Structural Polymorphism Kinetics Promoted by Charged Oxygen Vacancies in HfO_{2}.

Defects such as oxygen vacancy are widely considered to be critical for the performance of ferroelectric HfO_{2}-based devices, and yet atomistic mechanisms underlying various exotic effects such as wake-up and fluid imprint remain elusive. Here, guided by a lattice-mode-matching criterion, 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 Pca2_{1} 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 HfO_{2}-based thin films as well as wake-up, fluid imprint, and inertial switching.