# Structure and Optical Properties of Transparent Cobalt-Doped ZnO Thin Layers
## 3 Results and Discussion  
### The Crystal Structure  
X-ray diffractograms of the ZnO:Co nanostructured layers are shown in Figure 1. All layers show polycrystalline structures. The observed reflections come from hexagonal ZnO wurtzite P63 mc group (JCPDS No. 00-89-0510). A strong diffraction peak (100) at 34.40 \({}^{\circ}\)20 indicates that the grain growth mechanism of the c-axis mainly parallel to the substrate surface along the crystallographic axis was observed [35; 36]. Other pure reflections (101), (102), (110), (103), and (112) corresponding to the ZnO orientation planes are also visible. The peak near 44.4 \({}^{\circ}\)20 comes from cobalt (111) JCPDS No. 00-001-1259. The peaks around 55 \({}^{\circ}\)20 are from impurities in the substrate, such as barium, which is a standard additive that increases acceptor conductivity in silicon substrates. Decreases in the value of the interplanar distance \(d\) (from the well-known Bragg's law [37]) leads to the observation of a shift of the peak (100) toward the higher 20 values. We observe a shift of the reflections towards higher values of 20, which was also observed for ZnO layers with a 3% concentration of Co ions by [PERSON] et al. [38]. Thus, this indicates that the applied growth technique is effective in generating uncontaminated ZnO layers.  
The texture of a certain plane is represented by the factor \(F\). The degree of crystalline orientation can be determined from the XRD patterns using the Lotgering \(F\) factor (Equation (1)), given as follows:  
\[F=\frac{p-p_{0}}{1-p_{0}} \tag{1}\]  
\[p_{0}=\frac{\sum I_{0}(h00)}{\sum I_{0}(hkl)} \tag{2}\]  
\[p=\frac{\sum I(h00)}{\sum I(hkl)} \tag{3}\]  
Figure 1: XRD diffractograms for thin layers of ZnO:Co grown in vacuum (on a silicon) at substrate temperature 30 \({}^{\circ}\)C, without annealing and annealed in air (at temperature 300 \({}^{\circ}\)C). JCPDS No. 00-036-1451 was also placed. In the inset, the peak width at half the maximum (FWHM) for the (110) peak is shown for both samples.  
where _I(hkl)_ and _I0(hkl)_ are the intensity of the XRD peaks for the sample and reference. The parameters p and p\({}_{0}\) represent the relative contribution of the reflections (_h_00) to the sum of all the reflections for the sample and reference. If _F_\(\approx\) 1, it corresponds to the desired height. The \(F\) values were calculated for the first three major peaks. The degree of crystalline orientation to the plane (100) increases from \(F\) = 0.08 (before annealing) to \(F\) = 0.94 (after annealing at 400 \({}^{\circ}\)C). All ZnO:Co layers showed an increased intensity in the diffraction pattern corresponding to the reflection of (100) compared to (002) and (101). This indicated a dominant orientation along the axis \(\phi\), which was related to the minimum value of surface free energy (100). The crystallite size \(D\) of the ZnO:Co layers was determined from the FWHM, for the first three peaks, using the well-known Scherrer equation \(D\) = 0.9_\(\lambda\)/(_g_-cos\(\theta\)), where \(g\) = FWHM. The equation gave an estimate of the grain size \(D\) = 110 \(\div\) 140 nm. An increase in grain size of 27% and a change in orientation during annealing at 300 \({}^{\circ}\)C were observed. [PERSON] et al. observed a similar effect [PERSON] et al. (2019). The annealing temperature has little influence on the grain size (which was also observed for pure ZnO layers [PERSON] et al. (2019)). In addition, changing the temperature of the substrate has a very strong effect on the quality of the resulting layer [PERSON] et al. (2019). The layers had a predominantly _a_-axis oriented phase. At high concentrations of cobalt ions, precipitation of cobalt metal appears (corresponding to the peak near 44.4 \({}^{\circ}\)20).