Effect of Ta-substitution on the deposition of (K,Na)(Nb,Ta)O₃ films by hydrothermal method

Ta substituted (K,Na)NbO₃ films have been deposited at 200 °C on (001)La:SrTiO₃ substrates by hydrothermal method. Film composition continuously changed with increasing the concentration of Ta₂O₅ in the raw material powder, and X-ray diffraction measurement showed that {001}_c-oriented epitaxial (K_xNa_1-x)(Nb_1-yTa_y)O₃ films were obtained for all compositions. Microstructural analysis using scanning electron microscopy and transmission electron microscopy revealed that Ta-substitution made {001}_c planes dominate on the surface of the film, leading to an improvement in average surface roughness. Film thickness increased with deposition time and eventually saturated. However, the saturated film thickness decreased with increasing Ta/(Nb + Ta) ratio. By evaluating the residual powders synthesized simultaneously with the films, it was found that the formation of the perovskite phase had already started in the very early stage of the deposition process, provided that Ta₂O₅ powder was present in the source solution.