## 3 Results and Discussion
PL spectra were employed to investigate the emission characteristics of the as-received ZnO thin films grown on the quartz glass and sapphire (001) substrates, as illustrated in Figure 6. As can be observed clearly, both ZnO thin films revealed a strong UV peak around 350 nm, which corresponds to the near band edge emission (NBE) and zinc (Zn)-related defect luminescence peaks [36], such as Zn vacancies (V\({}_{\mathrm{Zn}}\)) and Zn interstitials (Zn\({}_{\mathrm{i}}\)). More clearly, the NBE is the recombination of free excitons through an exciton-exciton collision progress. There is a blue shift in the UV peak position compared with a previous report [37]. Furthermore, the UV peak intensity of the ZnO thin films on the quartz glass substrates was almost half of that on the sapphire (001) substrate. Apart from the UV peak, there was a wide and weak peak observed at 721 nm for the ZnO thin films on the quartz glass substrate, while two sharp peaks around 615 and 703 nm existed in the ZnO thin films on the sapphire (001) substrate. According to previous reports, the red peaks located at 721 or 703 nm were confirmed to be the contribution of the Zn vacancies (V\({}_{\mathrm{Zn}}\)) and/or complex defects involving V\({}_{\mathrm{Zn}}\)[38, 39]. Meanwhile, the yellow-orange peak of 615 nm was verified to correspond to oxygen interstitials (O\({}_{\mathrm{i}}\)) [40, 41, 42] and OH groups [43]. Consequently, the ZnO thin film on the quartz glass substrate mainly possessed the Zn vacancies/defects and some oxygen vacancies/defects, which is consistent with the XPS result (Figure 3c). Notably, the as-received ZnO thin film on the sapphire (001) substrate had relatively few Zn vacancies/defects and many oxygen vacancies/defects, which is well matched with the XPS measurement (Figure 3d). Raman spectra were introduced to detect the residual stress in the as-received ZnO thin films grown on the quartz glass and sapphire (001) substrates at room temperature, as displayed in Figure 7. For the as-received ZnO thin film on the sapphire (001) substrate, four characteristic Raman peaks  
Figure 5: (_ahv_)\({}^{2}\) vs. photon energy spectra for the ZnO thin films grown on the (**a**) quartz glass and (**b**) sapphire (001) substrates, respectively. Figure 6: Photoluminescence (PL) spectra of the as-received ZnO thin films grown on the quartz glass and sapphire substrates. named E\({}_{2}\)(low), A\({}_{1}\)(TO), E\({}_{2}\)(high), and A\({}_{1}\)(LO) were found, excluding the two characteristic Raman peaks (marked by points) of the sapphire substrate. In contrast, for the ZnO thin film on the quartz glass substrate, three peaks, E\({}_{2}\)(low), E\({}_{2}\)(high) and A\({}_{1}\)(LO), were observed. According to previous reports, the E\({}_{2}\)(high) mode located at appropriately 437 cm\({}^{-1}\) only corresponds to the oxygen atoms in the ZnO thin films. This clearly means that the residual stress of the wurtzite ZnO thin films can be obtained from the E\({}_{2}\)(high) mode [26], owing to which it is significantly sensitive and dependent on the residual stress. More deeply, the residual stress is normally calculated from the E\({}_{2}\)(high) mode based on the following equation [26]:  
\[\sigma=\frac{\Delta\omega\left(\text{cm}^{-1}\right)}{4.4}\ (\text{GPa}) \tag{4}\]  
where, \(\sigma\) and \(\Delta\omega\) are the residual stress and the shift compared with the standard E\({}_{2}\)(high) mode of the stress-free ZnO bulk [4; 26], respectively. The E\({}_{2}\)(high) mode of the stress-free ZnO bulk is located at 437 cm\({}^{-1}\). In this work, the E\({}_{2}\)(high) mode values of the as-received ZnO thin films on the quartz glass and sapphire (001) substrates were 441.0 cm\({}^{-1}\) and 447.1 cm\({}^{-1}\), suggesting the shifts of 4.0 cm\({}^{-1}\) and 10.1 cm\({}^{-1}\), respectively. Based on Equation (4), the residual stress values of the as-received ZnO thin films on the quartz glass and sapphire (001) substrates are 0.909 GPa and 2.295 GPa, respectively. These results are smaller than those detected by XRD.