Ab initio study of the two-dimensional metallic state at the surface of SrTiO3: importance of oxygen vacancies

TL;DR

This study uses density functional theory to analyze how oxygen vacancies induce a two-dimensional electron gas at the surface of SrTiO3, explaining recent experimental observations of metallic surface states.

Contribution

It demonstrates that oxygen vacancies cause surface reconstruction and charge redistribution, leading to a 2DEG formation, providing a theoretical explanation for experimental ARPES results.

Findings

Oxygen vacancies cause significant surface reconstruction.

Charge from vacancies redistributes and depletes rapidly from the surface.

A 2DEG forms independently of oxygen vacancy concentration.

Abstract

Motivated by recent angle-resolved photoemission spectroscopy (ARPES) observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO3 and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state (Ref 1), we analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO3 surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies --independently 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.