Atomic-Scale Phase Transition and Polarization Switching Mechanism of Fluorite Oxide Ferroelectrics by In-Situ Scanning Transmission Electron Microscopy

The discovery of ferroelectricity in fluorite oxides, exemplified by Hf_xZr_1-xO₂ (HZO), presents exceptional potential for silicon integration and robust electric polarization down to the thickness of several unit cells, which is beneficial for silicon-compatible and scalable electronics. Nevertheless, fluorite oxides exhibit various polymorphs, and the desirable ferroelectric orthorhombic (O) phase is metastable. Therefore, stabilizing O phase instead of nonpolar ground-state monoclinic (M) phase in thin films remains a persistent challenge. A comprehensive understanding of the atomic-scale mechanisms governing phase transitions is crucial for rational design of fluorite oxide devices [1]. In addition, the polarization switching process of fluorite oxides is invariably coupled with multiple phase transitions [2]. However, the specific switching pathway in the polar O phase and the correlated nonpolar phases remains elusive, necessitating direct imaging of in-situ structural evolution.

In this work, we investigate the atomic-scale O-M phase transition mechanism and polarization switching processes in ZrO₂ freestanding thin films using in-situ scanning transmission electron microscopy (STEM). We reveal that the phase transition from polar O phase to non-polar M phase occurs through a reversible shear deformation pathway, which can be protected by 90° ferroelectric-ferroelastic switching. Nevertheless, the accumulation of localized tensile strain can disrupt reversibility, leading to ferroelectric fatigue. Furthermore, our study reveals bidirectional transitions between antiferroelectric and ferroelectric orders through 180° switching, accompanied by several types of phase transitions including O, M, and tetragonal (T) phases. Finally, four-dimensional scanning transmission electron microscopy (4D-STEM) enables comprehensive, large-scale analysis of the crystal relationship and strain distribution within the thin film [3]. Through in-situ atomic-scale observation of structural evolution in fluorite oxide ferroelectrics, the uncovered phase transition and switching mechanisms offer fundamental insights for the design of next-generation ferroelectric devices.

References:
1. X. Li, et al., Nat. Mater., 23, 1077 (2024).
2. X. Li, et al., Adv. Mater., 35, 2207736 (2023).
3. Shi C, et al., Nat. Commun., 14, 7168 (2023).

X.L. acknowledges the support from the Rice Advanced Materials Institute (RAMI) at Rice University as a RAMI Postdoctoral Fellow. The authors acknowledge the support from NSF (FUSE-2329111 and CMMI-2239545) and Welch Foundation (C-2065-20210327).