Thermal stability of self-polarization in a (K,Na)NbO₃film prepared by the hydrothermal method

Herein, the systematic change in the self-polarization behavior against the post-heat treatment after film deposition was investigated to understand the origin of self-polarization in hydrothermally prepared (K x Na1-x )NbO3 films with x = 0.88 at 240 C below the Curie temperature. The transverse piezoelectric constant, e 31,f , was evaluated by the cantilever method. The as-prepared films and heat-treated films at 300 C had an almost perfectly self-polarized state and showed e 31,f of ∼-5.0 C m-2 without applying an electric field as a poling treatment. Also, the piezoelectric response shows good linearity with the asymmetric piezoelectric butterfly curves, showing imprint characteristics. The self-polarized states were maintained until the heat treatment above the Curie temperature. Moreover, the self-polarized film was successfully deposited on the post-heat-treated film without the self-polarized state. This result suggests that the self-polarized state is generated regardless of the polarized state of the underlying layer by the hydrothermal synthesis in which deposition temperature is below the Curie temperature.