Acoustic properties of the film/plate layered structure

A new propagation medium-a layered structure composed of a film and a plate-is suggested and studied, using c-oriented ZnO and AlN films on (001), {100}-Si plate, as two opposite examples of slow-on-fast and fast-on-slow material combinations. For both structures, the modes belonging to Lamb, quasi-longitudinal (QL), and Anisimkin Jr.´ (AN) families are found. For each family, the velocities v_n, displacement profiles, and electromechanical coupling coefficients K_n² for 4 electrode configurations are numerically calculated by the matrix method as a function of the mode order n = 0 to 8, plate thickness H/λ = 0 to 2.0, and film thickness h/λ = 0.02 to 0.04 (H and h are thicknesses; λ is the wavelength). Some high-order modes in the structure have K_n² = 0 for any H/λ, h/λ, and electrode configuration. Other modes possess variable K_n² with a maximum value larger than the coupling coefficient for the Rayleigh SAWs in ZnO and AlN single crystals or layered structures using the same films and semi-infinite silicon substrate. There are also QL-modes having high velocity v_n, large K_n², and low propagation loss caused by liquid loading. These modes are well suited for liquid sensors.