Insights into the origin of robust ferroelectricity in HfO2 -based thin films from the order-disorder transition driven by vacancies

HfO₂-based ferroelectrics (FE-HfO₂) have triggered extensive research interest because they are promising materials for beyond-Moore electronic device applications. However, the origin of ferroelectricity in HfO₂-based ferroelectrics remains elusive. Here, we demonstrate that the ordered oxygen vacancies (VO) in FE-HfO₂ thin films contribute to ferroelectricity through the formation of a stable-polar phase. In FE-HfO₂ thin films, oxygen vacancy (VO) defects induce local electric dipoles and the ordered VO distribution results in the stable-polar phases. Using first-principles calculations, a thermodynamically stable-polar phase M₁ with the spontaneous polarization of 27 {}C/cm² is predicted in monocliniclike Hf₄Zr₄O₁₅ structures. Moreover, the predicted M₁ structures are directly visualized in Hf0.5Zr0.5O_2-δ (HZO) ferroelectric thin films using spherical aberration (Cs)-corrected scanning transmission electron microscopy. The proposed mechanism for VO-induced order-disorder ferroelectricity in FE-HfO₂ opens a new pathway to explore the underlying physics of the ferroelectric characteristics in FE-HfO₂.