Physical origin of depolarization effects in HfO 2 films

${\mathrm{HfO}}₂$-based ferroelectric films have demonstrated great potential in nonvolatile memory, sensors, and energy harvesting applications. Although the ferroelectricity of ${\mathrm{HfO}}₂$ thin films can persist at the nanoscale, depolarization effects often lead to polarization degradation or even complete suppression. The underlying physical mechanisms governing this phenomenon remain controversial. Here, the depolarization mechanism of ${\mathrm{HfO}}₂$ films was thoroughly explored based on first-principles calculations, and results reveal that surface phase transitions play a vital role in the depolarization process. Although ferroelectric bulk phases for both $\mathrm{Pca}{2}₁$ and $\mathrm{Pmn}{2}₁$ are stable, out-of-plane ferroelectric (010) $\mathrm{Pca}{2}₁$ film is thermodynamically unstable and transforms into $\mathrm{P}{2}₁/\mathrm{c}$ spontaneously via an effectively barrierless twisting pathway. Notably, only out-of-plane electric polarization is reduced, while the in-plane one remains. Moreover, the tensile strain induced in ${\mathrm{HfO}}₂$ films by realistic substrates such as ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{\mathrm{x}}{\mathrm{MnO}}₃$ (x \ensuremath{\approx} 0.2 \ensuremath{\sim} 0.4) induces a transformation of the film from the paraelectric $\mathrm{P}{2}₁/\mathrm{c}$ phase to the antiferroelectric Pbcn one. Subsequently, during the wake-up process, the Pbcn phase can transition into the ferroelectric phase through a low-energy barrier. This study offers insights into the depolarization mechanisms in ${\mathrm{HfO}}₂$-based ferroelectrics and presents strategies for optimizing interfacial properties and tuning phase structures, thereby providing valuable theoretical guidance for enhancing phase stability in hafnium-based ferroelectric thin films.