Characterization of Ferroelectric SrBi₂Ta₂O₉ Thin Films Prepared from Alkoxide Solutions

SrBi₂Ta₂O₉ thin films prepared by chemical liquid deposition using alkoxide precursor solutions, followed by annealing at various temperatures, were characterized with a focus on the correlation between composition, microstructure, and ferroelectric properties. P-E hysteresis loops of the films exhibited well-defined shapes at temperatures above 700°C, and the leakage current decreased with increasing annealing temperature, except for a film annealed at 750°C. The crystal growth proceeded and ferroelectric properties of the films improved with increasing annealing temperature, whereas a large leakage current was observed for the film annealed at 750°C. Quantitative X-ray photoelectron specroscopic analysis and high-resolution transmission electron microscopic observation indicated the presence of a substantial amount of metallic bismuth in all films, predominantly in microcrystallite regions and on film surfaces, including the inner surface of cavities The atomic fraction of metallic bismuth increased upon slight argon etching; however, the total bismuth content was significantly reduced. The metallic bismuth was amorphous and detected as vibrating spherical particles measuring approximately 5 nm under electron irradiation. The discontinuity in morphology, such as cavities at the interface between large single-crystal grains and microcrystallite regions, was most remarkable in the film annealed at 750°C. The condensation of metallic bismuth at the cavities is believed to increase the leakage current when the path formed by cavities penetrates through the film.