Development of transparent single-crystalline KNbO3 thin film by LPE technique

Single-crystalline KNbO3 thin film has been successfully formed on SrTiO3 substrate from high-temperature K2CO3-Nb2O5 solution by the liquid phase epitaxy (LPE) technique. The growth morphology was strongly influenced by the melt composition and film growth temperature. The starting material for the film preparation was a powder mixture of K2CO3 and Nb2O5. The oxides were mixed in non-stoichiometric proportion with excess K2CO3 as flux. Under the optimized film growth conditions using melt compositions including K2CO3/Nb2O5=52.5/47.5, 60.0/40.0 and 65.0/35.0, transparent single-crystalline KNbO3 thin films could be obtained. The synthesized KNbO3 thin film was subjected to precession X-ray photography in order to evaluate the crystallographic relationship with SrTiO3 substrate, and the result was compared with a simulated diffraction pattern. The precession X-ray photography clearly indicated that the [010]KNbO3 is not placed on the same diffraction line as [010]SrTiO3 but is slightly shifted with a difference in angle of approximately 3°, while the [100] and [001] agree in direction for KNbO3 and SrTiO3. The observed lattice parameter c of KNbO3 film was calculated to be 4.043 Å which was slightly (1.7%) larger than 3.974 Å reported for KNbO3 bulk crystal. In-plane rotation and elongation toward substrate normal for KNbO3 lattice on SrTiO3 substrate were discussed from the viewpoint of release of elastic energy accumulated by lattice mismatch on the substrate.