Structural Characteristics

4.3.1. Effect of RF Power

Figure 4.7 and Fig. 4.8 display the XRD pattern of ZnO thin film for various RF

powers. All these XRD patterns observed several peaks located at 31.6°, 34.3°, 36.1°, and 47.3°, corresponding to ZnO (10 1 0), ZnO (0002), ZnO (10 1 1), and ZnO (10 1 2), respectively. But only the intensity of (0002) diffraction peak in the ZnO thin film was fairly strong, while the other intensities of diffraction peaks were weaker. Hence, all of these ZnO thin films are in the poly-crystalline phase with a hexagonal structure and the major orientation is (0002). It is also indicated that these ZnO thin films belong to the c-axis orientation of the film (the powder-diffraction file No.79-0208). Figure 4.7 exhibits the thickness effect of ZnO thin films. The detail film thickness for different deposition times with a constant RF power is shows in Table 4.5. Results clearly show that the ZnO thin films grown less than 10 min. (smaller than 50nm in thickness) are almost in the amorphous phase [88]. It is quite likely that the ZnO film began to accumulate and form on the substrate because ZnO particles were still able to move to a

suitable site in this stage. Consequently, there were not enough period arrangement/diffraction patterns to reveal the crystalline structure of the XRD pattern.

As the ZnO thin film grows over 20 min. (larger than 60 nm in thickness), a crystalline structure of (0002) diffraction peak begins to appear and become dominant in the film structure with further increasing of deposition time [81, 85, 87].

In addition, Fig. 4.8 presents the power effect on grown ZnO thin films with approximately 60 nm in thickness. Results show that ZnO thin films with enough film thickness are dominated by the (0002) orientation no matter what the RF power is, although the intensity of the diffraction peak increases significantly by increasing RF power. At the same time, the relative intensities of diffraction peak (10 1 0) and (10 1 1) reduce gradually with increasing RF power. According to Gang et al. [167], when the leading edge of steep crystal touches the lateral of vertical crystal it thereafter stops growing, whereas the vertical crystal keeps growing. This corresponds to the phenomenon of “evolutionary selection” [167] or “survival of the faster” [86], which was first proposed by Van der Drift [168] to explain the preferred orientation of a vapor-deposited PbO layer. As mentioned above, it seems possible to elucidate why the intensity of diffraction peak (10 1 0) and (10 1 1) gradually decreased. On the other hand, the RF power also enhanced the substrate temperature during the deposition process by the ion bombardment energy. So the surface mobility increased with enough

temperature (thermal energy). For this reason, the quality of the ZnO thin film was increased when the grain size increased and the amount of defect decreased. Detailed descriptions of quality and defect with grain size will be examined later again. In conclusion, the thickness and power effect were found to enhance crystalline when the deposition time (thickness) and RF power increased.

Figure 4.9 shows the FWHM and the calculating average grain size (crystalline

size) as a function of applied RF power. The deposition rate increases with increasing RF power, which is strongly correlated with incident ion flux towards the substrate as shown in Fig. 4.3. Based on the Scherrer equation (see Eq. (2.49)), the grain size is calculated using the FWHM data by XRD pattern. Grain size decreases from 34 nm at 50 W down to 26.4 nm at 100 W and 200 W, and then increases gradually up to 29.6 nm at 400 W. In addition, Fig. 4.10 presents the SEM images to confirm the calculating average grain size scale by the Scherrer equation for RF power effect. The results show that the average grain size is very close to the grain size of SEM images. However, we could not observe the slight difference from the SEM images. In contrast with the almost monotonously increasing deposition rate, ion bombardment energy, and grain size with increasing RF power, we attribute the observed unusual trend of grain size to the three distinct power regimes as detailed next.

In the low-power regime (50 W), the largest grain size appears corresponding to the slowest deposition rate because of the lowest ion flux (see Fig. 4.1, Fig. 4.3 and Fig.

4.9). Thus, crystal growth is mostly deposition rate controlled, in which the ion

bombardment energy and plasma density are both small. The medium-power regime (100-200 W), presents higher ion flux (and higher plasma density) along with still low ion bombardment energy (8.7-10.2 eV) and thus higher ZnO particle flux towards the substrate causes adverse effects in crystal growth since there is insufficient time for adatom migration on the substrate. Hence, the lowest grain size is observed in this regime. Finally, in the high-power regime (300-400 W), very high ZnO particle flux migrates towards the substrate, but with very high ion bombardment energy (12.9-15.8 eV), which can heat up the substrate and enhance thermal diffusion of the adatoms on the substrate, and thus favors the crystal growth of the ZnO thin film [87]. Therefore, grain size increases with increasing RF power in this regime, and is mainly ion bombardment energy controlled.

4.3.2. Effect of Gas Ratio (Ar / O2)

Figure 4.11 and Fig. 4.12 show the XRD data of ZnO thin film for all combinations of O2/(Ar+O2) ratios, while Fig. 4.13 shows that the FWHM of (0002) as well as estimated grain size [162] as a function of the O2/(Ar+O2) ratio. Drawing from

the XRD pattern, all of these results are similar to the various RF power cases, whatever the thickness effect or the O2/(Ar+O2) ratio effect.

According to our observation, the thickness effect of RF power and O2/(Ar+O2) ratio demonstrated an identical trend, as shown in Fig. 4.7 and Fig. 4.11. The detail film thickness shown in Table 4.6 reports different deposition times with a constant O2/(Ar+O2) ratio. Consequently, the amount of atomic layers (film thickness) is an important factor in the strength of ZnO (0002) peak intensity under a constant deposition condition. Next, for the O2/(Ar+O2) ratio effect, results indicated that XRD patterns are very similar to each other as shown in Fig. 4.12. However, the peak intensity has slight variations with different O2/(Ar+O2) ratios. Therefore, the FWHM and grain size help to understand and explain the various O2/(Ar+O2) ratio change in ZnO thin film structural. A minimal value of FWHM was obtained at ~0.3 of the O2/(Ar+O2) ratio where the corresponding grain size was the largest (35.68 nm), as shown in Fig. 4.13. Bachari et al [169] argued that in the film structure, less oxygen will cause an increase in crystallographic defects, while more oxygen will destroy the stoichiometry due to the lower surface mobility of the deposited atoms and lower kinetic energy for surface diffusion. In addition, adding more oxygen into the discharge might produce more neutral oxygen atoms which diffuse into films without enough energy [161-162, 164], thus creating additional unexpected defects. Furthermore, grain

size increases from 29.28 nm at 0.1 of O2/(Ar+O2) ratio up to 35.68 nm at 0.3 of O2/(Ar+O2) ratio, and then decreases gradually until it eventually gets down to 30.86 nm at 1.0 of O2/(Ar+O2) ratio. At the same time, we also observed similar results from SEM images as shown in Fig. 4.14. In other words, the maximum grain size also occurs at 0.3 of O2/(Ar+O2) ratio that is consistent with the lowest FWHM of ZnO thin film observed in this study.