ZnO on diamond: SAWs and pseudo-SAWs

The development of low-loss UHF SAW filters has led to devices using the higher phase velocities of pseudo-surface-acoustic-waves and shear-horizontally polarized waves. Another way of obtaining higher velocities is to use higher-velocity materials. Diamond, for example, has a SAW velocity near 11 km/s and a longitudinal velocity close to 18 km/s. In this paper the dispersion characteristics and the SAW coupling constants of the ZnO on diamond structure are described for the two lowest order modes and for the high-velocity pseudo-SAW mode. The SAW coupling constants are given for the four possible transducer geometries. At constant thickness-frequency, for the two lowest order modes, an unexpectedly large coupling (2Δv/v) is found over a certain range of layer thickness-frequency product; 23% for the first mode and 15% for the second. Because of dispersion, calculating coupling at constant thickness-wavenumber (as is done frequently) produces results which are nearly four times smaller than those calculated at constant thickness-frequency