Synthesis of Potassium Niobiate (KNbO₃) Thin Films by Low-Temperature Hydrothermal Epitaxy

Orthorhombic KNbO3 thin films were deposited on SrTiO3 (100) and LiTaO3 (001) substrates by low-temperature hydrothermal epitaxy at 180−210 °C using aqueous solutions containing Nb2O5 and KOH. The films were thoroughly characterized by X-ray diffraction, Rutherford backscattering, and scanning electron microscopy. The KNbO3 films were uniformly distributed on the entire substrate area, exhibited high stoichiometry, and did not show evidence of interdiffusion or chemical reaction at the film/substrate interface. Their microstructures were determined by the substrate used, the synthesis temperature, and the concentration of the reactants. The KNbO3 films on SrTiO3 (100) substrates with thickness of 45 nm to 1.28 μm were smooth and transparent, with microstructures strongly suggesting single crystal growth mechanism. They were pure orthorhombic phase with (011) orientation and probably single-domain. Conversely, the KNbO3 films with higher thickness (>3.5 μm) exhibited very rough microstructure with textured columnar grains. Such films were also orthorombic but with mixed (100)/(011) orientation. This work demonstrated for the first time deposition of heteroepitaxial KNbO3 films on single-crystal substrates using low-temperature hydrothermal epitaxy in a well-defined range of synthesis conditions. The low synthesis temperatures, which are below the 225 °C phase transition temperature of KNbO3, resulted in formation of films that appear to exhibit improved features as compared to the films prepared by high-temperature processing.