Annual Review 2024-2025

D I A M O N D L I G H T S O U R C E L I M I T E D 33 Unlocking the secrets of hafnia: a new era in ferroelectric materials Ferroelectric materials exhibit a unique property called spontaneous polarisation. Their built-in electric dipole moment can be switched between different directions by applying an external electric field. This makes them incredibly useful for a wide range of applications, including memory storage devices, sensors, and energy harvesters. Hafnia displays unusual behaviour in that its ferroelectricity becomes stronger as the material gets thinner, and one theory suggests that the electrochemical state within the hafnia film is directly linked to its polarisation and responsible for the unique size-dependent properties. A research team from the University of Cambridge focused on two specific compositions, Hf0.5Zr0.5O2 (HZO) and Hf0.88La0.04Ta0.08O2(HLTO), both in the form of single-phase epitaxial films. The first step was to meticulously characterise the structure and ferroelectric properties of the HLTO and HZO films using a combination of techniques. They used X-ray Diffraction (XRD) to determine the crystallographic phase and orientation of the films, Piezoresponse Force Spectroscopy (PFS) and Microscopy (PFM) to confirm the presence of ferroelectricity. These initial characterisations confirmed the presence of the desired ferroelectric phases in both HLTO and HZO and identified 24 areas on the samples, two sets of each specific polarisation state (P-up, P-down, or as-grown), to analyse using depth-resolved XPS on the I09 beamline. During the XPS experiments, the researchers discovered a surprising difference in the electrochemical behaviour between HLTO and HZO. These findings suggest that the polarisation state is not solely responsible for the changes in oxygen electrochemistry in these materials. Instead, the electric field used to switch the polarisation plays a crucial role. As irreversible electrochemical changes that occur within ferroelectric materials are a significant factor in device degradation, leading to performance decline over time, understanding and controlling the electrochemical processes in these materials is crucial for improving device performance and longevity. The study’s findings, therefore, have significant implications for the future development of hafnia-based ferroelectric devices. DOI: 10.1002/adma.202408572 S T R U C T U R E S A N D S U R FA C E S G R O U P A C D Variable energy XPS for HLTO/HZO stacks. a) Schematic of XPS measurement and HLTO device stack with a focused X-ray probe aligned between W markers (grey) to probe PFM-poled regions of the HLTO/HZO. b) O-1s spectra for P-up (red line), P-down (blue line), and as grown (grey line) PFM conditions, c) Hf-4f and d) Ta-4f spectra for P-up, P-down, and AG conditions. O-1s peaks marked: (Hf/Ta)-O (solid line), NL-O (dotted line), and La-O (dashed line). Core-level shifts are highlighted by red arrows. B