PHONON MODES AND METAL-INSULATOR-TRANSITION IN GAN CRYSTALS UNDER PRESSURE

Close inspection of experimental results given by Perlin and co-worker s [Phys. Status Solidi B 198, 223 (1996); Phys. Rev. B 45, 83 (1992)] shows that three phenomena were observed in that work: optical-phonon shift and splitting under pressures, which can be explained in a symme try consideration for the Gamma point of hexagonal crystals; inhomogen eous broadening and shift of phonon frequencies due to strain fluctuat ions which are described in the present paper using Dyson's equation f or the phonon Green's function; phonon hardening and decreasing of wid th in the metal-insulator transition in GaN under pressure of about 22 GPa. The last effect results from the interaction between electrons a nd optical phonons, but this interaction makes no impact on the line s hape (Fano effect). We find that the phonon line shape in semiconducto rs with small carrier concentration is determined by strain fluctuatio ns or imperfections. Estimates show that the electron-phonon interacti on is the reason why optical phonons are not detected in typical metal s.