Inevitable high density of oxygen vacancies at the surface of polar-nonpolar perovskite heterostructures LaAlO_3/SrTiO_3

摘要

The density of polarity-induced oxygen vacancies (V_Os) at the LaAlO_3 (LAO) surface of LAO/SrTiO_3 (STO) (001) heterostructures (HSs) and the density of induced carriers at the interface are quantitatively investigated. Using chemical equilibrium conditions combined with density-functional-theory calculations of total energy, we set up a model for the densities of the V_Os and the carriers, which are a function of the thickness of the LAO film, oxygen pressure, and temperature during the LAO/STO HSs growth. For the HSs with over three LAO unit-cell layers, our results show that the presence of a high density (~10~(14)cm~(-2)) of the V_Os is inevitable, even for the HSs grown at high oxygen pressures, and the densities of the V_Os and the carriers mainly depend on the LAO thickness and slightly on the oxygen pressure and temperature. Our results also demonstrate that the intrinsic doping cannot occur. The stability of the V_Os under high oxygen pressures is attributed to the release of electrostatic energy from the polar electric field in the LAO film.