Stabilization of competing ferroelectric phases of HfO${}_2$ under epitaxial strain

Abstract

Hafnia (HfO${}_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO${}_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO${}_2$ have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO${}_2$ are used to discover that imposing an in-plane shear strain on the tetragonal phase induces a nonpolar to polar phase transition. This in-plane shear-induced polar phase is shown to be an epitaxial distortion of a known metastable ferroelectric $Pnm{2}_1$ phase of HfO${}_2$. It is proposed that this ferroelectric $Pnm{2}_1$ phase can account for the recently observed ferroelectricity in the (111)-oriented HfO${}_2$-based thin film [Nature Materials 17, 1095-1100 (2018)]. Further investigation of this second functional ferroelectric phase in HfO${}_2$ could potentially improve the performances of HfO${}_2$-based films in logic and memory devices.

6 pages, 4 figures