Deterministic influence of substrate-induced oxygen vacancy diffusion on Bi₂WO₆ thin film growth

In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered as the most critical factors that control the phase stability of an oxide thin film. Here, we report an unusual case wherein diffusion of oxygen vacancies from the substrate overpowers the growth temperature and oxygen partial pressure to deterministically influence the phase stability of pulsed laser deposition grown Bi₂WO₆ thin film. We show that when grown on an oxygen-deficient SrTiO₃ substrate, the Bi₂WO₆ film exhibits a mixture of (001) and (100)/(010)-oriented domains alongside (001)-oriented impurity WO₃ phases. Oxygen annealing of the substrate or using a few monolayer-thick SrRuO₃ as the blocking layer for oxygen vacancy diffusion enables growing high-quality single-crystalline Bi₂WO₆ (001) thin film exhibiting an atomically smooth film surface with step-terrace structure. We propose that large oxide-ion conductivity of Bi₂WO₆ facilitates diffusion of oxygen vacancies from the substrate during the film growth, accelerating the evaporation of volatile Bismuth (Bi), which hinders the epitaxial growth. Our work provides a general guideline for high-quality thin film growth of Aurivillius compounds and other oxide-ion conductors containing volatile elements.