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The aim of this paper is to illustrate the dependence of DLTS characteristics and degradation of InGaN-based laser diodes (LDs) on the density of dislocations. Three groups of multi-quantum well LDs with different dislocation densities were submitted to constant current stress, at room temperature: the analysis is based on combined electrical-optical measurements and Deep-Level Transient Spectroscopy (DLTS) investigation was made before and after stress. DLTS results show the presence of a hole trap in all the samples, whose intensity is related to the dislocation density. Constant current stress induces a significant decrease in the optical power (subthreshold regime), not related exclusively to the dislocation density, and the appearance of a new deep level for electrons (point defect generated after stress).

Theoretical predictions of high hydrogen diffusivity in p-GaN layers were confirmed in many experiments for samples grown by Metalorganic Vapor Phase Epitaxy (MOVPE). In this technology, p-GaN has a high concentration of hydrogen incorporated during the growth. In this work, we confirm that also in hydrogen-free p-GaN grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE) the hydrogen diffusion during H2+NH3 annealing is much higher than in n-type layers. Additionally, we have compared hydrogen diffusion for samples of different dislocation density and we have found no effect of these defects. The photoluminescence of the PAMBE-grown p-GaN exhibited the following change after annealing in H2+NH3 atmosphere: the blue luminescence decrease and yellow luminescence increase.

In this work, we investigate the role of the electron blocking layer (EBL) in laser diodes based on a graded index separate confinement heterostructure. We compare two sets of devices with very different EBL aluminum composition (3% and 12%) and design (graded and superlattice). The results of electro-optical characterization of these laser diodes reveal surprisingly modest role of electron blocking layer composition in determination of the threshold current and the differential efficiency values. However, EBL structure influences the operating voltage, which is decreased for devices with lower EBL and superlattice EBL. We observe also the differences in the thermal stability of devices – characteristic temperature is lower for lasers with 3% Al in EBL.

In metalorganic vapour phase epitaxy (MOVPE) hydrogen is not commonly used in the carrier gas during growth of InGaN layers since it interferes with indium incorporation. However, hydrogen leads to a smoother surface morphology of the growing layers which is beneficial in many optoelectronic devices. The influence of hydrogen on indium incorporation was studied here. This study concludes that hydrogen alters the state of the GaN surface to the one, that does not favor indium incorporation. However, this change of the surface state by hydrogen is not immediate but occurs during the exposition of the GaN surface to H 2 flow and leading to a significant In-content gradient in the InGaN layer grown with 1% (of total flow) of hydrogen in the carrier gas. We showed, that indium content on the GaN/InGaN interface is different in the case, when hydrogen is present or absent during pre-growth vent. For the InGaN sample grown without hydrogen in the carrier gas, In-content gradient was not observed.

In this study, we theoretically studied the enhancement of carrier injection by employing heterostructure layers before p-GaN in green light-emitting diodes. It is evident from the simulation results that improved carrier injection results in high internal quantum efficiency and light output power in the structures using heterostructures on the p-side. In comparison to conventional structure, reduced efficiency droop is observed suggesting promising high-performance green light-emitting diodes.

Betavoltaics is a potentially attractive solution for ultra-long lifetime power sources. However, reliability of such devices depends on the radiation hardness of the semiconductor system. III-N semiconductors (nitrides) are known to be unusually strongly bonded materials, able to withstand relatively high doses of radiation. Gallium nitride structures can be grown either on sapphire templates with high dislocation density or on GaN substrates which ensures much better quality and perhaps performance of the final device. Therefore, the interest of this paper is to explore. p-i-n GaN junctions grown on various types of substrates and to find the best system for future betavoltaics batteries. Within the present work we demonstrate the design, modelling, and the tests of betavoltaic structures based on GaN p-n junction. The structures grown on high quality GaN substrates showed better efficiency at small excitation than counterparts grown on sapphire with power conversion efficiencies up to 3%. We do not observe any degradation in the studied structures after reliability testing, that consisted of electron irradiation in a scanning electron microscope or directly using Ni-63 radioisotope and also in an electron accelerator. All experiments indicate suitability of GaN for the construction of betavoltaic cells.

Nano-scale non-homogeneities in MOCVD- and HVPE-grown gallium nitride were revealed by means of photo-etching in K 2 S 2 O 8 –KOH solution and by chemical etching in hot KOH solution. These non-homogeneities result in formation of protruding etch features and in-depth elongated nano-pits, respectively. High spatial resolution cathodoluminescence evidenced a non-homogeneous distribution of non-radiative recombination centers, which correlates well with photo-etched pattern of protruding etch features. The density of these features is two orders of magnitude higher than this of threading dislocations. A possible association between the non-uniform in nano-scale distribution of native point-type defects, such as V Ga , O N and related V Ga –O N complexes, and the observed etch features is presented. Existence of such fluctuations may have a large influence on the reliability of nitride-based electronic and optoelectronic devices.

We describe surface patterning as a method to obtain discrete regions of different local miscut angles, δ, in semipolar ( 20 2 ¯ 1 ) GaN substrates. During patterning the region angle δ was varied between 0° and ±2° with respect to the growth plane. We chose two nonequivalent miscut directions: a 〈 1 ¯ 2 1 ¯ 0 〉 and c ׳ 〈 1 ¯ 01 4 ¯ 〉. On such patterned substrates In x Ga 1− x N/GaN quantum wells were grown by a Metalorganic Vapor Phase Epitaxy method. Angled regions were not planarized during growth and their initial miscut angles remained. We studied structural, morphological and optical properties of In x Ga 1− x N/GaN quantum wells as a function of δ . The intended In concentration, x , was about 13% on exact oriented ( 20 2 ¯ 1 ) planes. For miscut towards the a -direction, indium content decreases and luminescence energy increases with δ . For miscut towards c ׳, we observe quite different behaviors, indium content and quantum well width are constant and luminescence energy increases with δ . We propose the explanation of these effects on the basis of sample morphology, In-content and built-in electric field. The obtained results are important for understanding the role of miscut (intentional and unintentional) in basic properties of semipolar InGaN/GaN quantum well structures. It is also worth to note that, the spectral width of luminescence was the smallest for regions miscut 2° towards c ׳ direction, which can be important for semipolar optoelectronic devices.

InGaN/GaN multiple quantum well structures were grown on bulk GaN and on sapphire substrates using the metalorganic vapor-phase epitaxy in order to study the influence of hydrogen during the growth of the GaN barriers. This hydrogen flow had the following effects on the structures: (i) the thickness of the QWs was reduced, (ii) the indium concentration in the QWs was decreased and (iii) the growth rate of the quantum barriers was increased.

The InGaN layers were grown using the Metalorganic Vapour Phase Epitaxy on bulk GaN substrates in a stop-and-go-mode (30 s growth, 30 s stop) and then examined using X-ray Diffraction (XRD), Photoluminescence (PL), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). The experiment was done in order to study an influence of hydrogen and TMIn flows during the growth-breaks on the InGaN layer. It was found that the presence of hydrogen during the breaks removes indium atoms from the already grown InGaN layer and delays In-incorporation into the subsequent one. As a result, instead of having about 18% of In in the continuously grown InGaN, we have only about 6% (average In-content) and the layer is thinner by more than 20%. In the case of having simultaneously H 2 and TMIn flows on during the breaks, we get three times less of In-atoms torn away and the layer has the thickness unchanged. The simultaneous presence of TMIn and H 2 also gave much smaller surface roughness as compared to the situation when only H 2 was on during the breaks.

We present recent progress in the growth of nitride based laser diodes (LDs) made by Plasma Assisted Molecular Beam Epitaxy (PAMBE). In this work we demonstrate continuous wave (cw) LDs grown by PAMBE operating in the range 430–460 nm. The LDs were grown on c -plane bulk GaN substrates with threading dislocation densities (TDDs) ranging from 10 3 cm −2 to 10 7 cm −2 . The low TDDs allowed fabrication of cw LDs with the lifetime exceeding 2000 h at 10 mW of optical output power. The maximum output power for these LDs was 80 mW. We used AlGaN cladding-free design of LDs with InGaN waveguides. The key element to achieve lasing for wavelengths above 450 nm was substantial increase of the nitrogen flux available for growth in PAMBE. The increase of N flux is beneficial for growth of efficient InGaN QWs, which allowed demonstration of optically pumped lasing from single quantum well InGaN laser structures at 501 nm.

The properties of standard Ni–Au contacts to p-type GaN were intensively investigated using various microscopic techniques: scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results of these investigations allowed the basic structural properties to be established after the typical annealing procedures. The distribution of chemical species in the contact structure was investigated using in-depth X-ray photoelectron spectroscopy (XPS). It has been observed that during annealing in an ambient atmosphere containing oxygen and nitrogen, two types of contact structure may develop, depending on the morphology of the (0 0 0 1) GaN surface: (i) relatively low resistivity contact, obtained on a “finger-like” surface and (ii) high resistivity contacts on the GaN surface covered with hillocks containing dislocations. These studies were used to formulate a model, which explains the basic mechanism of contact formation. It is also shown that the non-uniformity in the Au layer could be the principal source of high resistivity of this type of contact.

In this paper we will discuss the application of almost dislocation-free, high-pressure grown gallium nitride bulk crystals as a substrate for the epitaxy of GaN/InGaN/AlGaN laser diode structures. We show that these laser diodes may have very low dislocation densities (even down to 8×10 4 cm −2 ). These dislocations appear during the growth of the laser structure as a result of the combined effect of strain and perturbation of the atomic step flow. We show also that the lifetime of these devices seems to be dependent on operating current via the increasing junction temperature.

Plasma-assisted molecular beam epitaxy (PAMBE) has recently emerged as a viable tool for production of nitride blue-violet laser diodes operating at room temperature in continuous wave mode and high output powers [C. Skierbiszewski, P. Wisniewski, M. Siekacz, P. Perlin, A. Feduniewicz-Zmuda, G. Nowak, I. Grzegory, M. Leszczynski, S. Porowski, Appl. Phys. Lett. 88 (2006) 221108]. The present work reviews the current state of the art in this program as well as discusses its future directions. Two elements are given particular attention: (1) the epitaxial growth in metal-rich conditions, which enables effective lateral diffusion of N adatoms at low growth temperatures and (2) the role of threading dislocations in destabilizing the growth front. Low-temperature growth by PAMBE on dislocation-free GaN substrates is instrumental in achieving high performance of optoelectronic structures. The inherent to this process capability of sustaining two-dimensional step-flow growth mode (with straight and parallel atomic steps) at low growth temperatures opens up the way to the growth of strained multilayer structures with no compositional fluctuations and with flat interfaces.

The rapidly growing gallium nitride device industry continues to be in dire need of high quality, cost effective native substrates. Considerable progress has been made with pseudo-bulk substrates synthesized by hydride vapor phase epitaxy (HVPE), but true bulk methods promise both superior quality and reduced cost. We give an overview of the high-pressure ammonothermal method developed by GE, based on adaptation of high-pressure apparatus developed for diamond growth, together with appropriate raw materials and methods. We describe recent progress, including characterization of and reductions in impurity concentrations, wafering, and successful fabrication of homoepitaxial laser diodes on GE substrates.

High-pressure growth from solution is at present the only method able to provide true bulk GaN monocrystals. In this paper, we would like to demonstrate that in spite of their small, centimeter range size, they can become a technological platform for the realization of high-quality violet and near-UV laser diodes. We used MOVPE technique to deposit InGaN/AlGaN/GaN layers forming a separate confinement heterostructure laser diode. These devices are characterized by a low density of dislocations (∼10 5 cm −2 ) and a high optical output power of 1.9 W, measured in short pulses (30 ns) to prevent the structure from overheating. We will briefly discuss the challenges we face during the process of optimization of these devices.

Spontaneous and stimulated luminescence in highly excited InGaN/GaN multiple quantum wells grown by MOCVD over sapphire and bulk GaN substrates is reported. By using high-power laser pulses of 20-ps duration, free-carrier densities as high as 1 0 19 cm - 3 were achieved, which resulted in almost screened built-in field. Therefore, dynamics of spontaneous and stimulated emission due solely to band potential fluctuations were revealed. In comparison with the heteroepitaxial counterparts, structures grown over bulk GaN exhibited strong stimulated emission within the first 100 ps of relaxation, a reduced intensity of spontaneous emission on the later stage of relaxation, and domination of delocalized carriers in the spontaneous spectra. The obtained results imply a significant reduction of band potential fluctuations in the InGaN/GaN structures grown over bulk GaN.

The surface and electronic structure of Ga0.92In0.08N layers grown by metal organic chemical vapor deposition (MOCVD) have been investigated by means of photoemission. Stability of chemical composition of the surface subjected to Ar+ ion sputtering was proven by means of X-ray photoemission spectroscopy. The analysis of the relative intensities of In 3d, Ga 3p, and N 1s peaks showed that argon ion bombardment does not change significantly the relative contents of the layer constituents. Simultaneous efficient removal of the main contaminants (O and C) was observed during the sputtering procedure, proving that argon sputtering can be used as a method for preparation of clean Ga1−xInxN surfaces. For a clean (0001)–(1×1) surface prepared by repeated cycles of Ar+ ion sputtering and annealing, electronic structure was investigated. The band structure was explored along the Γ–A direction of the Brillouin zone, measuring angle-resolved photoemission spectra along the surface normal. A similar set of data was also acquired for the same surface of GaN layer. Comparison of the collected data revealed an additional feature at the valence band edge, which can be ascribed to the presence of In in the layer.

High-pressure-grown bulk GaN crystals have the best reported structural quality and the lowest defect density among all available GaN substrates. We report on MOVPE growth, device processing and properties of high-power laser structures on such substrates. In this work, we describe very high optical output power per laser facet of 1.9 W from single LD device with 15 μm×500 μm cavity. The measured value is up to our knowledge the highest nitride laser power reported. Also, the electrical and optical power densities, yet pulsed, are in the range of the highest reported for broad area lasers what confirms that nitride-based wide bandgap structures are capable of handling very high power. From thermally accelerated electrical ageing tests we extracted the activation energy of one of degradation mechanisms to be 0.32 eV. Catastrophic optical damage was not observed up to power density of 40 MW/cm 2 . Demonstrated results show that the availability of very low defect density GaN substrates is clearly the key issue in fabrication of highly reliable devices and high power LDs.

We present results of temperature-dependent, time-resolved photoluminescence (PL) measurements on a set of Ga 1− x In x N ( x ≅0.08) layers grown at the same conditions on three different substrates: bulk (high pressure grown) GaN, SiC and Al 2 O 3 . At 9 K, the maximum of the PL occurs at about 3.1 eV. The linewidths are 48, 80 and 90 meV for the layers grown on GaN, Al 2 O 3 and SiC, respectively. The smallest width suggests the highest homogeneity of the GaInN/GaN. The PL lifetimes (about 0.8 ns) and intensities are similar in all three samples. The spectral diffusion of the PL (shift of the PL peak energy with time) is thermally activated. The highest shift (Δ E =90 meV) and an activation energy E a =80 meV are observed in the GaInN/SiC sample, the lowest (Δ E E (interpreted as a conduction and valence band fluctuation amplitude) found in the layer grown on GaN is due to the low dislocation concentration resulting from the good lattice matching.

Optical transmission measurements were performed on GaN bulk single-crystal platelet at temperatures between 293 and 1237 K. The energy bandgaps were determined from the corresponding optical absorption spectra. The bandgaps can be fit well as a function of temperature using the Varshni expression as E g ( eV )=3.556−9.9×10 −4 T 2 (T+600). The shapes of the measured absorption edges were found to be dependent on the thermal treatments of the sample.

The single crystals of GaN were grown at a high pressure of 10–20 kbar and a high temperature of 1500–1800°C. The crystals have the form of hexagonal platelets of size up to 100 mm 2 and dislocation density smaller than 10 3 cm −2 . These crystals were used as substrates for epitaxy of III-N compounds. Measurements indicate that the crystallographic structure of the substrates is reproduced in the epi-layers of the ternary compounds, AlGaN and InGaN, of compositions and thicknesses used in most of the nitride-based devices, including a blue-laser structure. The very low dislocation density in these layers and their structures is documented by X-ray diffraction, atomic force microscopy and high-resolution transmission electron microscopy. The multiple quantum wells of InGaN/GaN have very sharp interfaces not disturbed by the presence of dislocations.

Highly resistive GaN : Be was obtained by means of synthesis of Ga+Be with atomic nitrogen under high nitrogen pressure. Activation energy of resistivity is about 1.5 eV. This material exhibits features very different from those observed in highly resistive bulk GaN : Mg. Up to 300 K strong yellow band dominates photoluminescence spectrum in resistive GaN : Be crystals. Positron annihilation studies point to the presence of gallium vacancies, V Ga . In highly resistive GaN:Mg neither yellow band with considerable intensity, nor detectable concentration of V Ga was found. We also discuss the puzzling findings in highly resistive bulk GaN : Be of morphological features typical for highly conducting bulk n-GaN material.

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0 % 1)N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1mm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen. # 2001 Elsevier Science B.V. All rights reserved.

We performed a degradation study of high-brightness Nichia single-quantum well AlGaN/InGaN/GaN green light-emitting diodes (LEDs). The devices were subjected to high current electrical stress with current pulse amplitudes between 1 and 7 A and voltages between 10 and 70 V with a pulse length of 100 ns and a 1 kHz repetition rate. The study showed that when the current amplitude was increased above 6 A, a fast (about 1 s) degradation occurred, with a visible discharge between the p- and n-type electrodes. Subsequent failure analysis revealed severe damage to the metal contacts, which lead to the formation of shorts in the surface plane of the diode. For currents smaller than 6 A, a slow degradation was observed as a decrease in optical power and an increase in the reverse current leakage. However, a rapid degradation occurred between 24 and 100 h which was similar to the rapid degradation observed at higher currents. The failure analysis results indicate that the degradation process begins with carbonization of the plastic encapsulation material on the diode surface, which leads to the formation of a conductive path across the LED and subsequently to the destruction of the diode itself.

We report on high-electrical-stress testing of Nichia GaN/InGaN/AlGaN single-quantum-well (SQW) light-emitting diodes. In contrast to our earlier experiments with double-heterostructure LEDs, the present SQW devices have been improved to the point that the encapsulating plastic fails under high electrical stress earlier than the diode itself.

Results of our comprehensive studies of Nichia blue and green SQW LEDs are presented. These include measurements of the emission spectra over a wide range of currents, temperatures, and pressures. The observed anomalous blue shift with temperature and rapid current-induced blue shift at low excitation levels can be explained using a simple model of recombination between Gaussian band-tail states.

We propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16 meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. The pressure shift of the luminescence peaks in blue and green InGaN-based emitters predicted by this model is close to 14 meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.

The interband absorption of strained InGaAs/GaAs multiple quantum wells has been studied at 300 K and at 20 K for pressures up to 6 GPa. Three absorption lines have been attributed to the excitonic transitions hh1-e1, 1h1-e1 and hh2-e2. At 20 K we also measured a photoluminescence line which showed a small Stokes shift with respect to the hh1-e1 transition. The lines were visible until the Γ-X crossover pressures. At 300 K the lines disappeared around 5 GPa while at 20 K this happened around 4.5 GPa. The possible explanation could be that the Γ-X energy separation varies substantially with temperature. The pressure coefficients of the lines are the same at both temperatures and are close to the pressure coefficient of the GaAs band gap.

We have studied photoluminescence in two CdTe / Cd 1− x Mn x Te multiple quantum-well samples with well thicknesses from 9.7 A to 100 A and with barrier compositions x = 0.5 and x = 0.68. The following effects have been observed after hydrostatic pressure up to 3 GPa was applied: (i) decrease of the luminescence intensity from wider wells, which can be related to the stress-induced deterioration of initially strained CdTe layers; (ii) transfer of photoexcited electrons from thinner wells into the barriers where Mn-related recombination occurs; (iii) large negative pressure coefficient (≈ −80 meV/GPa) of the Mn-related photoluminescence; (iv) positive pressure coefficients for CdTe well luminescence, which do not practically depend on the well thickness, while their magnitude decreases with increasing Mn concentration in the barriers (75 meV/GPa for Cd 0.50 Mn 0.50 Te and 60 meV/GPa for Cd 0.32 Mn 0.68 Te).

We studied the pressure shift of direct PL lines in two different GaAs/AlGaAs quantum-well samples in the diamond anvil cell and in the gas cell. We found that up to the Γ-X crossover pressure the shift of the lines agrees with the effective-mass calculation and exhibits small dependence on the well parameters. When the X minima in the barriers become lower than the Γ level in the well, the pressure coefficient of the direct line is lowered. We attribute the Γ-X mixing to two factors: (i) boundary conditions at the interface and (ii) electron-optical phonon interaction. We believe that the first factor is more important than the second because the Γ-X resonant effects are much stronger in quantum wells than in the bulk. Our results imply that the pressure shift of the PL lines in quantum wells should not be fitted with a single linear or quadratic dependence below and above the crossover.

It is demonstrated that the interband absorption in InGaAs/GaAs quantum wells can be used for accurate pressure calibration up to 5 GPa (at room temperature) and up to 4 GPa at 20 K. By depositing the GaAs or InGaAs quantum-well layers on a thin (20–30 μm) GaAs diaphragm the effects of biaxial deformations on the quantum-well spectra can be studied. This might be useful for the optical characterization of the strain fields in Si diaphragms used as piezoresistive low-pressure sensors.

The luminescence of layered GaAs/AlGaAs MQW (multiple quantum wells) grown by molecular beam epitaxy has been studied. Under hydrostatic pressure the narrow excitonic line shifts to higher energies; thus it can be used to calibrate the pressure. The shift of the line is about 11 meV/kbar and does not seem to vary with temperature. Samples with different widths and depths of the wells have been investigated in order to increase the intensity of the line. Measurements are performed at various temperatures (from 12 to 400 K), powers and polarizations of the exciting light. The upper pressure limit of the device is determined by the change from direct to indirect optical transitions. The comparison with ruby fluorescence shows that our MQW sensors are over an order of magnitude more accurate in the 0–40 kbar pressure range.

We report a synthesis of new wide band gap semiconductor – Ga 0.25 Mg 0.25 N 0.5 by means of high-pressure, high-temperature method. Stoichiometry of the monocrystalline samples was determined using an energy dispersive X-ray (EDX) microprobe. We characterized the material obtained by X-ray diffraction revealing, that it crystallizes in the hexagonal structure with unit cell of P3 symmetry and a =3.277A, c =16.065A (almost three time larger than c -lattice constant of GaN). The measurements of the absorption edge showed that the energy gap of this material is substantially reduced in comparison with GaN and is close to 3.0 eV at room temperature. Measurements of thermal expansion coefficient of this material indicates the appearance of large anisotropy.

Blue-violet (2.9 eV) zinc-related photoluminescence was studied in GaN:Zn at pressures up to 5.5 GPa and in the temperature range 80–300 K. Substantial differences were revealed between pressure and temperature behavior of this luminescence. When pressure is applied the luminescence peak shifts toward higher energies with a rate practically identical to that of the energy gap, whereas neither energy nor the shape of the luminescence band changes with temperature. This behavior enables us to eliminate those models of radiative recombination processes which assumed an involvement of conduction band electrons, electrons originating from a hydrogenic donor state, or assumed intracenter recombination within the zinc-related defect complex.

Gallium and aluminum nitrides have several similarities, e.g. almost identical lattice constants and Phillips ionicities. In spite of that, they differ considerably in their structural stability and lattice dynamics. We discuss this problem on the basis of our recent experimental data.

Self-consistent Linear Muffin-Tin Orbital method is used to investigate the structural properties of AlN under high pressure. There is found the phase transition to the rocksalt structure at the pressure 16.6 GPa. We report a direct experimental observation of the phase transition in A1N. Our experiment locates the phase transition between 16 and 17 GPa which agrees perfectly with the above calculations.