Reversible transition between the polar and antipolar phases and its implications for wake-up and fatigue in HfO₂-based ferroelectric thin film

Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (O_I and O_II) within the unit cells of a ferroelectric (Hf_0.5Zr_0.5)O₂ thin film. A reversible transition between the polar Pbc2₁ and antipolar Pbca phases, where the crystal structures of the 180° domain wall of the Pbc2₁ phase and the unit cell structure of the Pbca phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar P4₂/nmc phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.