Unexpected Termination Switching and Polarity Compensation in LaAlO3/SrTiO3 heterostructures

TL;DR

This study investigates the unexpected surface termination and polarity compensation mechanisms in LaAlO3/SrTiO3 heterostructures, revealing asymmetries that impact electronic properties and atomic control of oxide interfaces.

Contribution

It uncovers an unexpected AlO2 surface termination in LaAlO3 films and analyzes its effects on band alignment and polarity compensation using experiments and first-principles calculations.

Findings

Surface termination remains AlO2 regardless of substrate termination.

Asymmetry affects band alignment and electronic properties.

Polarity compensation mechanisms are fundamentally limited by this asymmetry.

Abstract

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically. An electronic asymetry has long been observed in the LaAlO3/SrTiO3 system. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first principles calcula- tions to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.