Stabilization of competing ferroelectric phases of HfO2 under epitaxial strain
Abstract
Hafnia (HfO2)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO2 has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO2 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 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 Pnm21 phase of HfO2. It is proposed that this ferroelectric Pnm21 phase can account for the recently observed ferroelectricity in the (111)-oriented HfO2-based thin film [Nature Materials 17, 1095-1100 (2018)]. Further investigation of this second functional ferroelectric phase in HfO2 could potentially improve the performances of HfO2-based films in logic and memory devices.