Microstructure and microstructural evolution in BaTiO₃ films fabricated using the precursor method

Pulsed laser deposition of TiO₂ and BaF₂ layers at room temperature and subsequent annealing in flowing oxygen were used to form homogeneous epitaxial BaTiO₃ films on LaAlO₃. This oxide film synthesis method, known as the precursor technique, is frequently used for making combinatorial libraries. In this paper, we investigated the microstructures of the films at different stages of annealing using cross-sectional transmission electron microscopy, high-resolution imaging, and electron energy loss spectroscopy. It was shown that epitaxial BaTiO₃ thin films with large grains could be formed on a LaAlO₃ substrate. Their formation process consists of the following stages: At 200°C, the BaF₂ layer is partially oxidized. At 400°C, the amorphous TiO₂ layer crystallizes, further transformation of BaF₂ into BaO takes place, and interdiffusion begins. At 700°C, the formation of a polycrystalline structure with different Ba-Ti oxides occurs, epitaxial BaTiO₃ grains nucleate on the film/substrate interface, and significant interdiffusion takes place. Finally, at 900°C, the interdiffusion is completed, and the epitaxial BaTiO₃ grains coalesce and grow. The presence of nonepitaxial polycrystalline regions in fully annealed films can be explained as the following: (i) stoichiometric transient regions not yet consumed by recrystallization of BaTiO₃; (ii) nonstoichiometric regions resulting from inhomogeneous deposition of BaF₂.