## 3 Results and Discussion
The two thicker SiO\({}_{2}\) layers surrounding the ZnO layer, as shown in Figure 2, can make it harder for electrons to flow into and out of the ZnO layer. This would suppress the screening effect while forming potential barriers, leading eventually to the elevation of the output voltage. On the other hand, when the SiO\({}_{2}\) film is placed as an interlayer between the top and bottom ZnO layers, as shown in Figure 3, there will be a critical or optimum thickness of the SiO\({}_{2}\) film layer which acts as a barrier that helps to elevate the output voltage while suppressing the screening effect. Moreover, a thinner SiO\({}_{2}\) layer may allow relatively more electrons to flow through the oxide film. If the SiO\({}_{2}\) layer is thicker than the critical or optimum thickness, it may more effectively block the flowing of the electrons generated by the piezoelectric effect, thus reducing the net output voltage. The more detailed descriptions on the precise physical/electrical mechanism of our multilayered thin film devices remain our future work. Table 2 shows the thicknesses of individual film layers. The SiO\({}_{2}\) film thickness was about 52 nm when the top ZnO film was 153 nm thick and the bottom one was 116 nm thick. Figure 7 shows a cross-sectional view of the SiO\({}_{2}\) layer sandwiched between the top and bottom ZnO layers. The inset in the figure shows a high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern of the ZnO film, indicating that the ZnO film has a crystalline form [15]. The distance between the (0002) planes was found to be 0.26 nm. This observation is meaningful because the crystallinity of the ZnO film can have an influence on its ionic polarization, eventually affecting its piezoelectricity. Comparing the output voltages of the SZS and ZSZ devices shown in Figures 5 and 6, the ZSZ devices were found to exhibit better performance than the SZS devices. From this observation, we were inspired to further investigate the ZSZ devices in the hope of enhancing the device performance even further. First, we investigated the effects of working pressure on the output voltages of the ZSZ devices. We fabricated five ZSZ-MG samples with SiO\({}_{2}\) films deposited under various working pressures of 5 \(\times\) 10\({}^{-1}\), 1 \(\times\) 10\({}^{-1}\), 5 \(\times\) 10\({}^{-2}\), 1 \(\times\) 10\({}^{-2}\), and 5.1 \(\times\) 10\({}^{-3}\) Torr. Figure 8 shows the variations in the peak to peak (P-P) output voltages of these ZSZ-MGs with SiO\({}_{2}\) thin films deposited under different working pressures, indicating that there is a strong dependence of the output voltage on the working pressure during SiO\({}_{2}\) film deposition. Table 3 summarizes the deposition conditions of both the ZnO and SiO\({}_{2}\) thin films used to fabricate these ZSZ-MGs. As shown in Figure 8, the ZSZ device with an SiO\({}_{2}\) film deposited for 15 min with an RF power of 200 W at working pressure of 5 \(\times\) 10\({}^{-2}\) Torr produced the best output performance (V\({}_{\text{P-P}}\) = 4.5 mV) among the five samples. As shown in Figure 9, the surface and grain morphologies of the deposited SiO\({}_{2}\) films seem to be affected by the working pressure, which may eventually have an influence on the output voltage. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Film** & **ZnO** & **SiO\({}_{2}\)** \\ \hline Power (W) & 200 & 200 \\ Pressure (Torr) & 1.0 \(\times\) 10\({}^{-2}\) & 1.0 \(\times\) 10\({}^{-2}\) \\ Gas Flow (sccm) & N\({}_{2}\)O (12) + Ar (18) & Ar (20) \\ Substrate Temperature (\({}^{\circ}\)C) & 29 & 29 \\ Target-Substrate Distance (cm) & 12 & 12 \\  & SZS-MG: 60 & 5, 15, 30 \\  & ZSZ-MG: 30 (upper layer)/ & 5, 15, 30 \\  & 30 (lower layer) & 30 (lower layer) \\ \hline \hline \end{tabular}
\end{table}