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

The discovery of ferroelectricity in HfO2-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 the exploration of ferroelectricity in the fluorite-type oxide HfO2 films. Here we have investigated the structures and strain-induced ferroelectric transition in the different phases of few-layer HfO2 films (layer number N=1-5). It is found that HfO2 films for all phases are more stable with increasing films thickness. Among them, the Pmn2 1 (110)-oriented film is the most stable, and the films of N=4, 5 occur a P2 1 ferroelectric transition under tensile strain, resulting in polarization about 11.8 μC/cm2 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 HfO2 2D structures are discovered, together with computed electronic structures, mechanical, and dielectric properties analyses. This work provides guidance for theoretical and experimental study of the new structures and strain-tuned ferroelectricity in freestanding HfO2 films.