## 3 Results
### Top Electrodes  
Next, we investigated how the material type and thickness of the top electrodes influenced the properties of the quasi-Rayleigh mode. As Figure 4 shows, with the growth of top electrode thickness, the \(V_{\rm P}\) value decreases while the \(K^{2}\) value increases monotonically for most electrode materials. The \(K^{2}\) curve of Au as a function of top electrode thickness first exhibits an increasing trend and then a decreasing trend. When the height of the electrode is fixed, \(K^{2}\) increases with increasing electrode thickness, which is accompanied by the reduction of \(V_{\rm P}\). As shown in Figure 4, top Pt electrodes exhibit the strongest loading effect. They lower the \(V_{\rm P\;the}\) most sharply and enhance the \(K^{2}\) the most significantly as their thicknesses increase. When the thickness of the Pt electrode \(t_{1}\) reaches 0.12\(\lambda\), its \(K^{2}\) is rather constant at around 10.60% with increasing electrode thickness. As for Pt, Au and W and Cu and Mo, their \(V_{\rm P}\) and \(K^{2}\) curves are also very close. This is owing to the fact that the mass loading effect of different electrode materials is in full proportion to their densities [32]. Cu possesses superior electrical conductivity and low cost, while maintaining a high \(K^{2}\) simultaneously. The utilization of Cu electrodes is often reported in wideband SAW filters [8; 30] and TCSAWs [33; 34]. Thus, it was chosen for the subsequent optimization of quasi-Rayleigh. Since the top Cu electrode thickness \(t_{1}/\lambda\) is 0.07, the \(V_{\rm P}\) is 2021.1 m/s and the \(K^{2}\) is 9.32%. Figure 3: (**a**,**c**) \(V_{\rm P}\) and (**b**,**d**) \(K^{2}\) of the quasi-Rayleigh mode (**a**,**b**) and quasi-Sezawa mode (**c**,**d**) as functions of different normalized ZnO thickness \(h/\lambda\) and etching ratio \(d/h\) simulated by FEM, when the top Al electrode thickness \(t_{1}=0.05\lambda\). Figure 5 shows the change of the \(V_{\rm P}\) and \(K^{2}\) of the quasi-Sezawa mode with the normalized top electrode thickness when the etching ratio is \(d/h\) = 0.7 and the ZnO thickness is \(h/\lambda\) = 0.6. For most electrode materials, similar with Figure 4a, the \(V_{\rm P}\) decreases monotonically with the increase of electrode thickness. As for the \(K^{2}\), an increasing trend can be seen at first, followed by a decreasing trend, as shown in Figure 5b. Nevertheless, the \(V_{\rm P}\) and \(K^{2}\) dispersion curves of Au both exhibit an abnormal fluctuation as the functions of the top electrode thickness. Meanwhile, when Au is utilized as the top electrode material, its \(V_{\rm P}\) and \(K^{2}\) change more dramatically with the top electrode thickness compared to other electrode materials. The maximum \(K^{2}\) appears at different electrode thicknesses for individual electrodes, but each \(K^{2}\)\({}_{\rm max}\) value is about 6.80% similar. Among them, when the top electrode material is Al, the \(V_{\rm P}\) decreases slowly as its thickness increases. Simultaneously, \(K^{2}\) varies in a small range but keeps a relatively high value with changes in thickness to the top Al electrode. As the top Al electrode thickness \(t_{1}/\lambda\) is 0.11, the \(V_{\rm P}\) is 5407.3 m/s and the \(K^{2}\) is 6.78%, which is quite an exciting value compared to the conventional ZnO/SiC structure using Al electrodes. In the previous structure, the \(K^{2}\) of the Sezawa wave is only \(\sim\)3.6% when the \(V_{\rm P}\) is of the same value as the optimized configuration, indicating that partially etching the structure is an effective way to enhance the \(K^{2}\) of the ZnO/SiC substrate and a promising solution for the fabrication of SAW devices using high frequencies and wide bands. Figure 4: \(V_{\rm P}\) (**a**) and \(K^{2}\) (**b**) of quasi-Rayleigh waves as functions of top electrode thickness \(t_{1}/\lambda\) based on the partially etched ZnO/SiC structure when the etching ratio \(d/h\) = 1 and ZnO thickness \(h/\lambda\) = 0.5.