electroreflectance (CER) spectroscopy in a very powerful tool to investigate the built-in electric field in semiconductor heterostructures as well the Fermi-level position on semiconductor surface. In this work we will present our recent progress in the application of CER spectroscopy to study these issues in GaN-based (hetero)structures. To determine the Fermi level position at the GaN surface we perform CER measurements on specially designed UD+ structures [GaN(undoped)/GaN(doped)/substrate] doped by Si and Mg. Measuring the built-in electric field in the undoped (U) layer it is possible to determine the Fermi level position at the surface. In this way we have observed that the surface Fermi level of Ga-polar GaN exhibits a bistable behavior allowing it to be located at two distinct energetic positions: ~0.6 and ~1.7 eV below the conduction band for n-type and p-type structures, respectively. Next CER results for GaN(cap)/AlGaN/GaN heterostructures will be presented. In this case a decrease of built-in electric field in GaN(cap) layer and an increase of electric field in AlGaN layer has been observed with the increase in thickness of GaN(cap) layer, while a decrease of electric field in AlGaN layer and an increase in electric field in GaN(cap) layer has been noticed with the increase in thickness of AlGaN layer. The observed results are explained solving the Schrodinger and Poisson equations in self-consistent manner for the proper surface boundary condition.

Resume : In this talk, the properties of highly efficient AlGaN-based metal-semiconductor-metal photodetectors for the UVC and UVB spectral region will be reviewed in terms of experimental and simulated data. One aspect will be the illumination direction either from the top (absorber) or bottom (substrate/buffer) side. Another aspect is the role of internal polarization fields in MOVPE-grown AlGaN/AlN/c-sapphire structures and their interplay with an externally applied electric field. This explains the saturation of the external quantum efficiency above a certain bias voltage, which is determined by the AlGaN absorber layer thickness. However, this dependence is cancelled for detectors with an increased density of specific macroscopic crystal defects that form current transport paths through the absorber layer resulting in a thickness-independent saturation voltage. Furthermore, ageing experiments of UVC MSM detectors will be discussed, in order to reveal field-induced surface reactions as the root cause of degradation of un-passivated devices.

Resume : AlN has been used as the substrate for deep-ultraviolet light-emitting devices owing to its wide bandgap and high chemical stability. However, high-density of threading dislocations exists in AlN films grown on non-native substrates. Recently, we developed high-temperature face-to-face annealing (FFA) technique that can drastically improve the crystal quality of AlN films on sapphire substrates [1]. In this work, we fabricated high quality AlN films on sapphire substrates using face-to-face annealing. AlN films were grown on c-plane sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). The film thickness was 20 – 1000 nm. The growth temperature and the reactor pressure were maintained at 1400 oC and 17.3kPa, respectively. Then, the AlN films were thermally annealed at 1700 oC in an ambient N2 using FFA technique. The surface roughness was increased with increasing film thickness 20 – 300 nm, whereas it decreased with increasing film thickness 300 – 1000 nm. The AlN films formed a step and terrace surface morphology with pits for 1000-nm-thick AlN film. The full width at half maximum (FWHM) of X-ray rocking curve (XRC) for AlN(0002) and AlN(10-12) reflection were 74″ and 754″, respectively. After thermal annealing for 1000-nm-thick AlN films, the smooth step and terrace surface without pits was formed. The crystallinity was drastically improved with FWHM values of XRC for 57″ for AlN(0002), 138″ for AlN(10-12). [1] H. Miyake et al., J. Cryst. Growth 456 (2016) 155.

Resume : Heterostructures based on polar GaN have the strong intrinsic electric field caused by piezoelectric and spontaneous polarizations due to mismatch of the crystal cells of adjacent heterostructure layers. In present report the internal electric fields in active region of LEDs structures based on multiple InGaN/GaN QWs were investigated by electroreflectance (ER) spectroscopy. The sample under investigation is royal blue LED heterostructure with 5 QW in active area grown along [0001] direction on sapphire substrate by MOCVD technology. ER spectra were obtained at room temperature in the range of 2.2-3.5eV with different and DC bias voltages. Observed spectra were modified by Kramers–Kronig relations and approximated by sum of Lorenz functions. Obtained dependences of amplitude and peak position from the DC voltage allow to determine the inner electric field in different QWs with respect to applied bias voltages. The energies of two QWs showed independence from the external electric field and inner one is 3.0-3.5MV/cm. Other ones showed splitting in energies 75meV with changing the voltage from 0 to -4.5V which corresponds to the difference in electric field 1.8MV/cm. Such behavior of distribution of electric field is connected with the straight influence of external electric field on the two QWs and nonlinear impact on the other ones whereas internal electric field without bias voltage has the contrary dependence from the coordinate in growth direction in active area.

Resume : We present work from two recent areas of nitride MBE Role of calcium in the efficiency of MBE LEDs: Ca as an unintentional impurity has been investigated in III-nitride layers grown by molecular beam epitaxy (MBE). It is found that Ca originates from the substrate surface, even if careful cleaning and rinsing procedures are applied. The initial Ca surface coverage is ∼1012 cm−2. At the onset of growth, the Ca species segregates at the growth front while incorporating at low levels. The incorporation rate is strongly temperature dependent: ~0.03% at 820 °C and increases by two orders of magnitude when the temperature is reduced to 600 °C, which is the typical growth temperature for InGaN alloy. Consequently, [Ca] is as high as 1018 cm−3 in InGaN/GaN quantum well structures. Such a huge concentration might be detrimental for the efficiency of LEDs if one considers that Ca is potentially a source of Shockley-Read-Hall (SRH) defects. We thus developed a specific growth strategy to reduce [Ca] in the MBE grown LEDs, which consisted of burying Ca in a low temperature InGaN/GaN superlattice (SL) before the growth of the active region. [Young et al., APL 109, 212103 (2016)] Role of disorder in LED behavior: We review recent experimental work on unipolar analogs to LEDs to demonstrate the important role of alloy disorder in transport.

Resume : Comprehensive analysis of GaN(0001) surface under ammonia/hydrogen mixture was undertaken using results of density functional theory calculations. It was shown that the adsorption processes depend on the electronic properties of the surface. The adsorption energy of the same species may vary by several electronvolts depending on the Fermi level position at the surface. Its location is dependent, inter alia, on the surface reconstruction. There are two classes of surface: the first, wherein the Fermi level is pinned at surface states located in the bandgap (energy bands are bent), and the second, wherein Fermi level is free (energy bands are almost flat). Depending on the type, the different potential barriers at the surface exist. This could particularly affect the incorporation energy of impurities and dopants. Based on this phenomenon, we explain, for example, some unclear aspects of carbon incorporation during GaN growth by MOVPE. For practical use it is important to define the surface state in individual growth conditions, beyond the vague definition of N-rich and Ga-rich conditions. Therefore, we proposed a revised, more accurate approach, which includes also contribution derived from vibrational motion of the solid phase. This allows to calculate the thermal change of vibrational energy and entropy for differently reconstructed surfaces. The chemical potential difference contribution of the following terms: vaporization enthalpy, vaporization entropy, thermodynamic temperature related entropy change, the thermal enthalpy change and mechanical pressure was determined. Obtained results can be used to predict the true state of the hot surface.

Resume : AlN thin films are widely used in electronic, optoelectronic and sensor applications due to their outstanding properties and compatibility with standard CMOS process technology. In 2009 it was shown that the piezoelectric properties can be improved tremendously by alloying AlN with ScN [1], making AlScN a promising candidate for future electronic as well as optoelectronic devices and sensors. AlScN might also have superior properties for pyroelectric sensors [2]. However, no investigation was conducted on the thermal dependence of the pyroelectric current, that would determine the suitability of AlScN for high-temperature pyroelectric sensor applications. In this work, 1 µm thick pyroelectric Al1-xScxN films (x = 0 to 0.25) were deposited on Si(100) substrates by reactive pulsed dc magnetron co-sputtering. The X-ray diffraction measurements revealed that all films have wurtzite type crystal structure and are highly c-axis oriented. To measure the pyroelectric current in the temperature range of 20 °C to 95 °C a low-frequency temperature wave dynamic method was employed. The effective pyroelectric coefficient of AlScN increased by 50% in the investigated composition range. Moreover, a decline of the effective pyroelectric coefficient with rising temperature was observed for all samples. We explain this observation qualitatively by taking into account the contribution of primary and secondary pyroelectric effect as well as the substrate clamping. [1] M. Akiyama, et al., Advanced Materials, 21(5), 2009. [2] V. Vasilyev, et al., MRS Advances, 1 (39), 2016.

Resume : Damage-free precision etching of group III nitride semiconductors is an attractive technology expected to be applied for removing the dry etching damage and precise size tuning for the next-generation nanostructure-based LEDs, solar cells, transistors, and so on. We propose a new digital wet etching technique using surface oxidation by saturated ozone water (SOW) treatment and oxide film etching with buffered hydrofluoric acid (BOE). In this etching, the side facet of InGaN/GaN nanostructure formed on the (0001) plane wafer was etched with good controllability at 0.8 nm/cycle. For the evaluation of the etching characteristics, the InGaN/GaN nanostructures with 100~240 nm in width and 140 nm in height formed by the low-damage etching technique called hydrogen environment anisotropic thermal etching (HEATE) [1] was used. The SOW was made by bubbling ozone gas into deionized water. The sample was subjected to 75 cycles of SOW treatment, deionized water rinsing (1 min), BOE (10 min), and deionized water rinsing (1 min) at room temperature. The SOW treatment time was changed into 10, 20, and 30 min. The etching rate was estimated every 25 cycles from the reduction of nanostructure width via a field emission scanning electron microscope (FE-SEM) observation. Despite of the SOW treatment (oxidation) time change, the etching rate was almost kept at 0.8 nm/cycle (~3 ML/cycle). This simple etching has a feature of highly controllable sub-nm order digital etching technique. [1] R. Kita, R. Hachiya, T. Mizutani, H. Furuhashi, and A. Kikuchi, Jpn. J. Appl. Phys. 54, 046501 (2015).

Resume : 3D GaN nanostructures with well-controlled geometry, vertical architecture, and high aspect ratio are very attractive because they are expected to be an exciting new route towards extending the freedom for device design in GaN technology. Such building blocks provide large surfaces, defect free high-quality material, non-polar surface orientations, and the option to use very large area foreign substrates without implementing large area strain. All these aspects are difficult to be obtained in planar thin film approaches. To fabricate vertical 3D GaN nanoarrays, both bottom-up and top-down methods involving different nanolithography techniques can be employed offering good homogeneity. Additionally, 3D processing needs to be developed to integrate them into nanodevices for various optoelectronic sensing and electronics applications (e.g., biomedical/chemical sensors and vertical transistors). By adjusting the orientations, width and pitch of the hole/line openings, and the growth/etching parameters, different structural qualities can be evolved. In terms of the geometry, nanofins potentially offer more advantages than vertical nanowires as they can reduce edge effects, have a higher gain in effective area and be much easier accessible for material analysis. The MOCVD growth and hybrid etching results for both nanofin and nanowire structures will be compared, 3D processing will be described, and experimental results of the developed nanodevices will be presented.

Resume : Development of GaN-based optoelectronic and electronic devices is closely linked to ongoing work devoted to crystallizing bulk GaN. Hydride vapor phase epitaxy (HVPE) is the most popular method for obtaining commercial-grade substrates. Significant advantages of HVPE are high growth rate and high purity of new-grown GaN. Using ammonothermal GaN substrates of high crystallographic and structural quality as seeds enables growth of HVPE-GaN of the same high quality. Without intentional doping the material has a very low concentration of impurities and n-type conductivity comes from silicon with concentration close to 1×1017 cm-3. Free carrier concentration is at the level of 3-5×1016 cm-3. Controllable doping of HVPE-GaN to prepare substrates of specific parameters is still a challenge. In this work influence of different dopants on optical and electrical properties of GaN is presented. Results showing highly conductive n-type HVPE-GaN doped with silicon and germanium and semi-insulating material doped with carbon and iron with manganese will be presented. Characterization of the crystals includes their structural (X-ray diffraction, defect selective etching), optical (Raman spectroscopy, photoluminescence) and electrical (Hall measurements, Capacitance–voltage profiling) properties. Concentrations of dopants will be examined by Secondary Ion Mass Spectrometry (SIMS).

Resume : High-temperature vapour phase epitaxy (HTVPE) [1] is a physical vapour transport based technique for the growth of GaN, which uses thermally evaporated, elemental gallium and ammonia as precursors. This method allows the deposition of GaN layers on (0001)-oriented sapphire substrates exhibiting a crystal and structural quality comparable to conventional growth methods. In this work, we present results on structural and optical characterization of GaN layers grown by HTVPE at various growth conditions. The resulting layers were examined by high-resolution X-ray diffraction. We find, that the FWHM of 0002 X-ray reflections are below 200 arcsec. Additionally, we assessed the threading dislocation density analysing reciprocal space maps. In order to monitor residual stress within the GaN layers with high lateral and spatial resolution we performed confocal Raman spectroscopic measurements at room temperature. We discuss the spectral position of the non-polar E2(high) Raman mode indicating compressive in-plane strain, which is reduced with increasing GaN layer thickness. Low-temperature photoluminescence spectra of HTVPE layers grown at atmospheric pressure show a suppressed near band edge emission (NBE) in comparison to the ultraviolet luminescence. The NBE intensity increases about two orders of magnitude for layers deposited at reduced pressure and with elevated growth rates indicating an improved layer quality. [1] G. Lukin et al., physica status solidi (c), 11, 491 (2014).

Resume : This report presents preliminary results of the study of crystallization of gallium nitride from Fe-Ga-N solution under N2 pressure of 1 GPa. Motivation behind choosing the Fe-Ga-N system for investigating the possibilities of GaN crystallization is based on the information about high solubility of nitrogen in iron (Geochem. Int. 49 (2011) 429-438) under the conditions identical to those, applied in classical GaN growth method from nitrogen solution in gallium (J. Phys. Chem. Solids 56 (1995) 639–644). Performed experiments allowed to determine low limit of the temperature range available for the crystallization of GaN in the abovementioned system at 1 GPa N2 pressure. This temperature is limited by iron melting temperature which is found by DTA method to be as low as 1380 °С. This value is above 200 °С below from the one reported for pure Fe iron for 1 GPa pressure (Yu. Tonkov). Temperature dependencies of nitrogen and gallium nitride solubility in Fe were studied in the range of 1380-1480 °С and at 1 GPa N2 pressure. The possibility of growing GaN single crystals at cooling of N solution in Fe80%Ga20% alloy heated up to Т = 1450 °С at 1 GPa is demonstrated. Micro-Raman and micro-photoluminescence spectra of newly grown crystals were measured and analyzed. Based on these data, experiments on crystallization of gallium nitride from Fe-Ga-N solution at 1 GPa under temperature gradient are designed. Results of these experiments will be discussed as well.