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.