Ab initio study of high pressure phase transition in GaN

Abstract

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 3 d 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 3 d and N 2 s states with the ordering as Ga 3 d < N 2 s < N 2 p in all the phases. The band gap is direct at Γ 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 Γ point.