## 3 Results and Discussion  
XRD was employed to determine the structures of the as-annealed ZnO thin films grown on quartz glass and sapphire (001) substrates, as shown in Figure 1. As can be seen clearly, there were only two peaks located at 35.15\({}^{\circ}\) and 73.20\({}^{\circ}\) for the annealed ZnO thin films grown on the quartz glass substrate, which contributed to the ZnO (002) and ZnO (004) planes, respectively. Due to the quartz glass being amorphous, there was no peak ascribed to the quartz glass substrate. Similarly, ZnO (002) and ZnO (004) peaks were also found in the case of the annealed ZnO thin films grown on sapphire (001) substrate. Meanwhile, excluding the ZnO (002) and ZnO (004) peaks, only one sapphire (006) peak exists, which is accordingly assigned to the sapphire substrate. These results obviously indicate that the annealed ZnO thin films grown both on quartz glass and sapphire (001) substrates were of the preferred (002) orientation. It should be noted that the average crystalline grain size '\(D\)' can be extracted from the XRD results through the Scherrer's formula as follows [29]:  
\[D\ =\frac{k\lambda}{\beta\cos\theta} \tag{1}\]  
where, \(k\) represents 0.9, \(\lambda\) means the wavelength of CuK\({}_{\alpha}\) radiation (1.5406 A), \(\beta\) stands for the full width at half maximum (FWHM) of the (002) plane, and \(\theta\) is the Bragg's diffraction angle. Additionally, the crystalline grain \(D\) is believed to be associated with the amount of defects in the ZnO thin films and is considered an effective way to measure the dislocation density (\(\delta\)) through the following relation \(\delta\) = 1/\(D^{2}\)[30]. Herein, \(\delta\) is defined as the length of dislocation lines per unit volume of the crystal. Furthermore, the residual stress (\(\sigma\)) of the as-received ZnO thin films can be calculated through the following formula:  
\[\sigma\ =\ -233\bigg{(}\frac{c-c_{\circ}}{c_{\circ}}\bigg{)} \tag{2}\]  
Herein, \(c\) is the lattice constant of porous ZnO thin films, which is equal to 2\(d\) (\(d\) stands for interplanar distance/spacing), while \(c_{\circ}\) is the lattice constant of the perfect ZnO crystal (0.5206 nm). The calculated values of \(D\), \(\delta\), and \(\sigma\) are listed in Table 1. Both \(D\) and \(\sigma\) of the as-received ZnO thin films grown on the quartz glass substrate are slightly larger than those on the sapphire (001) substrate. The FWHM and \(\delta\) values of the as-received ZnO thin films grown on the quartz glass substrate are slightly smaller than those on the sapphire (001) substrate, suggesting that the ZnO thin films grown on the quartz glass substrate have better crystalline quality than those on the sapphire (001) substrate. Figure 1: X-ray diffraction (XRD) patterns of the as-grown ZnO thin films on quartz glass and sapphire (001) substrates annealed at 600 \({}^{\circ}\)C for 1 h, respectively. SEM and AFM were employed to investigate the morphological properties of the as-received ZnO thin films grown on the quartz glass and sapphire (001) substrates. Figure 2a displays the SEM image of the ZnO thin films grown on the quartz glass substrate. As can be observed clearly, the film grown on the quartz glass is composed of ZnO nanoparticles. Besides, distinct and shallow pits turn up on the surface of the ZnO thin film. This result is comparable to the ZnO thin films annealed at 800-1000 \({}^{\circ}\)C by means of sol-gel method [31]. Furthermore, the cross-section SEM image indicates that the thickness of the as-received ZnO thin films on the quartz glass substrate was measured to be 102.8 nm, as shown in Figure 2c, which matches with the result (100.1 nm) detected by the profilometer. Interestingly, the surface root mean square (RMS) roughness of the ZnO thin film on the quartz glass substrate was measured to be 0.50 nm by the AFM, as illustrated in Figure 2e.