Charged Oxygen Vacancy Induced Ferroelectric Structure Transition in Hafnium Oxide

TL;DR

This paper reveals that doubly positively charged oxygen vacancies are crucial in stabilizing the ferroelectric orthorhombic phase in hafnium oxide, offering a new defect-based mechanism for ferroelectricity.

Contribution

It introduces a novel defect-driven mechanism involving charged oxygen vacancies to explain ferroelectric phase stabilization in HfO2.

Findings

Doubly positively charged oxygen vacancies stabilize the ferroelectric phase.

Charge state of oxygen vacancies influences phase stability in wide-band-gap oxides.

First-principles calculations support the defect stabilization mechanism.

Abstract

The discovery of ferroelectric HfO2 in thin films and more recently in bulk is an important breakthrough because of its silicon-compatibility and unexpectedly persistent polarization at low dimensions, but the origin of its ferroelectricity is still under debate. The stabilization of the metastable polar orthorhombic phase was often considered as the cumulative result of various extrinsic factors such as stress, grain boundary, and oxygen vacancies as well as phase transition kinetics during the annealing process. We propose a novel mechanism to stabilize the polar orthorhombic phase over the nonpolar monoclinic phase that is the bulk ground state. Our first-principles calculations demonstrate that the doubly positively charged oxygen vacancy, an overlooked defect but commonly presented in binary oxides, is critical for the stabilization of ferroelectric phase. The charge state of oxygen vacancy serves as a new degree of freedom to control the thermodynamic stability of competing phases of wide-band-gap oxides.