Ab initio study of substitutional nitrogen in GaAs

We investigate the atomic geometry, formation energies, and electronic structure of nitrogen occupying both arsenic and gallium sites in GaAs (N_As and N_Ga) using first-principles total-energy calculations. We find that both neutral defects induce impurity-like empty levels in the band gap acting as acceptors. While N_As shows a s-like a₁ level in the middle of the band gap, N_Ga shows a p-like t₂ level close to the bottom of the conduction band. The gap level of N_As gives theoretical support for the experimentally observed band-edge redshift on the GaAsN alloy for a N concentration ∼3%. Strong inward relaxations preserving the T_d symmetry characterize the N_As equilibrium geometry in all the charge states investigated. In contrast, N_Ga exhibits a structural metastability in neutral charge state and Jahn-Teller off-center distortions in negative charge states forming a negative-U center. Formation energies of competing N_As and N_Ga defects are also discussed.