Comparison of Rayleigh and Sezawa wave modes in ZnO-SiO2-Si structures

ZnO-SiO₂-Si surface acoustic wave (SAW) devices with polycrystalline ZnO films of high quality at a thickness h between 0.3 and 15 μm were fabricated using the RF-magnetron bias sputtering technique. X-ray diffraction, rocking curves, and SEM microscopy were used to characterize the ZnO layers. The authors studied the propagation loss and the coupling coefficient of three different types of devices: Rayleigh wave devices with ZnO layers h/λ≃0.4 and h/λ≃0.04, and Sezawa wave devices. Measurements were performed in the frequency range between 80 and 600 MHz. It was observed that Rayleigh wave devices with a thin ZnO layer have a much lower frequency dependence of the propagation loss than the other two types of SAW devices. At 550 MHz, devices with thin ZnO layers exhibit a propagation loss of only 7 dB/cm, compared to 24 dB/cm for Rayleigh wave devices with a thick ZnO layer, and 14 dB/cm for the Sezawa wave devices. Phase velocity and coupling coefficient as a function of film thickness were examined and are compared with theoretical values