Anisotropic, multi-carrier transport at the (111) LaAlO$_3$/SrTiO$_3$ interface

TL;DR

This study reveals strong low-temperature anisotropic electrical transport at the (111) LaAlO$_3$/SrTiO$_3$ interface, with contributions from electrons and holes, and effects amplified by UV exposure, expanding understanding of interface physics.

Contribution

It provides the first detailed characterization of anisotropic transport properties at the (111) LAO/STO interface, highlighting the role of holes and UV effects.

Findings

Transport anisotropy is much stronger in (111) samples than in (001) or (110)

Anisotropy appears only at low temperatures below liquid helium point

UV exposure enhances transport anisotropy along one crystal direction

Abstract

The conducting gas that forms at the interface between LaAlO$_3$ and SrTiO$_3$ has proven to be a fertile playground for a wide variety of physical phenomena. The bulk of previous research has focused on the (001) and (110) crystal orientations. Here we report detailed measurements of the low-temperature electrical properties of (111) LAO/STO interface samples. We find that the low-temperature electrical transport properties are highly anisotropic, in that they differ significantly along two mutually orthogonal crystal orientations at the interface. While anisotropy in the resistivity has been reported in some (001) samples and in (110) samples, the anisotropy in the (111) samples reported here is much stronger, and also manifests itself in the Hall coefficient as well as the capacitance. In addition, the anisotropy is not present at room temperature and at liquid nitrogen temperatures, but only at liquid helium temperatures and below. The anisotropy is accentuated by exposure to ultraviolet light, which disproportionately affects transport along one surface crystal direction. Furthermore, analysis of the low-temperature Hall coefficient and the capacitance as a function of back gate voltage indicates that in addition to electrons, holes contribute to the electrical transport.