## 3 Results and Discussion
As shown in Figure 3c and Table 2, for the ZnO thin film grown on the quartz glass substrate, the relative area ratio of the O2 peak was detected to be 4.40%, which was much smaller than that grown on the sapphire (001) substrate (33.16%). This result means that the quartz glass substrate is beneficial for reducing the O vacancies/defects in the ZnO thin films compared with the sapphire (001) substrate. It is worth noting that such a high relative area ratio value (33.16%) of the O2 peak strongly suggests a high number of internal holes in the as-received ZnO thin films grown on the sapphire (001) substrate. Therefore, the as-received ZnO thin film grown on the quartz glass substrate is much denser than that grown on the sapphire (001) substrate. Figure 3: (**a**) X-ray photoelectron spectroscopy (XPS) analysis spectra and (**b**) high-resolution spectra of Zn 2\(p\) region for the as-achieved ZnO thin films grown on quartz glass and sapphire (001) substrates annealed at 600 \({}^{\circ}\)C for 1 h, respectively. (**c**) and (**d**) are XPS O 1\(s\) core-level 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. UV-Vis spectra were utilized to study the optical properties of the as-received ZnO thin films grown on the quartz glass and sapphire substrates, as presented in Figure 4a,b respectively. These two ZnO thin film samples showed an optical transmittance value of approximately 83% in the wavelength range of 300-800 nm. This result is in the common level (75%-90%) of the optical transmittance for ZnO thin films [34; 35]. Meanwhile, in the wavelength range of 300-370 nm, the as-received ZnO thin film on the quartz glass substrate exhibited a transmission of about 20%, which was much lower than that on the sapphire (001) substrate (approximately 60%). The higher optical transmission of the ZnO thin films on the sapphire (001) substrate may be due to the great number of internal holes of the ZnO thin films mentioned above. According to a previous report, for the ZnO thin films, the optical band gap, \(E_{g}\), can be calculated by means of the following equation [25; 35]:  
\[\left(\alpha\hbar v\right)^{2}=\hbar v-E_{g} \tag{3}\]  
where, \(\alpha\) and \(\hbar v\) represent the absorption coefficient and photo energy, respectively. As shown in Figure 5a,b, the optical band gap values of the as-received ZnO thin films grown on the quartz glass and sapphire (001) substrates were calculated to be 3.19 eV and 3.08 eV, respectively. Clearly, the optical band gap of the former was somewhat larger than that of the latter, which is attributed to the decrease in band bending effect at the grain boundaries that originate from the size increase of the ZnO nanograins [25]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Substrate** & **Annealing** & **Peak** & **Position (eV)** & **FWHM (eV)** & **Area (\%)** \\ \hline Quartz glass & 600 \({}^{\circ}\)C & O1 & 529.96 & 1.23 & 36.32 \\  & & O2 & 530.86 & 0.85 & 4.40 \\  & & O3 & 531.46 & 1.73 & 59.28 \\ \hline Sapphire & 600 \({}^{\circ}\)C & O1 & 529.94 & 1.24 & 37.06 \\  & & O2 & 531.31 & 1.51 & 33.16 \\  & & O3 & 531.97 & 1.81 & 29.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Oxygen-related XPS characteristic parameters for the as-grown ZnO thin films on quartz glass and sapphire (001) substrates annealed at 600 \({}^{\circ}\)C for 1 h.  
Figure 4: Ultraviolet-visible (UV-Vis) transmittance spectra of the as-received ZnO thin films grown on the (a) quartz glass and (b) sapphire (001) substrates, respectively.