Progress in Computational Understanding of Ferroelectric Mechanisms in HfO$_2$

TL;DR

This review summarizes computational advances in understanding the complex ferroelectric mechanisms of HfO$_2$, highlighting the roles of structural phases, dopants, vacancies, and novel simulation techniques.

Contribution

It provides a comprehensive comparison of metastable phases, discusses stabilization factors, and explores machine learning methods to address unresolved issues in ferroelectric HfO$_2$.

Findings

Comparison of various polar phases and stabilization factors

Insights into the interplay of polymorphs, dopants, and vacancies

Potential of machine learning to enhance modeling and address high coercive fields

Abstract

Since the first report of ferroelectricity in nanoscale HfO$_2$-based thin films in 2011, this silicon-compatible binary oxide has quickly garnered intense interest in academia and industry, and continues to do so. Despite its deceivingly simple chemical composition, the ferroelectric physics supported by HfO$_2$ is remarkably complex, arguably rivaling that of perovskite ferroelectrics. Computational investigations, especially those utilizing first-principles density functional theory (DFT), have significantly advanced our understanding of the nature of ferroelectricity in these thin films. In this review, we provide an in-depth discussion of the computational efforts to understand ferroelectric hafnia, comparing various metastable polar phases and examining the critical factors necessary for their stabilization. The intricate nature of HfO$_2$ is intimately related to the complex interplay among diverse structural polymorphs, dopants and their charge-compensating oxygen vacancies, and unconventional switching mechanisms of domains and domain walls, which can sometimes yield conflicting theoretical predictions and theoretical-experimental discrepancies. We also discuss opportunities enabled by machine-learning-assisted molecular dynamics and phase-field simulations to go beyond DFT modeling, probing the dynamical properties of ferroelectric HfO$_2$ and tackling pressing issues such as high coercive fields.