Origin of Wake-Up Effect in Hafnia

TL;DR

This study uses density functional theory to explain the wake-up effect in hafnia, showing that epitaxial strain and electric field stabilize ferroelectric phases, clarifying experimental polarization enhancements.

Contribution

It demonstrates that epitaxial strain and electric field induce ferroelectric phases in hafnia, providing a theoretical explanation for the wake-up effect observed experimentally.

Findings

Polar ($Pca2_{1}$) phase stabilized by electric field and epitaxial strain.

Small electric fields can induce ferroelectric phase in (010)-oriented hafnia.

Epitaxial strain destabilizes non-ferroelectric structures, enabling ferroelectricity.

Abstract

We performed density functional theory (DFT) calculations on epitaxially strained hafnia. We demonstrate the stabilization of the ferroelectric ($Pca2_{1}$) phase from the antiferroelectric ($Pbcn$) in bulk hafnia in the presence of electric field. We found that the polar ($Pca2_{1}$) phase can be efficiently stabilized with an adequate choice of film orientation. We show that for a (010)-oriented Pbcn, the ferroelectric $Pca2_{1}$ phase can be reached with a relatively small electric field ($\mid\varepsilon\mid\geq 150$ KV/m). We thus provide a simple explanation to the experimental observation of polarization enhancement through electric field cycling, or wake-up effect, as a ferroelectric phase transition driven by electric field. We find, in contrast, that stress free pure hafnia does not become ferroelectric for any reasonable electric field. So we explain the wake up effect and stabilization of ferroelectric pure hafnia as coming from a combination of epitaxial strain under applied electric field perpendicular to the film. We find that strain (or doping) primarily destabilizes the baddeleyite structure, so that the antiferroelectric Pbcn and ferroelectric phases can form.