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. The discovery of ferroelectricity in nanoscale hafnia-based films has spurred extensive research to understand its origin and unlock its full potential. 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.
In work recently published in Advanced Materials, researchers from the University of Cambridge used depth-resolved X-ray Photoelectron Spectroscopy (XPS) at Diamond’s I09 beamline to investigate the intricate relationship between polarisation and electrochemical changes in hafnia-based ferroelectrics. The results suggest that the electrochemical state is not directly linked to polarisation, and that certain dopants can suppress the electrochemical changes that cause degradation without sacrificing polarisation, opening up exciting possibilities for engineering more robust and reliable ferroelectric devices.
The research team focused on two specific compositions, Hf0.5Zr0.5O2 (HZO) and Hf0.88La0.04Ta0.08O2(HLTO), both in the form of single-phase epitaxial films. These films were chosen to minimise the influence of grain boundaries and other structural complexities that could complicate the analysis. 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 and visualise the domain structure and Positive-Up Negative-Down (PUND) measurements to measure the remnant polarisation and coercive field, key parameters describing the ferroelectric behaviour.
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.
Dr Nives Strkalj explained:
Our hafnia samples were intended to be very similar in terms of polarisation, but we were expecting to see changes in their electrochemistry when we used an electric field to change the polarisation. We opted for the I09 beamline because it's a unique setting where you can change between X-rays that probe deep and shallow with just the click of a button. Usually, if you want to probe depth, you have to realign the incidence angle, then you have to realign the detector, and it's very time consuming. We had to check many areas of our samples, areas which were P-up, or which were P-down, and on I09 we can get depth probing very quickly.
During the XPS experiments, the researchers discovered a surprising difference in the electrochemical behaviour between HLTO and HZO. In the P-up state, HLTO showed an increase in non-lattice oxygen (NL-O) primarily at the surface, suggesting that the electric field was driving oxygen species from the atmosphere onto the film. In contrast, HZO displayed an increase in NL-O distributed throughout the bulk of the film, accompanied by reduction of the Hf and Zr cations. 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.
The study's results challenged the prevailing hypothesis that oxygen vacancy migration was the crucial driver of hafnia's unusual ferroelectric behaviour at the nanoscale. The research team concluded that the enhanced ferroelectricity in thin hafnia films is intrinsic and not solely a result of electrochemical changes. They propose that other factors, such as coupling to additional phonon modes, may be responsible for this unique behaviour.
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.
Dr Strkal said:
We were able to show that some dopants essentially suppress electrochemical changes without sacrificing polarisation. Our results offer hints for device designers that there might be good dopants to reduce electrochemical degradation. Our group studies nanoscale ferroelectricity, so not just in hafnia, but also in classical ferroelectric perovskites such as barium titanate. It's hard to do such depth-dependent studies in those materials because they lose ferroelectricity when they're very thin and X-rays can’t probe deep enough. The discovery of ferroelectricity in nanoscale hafnia-based films has allowed us to create thin-enough samples for XPS measurements, and I’m hopeful that the results will also be applicable to more ferroelectric materials, as we have shown there’s nothing particularly special about hafnia in terms of the relationship between polarisation and electrochemistry.
To find out more about the I09 beamline or discuss potential applications, please contact Principal Beamline Scientist Tien-Lin Lee: [email protected].
Hill MO et al. Depth‐Resolved X‐Ray Photoelectron Spectroscopy Evidence of Intrinsic Polar States in HfO2‐Based Ferroelectrics. Advanced Materials (2024): 2408572. DOI:10.1002/adma.202408572.
Image credits: this article 10.1002/adma.202408572 under license Creative Commons CC-BY 4.0
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