## 3 Results and Discussion
On the contrary, the as-received ZnO thin film grown on the sapphire (001) substrate was also composed of smaller ZnO nano-particles but without any shallow pits on the surface (Figure 2b), suggesting a dense surface structure. As shown in Figure 2d, the thickness of the ZnO thin film on the sapphire (001) substrate was 108.6 nm, which is very closed the profilometer's result (105.3 nm). Moreover, AFM measurement revealed that the surface RMS roughness of the ZnO thin film on the sapphire (001) substrate was 0.61 nm. From what is mentioned above, it can be concluded that the as-received ZnO thin films grown on the quartz glass substrate was comparable to that on sapphire (001) substrate in terms of its morphological properties. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Substrate** & **2\(\theta\)/\({}^{\circ}\)** & _c(002)/nm_ & **FWHM/\({}^{\circ}\)** & _D/nm_ & _\(\delta\)/(\(\times\)10\({}^{-3}\) nm)_ & _\(\sigma\)/GPa_ \\ \hline Quartz glass & 35.151 & 0.5132 & 0.271 & 30.8 & 1.05 & 3.312 \\ Sapphire (001) & 35.020 & 0.5137 & 0.299 & 27.9 & 1.28 & 3.088 \\ \hline \end{tabular}
\end{table} 
Table 1: Peak position (2\(\theta\)), lattice constant (\(c\)), full width at half maximum (FWHM), calculated grain size (\(D\)), dislocation density (\(\delta\)), and stress values (\(\sigma\)) of the as-received ZnO thin films grown on the quartz glass and sapphire (001) substrates with an annealing treatment of 600 \({}^{\circ}\)C for 1 h.  
XPS has been demonstrated to be a powerful technology for confirming the chemical composition and electronic structure of the as-received ZnO thin films [32], as depicted in Figure 3a-d. Figure 3a represents the XPS analysis spectra of the as-achieved ZnO thin films grown on quartz glass and sapphire (001) substrates annealed at 600 \({}^{\circ}\)C for 1 h, respectively. Several peaks can be seen in Figure 3a, which correspond to the elements of Zn, C, and O, respectively. Considering that C is the standard reference, the as-received ZnO thin films are of very high purity. Furthermore, Figure 3b evaluates that the Zn 2p3/2 core levels were located in the range of 1021.24-1021.81 eV, which are assigned to the Zn\({}^{2+}\) ions in the ZnO thin films. As displayed in Figure 3b, two peaks located at 1021.1 and  
Figure 2: (**a**,**b**) are SEM images of the as-grown ZnO thin films on quartz glass and sapphire (001) substrates annealed at 600 \({}^{\circ}\)C for 1 h, respectively. (**c**,**d**) are their cross-section SEM images, respectively. (**e**,**f**) are their corresponding AFM images, respectively. 1044.1 eV can be obviously seen, which can be attributed to the Zn 2p\({}_{3/2}\) and Zn 2p\({}_{1/2}\) electronic states, respectively. The interval between the Zn 2p\({}_{3/2}\) and Zn 2p\({}_{1/2}\) electronic states was calculated to be 23.0 eV, which was generated from the spin-orbit splitting and is the same as the previously reported value [32]. In addition, the binding energy of the Zn 2p\({}_{3/2}\) electronic states was slightly lower than the reported value (1021.8 eV) of the bulk ZnO [33]. XPS O 1\(s\) core-level spectra were performed to further understand the oxygen-related defect states in the as-received ZnO thin films, as represented in Figure 3c,d. Normally, the O 1\(s\) peak is composed of three Gaussian peaks: O1 (529.96 eV), O2 (530.86 eV), and O3 (531.46 eV). The low binding energy O1 peak around 529.96 eV is believed to be the response of the lattice oxygen of ZnO, Zn-O bonds. The second peak, O2, is usually located at -530.86 eV, which is generated from the O vacancies or defects in the ZnO thin films. The third peak, O3, with a binding energy of approximately 531.46 eV is ascribed to a contribution of OH\({}^{-}\) group made from the H\({}_{2}\)O in the environment. As a result, the O2 peak can be taken as an index for the O vacancies/defects in the ZnO thin films.