A study of fast surface bounded plate modes with high coupling efficiency in ZnO/Si structures

Fast acoustic modes with phase velocities of 6000 to 8500 m/s and with coupling efficiencies up to 4.5% may be observed in silicon substrates using ZnO interdigital transducers. A computational study of the acoustic wave propagation in silicon plates covered by a layered structure containing ZnO, SiO₂ and metallization layers is conducted to obtain a complete characterization of this type of acoustic mode. It follows from the comparison with the experimental results that these modes are higher-order plate waves of the silicon substrate, extremely modified due to the waveguide nature of the ZnO transducer. Most of the modes have a frequency region of high coupling efficiency with midfrequencies shifted from mode to mode. There is a considerable sensitivity of midfrequencies to changes in thickness of any layer (ZnO, SiO₂ or metal). As seen from amplitude profiles, the plate mode character is lost in the high-coupling region because the energy is concentrated at the coated surface. At a given frequency, the phase velocities exhibit steplike variation with layer thickness