Growth and Atomically Resolved Polarization Mapping of Ferroelectric Bi2WO6 Thin Films

Abstract

Aurivillius ferroelectric Bi2WO6 (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) SrTiO3 substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. An atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 +/- 4 mu C cm(-2), which is in good agreement with the bulk polarization value. Furthermore, we found that the pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90 degrees domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180 degrees DWs. Application of an electrical bias led to in-plane 180 degrees polarization switching and 90 degrees polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain.