Deterministic Orientation Control of Ferroelectric HfO₂ Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode

The scale-free ferroelectricity with superior Si compatibility of HfO₂ has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal–oxide–semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO₂ remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO₂ thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO₂ through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO₂ layer. By stabilizing different polarization directions of HfO₂ through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO₂.