Optical Characteristics

In this section, we will discuss the optical characteristics of transmittance (T%) and absorption (abs.) under the following topics:

(A) The thickness effect for a constant RF power;

(B) The power effect for a constant thickness at 60 nm;

(C) The thickness effect of ZnO/glass (ZnO thin film coated on glass substrate) for a constant RF power;

(D) The power effect of ZnO/glass for a constant thickness at 60 nm;

(E) The thickness effect for a constant O2/(Ar+O2) ratio;

(F) The O2/(Ar+O2) ratio effect for a constant thickness at 60 nm;

(G) The thickness effect of ZnO/glass for a constant O2/(Ar+O2) ratio;

(H) The O2/(Ar+O2) ratio effect of ZnO/glass for a constant thickness at 60 nm.

4.5.1. Effect of RF Power

Topic (A) used the thickness effect of pure ZnO thin film to investigate the transmittance and absorption spectra in the visible (400-700 nm) and UV (280-400 nm) region as shown in Fig. 4.20 and Fig. 4.21, respectively. Figure 4.20(a), shows that the average transmittance of ZnO thin film (TZnO,ave%) decreases from 98% to 86% with increasing the deposition time from 5 min. to 60 min. The relative thicknesses were from 31.5 nm to 171.7 nm in the visible region [73-75, 80]. These results indicate that

the method proposed in this study achieved excellent optical transparency. It is an important property for optical device applications, especially in transparent conductive oxide (TCO) and glass industries. Correspondingly, Fig. 4.20(b) clearly shows that the TZnO,ave% decreases from 80% down to 5% by increasing the deposited time in the UV region. This phenomenon means that thicker ZnO film was accompanied with better UV-shielding characteristics. In addition, for absorption spectra, all of the specimens demonstrated lower absorption in the visible region as illustrated in Fig. 4.21(a). But the most significant absorption phenomenon appeared in the UV region due to ZnO characteristics. Therefore, the following figures of absorption are only present in the UV region. According to Fig. 4.21(b), the degree of absorption increases when the ZnO film thickness becomes increases. Therefore, the film thickness plays a key factor in UV sunlight absorption. Based on the observations of this study, the optimal UV-shielding level of ZnO thin film thickness is about 170 nm. In conclusion, we deduce that the complete UV-shielding characteristics of ZnO thin film may be obtained when the thickness is over 200 nm.

The aim of topic (B) was to investigate the transmittance and absorption spectra in the visible and UV region for the power effect of pure ZnO thin film as shown in Fig.

4.22 and Fig. 4.23, respectively. To understand the power effect, we maintained a constant ZnO thin film thickness of 60 nm, with various RF powers. All of the

transmittance spectra trends were nearly identical due to the constant thicknesses.

Furthermore, the results indicate that the TZnO,ave% decreases from 90% down to 86% in the visible region and from 50% down to 41% in the UV region (see Fig.4. 22(b)) with increasing RF power, respectively. However, both transmittance and absorption spectra (see Fig. 4.23) exhibit exponential decreases when the RF power increases, but the decay trends gradually diminish until reaching RF 400 W. Thus, the RF power still influences the optical properties of ZnO thin film. Kim et al. [76] also observed the similar result. Appropriate RF power seems to effectively create better UV-shielding characteristics. In this study, the best transparency with good UV-shielding characteristics was ultimately obtained at RF 400 W. From what has been discussed above, we can conclude that the main influencing factor of UV-shielding characteristics is thickness regardless of RF power. However, RF power is also able to affect the UV-shielding characteristics if all the ZnO thin films are fixed at the same thickness.

The difference between this result and the thickness effect is a matter that remains to be discussed further.

Next, it is important to present the overall sample of degree of transparency and absorption, and to explain why it is critical for practical applications. For this reason, it is necessary to focus on the influence of ZnO thin film coated on glass substrate by following topics (C) and (D). To understand the difference between pure ZnO thin film

and ZnO thin film coated on glass substrate, we aimed at the blank glass to measure the transmittance and absorption, respectively. The results show that the UV-shielding characteristics of blank glass are only good below a certain UV wavelength (~280 nm).

It indicates that blank glass has the highest transparency and well UV-shielding characteristics, as shown in Fig. 4.24 to Fig. 4.27. By combining ZnO thin film with blank glass, we have demonstrated that high transmittance (80-91%) in the visible region can still be obtained, and better UV-shielding characteristics are found with increasing deposition time as shown in Fig. 4.24(a). For example, less than 10% of optical transmittance can be obtained for the wavelength below 350 nm.

The following section will be addressing the glass substrate effect. In Fig. 4.24(a) and Fig. 4.26(a), it was observed that the transmittance spectra seem combined with blank glass spectra and pure ZnO thin film spectra by thickness effect and power effect, respectively. Nevertheless, the absorption spectra also obtain a similar tendency (please refer to Fig. 4.21 to Fig. 4.27). Compared with absorption of pure ZnO and ZnO thin film coated on glass substrate, there is enough evidence to show that the absorption of ZnO thin film coated on glass substrate is higher than the absorption of pure ZnO thin film in the same conditions. Regardless of thickness effect or power effect, the average optical transmittance still keeps high transparency over 86% in the visible region with

ZnO thin film coated on glass substrate; however, the average optical absorption of UV region is clearly better than pure ZnO thin film.

To summarize briefly, effect of varying the RF power with a constant ZnO thickness on the optical transmittance from UV to visible is relatively minor [76] as compared to that of varying the film thickness, especially in the UV region. However, for transparency, the pure ZnO and ZnO thin film coated on glass substrate were exhibiting high transmittance (over 85%) in visible region regardless of the thickness effect or power effect. The best transparency and UV-shielding characteristics were obtained at RF 100 W with 170 nm thickness for thickness effect and RF 400 W with 60 nm thickness for power effect, receptively.

4.5.2. Effect of Gas Ratio (Ar / O2)

Figure 4.28 and Fig. 4.29 illustrate the transmittance and absorption of pure ZnO

thin film at a constant O2/(Ar+O2) ratio with different film thicknesses (for topic (E)), respectively. According to Fig. 4.28, it was also determined that the TZnO,ave% (transparent) decreases from 98.3% to 87.7% with increasing deposition time from 5 min. to 90min. in the visible region [73, 80]. Correspondingly, the TZnO,ave% (UV-shielding characteristics) decreases from 78.3% down to 13.1% with increasing deposition time in the UV region [174]. Likewise, the absorption of ZnO thin film in the UV region increases when the thickness increases, as shown in Fig. 4.29. Regardless of

transmittance or absorption, both results present the identical tendency under a constant condition for thickness effect.

Next, we explored the O2/(Ar+O2) ratio effect with the thickness fixed at 60 nm as displayed in Fig. 4.30. Figure 4.30(a) shows that all of the TZnO,ave% in the visible region exhibit higher transparency (over 86.5%) and the TZnO,ave% in the UV region display UV-shielding characteristics (from 50% to 30%) with the fixed film thickness at 60nm using different O2/(Ar+O2) ratios. Results clearly revealed that the transmittance spectra have no obvious variation with the O2/(Ar+O2) ratio variance, although there was still a slight difference in the UV region as illustrated in Fig. 4.30(b). The similar phenomena were discovered in the absorption spectra as shown in Fig. 4.31. We do not have enough evidence to explain non-regular tendency at this point. According to several reports, the oxygen-rich condition improves optical transmittance, but the oxygen-deficient condition will induce the lower transmittance [73]. This is due to an improvement of the stoichiometry of ZnO thin film which decreases loss in light scattering [164, 167, 175]. Nonetheless, we imagine that a slight variance will influence the optical characteristics, such as film thickness, grain size, crystalline, composition, etc.

Topic (G) and topic (H) both point out overall transparency (combined with glass substrate) for practical applications. Moreover, we have demonstrated that high

transparency (80-90%) can still be acquired, and better UV-shielding characteristics are found with increasing deposition time (film thickness) when the ZnO thin film coated on glass substrate (TZnO/Glass%) as presented in Fig. 4.32. Figure 4.33 also points out the high absorption in the UV region when the film thickness gradually increases. Likewise, Fig. 4.34 and Fig. 4.35 demonstrate the same consequences. These results reveal that

very good UV-shielding characteristics with excellent high transparency can be easily obtained on glass, if the ZnO thin film thickness is large enough.

In summary, when dealing with either pure ZnO thin film or ZnO thin film on glass, results show that the film thickness plays a more important role both in transparency and UV-shielding characteristics than RF power effect or O2/(Ar+O2) ratio effect. We expect that the optimum thick film thickness will produce complete UV-shielding with highly transparent ZnO thin film in some applications. Therefore, ZnO thin film is an excellent candidate material for high transparency and UV-shielding applications in optoelectronics and the glass industry, etc. Incidentally, color characteristics can be observed by these transmittance spectra [176]. For instance, according to Fig. 4.22(a), the strong absorption that appears near the edge between blue and violet and leads to couple with red and green exhibits slightly yellow characteristics in all approximately 60 nm ZnO thin films.