Structural and electrical characterization of hydrothermally deposited piezoelectric (K,Na)(Nb,Ta)O₃ thick films

(K_0.89Na_0.11)(Nb_0.85Ta_0.15)O₃ thick films were epitaxially grown at 200 °C on (001)La:SrTiO₃ and (001)_cSrRuO₃//(001)SrTiO₃ substrates by hydrothermal method, and their crystal structures and electrical properties were investigated.Film thickness increased with deposition time and reached 6 µm in 10 h. High-temperature X-ray diffraction measurement showed that successive phase transitions from orthorhombic to tetragonal and from tetragonal to cubic phases take place at 120 and 400 °C, respectively. Microstructure analyses were performed by using electron microscopy, which revealed the existence of two types of stripe patterns with a width of 100 nm or less. In addition, scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy elemental mapping showed that Nb/(Nb + Ta) ratio of the deposited films abruptly changed around 700 nm in thickness. Annealing at 500 °C led to the reduction in leakage current density from 10² to 10^–5 A/cm² at 30 kV/cm, showing that annealing is an effective way to improve insulation. Relative dielectric constant (ε_r) decreased linearly with increasing frequency, reaching 450 at 10 kHz. Polarization–electric field hysteresis loop and field-induced stain curve were measured by piezoelectric force microscopy, which showed remanent polarization (P_r) of 30 µC/cm² and piezoelectric constant (d_33,PFM) of 70 pm/V. These results demonstrate that (K,Na)(Nb,Ta)O₃ thick films with superior electrical properties can be fabricated by the low-temperature deposition technique.