Structural properties and energetics of oxygen impurities in GaAs

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 (Formula presented) and various interstitial sites (Formula presented) were considered. For the (Formula presented) defect in negative charge states we find off-center configurations with (Formula presented) symmetry as reported experimentally. Our results for the formation energies reveal a negative-(Formula presented) behavior for the (Formula presented) defect, in which the paramagnetic (Formula presented) charge states is never stable. For the (Formula presented) 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 (Formula presented) charge state is never stable, suggesting that the interstitial defect also exhibits a negative-(Formula presented) behavior. Based on our results, we suggest that the (Formula presented) and B bands of the local-vibrational-mode absorption spectrum of oxygen in GaAs are due to the off-center (Formula presented) defect. Also we show that this spectrum cannot be associated with an interstitial-oxygen configuration as previously proposed.