Results of Rietveld refinement for indium nitride data collected in the temperature range 105–295 K are presented. Acicular microcrystals of indium nitride prepared by reaction of liquid indium with nitrogen plasma were studied by X-ray diffraction. The diffraction measurements were carried out at the Swiss-Norwegian Beamline SNBL (ESRF) using a MAR345 image-plate detector. Excellent counting statistics allowed for refinement of the lattice parameters of InN as well as those of the metallic indium secondary phase. In the studied temperature range, the InN lattice parameters show a smooth increase that can be approximated by a linear function. Lattice-parameter dependencies confirm the trends indicated earlier by data measured using a conventional equipment. The relative change of both the a and c lattice parameters with increasing the temperature in the studied range is about 0.05%. The axial ratio slightly decreases with rising temperature. The experimental value of the free structural parameter, u=0.3769(14), is reported for InN for the first time. Its temperature variation is found to be considerably smaller than the experimental error. The thermal-expansion coefficients (TECs), derived from the linearly approximated lattice-parameter dependencies, are αa=3.09(14)×10−6 K−1 and αc=2.79(16)×10−6 K−1. The evaluated TECs are generally consistent with the earlier data. For the present dataset, the accuracy is apparently higher for both, the lattice parameters and thermal-expansion coefficients, than for the earlier results. The refined lattice parameter cIn of the indium secondary phase exhibits the known strongly nonlinear behavior; a shift (ΔT equal about −50 K) of the maximum in cIn(T) dependence is observed with respect to the literature data.

In the growth of GaN from nitrogen dissolved in Ga under high N2 pressure, two main habits are observed: plate-like and needle-like. The plate-like crystals can be divided into those having (0001), (000) and {100} faces and those with the additional {101} and {102} faces. The needle-like crystals belong to three classes: with or without (0001) faces and a third with unusual, star-like needles. The plate-like and needle-like habits and transformation between these habits are discussed in greater detail. It is shown that it is possible to evaluate the relative growth rates corresponding to such transitions.

GaN is recently considered as the most important material for blue and ultraviolet optoelectronics. The device structures are usually grown on foreign substrates which results in high density of dislocations above 108cm−2. The application of high N2 pressure gives a unique possibility of growing of GaN single crystalline substrates which allows to lower dislocation density in epitaxial layers by 3–4 orders of magnitude.

In this paper, the results of high nitrogen pressure study of properties of Al-N, Ga-N and In-N systems are presented. The results include the phase diagrams in large range of pressures and temperatures (up to 2 GPa and 2000K) and also growth of GaN single crystals from atomic nitrogen solution in liquid gallium. The kinetic limitations of dissolution of N2 in liquid Al, Ga and In will be discussed. It follows, that the best conditions for crystal growth at available pressures and temperatures can be achieved for GaN.

The high nitrogen pressure experimental system equipped with multi-zone internal furnace was used for growth of high quality GaN crystals. At present both n-type and semi-insulating substrate quality GaN crystals with surface area up to 1cm2, with dislocation density below 105 cm-2 are routinely obtained and successfully used for homoepitaxy.

Some results concerning homoepitaxial growth by the MOCVD and MBE methods are shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

The recent progress in high pressure crystallization of GaN is reported. The results of the growth from the solutions of atomic nitrogen in pure Ga and in its alloys with Mg, Ca and Zn are discussed. It is shown that the growth mechanisms and the physical properties of the crystals depend on the type of dopant added into the solution. In particular, high resistivity (104-106Ωcm) GaN crystals of improved structural quality can be grown from solutions containing Mg. It was also observed that the addition of Mg, Ca and Zn suppress yellow photoluminescence commonly observed in GaN crystals grown without an intentional doping. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechanochemical polishing is discussed. It is shown that atomically flat, thermally stable surfaces are possible to obtain by the applied procedures.

Some most interesting results concerning homoepitaxial growth by MOCVD and MBE is shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

GaN single crystals have been grown from Ga solution. The crystals grow in the form of platelets with their basal plane perpendicular to the c-axis. The two opposite crystal surfaces are not equivalent since one is N- and the other Ga-terminated. Atomic force microscopy has been applied to study surface morphology on both surfaces. It was found that one side is atomically flat. The other side consists of pyramid-like structures about 25 nm in size.

The influence of annealing in an NH3+H2 atmosphere in the temperature range from 600°C to 900°C was investigated. Depending on crystal face the results were drastically different. It was found that on the rough side, annealing yields an atomically flat surface with terraces of monolayer height. The size of the terraces depends on the temperature of the annealing. On the originally flat side the surface becomes rougher after annealing. The transformation of surface morphology begins at temperatures below 700°C. Preliminary results of annealing in a hydrogen atmosphere are also reported. These findings are crucial for the understanding and development of GaN homoepitaxy.

The theoretical analysis of high pressure influence on the conditions of crystal growth of GaN is presented. High pressure influence on the transport and equilibrium properties of nitrogen is estimated using scaling approach. Nitrogen gas properties are used in the finite element calculation of the thermal conditions in the high pressure chamber. The temperature distribution during GaN growth in the vertical temperature-gradient configuration is obtained.

GaN single crystals are grown from the solution of atomic nitrogen in liquid Ga at N2 pressure up to 20 kbar. The crystals reaching dimensions of 1cm, with dislocation density of 103 - 105 cm-2 are currently obtained and successfully used for homoepitaxy by MOCVD and MBE.

The increase of the maximum size of GaN crystals with stable morphology requires the increase of the volume of the crucible. Beside the obvious geometric factors, the increase of the volume of the furnace and crucible allows to achieve much better control of supersaturation profiles in the solution and therefore better control of growth process. It helps to avoid morphological instabilities and leads to the growth of transparent, inclusion free crystals.

In the paper, the results of crystallization of substrate quality GaN crystals obtained with the use of large volume high pressure reactor will be presented. The crystals were characterized by High Resolution X-ray Diffraction and Atomic Force Microscopy. It will be shown that the quality of GaN crystals does not deteriorate with the increasing size and that epi-ready surfaces of GaN substrates can be obtained. Two-dimensional growth by propagation of monoatomic steps of GaN homoepitaxial layers will be presented as a verification of the quality of GaN substrates.

We present recent results on bulk GaN crystallization. The best quality GaN crystals grown from the solution at high N2 pressure without an intentional seeding are single crystalline platelets of stable morphology reaching dimensions up to 10 mm. The fastest growth direction for such crystals is [1 0 0], perpendicular to the GaN c-axis. The maximum stable growth rate perpendicular to crystal c-axis is determined from the experiment and used for an estimate of the effective supersaturation for the {10 0} face assuming two dimensional layer growth. The heat of GaN disssolution, determined from experimental solubility data, is used for the estimation of the edge energy of 2-D nuclei on the growing {10 0} face. Bulk crystal growth seeded by a single hexagonal needle with well developed {10 0} faces is also reported. The crystallization mechanisms and morphological stability in seeded growth of GaN are discussed on the basis of experimental results. The physical properties of the GaN crystals and homoepitaxial layers grown on them are briefly reviewed.

GaN epitaxial layers on GaN single crystals were grown using molecular beam epitaxy with an NH3 source. The deposited layers were examined by high resolution x-ray diffraction and photoluminescence (PL) spectroscopy. We observed strong and extremely narrow (half-widths of 0.5 meV) lines related to the bound excitons. In the higher energy range we observed three strong lines. Two of them are commonly attributed to free exciton transitions A (3.4785 eV) and B (3.483 eV). Their energetic positions are characteristic of strain-free GaN material.