Precise structure and polarization determination of Hf0.5Zr0.5O2 with electron ptychography

TL;DR

This study uses advanced electron ptychography to precisely determine the atomic structure and polarization of Hf0.5Zr0.5O2, revealing its ferroelectric orthorhombic phase, oxygen vacancy distribution, and polarization behavior at the atomic level.

Contribution

It introduces multislice electron ptychography for atomic-scale analysis of Hf0.5Zr0.5O2, providing detailed insights into its ferroelectric phase and defect structures.

Findings

Orthorhombic phase is ferroelectric with ~34 μC/cm2 polarization.

Oxygen vacancies are distributed preferentially, affecting polarization.

Polarization is suppressed near grain boundaries, unchanged at domain walls.

Abstract

Hf0.5Zr0.5O2 (HZO) is a promising candidate for next generation ferroelectric memories and transistors. However, its ferroelectricity origin is still under debate due to the complex of its phase and microstructure in practical samples. In this study, we investigate the atomic structure of substrate-free HZO freestanding film with multislice electron ptychography, for which the ultra-high space resolution (up to ~25 pm) and capability to simultaneously image the cation and oxygen allow us to precisely determine the intrinsic atomic structures of different phases and reveal subtle changes among them. We clarify that the orthorhombic phase is ferroelectric with spontaneous polarization ~34{\pm}4 {\mu}C/cm2 (corresponding to 56{\pm}6 pm in displacement) that is accurately measured through statistical analysis. Significant polarization suppression is observed near the grain boundary, while no distinguishable structural changes are detected near the 180{\deg} ferroelectric domain walls. Through the direct oxygen imaging of orthorhombic phase from the [111] zone axis, we quantify a substantial number of oxygen vacancies with a preferential distribution, which influences the polarization direction and strength. These findings provide fundamentals for HZO research, and thus lay a foundation for the design of high-performance ferroelectric devices.