Deterministic Influence of Substrate-Induced Oxygen Vacancy Diffusion on Bi<sub>2</sub>WO<sub>6</sub> Thin Film Growth
Saikat Das, Tadakatsu Ohkubo, S. Kasai, Yusuke Kozuka
Abstract
In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered 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 Bi2WO6 thin film grown by the pulsed laser deposition technique. 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. The (100)/(010)-oriented Bi2WO6 phases form a self-organized 3D nanopillar structure, yielding a very rough film surface morphology. 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 the 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.