Examples of microstructure-related properties of gallium nitride

The work provides a brief overview and the latest experimental results concerning the microstructure of gallium nitride. Because of the importance for the optoelectronic and electronic technologies, mainly problems related to the lattice mismatch between substrates and GaN layers are discussed. Three main substrates, sapphire, silicon carbide and high-pressure-grown bulk GaN crystals, are compared. Mosaicity, thermal strains and surface roughnesses of the GaN layers grown on those substrates are reported. The application of high-pressure technologies makes it possible to use temperatures higher by a few hundred degrees with respect to the atmospheric pressure for which the decomposition of gallium nitride occurs at temperatures below 1000 degrees C. Annealing at pressures higher than 10 kbar and temperatures up to 1550 degrees C causes modifications of the microstructure of GaN heteroepitaxial layers on sapphire. For example, their mosaicity decreases as observed by narrowing of the X-ray diffraction peaks. The implanted layers recover upon high-pressure annealing and give a strong dopant-related luminescence.