## 3 Results and Discussions
### Optical Properties  
#### 3.3.1 Transmittance Measurements  
Transmittance spectra (\(T\)) of films deposited on glass substrates are measured in the spectral range from 320 nm to 900 nm and are plotted in Figure 5 along with the spectra of bare glass substrates. Since the thicknesses of all doped films are similar (around 200 nm),  
Figure 4: TEM pictures of pure ZnO films (**a**) and films doped with 5% Al (AZO) (**b**), 5% Co (CZO) (**c**) and co-doped with 5% of Al/Co (CAZO) (**d**). The typical SAED pattern with indexed diffraction is displayed as an inset.  
it is appropriate to compare their transmittance spectra and make conclusions about the degree of transparency. It can be seen from Figure 5 that the films are transparent in the VIS-NIR range, although a small decrease in \(T\) is observed in doped films, especially cobalt-doped films. Two possible reasons for the observed drop in \(T\) in the UV-VIS spectral range are scattering losses and light absorption. To analyze the possibility of small scattering losses, we measured the surface roughness of the films using an optical profiler. The obtained values are presented in Table 2. It can be seen that for the doped films, the surface roughness is from 5 nm to 9 nm higher than the pure film. This could generate some scattering, resulting in the decrease in \(T\). The comparison of the \(T\) spectra of doped films shows that AZO film has the highest transmittance, which is very similar to the \(T\)-curve of the undoped films. The difference in the spectral position of the peaks is due to a small difference in the film thickness of both films. Therefore, the small increase in the roughness observed for the Al-doped films does not lead to a drop in transmittance of the films. This means that the observed decrease in \(T\) for cobalt-doped films is due more to absorption than to scattering. The comparison of the spectral positions of the fundamental absorption edges indicates a slight blue-shifting for the AZO films and noticeable band-tailing for the CZO and CAZO films. Similar band tails were observed by [PERSON] et al. [22] and [PERSON] et al. [37] for pulsed-laser-deposited CZO, and were explained by impurities, disorder and point defects [22; 37].  
Interference patterns due to the multiple reflections from the top and bottom sides of the films is conspicuous in the \(T\) spectra of the ZnO and AZO films (Figure 5) and suppressed to some extent in the spectra for the CZO and CAZO films. In the cobalt-doped samples, there are three absorption peaks in the spectral range 550-700 nm, indicated with arrows in Figure 5. These absorption peaks are centered at 560 nm (2.21 eV), 609 nm (2.04 eV) and 655 nm (1.89 eV), and perfectly match the peaks obtained by other groups--2.20 eV, 2.04 eV and 1.88 eV [21].  
The absorption peaks are characteristic peaks of Co\({}^{2+}\) ions assigned to charge transfers between donor and acceptor ionization levels located within the band gap of the ZnO host [16] and prove the existence of Co at the tetrahedral sites of the ZnO hexagonal wurtzite structure in the form of Co\({}^{2+}\)[16; 21; 28].  
Figure 5: Transmittance spectra of pure ZnO thin film and ZnO doped with Al (AZO), Co (CZO) and (Co/Al) (CAZO) with a concentration of 5%.