An investigation was made of the properties of O impurities by using first-principles total-energy calculations. Five charge states of O which occupied an As site, OAs, and various interstitial sites, Oi, were considered. In the case of OAs defects in negative charge states, off-center configurations with C2v symmetry were found; as reported experimentally. The results for the formation energies revealed a negative-U behavior for the OAs defect; in which the paramagnetic 2- charge state was never stable. In the case of the Oi defect, 3 equilibrium configurations were found for the O atom and were present in all of the charge states which were investigated. The stable configuration for the neutral defect placed the O atom between an As-Ga pair, to form the As-O-Ga structure. However, in negative charge states, the stable configuration placed the O atom exactly at the tetrahedral interstitial site; bonded to four Ga first-nearest neighbours. Furthermore, it was found that the 1- charge state was never stable; thus suggesting that the interstitial defect also exhibited negative-U behavior. On the basis of the results, it was suggested that the A, B' and B bands of the local vibrational mode absorption spectrum of O were due to the off-center OAs defect. It was also shown that this spectrum could not be associated with an interstitial-O configuration.

Structural Properties and Energetics of Oxygen Impurities in GaAs W.Orellana, A.C.Ferraz: Physical Review B, 2000, 61[8], 5326-31