Lattice parameters of gallium nitride were measured using high-resolution x-ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.

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We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.