Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3

TL;DR

This study uses density functional theory to analyze how multiple oxygen vacancies in SrTiO3 influence the localization or itinerancy of electrons, revealing vacancy clustering and distinct electronic behaviors based on vacancy position.

Contribution

It provides a detailed theoretical analysis of the lattice and electronic effects of multiple oxygen vacancies in SrTiO3, highlighting the difference between surface and subsurface vacancy states.

Findings

Vacancies prefer to cluster in the first two layers near the surface.

Surface vacancies lead to itinerant Ti t2g electrons.

Subsurface vacancies result in localized Ti eg electrons.

Abstract

Oxygen vacancies in strontium titanate surfaces (SrTiO3) have been linked to the presence of a two-dimensional electron gas with unique behavior. We perform a detailed density functional theory study of the lattice and electronic structure of SrTiO3 slabs with multiple oxygen vacancies, with a main focus on two vacancies near a titanium dioxide terminated SrTiO3 surface. We conclude based on total energies that the two vacancies preferably inhabit the first two layers, i.e. they cluster vertically, while in the direction parallel to the surface, the vacancies show a weak tendency towards equal spacing. Analysis of the nonmagnetic electronic structure indicates that oxygen defects in the surface TiO2 layer lead to population of Ti t2g states and thus itinerancy of the electrons donated by the oxygen vacancy. In contrast, electrons from subsurface oxygen vacancies populate Ti eg states and remain localized on the two Ti ions neighboring the vacancy. We find that both, the formation of a bound oxygen-vacancy state composed of hybridized Ti 3eg and 4p states neighboring the oxygen vacancy as well as the elastic deformation after extracting oxygen contribute to the stabilization of the in-gap state.