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

The total energy of GaN as a function of unit cell volume has been cal culated for the wurtzite, zinc-blende, and rocksalt phases by the ab i nitio all-electron periodic Hartree-Fock method. The gallium 3d levels were treated as fully relaxed band states, and the internal coordinat es c/a and u in the wurtzite phase were optimized. The calculated tran sition pressure between the wurtzite rocksalt phases comes out to be a bout 52 GPa at the Hartree-Fock level and about 35 GPa at the correlat ed level. The calculated electronic structure shows strong hybridizati on of Ga 3d and N 2s states with the ordering as Ga 3d < N 2s < N 2p i n 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.