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In this work, we study the thermal degradation of In-rich InxGa1–xN quantum wells (QWs) and propose explanation of its origin based on the diffusion of metal vacancies. The structural transformatio...

V-pit defects in InGaN/GaN were studied by numerical simulations of the strain field and X-ray diffraction (XRD) reciprocal space maps. The results were compared with XRD and scanning electron microscopy (SEM) experimental data collected from a series of samples grown by metal–organic vapor phase epitaxy. Analysis of the principal strains and their directions in the vicinity of V-pits explains the pseudomorphic position of the InGaN epilayer peak observed by X-ray diffraction reciprocal space mapping. The top part of the InGaN layer involving V-pits relieves the strain by elastic relaxation. Plastic relaxation by misfit dislocations is not observed. The creation of the V-pits appears to be a sufficient mechanism for strain relaxation in InGaN/GaN epilayers.

The aim of this paper is to give an experimental evidence that point defects (most probably gallium vacancies) induce decomposition of InGaN quantum wells (QWs) at high temperatures. In the experiment performed, we implanted GaN:Si/sapphire substrates with helium ions in order to introduce a high density of point defects. Then, we grew InGaN QWs on such substrates at temperature of 730 °C, what caused elimination of most (but not all) of the implantation-induced point defects expanding the crystal lattice. The InGaN QWs were almost identical to those grown on unimplanted GaN substrates. In the next step of the experiment, we annealed samples grown on unimplanted and implanted GaN at temperatures of 900 °C, 920 °C and 940 °C for half an hour. The samples were examined using Photoluminescence, X-ray Diffraction and Transmission Electron Microscopy. We found out that the decomposition of InGaN QWs started at lower temperatures for the samples grown on the implanted GaN substrates what provides a strong experimental support that point defects play important role in InGaN decomposition at high temperatures.

The drawback of the hydrothermal technique is driven by the fact that it is a time-consuming operation, which greatly impedes its commercial application. To overcome this issue, conventional hydrothermal synthesis can be improved by the implementation of microwaves, which should result in enhanced process kinetics and, at the same time, pure-phase and homogeneous products. In this study, nanometric zinc aluminate (ZnAl2O4) with a spinel structure was obtained by a hydrothermal method using microwave reactor. The average ZnAl2O4 crystallite grain size was calculated from the broadening of XRD lines. In addition, BET analysis was performed to further characterize the as-synthesized particles. The synthesized materials were also subjected to microscopic SEM and TEM observations. Based on the obtained results, we concluded that the grain sizes were in the range of 6–8 nm. The surface areas measured for the samples from the microwave reactor were 215 and 278 m2 g−1.

The aim of this work was to study the effect of the applied chemical reaction stimulation method on the morphology and structural properties of zinc oxide nanoparticles (ZnONPs). Various methods of chemical reaction induction were applied, including microwave, high potential, conventional resistance heater and autoclave-based methods. A novel, high potential-based ZnONPs synthesis method is herein proposed. Structural properties–phase purity, grain size–were examined with XRD methods, the specific surface area was determined using BET techniques and the morphology was examined using SEM. Based on the results, the microwave and autoclave syntheses allowed us to obtain the desired phase within a short period of time. The impulse-induced method is a promising alternative since it offers a non-equilibrium course of the synthesis process in an highly energy-efficient manner.

In this paper the reliability of III-nitride blue laser diodes grown by plasma-assisted molecular beam epitaxy on low threading dislocation density Ammono-GaN substrates is studied. It is found that defects formed in the heavily Mg-doped electron blocking layer (EBL) strongly affect the lifetime of the devices. These defects are identified as basal stacking fault domains which create threading dislocations. The effect of the EBL growth conditions on their generation and influence on lifetime of devices is presented. By optimization of the growth conditions of the EBL the lifetime of III-nitride laser diodes has been increased from 2000 up to 100,000 h.

InGaN/GaN quantum wells grown by Metal Organic Chemical Vapor Phase Epitaxy (MOVPE) were initially studied by optical measurements and X-ray diffraction measurements. The comparison of these two techniques indicated that indium is not distributed homogeneously, which was confirmed by transmission electron microscopy in the nanometer scale. The experimental results of Secondary Ion Mass Spectrometry (SIMS) measurements showed that this analytic method can provide specific information on In spatial distributions not accessible by other methods. SIMS data revealed that In fluctuations occur only in the lower part of 2 nm thick InGaN quantum wells, whereas the QW composition is quite uniform in the upper parts. From the experimental data, one may estimate a SIMS depth resolution of about 0.2 nm and of about 1 μm in lateral directions.

The present paper focuses on the high-pressure metal-organic vapor phase epitaxy (MOVPE) upside-down vertical reactor (where the inlet of cold gases is below a hot susceptor). This study aims to investigate thermo-kinetic phenomena taking place during the GaN (gallium nitride) growth process using trimethylgallium and ammonia at a pressure of above 2 bar. High pressure accelerates the growth process, but it results in poor thickness and quality in the obtained layers, hence, understanding the factors influencing non-uniformity is crucial. The present investigations have been conducted with the aid of ANSYS Fluent finite volume method commercial software. The obtained results confirm the possibility of increasing the growth rate by more than six times through increasing the pressure from 0.5 bar to 2.5 bar. The analysis shows which zones vortexes form in. Special attention should be paid to the transitional flow within the growth zone as well as the viewport. Furthermore, the normal reactor design cannot be used under the considered conditions, even for the lower pressure value of 0.5 bar, due to high turbulences.

Nitride-based light-emitting diodes (LEDs) are well known to suffer from a high built-in electric field in the quantum wells (QWs). In this paper we determined to what extent the electric field is screened by injected current. In our approach we used high pressure to study this evolution. In LEDs with a narrow QW (2.6 nm) we found that even at a high injection current a large portion of built-in field remains. In LEDs with very wide QWs (15 and 25 nm) the electric field is fully screened even at the lowest currents. Furthermore, we examined LEDs with a tunnel junction in two locations – above and below the active region. This allowed us to study the cases of parallel and antiparallel fields in the well and in the barriers.

An analysis of the main recombination modes in nitrides, based on new method of data treatment is proposed for the determination of the carrier recombination processes in optically excited matter measured by time-resolved photoluminescence (PL). The analysis includes basic recombination modes: nonradiative Shockley-Read-Hall (SRH), radiative and Auger recombination in relation to monomolecular, bi-molecular, and tri-molecular processes of optical relaxation. The method is based on the introduction of instantaneous PL decay rate r P L plotted as a function of the PL intensity or of the time. Such an approach provides deep insight into the time evolution of the recombination of the optically excited semiconductor systems and can be applied to the time evolution of a variety of optically excited systems. The demonstration of its strength is given by the application to III-nitride based systems, including nitride highly doped and semi-insulating thick layers, polar and non-polar multi-quantum wells (MQWs). At low temperatures (5 K), the mono- and bi-molecular processes determine the carrier relaxation, and the tri-molecular Auger recombination contribution is negligible. At room temperature the data indicate an important contribution of Auger processes. It is also shown that asymptotic (low excitation), one-exponential recombination rate has different character depending on the presence of the electric fields across the structure.

The hydrothermal method of obtaining nano zinc oxide doped with different contents of aluminum ions (III) was presented and discussed in this paper. Aqueous solution of Zn(NO3)2*6H2O and Al(NO3)3*9H2O salts mixture were used as the synthesis precursor. In order to reduce the process time all reactions were performed in a microwave reactor. The influence of process parameters and the content of impurity ions on the properties of synthesized nano zinc oxide were analyzed. In addition to zinc oxide doped with Al(III) ions, an additional spinel phase (ZnAl2O4) was obtained. The luminescent properties of nano zinc oxide as a function of the dopant ions were also discussed. Based on the luminescence measurements results, it was found that the luminescence intensity decreases with the increasing dopant content. The obtained materials are aimed to be implemented as luminescent materials in optoelectronic and sensors.

The formation and diffusion of point defects have a detrimental impact on the functionality of devices in which a high quality AlN/GaN heterointerface is required. The present paper demonstrated the heights of the migration energy barriers of native point defects throughout the AlN/GaN heterointerface, as well as the corresponding profiles of energy bands calculated by means of density functional theory. Both neutral and charged nitrogen, gallium, and aluminium vacancies were studied, as well as their complexes with a substitutional III-group element. Three diffusion mechanisms, that is, the vacancy mediated, direct interstitial, and indirect ones, in bulk AlN and GaN crystals, as well at the AlN/GaN heterointerface, were taken into account. We showed that metal vacancies migrated across the AlN/GaN interface, overcoming a lower potential barrier than that of the nitrogen vacancy. Additionally, we demonstrated the effect of the inversion of the electric field in the presence of charged point defects VGa3− and VAl3− at the AlN/GaN heterointerface, not reported so far. Our findings contributed to the issues of structure design, quality control, and improvement of the interfacial abruptness of the AlN/GaN heterostructures.

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.

Detailed comparison of optical quality of GaN layers grown homoepitaxially on bulk Ga-polar and N-polar substrates by plasma-assisted molecular beam epitaxy was performed. One order of magnitude lower photoluminescence (PL) intensity and decay time at room temperature was observed for layers grown on N-polar substrates, realized using gallium-rich conditions. This nonradiative recombination channel was very efficient also at liquid helium temperatures, what was evidenced by no improvement in PL time decay. Optical quality of GaN layer (PL intensity, lifetimes) grown on N-polar substrates was greatly improved by using nitrogen-rich growth conditions. We attribute this improvement to suppressed formation of nitrogen vacancy-related point defects. Finally, linewidth below 1 meV and lifetime above 0.1 ns (DX line, He temperature) were obtained for layers grown on N-polar substrate.

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.

The distance between the showerhead and the sample surface (GAP) is one of the main growth parameters of the commonly used research reactor, Close Coupled Showerhead. We examine its influence on the growth rate of GaN layers deposited under various conditions (growth temperature, carrier gas, V/III ratio and growth pressure). Regardless of other growth parameters, increasing the GAP value leads to a reduction in the growth rate.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 &micro, m and their height was 4 and 1 &micro, m. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 &micro, m high stripes and less pronounced for the shallower 1 &micro, m high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Diffusion of native point defects is examined in the interface region of the InN/GaN heterostructure grown in hexagonal c direction. Heights of migration energy barriers of single vacancies and their complexes with substitutional group-III element are calculated in bulk GaN and InN crystals for two diffusion mechanisms, as well as across the InN/GaN heterointerface. An impact of the in-plane strain, the influence of built-in electric field, as well as the presence of electric dipole on vacancy motion is analysed. The obtained results show that metal vacancies V Ga and V In can migrate across the InN/GaN interface overcoming a potential energy barrier lower than that of V N . Moreover, the smallest energy barrier is related to the in-plane migration of V In what can favor spatial segregation of In atoms near the InN/GaN interface. This phenomenon can significantly contribute to the thermal degradation process of InGaN/GaN QWs, often observed experimentally both during the operation of nitride devices, as well as at the stage of their growth.

Advanced Substrates consist of a thin GaN layer bonded to a carrier wafer. The layer is separated from starting material by Smart Cut™ technology. GaN on sapphire Advanced Substrates were successfully used as seeds for HVPE-GaN growth. Unintentionally doped and silicon-doped thick GaN layers were crystallized. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates. Free-standing HVPE-GaN with high free carrier concentration was obtained. A laser diode was built on the n-type doped HVPE-GaN grown on an Advanced Substrate.

Relaxed InGaN layers have recently gained a lot of interest as potential pseudo-substrates for InGaN-based devices. In this work we study PAMBE grown thick In0.2Ga0.8N layers relaxed through the formation of (a+c)-type misfit dislocations. We show that either Ga/N flux ratio (ΦGa/ΦN) below 0.6 or high nitrogen flux (ΦN) equal to 1.4 μm h−1 results in basal stacking fault (BSF) formation. Structural analysis has shown that stacking faults are I1-type in I3 and I4 configurations. We discuss the interactions between (a+c)-type dislocations and BSFs which depend on stacking fault configuration. Probably due to the lack of defect character of BSFs in I3 configuration, dislocations can cross them. Whereas BSFs in I4 configuration stop the dislocations a few nanometers above them, possibly due to the repulsive interactions between the descending dislocation and the Shockley partial dislocations surrounding the BSFs.

Studies of internal electric fields in GaN/AlxGa1-xN (3 nm/4 nm) multi-quantum-wells (MQWs) with x = 0.25, 0.5, and 1 are presented. The structures were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy. Optical properties of these structures were investigated by ambient and high-pressure photoluminescence (PL) measurements. They were strongly affected by polarization-induced electric fields giving rise to a quantum confined Stark effect, depending on the composition of the AlGaN barrier. The optical emission energy redshifts by > 100 meV when the Al content in the barrier increases from x = 0.25 up to x = 1, while the pressure coefficients of the PL energy are significantly reduced in comparison with bulk GaN. The transition energies and their pressure dependencies are modelled 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. Additionally, influence of blurring of well-barrier interface on transition energy has been modelled. A good agreement between experimental results and theoretical analysis 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, and that these effects are well described by ab initio calculation procedures. Our results reflect the role of composition and geometry on the basic optical properties of nitride quantum wells.

International audience; The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

In this paper, we study the plastic relaxation of InGaN layers deposited on (0001) GaN bulk substrates and (0001) GaN/sapphire templates by molecular beam epitaxy. We demonstrate that the InGaN layers relax by the formation of (a+c)-type misfit dislocations gliding on pyramidal planes in the slip system ⟨11 2 ¯3⟩{11 2 ¯2} down to the interface where they form a trigonal dislocation network. Combining diffraction contrast and large angle convergent beam electron diffraction analyses performed using a transmission electron microscope, we reveal that all (a+c)-type dislocations belonging to one subset of the network exhibit Burgers vectors with the same c-component. This relaxation mechanism leads to partially relaxed InGaN layers with smooth surfaces and threading dislocation densities below 109 cm−2. Such layers are of potential interest as pseudo-substrates for the growth of InGaN heterostructures.

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.

In the case of nearly perfect semiconductor layers (as AlGaAs on GaAs), the experimental data of X-ray Diffraction are well simulated using the Dynamical Theory of X-ray Diffraction. In the case of AlGaN and InGaN layers, intensity of their XRD peaks depend very strongly on the crystallographic quality of not only the layers, but also the substrates. In the paper, we show differences in XRD peak intensities for the AlGaN layers on GaN of the same thickness and Al-content, but of different dislocation density.

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.

In this work, the electrical and structural properties of Ti/Al contacts on AlGaN/GaN heterostructures with a different crystalline quality were investigated. In particular, the sample with a lower defects density required a higher temperature (800 °C) to obtain Ohmic contacts, while the more defective sample showed Ohmic behaviour at lower annealing temperature (500 °C). The susceptibility to oxidation of the metal stack upon annealing has been monitored by chemical analyses. Moreover, the temperature dependence of the contact resistance of the Ti/Al annealed contacts revealed that the interface current transport mechanism depends on the material quality. These results were attributed to the presence of V-shaped defect close to the surface of the AlGaN barrier layer. A nanoscale electrical analysis demonstrated a preferential current conduction through these defects, which indeed plays a relevant role for the formation of the Ohmic contact.

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.

The XPS technique combined with low energy Ar + ion sputter depth profiling was used to obtain information about chemical composition and reproducibility of the growth procedure by the MOVPE method of the InGaN multi quantum wells (MQWs). A good in-depth resolution of the XPS depth profiling technique, allows observation of InGaN quantum wells with thickness down to 2 nm. However quantitative characterization of very narrow QWs is limited by the elemental detection range of the XPS analysis. Both the InGaN QWs and GaN barriers parameters were well characterized using the model MQWs samples with various QW thickness. Inter-diffusion of indium within the GaN barrier layers, induced by high-temperature annealing, has been detected. This work evidences the successful application of the XPS depth profiling analysis as a very useful tool to optimize the growth parameters and thermal stability of InGaN/GaN MQWs.

In this work we investigate InGaN/GaN short period superlattices (SPSLs) grown on (0001) GaN and on relaxed In0.2Ga0.8N buffers with varying degree of plastic relaxation. The SPSLs were fabricated by plasma assisted molecular beam epitaxy (PAMBE). Despite the nominal SPSL structure consisted of InN monolayers (MLs) embedded in GaN barriers, we observed single monolayer InGaN layers instead of InN. We study the photoluminescence (PL) of such SPSLs as a function of the substrates lattice constant. Increase of the substrate lattice constant from a = 3.183 A (for GaN grown on sapphire) to a = 3.216 A resulted in a peak emission shift from 379 nm to 419 nm. We attribute the PL red-shift to the increased In incorporation in the ML-thick InGaN on relaxed buffers as a consequence of the reduced lattice mismatch between the substrate and the film. Our experiment confirms that coherent growth of InN is prohibited on GaN and points out the important role of substrate lattice constant engineering for realization of InN/GaN SPSLs.

The influence of waveguide design on performance of nitride based laser diodes (LDs) grown by plasma assisted molecular beam epitaxy is studied. A large improvement in threshold current density and slope efficiency of LDs is observed when an InGaN interlayer with 8% In content is introduced between multi-quantum-well region and electron blocking layer. This dependence is attributed to reduction of internal losses due to lower optical mode overlap with highly absorptive Mg-doped layers. This led to demonstration of blue LD operating at λ =450 nm with high optical power of 500 mW per facet.

In the reported work, the combination of severe plastic deformation (SPD) and annealing under high hydrostatic pressure was used to enhance the ductility whilst maintaining the high mechanical strength possessed by the nanostructured 316LVM stainless steel. The nanostructure was obtained by a multi-step hydrostatic extrusion process to a total true strain of 1.8. This process produced a microstructure consisting of nanotwins and shear bands. The extruded samples were annealed at 700 and 900 °C for 10 min under atmospheric or hydrostatic pressures of 2 or 6 GPa. The resulting microstructures were examined using TEM and FIB techniques. The microstructural observations and X-ray measurements were used to estimate the crystallite sizes. The mechanical properties were determined by microhardness and tensile tests. It was established that annealing under high pressure improved the ductility of the material whilst retaining its high ultimate tensile strength.

The first part of the paper shows the advantages and disadvantages of using GaN and SiC substrates with an off-cut up to 2 degrees. The following experimental observations were made: i) higher efficiency in p-doping with magnesium, ii) higher critical conditions for AlGaN cracking, iii) trigonal deformation of AlGaN and InGaN unit cells. A successful growth of AlGaN/GaN HEMT epi structure on 2 degree-off SiC substrates will be shown, what paves the way to a monolithic integration of GaN-based and SiC based devices on a common SiC substrate. In the second part of the paper, we show the experimental results on AlGaN/GaN degradation upon prolonged annealing at 600oC. In the case of epi structures on Si and sapphire, the parasitic parallel conductivity appeared, whereas in the case of SiC substrates, annealing introduced no distinct changes.

Advanced Substrates consist of a 200-nm-thick GaN layer bonded to a handler wafer. The thin layer is separated from source material by Smart CutTM technology. GaN on Sapphire Advanced Substrates were used as seeds in HVPE-GaN growth. Unintentionally doped and silicon-doped GaN layers were crystallized. Free-standing HVPE-GaN was characterized by X-ray diffraction, defect selective etching, photo-etching, Hall method, Raman spectroscopy, and secondary ion mass spectrometry. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates.

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape o...

Three-component luminescent material consisting of silica xerogel as a support with immobilized ZnS:Mn 2+ nanocrystals and Tb 3+ ions was compared with such two-component materials as the silica support with ZnS:Mn 2+ as well as the support with Tb 3+ . In each case the nanocrystals and the lanthanide ions were immobilized at silica surface by impregnation procedure. Size of the ZnS quantum dots doped with Mn 2+ were estimated by Scherrer method from the X-ray diffraction (XRD) pattern. The materials have been characterized by EPR and optical spectroscopy techniques. EPR spectra allow to distinguish two different Mn 2+ sites: the first is assigned to isolated Mn 2+ substitutionally and incorporated into cubic ZnS lattice and the second is ascribed to the Mn 2+ situated near the nanocrystal surface. From the optical spectra we have found that in the three-component material, energy transfer from excited ZnS:Mn 2+ nanocrystals to Tb 3+ ions takes place. The different mechanisms of such transfer are discussed.

ZnO nanopowders doped with Mn2+, Ni2+, Co2+ and Cr3+ ions have been synthesised for the first time using a solvothermal reaction with microwave heating. The nanopowders were produced from a solution of zinc acetate and manganese (II), chromium (III), nickel (II) and cobalt (II) acetates, using ethylene glycol as a solvent. The content of Ni2+, Co2+ and Cr3+ ions in the solution and in the solid phase were close to each other up to 5 mol%. The doping level of Mn2+ ions in the solid is about 50% of that in the solution. No phases or compounds other than ZnO were detected by X-ray diffraction with Mn2+, Co2+ and Ni2+ doping. With Cr3+ ions a small amount of chromium oxide was found. None of the powders displayed any luminescence after doping. The Mn2+-doped powder displayed a paramagnetic behaviour. ESR and magnetisation investigations have revealed that no clustering of Mn2+ ions occurred up to a doping level of 3.9 mol%. The average grain size of powders doped with Ni2+, Cr3+, Co2+ and Mn2+ for a 10 mol% ion content in the solution was about 20 nm and the grain size dispersion 30%. With increasing dopant content the grain size decreased. It appears that the solvothermal process employed allows relatively high doping levels of the transition metal ions to be achieved without any dopant clustering or oxide precipitation.

The ammonothermal technique was used to obtain GaN powders doped with various transition metal ions—Mn, Fe, Cr. The procedure of ammonothermal growth and morphology, structural, magnetic and optical investigations of magnetic GaN is described. The largest solubility limit ( x 2+ centers are responsible for the paramagnetic behavior of GaMnN. X-ray diffraction study of GaMnN revealed presence of different Mn x N y compounds, in particular ferromagnetic Mn 4 N which may be the source of additional ferromagnetic contribution. The results show substantial difficulties to obtain hole induced ferromagnetism in GaMnN. Mn reduces the luminescence efficiency of ammonothermal GaN. In the case of Fe doping we are dealing with a spectrum of different Fe centers. Ammonothermal GaN:Cr is ferromagnetic but its origin is unrecognized yet.

In this review work we discuss applicability of Bragg scattering to examination of nanocrystals. We approximate the structure of nanograins by a commonly accepted core-shell model. We show that, for principal reasons, the Bragg equation is not applicable directly to nanocrystals. We use the Bragg relation through application of the apparent lattice parameter (alp) concept which we use to evaluate quantitatively the core-shell model. We also introduce a new parameter of the structure, Equivalent Cubic Lattice Parameter (EClp), which quantifies deviation of the real (trigonal) lattice from its parent fcc structure due to the lattice deformation (e.g. by the stacking faults). We show examples of an analysis of experimental X-ray and neutron diffraction data based on the alp methodology and on the theoretical patterns calculated for various core-shell models.