Mapping of the full polarization switching pathways for HfO2 and its implications

Abstract

The discovery of ferroelectric phases in HfO2 offers insights into ferroelectricity. Its unique fluorite structure and complex polarization switching pathways exhibit distinct characteristics, challenging conventional analysis methods. Combining group theory and first-principles calculations, we identify numerous unconventional electric polarization switching pathways in HfO2 with energy barriers of 0.32 to 0.57 eV as a function of the different shift in the suboxygen lattices. In total, we identify 47 switching pathways for the orthorhombic phase, corresponding to the left cosets of the Fm3m group with Pca21 group. Contrary to the conception that the tetracoordinated oxygen (OIV) layers are inactive, our result demonstrates that both the tricoordinated oxygen (OIII) and OIV can be displaced, leading to polarization switching along any axial direction. The multiple switching pathways in HfO2 result in both 180° polarization reversal and the formation of 90° domains observed experimentally. Calculations show that specific switching pathways depend on the orientation of the applied electric field relative to the HfO2 growth surface. This allows HfO2 to automatically adjust the in-plane polarization direction under an out-of-plane electric field, thereby maximizing the out-of-plane component and contributing to the wake-up process. These findings redefine the roles of OIII and OIV layers, clarify unconventional switching pathways, and enhance our understanding of electric field response mechanisms, wake-up, and fatigue in ferroelectrics.