Structural polymorphism kinetics promoted by charged oxygen vacancy in HfO₂

Defects such as oxygen vacancy are widely considered to be critical for the performance of ferroelectric HfO₂-based devices, and yet atomistic mechanisms underlying various exotic effects such as wake-up and fluid imprint remain elusive. In contrast to conventional perovskite ferroelectrics such as BaTiO₃, the local atomic displacements of polar oxygen atoms in ferroelectric HfO₂ are insensitive to charge-carrier doping. This feature suggests that the polar phase can support a substantial amount of charged defects while maintaining the polar structure, and understanding the impacts of charged defects on polymorphism kinetics is expectedly important.

We systematically study the phase transitions between different polymorphs of hafnia under the influences of neutral and positively charged oxygen vacancies using a first-principles-based variable-cell nudged elastic band technique. We find that the positively charged oxygen vacancy can promote the transition of various nonpolar phases to the polar phase kinetically, enabled by a transient high-energy tetragonal phase and extreme charge-carrier-inert ferroelectricity of the polar Pca2₁ phase. The intricate coupling between structural polymorphism kinetics and the charge state of the oxygen vacancy has important implications for the origin of ferroelectricity in HfO₂-based thin films as well as wake-up, fluid imprint, and inertial switching.