Ferroelectricity in [111]-oriented epitaxially strained HfO₂ from fist principles

First principles calculations are used to investigate the effect of (111) epitaxial strain in the structural and ferroelectric properties of Hafnia (HfO₂), a silicon compatible high-k dielectric that is already used in high volume semiconductor manufacturing applications. We find that [111]-oriented epitaxial strain lowers the symmetry of the bulk cubic Fm-3m and tetragonal P4₂/nmc phases of HfO₂ to rhombohedral R-3m and monoclinic P2₁/m, respectively. The polar orthorhombic Pca2₁ phase stabilizes a ferroelectric monoclinic P1 structure above -2% (111)-strain, not present in either bulk or (001)-strained form. Under (111)-strain, ferroelectricity displays an out of plane polarization component P~ 35μC/cm². At large compressive strain (>2%), [111]-oriented thin films are paraelectric. We further explore the energy landscape of (111) strained HfO₂ and possible pathways for the stabilization of ferroelectricity.