## 3 Simulation Performance Analysis
### Piezoelectric and SiO\({}_{2}\) Film Effects on the Properties of SAW Propagation  
To investigate the effects of ZnO and SiO\({}_{2}\) films on the properties of SAW propagation, the \(k^{2}\) curves of the Rayleigh and Sezawa waves in the ZnO/SiO\({}_{2}\)/diamond structure are shown in Figure 3. Here, the thickness of the Al electrode is negligible, where the normalized film h1/\(\lambda\) = 0.1, 0.2, 0.3, 0.4, 0.5. From Figure 3a, it can be concluded that the \(k^{2}\) of the Rayleigh wave gradually increases with the growth of h1/\(\lambda\), and the value of \(k^{2}\) is approximately 1.2% when h1/\(\lambda\) = 0.5. When the thickness of the ZnO is determined, there is a smaller increase in \(k^{2}\) than the growth of h2/\(\lambda\). As can be seen in Figure 3b, the \(k^{2}\) of the Sezawa wave increases monotonically and then decreases as h1/\(\lambda\) increases, and it reaches a maximum value near 4% when h1/\(\lambda\) = 0.4. This phenomenon is due to the displacement characteristics of the Sezawa wave. When the piezoelectric film is thin, the energy of the Sezawa wave is more concentrated in the piezoelectric layer as h1/\(\lambda\) increases, and \(k^{2}\) increases. However, when h1/\(\lambda\) exceeds a suitable range, the advantage of the multilayer structure decreases, and the excitation strength of the Sezawa wave is weakened. When h1/\(\lambda\) is 0.1 and 0.2, \(k^{2}\) increases monotonically with the increase in h2/\(\lambda\), and \(k^{2}\) increases and then decreases with the increase in h2/\(\lambda\) when h1/\(\lambda\) is 0.3, 0.4, and 0.5. Figure 3: Rayleigh and Sezawa waves \(k^{2}\) as functions of h2/\(\lambda\) with different h1/\(\lambda\) values: (**a**) Rayleigh wave; (**b**) Sezawa wave. Figure 2: Resonant modes of Rayleigh and Sezawa waves: (**a**) Rayleigh wave; (**b**) Sezawa wave. Significantly, in the ZnO/SiO\({}_{2}\)/diamond structure, the \(k^{2}\) of the Rayleigh wave is always lower than the \(k^{2}\) of the Sezawa wave, especially when h1/\(\lambda\) = 0.4. The \(k^{2}\) of the Sezawa wave is 3.4 times larger than the \(k^{2}\) of the Rayleigh wave, and the \(v_{\rm p}\) of the Sezawa wave is 1.5 times larger than the \(v_{\rm p}\) of the Rayleigh wave. The findings presented in Figure 4 demonstrate that, although five distinct ZnO thicknesses are chosen for comparison, the \(v_{\rm p}\) of the Rayleigh wave and that of the Sezawa wave both diminish as h2/\(\lambda\) increases because the acoustic energy becomes more constrained within the SiO\({}_{2}\) layer. Nonetheless, it is evident that when h2/\(\lambda\) increases, the resonance frequency of the Rayleigh mode is more impacted, but the Sezawa mode's resonance frequency is less impacted. As h1/\(\lambda\) increases, the resonance frequencies for both wave modes fall. Still, the Sezawa mode easily maintains high frequencies in excess of GHz at relatively large \(\lambda\). In order to suppress the shift of the operating frequency of the SAW device with temperature changes, this paper investigates the temperature effects on the ZnO piezoelectric film and the SiO\({}_{2}\) temperature complementary layer in the Sezawa mode. Lower _TCF_ is also an extremely important performance index for high-frequency SAW devices. ZnO piezoelectric film is known to have a _TCF_ of approximately \(-28\) ppm/\({}^{\circ}\)C. The _TCF_ of the device can be significantly improved by adding SiO\({}_{2}\) with a positive _TCF_ between the piezoelectric film and the diamond substrate. Figure 5 shows the _TCF_ in ZnO/SiO\({}_{2}\)/diamond structures producing the Sezawa mode at different normalized thicknesses of ZnO and SiO\({}_{2}\). When h2/\(\lambda\) = 0.1, 0.2, 0.3, and 0.4, it is observed that the Sezawa mode shows positive _TCF_ values with increasing SiO\({}_{2}\) thickness; thus, a zero _TCF_ can be obtained. The effect of SiO\({}_{2}\) on the _TCF_ of the Sezawa mode decreases gradually for larger values of h2/\(\lambda\) because most of the acoustic energy is confined within the ZnO piezoelectric film.