Process window for growth of polar-axis-oriented tetragonal (Bi, K)TiO₃ epitaxial films on (100) _cSrRuO₃//(100)SrTiO₃ substrates by the hydrothermal method

(Bi, K)TiO₃ is a lead-free ferroelectric material that has large tetragonality at room temperature. High-quality films of (Bi, K)TiO₃ have been difficult to grow due to the high volatilities of the constituent Bi and K. Epitaxial tetragonal (Bi, K)TiO₃ films with large remanent polarization of about 84 µC/cm² were grown on (100)_cSrRuO₃//(100)SrTiO₃ substrates by the hydrothermal method. In this study, we systematically investigated the effects of the input amounts of the starting materials on the compositions, crystal structures, and ferroelectric properties of films grown on (100)_cSrRuO₃//(100)SrTiO₃ substrates. As a resultant, (001)-oriented (Bi, K)TiO₃ single-phase films were obtained with input Bi/(Bi + Ti) ratios of 0.056–0.450 at a KOH concentration of 10.0 mol/L. This indicates an approximately one-order-wide process window of the input Bi/(Bi + Ti) source ratio for (001)-oriented (Bi, K)TiO₃ single-phase films. On the other hand, these films were also obtained at KOH concentrations of 8.0 – 13.5 mol/L and with a fixed input Bi/(Bi + Ti) source ratio of 0.22. This suggests the high reproducibility of (001)-oriented (Bi, K)TiO₃ films even with fluctuations in the input source amount. In addition, all films deposited within this KOH concentration range showed large remanent polarization of about 84 µC/cm². Moreover, good insulation characteristics were achieved for the films deposited at higher KOH concentrations of 10.0 to 13.5 mol/L.