Structural and ferroelectric transition in few-layer HfO₂ films from first principles

The discovery of ferroelectricity in HfO 2 -based materials with high dielectric constant has inspired tremendous research interest for next-generation electronic devices. Importantly, films structure and strain are key factors in exploration of ferroelectricity in fluorite-type oxide HfO 2 films. Here we investigate the structures and strain-induced ferroelectric transition in different phases of few-layer HfO 2 films (layer number N = 1–5). It is found that HfO 2 films for all phases are more stable with increasing films thickness. Among them, the Pmn 2 1 (110)-oriented film is most stable, and the films of N = 4, 5 occur with a P 2 1 ferroelectric transition under tensile strain, resulting in polarization about 11.8 μC/cm 2 along in-plane a -axis. The ferroelectric transition is caused by the strain, which induces the displacement of Hf and O atoms on the surface to non-centrosymmetric positions away from the original paraelectric positions, accompanied by the change of surface Hf–O bond lengths. More importantly, three new stable HfO 2 2D structures are discovered, together with analyses of computed electronic structures, mechanical, and dielectric properties. This work provides guidance for theoretical and experimental study of the new structures and strain-tuned ferroelectricity in freestanding HfO 2 films.