Conductometric sensing capabilities of Rayleigh modes in ZnO/Si structures

The propagation of the fundamental and higher order Rayleigh modes in c-ZnO/Si(001)< 100 > is theoretically investigated by numerical calculations and 3D finite element model. The dispersion curves of the phase velocity and electroacoustic coupling coefficient K-2 were calculated for the first seven modes, and for two electroacoustic coupling configurations with the interdigital electrodes positioned at the ZnO/Si interface or onto the free surface of the piezoelectric layer. The acoustoelectric response of the Rayleigh modes, i.e. the modes relative frequency and propagation loss shifts induced by the ZnO sheet conductivity changes, was studied aimed at the design of a SAW sensor showing enhanced conductometric sensitivity. The sensitivity of the modes varies with the ZnO layer thickness h(ZnO) and reaches a peak whose magnitude increases from the fundamental Rayleigh wave to the third mode R3, then it decreases with increasing the mode order. The K-2 corresponding to the sensitivity peak is lower than the maximum K-2 value obtainable with the corresponding mode. The Sezawa and R3 waves show the largest sensitivity values among the modes studied. The peak sensitivity of the Sezawa and of the higher order modes occurs at a layer thickness corresponding to the minimum of the derivative of the phase velocity respect to h(ZnO).