Polarization of Ferroelectric Hafnia: Up or Down?

HfO₂-based ferroelectrics have attracted huge interest in the past ten years due to their good compatibility with CMOS technology and stable ferroelectricity at the nanometer scale. It has been unambiguously demonstrated that the intrinsic ferroelectricity in doped hafnia films is associated with the orthorhombic structural phase of the Pca2₁ space group [1]. However, the polarization switching mechanism remained ambiguous. Here, using a state-of-the-art solid-state generalized variable cell nudged elastic band (VCNEB) method, we predict that the most energetically favorable switching pathway in ferroelectric hafnia involves three-fold O atoms passing across the nominal unit cell boundaries bounded by the Hf atomic planes [2]. This finding is at odds with the commonly accepted polarization switching mechanism, where the displacement of the O atoms is limited by the nominal unit cell boundaries. Our results have significant implications for the understanding of ferroelectricity in HfO₂-based ferroelectrics, including (1) the assignment of polarization orientation to different ferroelectric domains, (2) domain-wall motion under an applied electric field, (3) the measurable magnitude of ferroelectric polarization, and (4) the longitudinal piezoelectric coefficient. These results are important for the interpretation of experimental data on HfO₂-based ferroelectrics and may be relevant to a broad range of ferroelectrics where the ionic polar displacements from the relevant centrosymmetric phase are large.   

 

Y. Yun at al., Nature Mater. 21, 903 (2022).

Y. Wu et al., Phys. Rev. Lett. 131, 226802 (2023).