Effect of deposition time on film thickness and their properties for hydrothermally-grown epitaxial KNbO₃ thick films

Deposition behavior and the crystal structure of the epitaxial KNbO3 films, grown at 240 =C on (100)cSrRrO3 == (100)SrTiO3 substrates using hydrothermal method, as well as their ferroelectric and piezoelectric properties were systematically investigated. Film deposition was detected to have begun after 2 h from the start, and the thickness increased with increasing the deposition time up to 4 h, but contrary decreased after 9 h due to the etching by KOH. X-ray diffraction (XRD) patterns of the films were basically the same regardless of the deposition time. However, the films deposited for 15 h introduces large amount of voids than those for 2 h. Ferroelectric and piezoelectric response of the films deposited for 2 and 15 h were measured to have similar film thickness, but the maximum applied electric field for the 2-h films was twice as that for the 15-h ones. The prototype ultrasonic transducer was fabricated using the KNbO3 films, which were deposited by hydrothermal method at 240 =C for 3 h, and proved to be able to transmit and receive ultrasonic signals above 90MHz in degassed water.