Non-Ferroelectric to Ferroelectric Phase Transition in epitaxial Y:HfO$_2$ via Rapid Thermal Annealing Induced Nitrogen Doping

Abstract: Oxygen vacancies are often essential for stabilizing the orthorhombic ferroelectric phase in HfO$_2$, with cationic doping widely employed to introduce such defects. In contrast, systematic studies on anionic doping to induce ferroelectricity remain largely in nascent stages. On epitaxial Y:HfO$_2$ grown on ITO buffered YSZ substrates that crystallize in a mixed monoclinic (non-polar) and orthorhombic (polar) phases, we introduce nitrogen doping via post deposition rapid thermal annealing (RTA) in N$_2$ atmosphere at 900~$^\circ$C. As the annealing time increases from 10s to 4min, the monoclinic phase fraction diminishes, enabling the emergence of well-defined ferroelectric loops in films annealed beyond 2~min. We clearly show that this is an effect of nitrogen incorporation (doping) into the samples through a suite of structure-property correlation measurements including x-ray photoelectron spectroscopy. These results reveal that nitrogen actively participates in the RTA-induced phase stabilization, enabling ferroelectricity in epitaxial Y:HfO$_2$ without sacrificing crystallographic coherence, providing a viable pathway for structure-property correlation studies and a model system to study opto-electronic devices integrated with ferroelectrics.