Film thickness dependence of ferroelectric properties in polar-axis-oriented epitaxial tetragonal (Bi,K)TiO₃films prepared by hydrothermal method

Tetragonal (00l)-oriented epitaxial (Bi,K)TiO3 films were grown at 240 °C on (100)cSrRuO3//(100)SrTiO3 substrates by the hydrothermal method. KOH aqueous solutions and Bi(NO3)3 · 5H2O and TiO2 powders were used as the starting materials. Film thickness was controlled from 33 to 1200 nm by changing the deposition time, and the Bi/(Bi+K) ratio in the A-site of perovskite ABO3 was almost constant for all film thicknesses. Polar-axis (00l)-oriented epitaxial (Bi,K)TiO3 films were obtained without a secondary phase and/or other orientation for all thickness ranges. Large ferroelectricity with the remanent polarization (Pr) of about 84 μC/cm2, comparable to previously reported lead-based ferroelectric films, was observed for (Bi,K)TiO3 films down to 33 nm in thickness. On the other hand, Ec increased with decreasing film thickness, but did not show strong film thickness dependence like other perovskite ferroelectric films. These data are very useful for understanding the degradation mechanism of ferroelectric thin films.