First-principles study of yttrium oxysulfide: bulk and its defects

Abstract First-principles electronic-structure calculations of yttrium oxysulfide (Y 2 O 2 S) and its intrinsic point defects have been performed for the first time. From the calculation of the bulk, it is found that the material has an indirect band gap and that the top of the valence bands shows anisotropic character which means anisotropic mass of the hole. From the calculation of the defects, it is found that the anion vacancies and the interstitial sulfur induce relatively deep levels in the band gap, whereas the interstitial oxygen induces relatively a shallow hole trap level. These findings are discussed with defect-related phenomena in the oxysulfides. Further, negative-U characters are found in the oxygen vacancy and the interstitial sulfur from the result of the defect formation energies. These defect properties can be explained from the viewpoint of covalency of the bonds newly appearing in the defect geometries.