Epitaxial ZnO piezoelectric thin films for saw filters

ZnO is a wide bandgap semiconductor material with high piezoelectric coupling coefficients. It can be used for making low-loss surface acoustic wave (SAW) filters operating at high frequency. We report MOCVD growth of epitaxial ZnO thin films on R-plane sapphire substrates. The crystallinity and orientation of the films, as well as the epitaxial relationship between the ZnO films and the R-plane Al2O3 substrate were studied using X-ray diffraction techniques. An atomically sharp interface structure was revealed by high-resolution TEM. Surface morphology was investigated using field emission SEM. SAW filters with 10 and 16 μm wavelength were fabricated. Low insertion loss and high piezoelectric coupling coefficient, up to 6%, were achieved. The acoustic velocities range from 4050 to 5800 m/s, varying as a function of ZnO film thickness.