# Recent Advances in the Growth and Characterizations of SILAR-Deposited Thin Films
## 5 Metal Oxide Thin Films
### Zinc Oxide Films  
Zinc oxide films were deposited onto various substrates, such as copper (Cu), silicon (Si), and glass slides [131]. Morphological studies revealed nano-prisms, small flowers, and rods were observed in glass, silicon, and copper substrates, respectively. In XRD patterns, it was found that two, three, and four peaks could be detected in silicon, glass, and copper substrates. However, all the films showed the wurtzite phase with a very sharp peak in the (002) plane. [PERSON] and co-workers [132] described the produced ZnO films (15 deposition cycles), treated under 450 \({}^{\circ}\)C for 60 min. These films showed lower percentages of absorbance and transmittance values in the visible region. [PERSON] and co-workers [133] highlighted the films deposited on soda lime glass showing three peaks (20 = 31.74\({}^{\circ}\), 34.4\({}^{\circ}\), and 36.21\({}^{\circ}\)) with the hexagonal wurtzite structure. Major peaks corresponded to the (101) plane for the post-annealed films (250 \({}^{\circ}\)C, 60 min, air atmosphere) deposited on copper foil. Morphological characterization confirmed bigger grain sizes for films grown on the soda lime glass (260-300 nm) than copper foil samples (200-230 nm). Optical characterization showed that a 35-65% transparency in the visible range could be observed (soda lime glass). Further, they explained that the transparency improved and the absorption edge shifted from 350 to 380 nm upon the post-annealing process. The flower-like morphology and zincite hexagonal structure (crystalline size = 10-11.85 nm, thickness = 510 nm) could be seen in the films deposited on silica glass substrates by using ZnSO\({}_{4}\), ammonia, and thiourea [134]. These films could be used in solar cell applications due to the suitable band gap (3.2 eV) and moderately high transmittance in the visible region. The growth of the ZnO film (by using ethanol or distilled water) was proposed by [PERSON] and co-workers [135] as highlighted in Equations (41)-(44).  
\[\mathrm{ZnCl_{2}+2 NH_{4}OH\to Zn(OH)_{2}+2 NH_{4}Cl} \tag{41}\]  
\[\mathrm{Zn(OH)_{2}+4 NH_{4}^{+}\to 2H_{2}O+2H^{+}+\left[Zn(NH_{3})_{4}\right]^{2+}} \tag{42}\]  
\[\mathrm{[Zn(NH_{3})_{4}]^{2+}+4H_{2}O\to Zn(OH)_{2}+2 OH^{-}+4 NH_{4}^{+}} \tag{43}\]  
\[\mathrm{Zn(OH)_{2}\to ZnO+H_{2}O} \tag{44}\]  
Experimental results showed that a higher band gap (3.32 eV) was observed in the films grown with ethanol due to an increase in the carrier concentration. Figure 8 indicates rougher and smoother surfaces in the random oriented grains (using distilled water (DW) and well aligned along the c-axis (using ethanol (C\({}_{2}\)H\({}_{5}\)OH), respectively. It was noted that the Fermi level successfully moved to a higher energy level (significantly above the minimum of the conduction band). The influence of the number of cycles on the morphology of films was reported by [PERSON] and co-workers [136]. Firstly, nuclei were observed in the sample. Then, nucleation and growth processes were carried out. Finally, a flower-like structure was observed They explained that the greater number of petals with bigger diameters could be seen with the increasing number of cycles (1, 3, 5, and 10 cycles).