Ferroelectricity in Simple Binary ZrO₂ and HfO₂

TLDR

ZrO₂ and HfO₂, widely studied binary oxides, are traditionally viewed as centrosymmetric dielectrics with limited functionality, despite their successful use in microelectronics. The ferroelectric behavior arises from a size‑driven tetragonal‑to‑orthorhombic transition in sub‑10 nm films, with the orthorhombic Pbc21 phase providing the noncentrosymmetric structure that supports spontaneous polarization. We discovered a field‑driven ferroelectric phase transition in sub‑10 nm ZrO₂ films and a composition‑ and temperature‑dependent stable ferroelectric phase in HfO₂–ZrO₂ mixtures.

Abstract

The transition metal oxides ZrO2 and HfO2 as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO2 thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO2–ZrO2 mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc21, whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.