Ab initio study of substitutional nitrogen in GaAs

We investigate the atomic geometry, formation energies, and electronic stru cture of nitrogen occupying both arsenic and gallium sites in GaAs (N-As an d N-Ga) using first-principles total-energy calculations. We find that both neutral defects induce impurity-like empty levels in the band gap acting a s acceptors. While N-As shows a s-like a(1) level in the middle of the band gap, N-Ga shows a p-like t(2) level close to the bottom of the conduction band. The gap level of N-As gives theoretical support for the experimentall y observed band-edge redshift on the GaAsN alloy for a N concentration simi lar to3%. Strong inward relaxations preserving the T-d symmetry characteriz e the N-As equilibrium geometry in all the charge states investigated. In c ontrast, N-Ga exhibits a structural metastability in neutral charge state a nd Jahn-Teller off-center distortions in negative charge states forming a n egative-U center. Formation energies of competing N-As and N-Ga defects are also discussed. (C) 2001 American Institute of Physics.