Physical Properties of the Bulk GaN Crystals Grown by the High-Pressure, High-Temperature Method

This chapter describes a number of important characteristics of GaN bulk, single crystals. Bulk GaN crystallizes in the hexagonal wurtzite structure. In wurtzitetype crystals, with 4 atoms per unit cell, there are 9 optical modes and 6 of them can be observed in the Raman experiment. The observation of LO phonon–plasmon coupled modes has been possible only in the bulk single crystals. The reason for that is likely related to high crystalline quality of the samples. Variation of the phonon frequencies with applied hydrostatic pressure supplies information about the strength of interatomic interactions and the lattice stability of the considered material. Usually with increasing pressure followed by a corresponding decrease in the inter-atomioc distances, phonon frequencies increase. Luminescence spectra for many GaN materials (bulk and epitaxial films) contain in addition to the band-edge luminescence, the band of a yellow luminescence.