A high-mobility electronic system at an electrolyte-gated oxide surface

TL;DR

This paper introduces a method using hexagonal boron nitride to protect electrolyte-gated oxide surfaces, significantly enhancing electron mobility and enabling high-density modulation while minimizing surface disorder and electrochemical reactions.

Contribution

The authors demonstrate a protective technique with BN that improves mobility and allows high-density electron accumulation in electrolyte-gated oxides, expanding the method's applicability.

Findings

Mobility improves over tenfold with BN protection.

High electron densities (~10^14 cm^-2) are achievable with minimal surface disorder.

BN protection is effective even with ultra-thin layers (~6 Å).

Abstract

Electrolyte gating is a powerful technique for accumulating large carrier densities in surface two-dimensional electron systems (2DES). Yet this approach suffers from significant sources of disorder: electrochemical reactions can damage or alter the surface of interest, and the ions of the electrolyte and various dissolved contaminants sit Angstroms from the 2DES. Accordingly, electrolyte gating is well-suited to studies of superconductivity and other phenomena robust to disorder, but of limited use when reactions or disorder must be avoided. Here we demonstrate that these limitations can be overcome by protecting the sample with a chemically inert, atomically smooth sheet of hexagonal boron nitride (BN). We illustrate our technique with electrolyte-gated strontium titanate, whose mobility improves more than tenfold when protected with BN. We find this improvement even for our thinnest BN, of measured thickness 6 A, with which we can accumulate electron densities nearing 10^14 cm^-2. Our technique is portable to other materials, and should enable future studies where high carrier density modulation is required but electrochemical reactions and surface disorder must be minimized.