AB-INITIO STUDY OF HIGH-PRESSURE PHASE-TRANSITION IN GAN

The total energy of GaN as a function of unit cell volume has been calculated for the wurtzite, zinc-blende, and rocksalt phases by the ab initio all-electron periodic Hartree-Fock method. The gallium 3d levels were treated as fully relaxed band states, and the internal coordinates c/a and u in the wurtzite phase were optimized. The calculated transition pressure between the wurtzite rocksalt phases comes out to be about 52 GPa at the Hartree-Fock level and about 35 GPa at the correlated level. The calculated electronic structure shows strong hybridization of Ga 3d and N 2s states with the ordering as Ga 3d < N 2s < N 2p in all the phases. The band gap is direct at Gamma in the wurtzite and zinc-blende phases and is indirect in the high pressure rocksalt phase where the valence band maximum is shifted away from the Gamma point.