A rhombohedral ferroelectric phase in epitaxially-strained Hf0.5Zr0.5O2 thin films

TL;DR

This study reports the discovery of a rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films, revealing a new structural mechanism for robust nanoscale ferroelectricity suitable for memory devices.

Contribution

It demonstrates the epitaxial growth of high-quality Hf0.5Zr0.5O2 films with a rhombohedral phase, differing from the typical orthorhombic phase, and proposes a model for this ferroelectric phase formation.

Findings

Large ferroelectric polarization up to 34 μC/cm² without wake-up cycling

Identification of a rhombohedral phase via structural characterization

Epitaxial strain stabilizes a new ferroelectric phase in Hf0.5Zr0.5O2

Abstract

After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 {\mu}C/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.