Electrochemical ferroelectric switching: The origin of polarization reversal in ultrathin films

TL;DR

This paper investigates the unexpected ferroelectric switching in ultrathin films, proposing electrochemical processes at the surface as the key mechanism behind polarization reversal and its effects on electronic properties.

Contribution

It introduces a first-principles analysis showing surface charge from electrochemical reactions enables ferroelectric polarization in ultrathin films.

Findings

Surface charge from electrochemical reactions induces polarization.

Redox processes at the surface explain tunneling electroresistance.

Polarization depends on electrochemical surface conditions.

Abstract

Against expectations, robust switchable ferroelectricity has been recently observed in ultrathin (1 nm) ferroelectric films exposed to air [V. Garcia $et$ $al.$, Nature {\bf 460}, 81 (2009)]. Based on first-principles calculations, we show that the system does not polarize unless charged defects or adsorbates form at the surface. We propose electrochemical processes as the most likely origin of this charge. The ferroelectric polarization of the film adapts to the bound charge generated on its surface by redox processes when poling the film. This, in turn, alters the band alignment at the bottom electrode interface, explaining the observed tunneling electroresistance. Our conclusions are supported by energetics calculated for varied electrochemical scenarios.