Abstract

We investigate the structural properties, formation energies, and electronic structure of oxygen impurities in GaAs using first-principles total-energy calculations. Five charge states of oxygen occupying an arsenic site (O-As) and various interstitial sites (O-i) were considered. For the OAs defect in negative charge states we find off-center configurations with C-2v symmetry as reported experimentally. Our results for the formation energies reveal a negative-ii behavior for the O-As defect, in which the paramagnetic 2- charge states is never stable. For the O-i defect, we find three equilibrium configurations for the O atom, which are present in all the charge states investigated. The stable configuration for the neutral defect shows the O atom between an As-Ga pair forming the As-O-Ga structure. However, for the negative charge states, the stable configuration shows the O atom exactly at the tetrahedral interstitial site, bonding with four gallium first neighbors. Further, we find that the 1- charge state is never stable, suggesting that the interstitial defect also exhibits a negative-ii behavior. Based on our results, we suggest that the A, B', and B bands of the local-vibrational-mode absorption spectrum of oxygen in GaAs are due to the off-center O-As defect. Also we show that this spectrum cannot be associated with an interstitial-oxygen configuration as previously proposed.