Ab initio study of the two-dimensional metallic state at the surface of SrTiO ₃: Importance of oxygen vacancies

Motivated by recent angle-resolved photoemission spectroscopy observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO ₃ and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state, we analyze by means of density functional theory the electronic structure of various oxygen-deficient SrTiO ₃ surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies-independent of the oxygen concentration-redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a 2DEG. We discuss the underlying model emerging from such observations.