Stabilization of Competing Ferroelectric Phases of HfO2 under Epitaxial Strain

Hafnia ( HfO2 )-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. Identifying and stabilizing the ferroelectric phases of HfO 2 have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO2 are used to discover that imposing an in-plane shear strain on the metastable tetragonal phase drives it to a polar phase. This in-plane-shear-induced polar phase is shown to be an epitaxial-strain-induced distortion of a previously proposed metastable ferroelectric Pnm21 phase of HfO2. This ferroelectric Pnm21 phase can account for the recently observed ferroelectricity in (111)-oriented HfO 2 -based thin films on a SrTiO3 (STO) (001) substrate [Nat. Mater. 17, 1095 (2018)]. Further investigation of this alternative ferroelectric phase of HfO2 could potentially improve the performances of HfO2 -based films in logic and memory devices.