Disentangling the ferroelectric phases of epitaxial hafnia

TL;DR

This study clarifies the existence of two distinct ferroelectric phases in epitaxial hafnia using advanced diffraction techniques, resolving previous ambiguities about their nature and properties.

Contribution

The paper provides definitive experimental evidence distinguishing the polar orthorhombic and rhombohedral phases of ferroelectric hafnia.

Findings

Confirmed the orthorhombic and rhombohedral phases are separate entities.

Compared temperature dependence and electrical responses of both phases.

Provided detailed 3D reciprocal space maps of the phases.

Abstract

Since its discovery, ferroelectric hafnia has been extensively studied due to its CMOS-compatibility and ability to remain polarized at sub-10 nm thicknesses. The ferroelectric behaviour is generally attributed to a polar orthorhombic (OIII) phase. However, a second polar phase with rhombohedral symmetry (R-phase) has also been reported in epitaxial films. The nature of the R-phase remains disputed due to the subtle differences with the OIII-phase when probed by standard thin film characterisation techniques. Given the functional properties of ferroelectrics are crucially determined by the crystal symmetry, resolving this matter is imperative. In this work, we settle the controversy through extensive 3D reciprocal space surveys made possible via synchrotron-based grazing incidence diffraction from epitaxial films of both phases. These experiments, together with direct comparison of their temperature dependence and electrical responses, conclusively establish them as two distinct phases and provide insight into their key characteristics.