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Polarization doping in a GaN-InN system with a graded composition layer was studied using ab initio simulations. The electric charge volume density in the graded concentration part was determined by spatial potential dependence. The emerging graded polarization charge was determined to show that it could be obtained from a polarization difference and the concentration slope. It was shown that the GaN-InN polarization difference is changed by piezoelectric effects. The polarization difference is in agreement with the earlier obtained data despite the relatively narrow bandgap for the simulated system. The hole generation may be applied in the design of blue and green laser and light-emitting diodes.

In this work, we study the emergence of polarization doping in AlxGa1-xN layers with graded composition from a theoretical viewpoint. We demonstrate that the charge conservation law applies for fixed and mobile charges separately, leading to nonlocal compensation phenomena involving bulk fixed and mobile charge and polarization sheet charge at the heterointerfaces. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities., Comment: 16 pages, 4 figures

Step pattern stability of the vicinal surfaces during growth was analyzed using various surface kinetic models. It was shown that standard analysis of the vicinal surfaces provides no indication on the possible step coalescence and therefore could not be used to elucidate macrostep creation during growth. A scenario of the instability, leading go macrostep creation was based on the dynamics of the step train. The critical is step motion at the rear of the train which leads to double and multiple step creation. The condition is that the step density ratio in and out of the train lower than 2 prevents double step formation irrespective of the kinetics. For higher step density ratio low density of the step promotes single step stability. Fast step kinetics from lower terrace stabilizes the single steps slow (high barrier) is promoting step coalescence. The incorporation kinetics from upper terrace role is close to neutral. The creation of double step creates slow the step in front to accelerate and catch the previous double step while those behind catch up the double step creating multistep structure. The multistep are not mobile as the alimentation leads to emission of single step which moves forward. The final structure consist of macrosteps and superterraces with the number of single steps moving forward. Thus the single step motion is essential crystal growth mode despite the presence of the macrosteps. The macrostep are prone to creation of the overhangs which results from surface dynamics coupling to impingement from the mother phase. The angular preferential access of the bulk material to the macrostep edge, leads to diffusive instability. Therefore it is expected that harmful influence of the macrosteps by creation of inclusions and dislocation is stronger during growth from the liquid phase., Comment: 6 figures, 24 pages

In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference, hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

Molecular nitrogen and molecular oxygen adsorption at the Al-terminated AlN(0001) surface was investigated using ab initio simulations. It was shown that both species undergo barrierless dissociation during attachment to the surface. The H3 adsorption site was identified as the most favorable for both O and N adatoms. The adsorption energies for O2 and N2 were found to be 14.8 and 6.0 eV, respectively. At the clean surface, separate N and O adatoms have their energies 4.37 eV lower than the NO admolecule. At normal nitrogen pressures, the AlN(0001) surface is fully covered by nitrogen adatoms. It was shown for this coverage that adsorption of oxygen leads to creation of the NO admolecule which is a necessary step for nitric monoxide synthesis. The energetics of NO molecules is related to electronic charge transfer: from the two possible configurations, it is shown that T4 and H3 are donors and acceptors, respectively. The resulting coverage is the mixture of both configurations, controlled by electron cha...

The dissolution of molecular nitrogen in Ga and Fe was investigated by ab initio calculations and some complementary experiments. It was found that the N bonding inside these solvents is fundamentally different. For Ga, it is between Ga4s and Ga4p and N2p states whereas for Fe this is by N2p to Fe4s, Fe4p and Fe3d states. Accordingly, the energy of dissolution of N2 for arbitrarily chosen starting atomic configurations was 0.535 eV/mol and −0.299 eV/mol for Ga and Fe, respectively. For configurations optimized with molecular dynamics, the difference between the corresponding energy values, 1.107 eV/mol and 0.003 eV/mol, was similarly large. Full thermodynamic analysis of chemical potential was made employing entropy-derived terms in a Debye picture. The entropy-dependent terms were obtained via a normal conditions path to avoid singularity of ideal gas entropy at zero K. Nitrogen solubility as a function of temperature and N2 pressure was evaluated, being much higher for Fe than for Ga. For T=1800 K and p=104 bar, the N concentration in Ga was 3×10−3 at. fr. whereas for Fe, it was 9×10−2 at. fr. in very good agreement with experimental data. It indicates that liquid Fe could be a prospective solvent for GaN crystallization from metallic solutions.

The $p\text{\ensuremath{-}}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{\ensuremath{-}}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.

Adsorption of molecular nitrogen and molecular hydrogen at an Al-terminated AlN(0001) surface was investigated using ab initio simulation methods. It was shown that both species undergo dissociatio...

Adsorption of atomic and molecular nitrogen at AlN(000–1) surface was investigated by ab initio calculations and thermodynamic analysis. According to earlier works{Kempisty et al. Appl. Surf. Sci. 2020, 532, 147,719} in equilibrium with Al vapor, the AlN(000–1) surface is thermodynamically stable in two states: low Al coverage θAl ≤ 1/3 ML and high Al coverageθAl ≅ 1 ML. In these two cases nitrogen adsorption is completely different. At low Al-covered surface the nitrogen atom is strongly bounded to N surface atom, creates the N2 admolecule that is finally detached leaving surface vacancy VN(s). This reaction chain energy gain is positive, Δ E D F T d e t ( N − N 2 ) = 3.50 e V . Therefore, the atomic nitrogen present in plasma assisted molecular beam epitaxy (PA-MBE) fluxes induces the surface decay. N2 is adsorbed molecularly at the bare surface with the coverage independent energy gain about 1 eV. At the fully Al-covered surface atomic nitrogen is adsorbed in T4 sites with no barrier and large energy gain Δ E D F T a d s − A l ( N ) = 8.68 e V . Molecular nitrogen dissociates with the energy gain, dependent on additional N coverage: Δ E D F T a d s − A l ( N 2 ) = 7.65 e V at low and Δ E D F T a d s − A l ( N 2 ) = 2.77 e V at high, respectively. This change is related to the reduction of electron transfer contribution, caused by Fermi level shift down due to electron transfer from Al to N surface states. The thermodynamic analysis shows incomplete N coverage above the adsorbed Al layer due to the above adsorption energy reduction effect. The resulting incomplete N coverage is responsible for creation of nitrogen vacancies during AlN physical vapor transport (PVT) growth and their coalescence into Al-rich inclusions.

Physical review letters 124(8), 086101 (2020). doi:10.1103/PhysRevLett.124.086101, The equilibrium atomic interface structure between Ga and GaN(0001) is shown to contain substrate surface vacancies followed by substrate-induced layering and preferential lateral ordering in the liquid. The uncovered presence of point defects, in the form of vacancies at both sides of the solid-liquid interface, is an important structural feature which governs the local physical properties. Our x-ray diffraction study reveals that the layering is very stable and persists up to a temperature of 1123 K and a nitrogen pressure of 32 bar. The Ga layer spacing agrees remarkably well with the Friedel oscillation period for this system., Published by APS, College Park, Md.

Results of growth of high structural quality gallium nitride single crystals doped with silicon are described in this paper. Dichlorosilane was used as precursor of silicon in the hydride vapor phase epitaxy method. Crystallization runs with different flows of dichlorosilane were performed and compared. One-inch free-standing HVPE-GaN crystals of high structural quality and high purity, previously grown on ammonothermal GaN substrates, were used as seeds. Structural, electrical, and optical properties of HVPE-GaN doped with silicon are presented and discussed in detail. A laser diode built on the homoepitaxially grown GaN is demonstrated.

Study on the sources of stress in HVPE-GaN layer crystallized on 1-in. ammonothermally grown GaN seed is presented in this paper. Characterization by means of X-ray diffraction and transmission electron microscopy is performed. HVPE-GaN samples of high quality and those with visible quality deterioration are investigated on c-plane and m-plane cross-sections. Special attention is paid to HVPE material growing in semi-polar and non-polar directions on the edges of the seed and the growing layer. It is shown that this material generates significant stress leading to a structural deterioration of HVPE-GaN growing in the c-direction.

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration ( N O =(2–4)×1019 cm −3 ) than that in the bulk matrix ( N O −3 ). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients D O ( T , P ). The obtained values of D O ( T , P ) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.

We studied the incorporation of Mg-related impurity centers in GaN crystals depending on the direction of the crystallization front. Two series of GaN crystals – (i) undoped and (ii) Mg-doped – were grown by High Nitrogen Pressure Solution (HNPS) method under otherwise identical conditions. Each series contained four samples with ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) orientations. The low-temperature photoluminescence (PL) spectroscopy was used for characterization of the obtained crystals. The observed differences in the PL spectra of GaN:Mg crystals suggested that Mg incorporation in GaN grown by HNPS method depends considerably on the orientation of crystallization front. The concentration of Mg impurity incorporated into the GaN crystals subsequently increases for the following sequence of planes: ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) . For ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) and ( 20 2 ¯ 1 ¯ ) planes the blue band is related only to ON – MgGa donor-acceptor pair (DAP) transitions, while for ( 20 2 ¯ 1 ) plane the incorporation of Mg-H complexes occurs additionally to the formation ON – MgGa DAP.

Gallium nitride (GaN) is considered to be one of the most important semiconductors nowadays. In this report a solution of the long standing puzzle regarding GaN decomposition and melting under high pressure and high temperature is presented. This includes the discussion of results obtained so far. The possibility of a consistent parameterisation of pressure (P) evolution of the melting temperature (Tm) in basic semiconductors (GaN, germanium, silicon…), independently from signs of d T m / d P is also presented.

We use structural and analytical transmission electron microscopy to study the length scale on which InGaN alloy fluctuations influence the optical properties of InGaN quantum wells. We show that compositional and layer thickness fluctuations can be present in InGaN thin layers, depending on the growth conditions and In concentrations. Spectrally and spatially resolved cathodoluminescence performed in the transmission electron microscope indicates that the optical properties of InGaN quantum wells depend on short range order on an atomic scale rather than on medium-range clustering due to lateral segregation.

It is demonstrated in this paper that 1.9-mm-thick gallium nitride grown by Hydride Vapor Phase Epitaxy (HVPE) on an ammonothermally grown GaN seed can reproduce the structural, in terms of defects, properties of the seed. The etch pit density and its correlation to the threading dislocation density in the ammonothermal GaN substrate and the HVPE-GaN layer is presented and analyzed. However, it has recently been observed that for HVPE-GaN thicker than 2 mm some additional defects are formed in the new grown material. Therefore, three HVPE growth runs were performed in the same experimental conditions, using three structurally identical ammonothermally grown GaN seeds of different thicknesses. The influence of the thickness of the seeds on the crystallization process and the properties of the HVPE-GaN layers is shown and discussed.

One of the main advantages of the hydride vapor phase epitaxy (HVPE) method for crystallizing bulk gallium nitride is the crystallization of GaN with a relatively high growth rate (>100 μm/h). In this paper various growth rates in the c-direction during crystallization of GaN by HVPE on ammonothermally grown GaN crystals are determined and examined. The influence of the highest (380 μm/h) and the lowest (40 μm/h) growth rate on the structural quality and purity of the HVPE-GaN crystals is analyzed. The optimal macroscopically stable growth rate (without cracks and pits) and the way of achieving it are presented and discussed.

The influence of periodic changes of carrier gas composition (H 2 and N 2 +10%H 2 ) on structural and physical properties of GaN crystals grown by Hydride Vapor Phase Epitaxy (HVPE) was studied using photo-etching, defect-selective etching (DSE), secondary ions mass spectroscopy (SIMS), low temperature photo-luminescence (LTPL) and Raman spectroscopy. The studies were performed on (1–100) cleavage planes of thick crystals grown with short (a few minutes) and long (one hour) periods of the carrier gas exchange. Markedly large differences between the level of impurities (Si, C, O 2 , H 2 ) were measured by SIMS in the GaN grown in [0001] direction and inside the V-shape defects, that well correlates with corresponding different carrier concentration levels revealed by Raman and photo-etching. Small differences in carrier concentration across the striations in the [0001] direction were clearly revealed by photo-etching, while PL and Raman appeared to be not sensitive to them.

The results of the Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal GaN seeds (Am-GaN) with various misorientations: 0.3, 0.5 and 1 degree to the [1 0 −1 0] and [1 1 −2 0] directions are presented. Growth rate and structural quality of the HVPE-GaN layers are analyzed. Morphology of the crystal growing surface at the beginning of the crystallization process (after one and two hours of growth) and at the end of it (after eight hours) is presented as a function of the Am-GaN seed misorientation. Based on these results a rough growth model is proposed and discussed in detail.

In this work homoepitaxial HVPE-GaN growth on non-polar and semi-polar GaN seeds was described. Two crystallization processes, in the same experimental conditions but using different carrier gases: N2 and H2, were performed. An influence of growth directions and growth conditions on the growth rate and properties (morphology, structural quality and oxygen and silicon contaminations) of obtained crystals were investigated and discussed. It was shown that the growth rate strongly depends on the growth direction and the carrier gas. It was demonstrated that for the semi-polar [20–21] direction it was possible to obtain high quality and highly conductive (without intentional doping) gallium nitride layers.

Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

Crystallization of GaN by the HVPE technique on an MOCVD-GaN/sapphire template with photo-lithographically patterned Ti mask and ammonothermally-grown GaN crystals (Am-GaN) was studied and compared. Structural and optical properties of the obtained free-standing HVPE-GaN revealed that Am-GaN seeds produced material of much higher quality in terms of etch pit density (EPD), X-ray rocking curves, and excitonic emission than the sapphire-based templates. The crystallization run on the Am-GaN seed, annealed before the growth in H2+NH3 atmosphere, resulted in the HVPE-GaN of an average EPD of 5×104 cm−2, (002) rocking curve width of 22″, and photo-luminescence as narrow as 130 µeV. However, small change in quality was observed when the Am-GaN seed was annealed in N2+NH3 atmosphere prior to growth. In turn, the Hall effect measurements showed that the HVPE-GaN grown on MOCVD-GaN/sapphire template possessed lower free carrier concentration and higher mobility than the HVPE-GaN grown on the Am-GaN seeds. These results were associated to the higher growth rate of the examined HVPE-GaN sample.

We present nitride based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy. In this work we investigate influence of epitaxial structure on LDs pa-rameters. In particular we compare LDs with and without AlGaN cladding layers operating at 430 to 460 nm. We show results of waveguide simulations which explain the increasing threshold current density of longer wavelength LDs. The PAMBE LD with InGaN waveguides and AlGaN bottom cladding has threshold current density of 4.3 kA/cm2 and slope efficiency of 0.56 W/A. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An influence of doping with iron and manganese on GaN crystallized by halide vapor phase epitaxy is described in this work. This is a continuation of previous work where solid iron was used as a metal precursor and semi-insulating GaN co-doped with manganese and iron was crystallized. In this paper, metals of the highest purity were used. The acceptors (iron and manganese) were incorporated separately in order to compensate unintentional donors present in undoped GaN crystals. Native GaN wafers were used as seeds. The crystallized doped samples were examined in terms of their structural and electrical properties. Concentrations of dopants and unwanted impurities were determined. Iron-doped and Mn-doped GaN, with resistivity at room temperature exceeding 107 Ωcm, was presented and compared. It was shown that, at room temperature, for the same acceptor concentration, GaN:Mn crystals present better structural quality and higher resistivity than GaN:Fe.

During crystallization of GaN in the c-direction by halide vapor phase epitaxy the formation of semi-polar facets as well as growth in semi-polar directions are observed. Crystallization process on these semi-polar facets is described in this paper in detail. Ammonothermal GaN crystals were used as seeds. Morphology, structural quality, growth rate, impurities concentration and free carrier concentration are analyzed and compared to data obtained for crystals grown in the c-direction.

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.

Gallium nitride layers were deposited on high-pressure-grown GaN substrates containing planar defects. The layer growth above the defects was disturbed and the layer contained terraces. The terraced region was investigated by the molten-bases defect-selective etching (DSE), atomic force microscopy (AFM), and transmission electron microscopy (TEM) methods. The AFM measurement showed significant differences in atomic steps densities in different parts of the layer. The TEM investigation revealed the presence of stacking faults and the DSE showed that the substrate planar defect influences the layer growth. The schema of the defect–layer interaction has been proposed.

GaN bulk crystals grown by Hydrate Vapor Phase Epitaxy (HVPE) contain regions with non-homogenous electrical properties. Kelvin Probe Force Microscopy (KPFM) was used for revealing and analysis of these defects in thick GaN layers grown by this method on top of GaN on sapphire templates. Such layers initially grow in the form of separate pyramids, which are later overgrown, creating microscopically flat crystallization front. Cross-sectional KPFM images, made just above the template surface, revealed a series of inverted dome-like features of significantly lower potential, indicating regions of high electron concentration. Inside the thick HVPE-grown layer the changes of surface potential are much smaller and indicate the existence of minor fluctuation in carrier concentration during bulk growth of GaN. Subsequent photo-etching, sensitive to carrier concentration, and measurements of etch depth supported this findings. Both KPFM and photo-etching confirmed the known preferential incorporation of impurities at sides of the overgrown pits (pinholes) during initial phase of HVPE growth. During subsequent HVPE growth the changes of the surface potential and of the etch depth are small and may be related to non-uniform incorporation of impurities due to rotation of the growing sample.

The growth and physical properties of GaN crystallized in a multi feed-seed (MFS) configuration by High Nitrogen Pressure Solution (HNPS) growth method are presented in detail. The conversion of free standing HVPE-GaN crystals to free standing HNPS-GaN is the basis of the MFS configuration. The influence of the experimental conditions (i.e. growth temperature, temperature gradient, etc.), the c-plane bowing of the initial substrate, the electrical properties of HNPS-GaN, and the rate and mode of growth from solution are analyzed. We show that the HNPS-GaN crystals have better structural quality than their HVPE-GaN seeds. The defect density decreases with increasing growth temperature, reaching 5×10 5 cm −2 for crystals grown at 1420 °C or higher. In contrast, the free carrier concentration in HNPS-GaN increases with increasing growth temperature, reaching 7×10 19 cm −3 for samples crystallized at 1440 °C. Thus the possibility to obtain good quality plasmonic GaN substrates for laser diodes can be realized.

Crystallization of GaN by High Nitrogen Pressure Solution method in multi-feed–seed (MFS) configuration without intentional doping [1] results in: (1) Growth of strongly n-type crystals with free electron concentration increasing with growth temperature in ranges of 2–6×10 19 cm −3 and 1350–1430 °C, (2) stable growth on Ga-polar surface and unstable growth on N-polar surface, crystals slightly brown, (3) improvement of (0001) crystallographic planes curvature (flattening) with respect to bowing of these planes in the seed crystals. The addition of magnesium into the growth solution causes strong compensation of free electrons in the crystals. Therefore, highly resistive GaN crystals can be grown. In this work, the crystallization of Mg doped GaN on flat ∼1 in. seeds (substrates) grown by HVPE in MFS configuration has been studied. It is shown that: (1) Highly resistive GaN:Mg crystals with resistivity higher than 10 7 Ω cm were grown, (2) the growth is stable on N-polar surfaces of the seeds whereas it is unstable on the Ga-polar surfaces, which is opposite to the HNPS growth of the n-type crystals. The GaN:Mg crystals are fully transparent with no visible color, (3) shape of (0001) crystallographic planes improves (flattens) with respect to bowing of these planes in the seed crystals (HVPE substrates).

The results of High Nitrogen Pressure Solution (HNPS) growth of GaN in multi feed-seed (MFS) configuration are presented. The idea of this configuration is based on the conversion of the free standing HVPE-GaN crystals (seeds) to the free standing HNPS-GaN of much higher quality than the seeds. A new arrangement of the seeds in the MFS configuration is shown. The influence of the experimental conditions (i.e. growth temperature, temperature gradient) on the growth rate, growth mode and electrical properties of HNPS-GaN is presented. The regrowth procedure is introduced in order to increase the lattice bowing radii of the HNPS-GaN crystals and finally obtain crystalographically flat (bowing radius of 30 meters) pressure grown GaN substrates (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied pressure dependence of photoluminescence in InGaN/GaN quantum wells (QWs). Universal dependence of the pressure coefficients dEPL/dp on the emission energy, at ambient pressure. EPL, in polar InGaN/GaN QWs has been found. This dependence is fulfilled for structures having different QWs, different composition of InGaN, and growth method. Calculations performed using k · p method combined with theory of elasticity and piezoelectricity support the experimental finding when an assumption on strain dependence of piezoelectric coefficients in nitride semiconductors is applied. We have also performed measurements of semipolar () InGaN/GaN QWs with different In-content. The same univocal dependence between EPL and dEPL/dp realized for polar structures has been also found in the studied semipolar QWs.

We discuss problem of Rashba field in bulk GaN and in GaN/Al x Ga 1− x N two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin–orbit coupling between bulk material and heterostructures. We observe coupled plasmon–cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in the literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin–orbit coupling in bulk material, not exceed 400 G, α BIA −13 eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/Al x Ga 1− x N heterostructures originates from properties of the interface.

In this paper the growth and physical properties of HNPS-GaN crystallized in "feed-seed" configuration is described. The idea of this configuration is based on the conversion of the free standing HVPE-GaN crystals to the free standing HNPS-GaN. The influence of the c-plane bowing in the initial substrate on quality, rate, and mode of growth from solution is analyzed.

Recent results of High Nitrogen Pressure Solution (HNPS) growth of GaN crystals deposited on and separated from 2 inch, and smaller, GaN substrates grown by Hydride Vapor Phase Epitaxy (HVPE) have been presented. The influence of the c-plane bowing in the initial substrate on quality, rate and mode of growth by HNPS method has been analyzed in details.

In this communication we will summarize the results of a complementary study of structural and chemical non-homogeneities that are present in thick HVPE-grown GaN layers. It will be shown that complex extended defects are formed during HVPE growth, and are clearly visible after photo-etching on both Ga-polar surface and on any non-polar cleavage or section planes. Large chemical (electrically active) defects, such as growth striations, overgrown or empty pits (pinholes) and clustered irregular inclusions, are accompanied by non-uniform distribution of crystallographic defects (dislocations). Possible reasons of formation of these complex structures are discussed. The nature of defects revealed by selective etching was subsequently confirmed using TEM, orthodox etching and compared with the CL method. The non-homogeneities were studied in GaN crystals grown in different laboratories showing markedly different morphological characteristics.

Samples of HVPE-prepared hexagonal gallium nitride GaN were subjected to high temperatures under ammonia in order to induce decomposition. Powder XRD, SEM, and solid-state 69Ga and 71Ga MAS NMR spectroscopy were used to characterize changes in structure and morphology. The major changes were found to include GaN sublimation and decomposition to the elements in the gas phase. No significant Knight shift effect was detected by gallium NMR in striking contrast to the behavior observed earlier in a similar study of GaN nanopowders. The latter could now be linked to crystal growth and recrystallization phenomena that operate efficiently in pyrolyzed nanopowders while being absent or negligible in heated

We have studied the properties of InGaN layers and quantum wells grown on gallium nitride substrates with intentional surface misorientation with respect to its crystalline c-axis. Misorientation varied in the range from 0 up to 2 degree. The indium content was changed by using the different growth temperature (between 750 °C and 820 °C) during metalorganic vapor phase epitaxy. With increasing misorientation angle the average indium content decreased significantly. This effect was accompanied by the strong increase of the emission line bandwidth suggesting more pronounced indium segregation. The results of cathodoluminescence measurements show that these effects correspond to different number of atomic steps/terraces existing on the surface of gallium nitride substrate. Very interesting result is also demonstrated concerning p-type GaN layers. With increasing misorientation, the free hole density drastically increases above 1018 cm–3. This improvement in p-type doping is not related to the increased Mg concentration but to the reduction in the compensating donor density. Using this advantage we demonstrate nitride light emitters with improved electrical properties. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer.

Luminescence properties of 100-μm thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3 ns and diffusion coefficient of 1 cm 2 /s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.

Over last few years, the above room temperature ferromagnetism in MBE_grown GaN layers weakly doped with Gd has been reported by different researchers. The estimated average value of the magnetic moment per Gd ion has been deduced to be as high as 500 times larger as that of the Gd atomic moment [1]. It has been argued that the observed enhancement of the average magnetic moment resulted from strong contribution of GaN host lattice to macroscopic magnetization of the whole material. The origin of such behavior remains puzzling and more studies of GaN:Gd are required to clarify this issue. Recently, we have shown that Gd 3+ ions could be successfully introduced into bulk GaN crystals, grown from the solution of nitrogen in liquid gallium under high (1.5GPa) pressure (HP) of N2 and at elevated temperatures of about 1500 0 C. [1] The presence of Gd in these crystals was confirmed by electron paramagnetic resonance (EPR) and photoluminescence (PL) studies. In EPR spectrum, sharp absorption lines were detected, which could be attributed to Gd 3+ (4f 7 ) state. PL studies of Gd-doped GaN crystals revealed new narrow transitions, close to the GaN band gap (3.1-3.6 eV) and in the range of 1.6-1.8 eV. This latter emission band consists of several lines with a dominant one at 1.77eV (as narrow as 5 meV) and it was tentatively attributed by us to the transitions between two Gd 3+ excited states 6 G7/2 and 6 PJ (J=7/2, 5/2, 3/2). The origin of the Gd-related emission lines observed in UV region is highly speculative. The corresponding energy positions seem to be too low to be assigned to the 6 P7/2→ 8 P7/2 transition involving the first excited and the ground state of the Gd 3+ (4f 7 ) ion, which has already been observed for AlN:Gd. [2] In order to clarify the nature of the observed transitions, time-resolved photoluminescence experiments on high quality bulk GaN doped with Gd were performed. It was found that the decay time of the transitions observed at ~1.77 eV exceeds 2 ms. Such a long decay time strongly supports the identification of this emission band as due to transitions between Gd 3+ (4f 7 ) levels, for which the decay time in the ms range is observed in the case of many compounds doped with Gd. On the other hand, the decay time obtained for Gd related transitions in the UV range was found to be much faster, lower than 1 μs, which is close to the limit of our experimental system. This remains in strong contrast to the decay time observed for 6 P7/2→ 8 P7/2 transition in AlN, which was found to be of about 0.69 ms, and suggests that the GaN:Gd related emission lines observed in the UV region could hardly be correlated with internal transitions within Gd 3+ (4f 7 ). It seems possible that Gd ions form some complexes, for example with oxygen, which give rise to excitonic-like transitions below the bandgap of GaN:Gd, and such transitions could be responsible for the discussed UV luminescence. This conclusion is also consistent with the micro-PL mapping, which showed that the 1.77eV emission was not spatially correlated with presence of the Gd related lines detected in the UV region.

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.

In this work, we report on the growth of InGaN layers and InGaN/InGaN multi-quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PAMBE). We show that the incorporation of indium in InGaN layers can be controlled either by the ratio of Ga to N flux or the growth temperature. A method to increase the internal quantum efficiency of MQWs emitting green light at 500 nm by optimizing the growth temperature for the In content of each individual layer is proposed.

Recent results of the bulk GaN crystallization by the high-pressure solution (HPS) growth method are presented. The new experimental configurations for seeded growth on HVPE free-standing GaN crystals are proposed. The growth rate, morphology and basic physical properties of the pressure-grown materials are reported. The finite element calculation is used for modeling the convective transport in the solution. The convectional flow velocity vectors in liquid gallium are determined from experimentally measured temperatures and by solving the set of Navier–Stokes equations.