Polarity-field driven conductivity in SrTiO$_3$/LaAlO$_3$: a hybrid functional study

TL;DR

This study uses hybrid functional DFT to investigate the mechanisms behind 2DES formation at SrTiO3/LaAlO3 interfaces, highlighting the role of donor defects and electric-field effects in the critical thickness phenomenon.

Contribution

It provides a detailed theoretical analysis of the competing mechanisms for 2DES emergence, emphasizing the importance of donor defect stabilization over Zener breakdown under typical growth conditions.

Findings

Oxygen vacancies and hydrogen adsorbates stabilize at smaller LAO thicknesses than Zener breakdown.

Donor defect stabilization is likely the primary mechanism for 2DES formation.

Hybrid functional DFT clarifies the role of electric fields in defect stabilization.

Abstract

The origin of the 2-dimensional electron system (2DES) appearing at the (001) interface of band insulators $\rm SrTiO_3$ and $\rm LaAlO_3$ has been rationalized in the framework of a polar catastrophe scenario. This implies the existence of a critical thickness of polar $\rm LaAlO_3$ overlayer ($4~\rm u.c.$) for the appearance of the 2DES: polar catastrophe for thick $\rm LaAlO_3$ overlayer is avoided either through a Zener breakdown or a stabilization of donor defects at the $\rm LaAlO_3$ surface, both providing electrons to dope the substrate. The observation of a critical thickness is observed in experiments, supporting these hypotheses. Yet, there remains an open debate about which of these possible mechanisms actually occurs first. Using hybrid functional Density Functional Theory, we re-examine these mechanisms at the same level of approximation. Particularly, we clarify the role of donor defects in these heterostructures, and argue that, under usual growth conditions, electric-field driven stabilization of oxygen vacancies and hydrogen adsorbates at the LAO surface occur at a smaller LAO thickness than required for Zener breakdown.