Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is described. The initial growth conditions for HVPE are determined and applied for further bulk growth. Smooth GaN layers up to 1.1 mm thick and of excellent crystalline quality, without cracks, and with low dislocation density are obtained. Preparation of the free-standing HVPE-GaN crystal by slicing and structural and optical quality of the resulting wafer are presented.

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 dTm/dP is also presented

The $p\text{−}T$ 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 $p\text{−}T$ 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.