## 3 Results and Discussion  
A basic piezoelectric device consisting solely of a single layer of ZnO was fabricated for use as a control (reference) sample, as shown in Figure 4. For comparison, we designed and fabricated various kinds of new piezoelectric micro-energy generator devices by stacking both ZnO and SiO\({}_{2}\) films in an effort to further enhance the output voltage. As shown in Figure 2, the SZS-MGs are composed of a ZnO layer sandwiched between two SiO\({}_{2}\) layers. In contrast, the ZSZ-MGs are composed of a silicon dioxide (SiO\({}_{2}\)) layer sandwiched between two ZnO layers, as shown in Figure 3. The ZnO layer for SZS-MGs was deposited for one hour while for ZSZ-MGs, both the upper and lower ZnO layers were deposited respectively for 30 minutes with an RF power of 200 W, resulting in a total thickness of about 270 nm. The SiO\({}_{2}\) layers were deposited by sputtering with an RF power of 200 W for 5, 15, and 30 min, respectively. The output voltages were measured and compared to find out which device configurations could generate higher output voltages. While the control MG device made up of a single layer of ZnO alone showed a relatively low output voltage of 0.7 mV, as shown in Figure 4, better output voltages were obtained from the ZnO/SiO\({}_{2}\)-stacked film devices, i.e., the SZS-MGs and ZSZ-MGs illustrated in Figures 2 and 3. This is considered to be mainly due to the inserted SiO\({}_{2}\) insulating layers. The measured output voltages of the three SZS-MGs illustrated in Figure 2 are plotted in Figure 5. In the case of the devices with SiO\({}_{2}\) films deposited for times in the range from 5 min to 30 min, they produced slightly better output voltages ranging from about 0.9 to 1.7 mV. This seems to suggest that the thicker SiO\({}_{2}\) layers lead to better output voltages, but the magnitude of the output voltage is still small. The measured output voltages of the three ZSZ-MGs shown in Figure 3 are plotted in Figure 6. The ZSZ-MGs showed output voltages about 9 times larger than those of the SZS-MGs. The ZSZ-MGs device with a SiO\({}_{2}\) film interlayered between both the top and bottom ZnO layers was observed to generate about 15 mV of peak to peak (P-P) output voltage where the SiO\({}_{2}\) film was deposited for 15 min. The deposition conditions of ZnO and SiO\({}_{2}\) thin films used for the SZS-MGs and ZSZ-MGs, mentioned respectively in Figures 5 and 6, are summarized in more detail in Table 1.  
Figure 4: (a) Schematic diagram of control MG with a single ZnO layer used as a reference sample; and (b) its output voltage variation (V\({}_{\text{P-P}}\) = \(\sim\)0.7 mV) (ZnO film thickness = 273 nm). Figure 3: Schematic diagrams of three ZnO/SiO\({}_{2}\)/ZnO micro-energy generators (ZSZ-MGs) with different SiO\({}_{2}\) thin films. (a) SiO\({}_{2}\) thickness = 17 nm; (b) SiO\({}_{2}\) thickness = 52 nm; (c) SiO\({}_{2}\) thickness = 141 nm. Figure 5: (**Left**) Peak to peak (P–P) output voltages of three SZS-MGs as a function of the SiO\({}_{2}\) deposition time. (Average deposition rate of SiO\({}_{2}\) = 7.05 nm/min, ZnO = 4.55 nm/min at 200 W deposition power). (**Right**) Output voltage variations of the SZS-MGs with SiO\({}_{2}\) thin films deposited for (**a**) 5 min, (**b**) 15 min, and (**c**) 30 min, respectively. Figure 6: (**Left**) Peak to peak (P–P) output voltages of three ZSZ-MGs as a function of the SiO\({}_{2}\) deposition time (Average deposition rate of SiO\({}_{2}\) = 3.85 nm/min, ZnO = 4.48 nm/min at 200 W deposition power). (**Right**) Output voltage variations of three ZSZ-MGs with SiO\({}_{2}\) thin films deposited for (**a**) 5 min, (**b**) 15 min, and (**c**) 30 min, respectively. Therefore, the ZSZ-MGs showed better output voltages than the SZS-MGs after 300 bending/unbending cycles. Additionally, among the ZSZ-MGs, the SiO\({}_{2}\) films deposited at 200 W for 15 min were of a desirable thickness, leading to the generation of the highest output voltages. This is believed to be due to the blockage of electrons.