Rapid Thermal Annealing as a Route to Nitrogen Incorporation and Ferroelectric Phase Stabilization in Epitaxial Y:HfO₂

Ferroelectricity in doped hafnium oxide originates from the stabilization of a metastable polar orthorhombic phase, typically induced by cation substitution and oxygen vacancies. However, the influence of anionic doping on ferroelectric behavior remains largely unexplored. In this work, we demonstrate that rapid thermal annealing (RTA) under a nitrogen atmosphere provides an effective route for nitrogen incorporation into epitaxial Y-doped HfO₂ (Y:HfO₂) thin films, transforming non-ferroelectric monoclinic phases into ferroelectric orthorhombic ones. Epitaxial Y:HfO₂ films were grown on indium tin oxide–buffered yttria-stabilized zirconia (110) substrates via pulsed laser deposition. By adjusting the laser fluence, we controlled the kinetic energy of adatoms and, consequently, the phase stability. Low fluence favored metastable orthorhombic phase formation, yielding ferroelectric hysteresis with a remanent polarization of ~12 μC/cm², whereas high fluence promoted the equilibrium monoclinic phase with purely dielectric response. Subsequent RTA in nitrogen at 900 °C induced a time-dependent structural transformation from the monoclinic to the orthorhombic phase, producing stable, wake-up-free polarization switching. X-ray photoelectron spectroscopy confirmed nitrogen incorporation through Hf–N bond signatures, with nitrogen content saturating upon extended annealing. Control samples annealed in argon showed no such transformation, underscoring the critical role of nitrogen chemistry. Substitutional N^3- doping introduces charge-compensating oxygen vacancies, forming (N^3-– oxygen vacancy) defect complexes that reduce lattice volume and drive the phase transition. This defect-mediated mechanism stabilizes ferroelectricity while preserving epitaxial film quality. The results establish nitrogen-assisted RTA as a controllable pathway for anionic doping in complex oxides, offering a generalizable strategy for phase and defect engineering in ferroelectric and optoelectronic devices.