Single crystal growth of KNbO₃ and application to surface acoustic wave devices

Potassium niobate (KNbO₃) has large piezoelectric constants. The surface acoustic wave (SAW) substrates with high couplings are very important for wide-band SAW filters and SAW devices. Therefore, the large size KNbO₃ single crystals are required for SAW device applications. In this paper, the techniques of a large size of KNbO₃ using the top seeded solution growth techniques (TSSG) with large size of crucible of 60 mmφ platinum are described. The results show that 50×50×15 mm³ single crystals are obtained. Also, the poling techniques of KNbO₃ crystal are investigated by using the nonlinear scanning dielectric microscope. The propagation characteristics of SAW and piezoelectric leaky surface wave in KNbO₃ single crystal were investigated theoretically. The results show that the electro-mechanical coupling coefficient of the surface wave propagating along the X-axis of the rotated Y-cut plane is extremely large: K² = 0.53, compared to K² of 0.055 for LiNbO₃. The experimental results of the above KNbO₃ substrates agree with the theoretical ones. Also the KNbO₃ substrates showed experimentally the zero temperature coefficients of frequency (TCF) around 20 °C.