Built-in and induced polarization across LaAlO$_3$/SrTiO$_3$ heterojunctions

TL;DR

This paper provides experimental evidence of a built-in electric potential across ultrathin LaAlO₃ films on SrTiO₃, revealing new insights into interface properties and potential for device tuning in oxide heterostructures.

Contribution

It demonstrates the existence of a measurable built-in potential in ultrathin LaAlO₃/SrTiO₃ heterostructures using electron tunneling and capacitance measurements, a phenomenon previously elusive.

Findings

Measured a built-in electric field of 93 meV/Å across LaAlO₃

Detected an induced dipole moment near the SrTiO₃ interface

Revealed the stability of ultrathin polar films without surface reconstruction

Abstract

Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. In ultra thin film form however the polar crystals are expected to remain stable without necessitating surface reconstructions, yet the built-in potential has eluded observation. Here we present evidence of a built-in potential across polar \lao ~thin films grown on \sto ~substrates, a system well known for the electron gas that forms at the interface. By performing electron tunneling measurements between the electron gas and a metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93 meV/\AA. Additionally, capacitance measurements reveal the presence of an induced dipole moment near the interface in \sto, illuminating a unique property of \sto ~substrates. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.