## 3 Results and Discussions
### Structure  
The structure of the films was studied both by X-ray diffraction (XRD) and selected area electron diffraction (SAED) using TEM. The XRD patterns of ZnO films with different dopants are shown in Figure 3. All films are polycrystalline, with diffraction peaks corresponding to those of polycrystalline wurtzite ZnO. The intensity of the (100) and (101) peaks decreases with Al doping, while Co doping leads to an increase in the intensity of all peaks, particularly in the (101) plane. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Grain Size (nm)** & \(D_{\mathbf{100}}\) (nm)** & \(D_{\mathbf{002}}\) (nm) & \(D_{\mathbf{101}}\) (nm) \\ \hline ZnO & 143 \(\pm\) 34 & 26 & 21 & 22 \\ AZO & 128 \(\pm\) 23 & 25 & 19 & 18 \\ CZO & 62 \(\pm\) 10 & 22 & 20 & 29 \\ CAZO & 71 \(\pm\) 10 & 26 & 21 & 22 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average grain size with standard deviations measured from SEM pictures (Figure 2) and calculated crystallite size \(D\) in indicated crystallographic planes. Figure 2: SEM pictures of pure ZnO thin films (**a**) and films doped with 5% of Al (**b**), 5% of Co (**c**) and co-doped with 5% of Al and Co (**d**). The scale bar is 1 μm. To check how doping impacts the crystallinity of the films, the crystallite size was calculated from the positions of the peaks and their full width at half maximum using the Debye-Scherrer formula. From the calculated data presented in Table 1, it can be seen that in general doping does not noticeably change the size of the crystallites, with one exception. In the case of Co doping, the size of the crystallites in the (101) direction increased from 22 nm for ZnO to 29 nm for CZO, an enhancement of more than 30%. The same trend of improved crystallinity was observed by [PERSON] et al. [17]. The authors assumed that cobalt in ZnO matrix would enhance the nucleation of particles and the growth of crystallites [17]. However, this trend was not observed for CAZO films that also contained Co. Considering that the net concentration of Co in CAZO is half that of CZO, we may conclude that a concentration threshold exists below which an enhancement is not observed. [PERSON] et al. [32] explained the preferential growth in a particular plane in terms of surface energy minimization: the preferred orientation is a result of self-ordering caused by the minimization of the crystal surface force energy. Obviously, more studies are needed to unveil the possible reasons behind the observed preferential growth in the (101) plane for CZO films. However, these are beyond the scope of the present manuscript. It can be seen from the SEM pictures (Figure 2) and Table 1 that for all studied films, the grain sizes are bigger than the crystallite size calculated from the XRD patterns (Figure 3). This means that the grains on the surface were built from few crystallites (about 6 for pure ZnO and AZO films and about 3 for CZO and CAZO films), so an agglomeration of crystallites took place. The decrease in the grain size in the case of cobalt doping (Figure 2c,d) could be associated with suppression of crystallite agglomeration, possibly due to the higher solubility of Co in the ZnO matrix as compared to Al. Similar suppression of aggregation is clearly evident in [10], but unfortunately no reason is discussed. On the other hand, the decrease in grain size leads to growth of the attraction forces between crystallites, causing them to aggregate slightly in the case of cobalt doping. Another possible explanation was given by [PERSON] et al. [5], who attributed the increase in crystallite size to the distortion in the ZnO host lattice by Co impurities, which stimulate the coalescence of the ZnO crystallites [5]. It should be noted that no additional peaks due to dopant impurities or dopant oxides are observed at a 5% doping concentration. This is in line with the results of other authors, who have confirmed the presence of metal Co only at concentrations higher than 10% [22].