Nives Štrkalj, together with colleagues from the University of Cambridge, published an article in the journal Advanced Materials, where they observed an indirect relationship between polarization and electrochemical state of ferroelectric films based on HfO₂. Films with lower polarization exhibit more significant electrochemical changes during polarization switching, providing evidence of intrinsic polar states.

Depth-Resolved X-Ray Photoelectron Spectroscopy Evidence of Intrinsic Polar States in HfO₂-Based Ferroelectrics

Megan O. Hill, Ji Soo Kim, Moritz L. Müller, Dibya Phuyal, Sunil Taper, Manisha Bansal, Maximilian T. Becker, Babak Bakhit, Tuhin Maity, Bartomeu Monserrat, Giuliana Di Martino, Nives Strkalj, Judith L. MacManus-Driscoll, Adv. Mater. 36, 2408572 (2024).

DOI: https://doi.org/10.1002/adma.202408572

The discovery of ferroelectricity in oxide films based on HfO₂ has sparked interest in understanding the origin of their polarization. Although it has been shown that polarization switching and electrochemical reactions co-occur, their interrelation remained unclear. This study employs depth-resolved X-ray photoelectron spectroscopy to examine changes in electrochemical states occurring during polarization switching. By comparing Hf_0.88La_0.04Ta_0.08O₂ (HLTO) and Hf_0.5Zr_0.5O₂ (HZO) films, which have equivalent structures and comparable average ionic radii, it was found that films with lower polarization exhibit more significant electrochemical changes during polarization switching. This suggests an indirect relationship between polarization and electrochemical state, providing evidence of intrinsic polar states.

The surprising difference between HLTO and HZO films lies in the redox reaction exhibited by HZO under application of an electric field, whereas HLTO does not exhibit such behavior. Our observations reveal that HLTO, despite having higher polarization than HZO, undergoes significantly less electrochemical modification under application of an electric field. The electric field induces oxygen reorganization, with the material’s chemistry, charge state, and device structure determining the resulting changes. This study highlights the reduced significance of oxygen reorganization in single-cycle polarization switching and provides evidence for the intrinsic nature of polar states in HfO₂-based ferroelectrics.

Figure 1: Schematic of the variable energy x-ray photoelectron spectroscopy (XPS) measurements performed to access different depths of the heterostructure. Schematic of electrochemical states in films of Hf_0.88La_0.04Ta_0.08O₂ (HLTO) i Hf_0.5Zr_0.5O₂ (HZO) with polarization pointing towards and away from the sample’s surface.