Anion Doping Driven Non-Ferroelectric-to-Ferroelectric Phase Transition in Epitaxial Y:HfO2
Oxygen vacancies are often essential for stabilizing the orthorhombic ferroelectric phase in HfO2, with cationic doping widely employed to introduce such defects. In contrast, systematic studies on anionic doping to induce ferroelectricity remains largely in nascent stages. Here, using epitaxial Y:HfO2 films grown on ITO-buffered YSZ substrates that initially crystallize predominantly in the monoclinic non-polar phase, we demonstrate that post-deposition rapid thermal annealing in N2 atmosphere at 900 {\deg}C enables nitrogen incorporation without disrupting epitaxy. As the annealing duration increases from 10 s to 2 min, the monoclinic phase diminishes, accompanied by the emergence of robust ferroelectric hysteresis and a corresponding increase in the orthorhombic phase fraction. Combining independent spectroscopic and compositional analyses, we experimentally establish that nitrogen preferentially incorporates into pre-existing neutral oxygen-vacancy sites, converting them into charged oxygen vacancies that drive the transformation from the non-polar monoclinic phase to the ferroelectric orthorhombic phase. Our epitaxial model platform therefore reveals an anion-mediated defect-engineering pathway for controlling ferroelectricity in Y:HfO2, establishing nitrogen incorporation not merely as a chemical dopant, but as a route to fundamentally reconfigure the defect thermodynamics governing phase stability in fluorite ferroelectrics.