## 3 Results & Discussions  
### Structural Properties of Cr-Doped NiO Thin Films  
Figure 2a shows the XRD patterns of Cr-doped NiO thin films with diffraction angles ranging from 20\({}^{\circ}\) to 70\({}^{\circ}\) at various doping times. The diffraction patterns of Cr-doped NiO films reveal two diffraction peaks at 43.3\({}^{\circ}\) and 62.5\({}^{\circ}\), respectively, corresponding to the NiO (200) and Ni (220) orientations. These XRD patterns suitably paired the standard documented XRD spectrum in the JCPDS card No. 00-047-1049. This result indicates that the crystallization of the deposited Cr-doped NiO layer forms a cubic structure. A similar \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **NiO Target** & **Cr Target** \\ \hline Purity & 99.99\% & 99.999\% \\ Diameter of the target & 50 mm & 50 mm \\ Thickness of the target & 5 mm & 5 mm \\ Preheat temperature & 100 \({}^{\circ}\)C & 100 \({}^{\circ}\)C \\ Preheating time & 90 min & 90 min \\ Substrate temperature & 100 \({}^{\circ}\)C & 100 \({}^{\circ}\)C \\ \multicolumn{2}{c}{Sputtering gas} & Argon & Argon \\ Deposition time & 90 min & 20–80 s \\ RF power & 50 W & 20 W \\ Argon gas flow & 4 sccm & 4 sccm \\ Base pressure & \(5\times 10^{-6}\) Torr & \(5\times 10^{-6}\) Torr \\ Operating pressure & 18 mTorr & 18 mTorr \\ Distance between target and substrate & 14 cm & 14 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Deposition parameters (RF Sputtering) of Cr-doped NiO. Figure 1: Schematic diagram of the sputtering chamber. trend has been observed in other studies in the doping of Cr in NiO [20]. Given that the doping amount is minimal, no phase corresponding to Cr is observed in the XRD pattern. The diffraction peak broadens with increasing doping time, and the amount of doping indicates that the crystallite size reduces with boosting doping time. Figure 2b shows the shift of the XRD peak with the diffraction angle from 42\({}^{\circ}\) to 45\({}^{\circ}\) along the (200) plane at different doping times. Given that the radii of Ni\({}^{+2}\) and Cr\({}^{+3}\) at a coordination number of 6 are 0.0690, 0.0615 nm, respectively, the diffraction peaks shift to the right as the amount of Cr doping increases. The diffraction peak along (200) plane shifts to 0.08\({}^{\circ}\) for a doping time of 80 s, compared with the undoped NiO. This result indicates that Cr\({}^{+3}\) can easily enter the NiO lattice [4]. Other structural parameters, namely, FWHM, crystallite size, microstrain, lattice constant, and dislocation density, were determined from the XRD analysis. Figure 3 shows the effect of crystallite size and FWHM of Cr-doped NiO diffraction peaks at (200) plane that corresponds to the different doping times. Crystallite size varies from 23.52 nm to 14.65 nm with the increasing doping time from 0 s to 80 s. Initially crystallite size of Cr doped NiO thin film increases with increasing doping time due to the creation of new nucleating center by the Cr atom. But at higher doping time, the crystallite size of Cr doped NiO thin film decreases and drastic change in grain size and grain boundaries. At higher doping concentration, it may be happened due to the different ionic radius of Cr\({}^{+3}\) and Ni\({}^{+2}\). Average crystallite size (\(L\)) was calculated by employing [PERSON]'s equation as given below [21]:  
\[L_{hkl}=0.9\lambda/(\beta cos(2\theta)) \tag{1}\]  
where, \(L\) is the crystallite size, \(\lambda\) is the X-ray wavelength, \(\beta\) is the full width at half maximum intensity of the main peak observed at 2\(\theta\) (in radian), \(\theta\) is the Bragg's angle. Variations in the macrostrain and dislocation density of Cr-doped NiO thin films at different doping times were calculated using Equations (2) and (3) respectively [22; 23; 24]. \[\varepsilon=\beta/(4\ tan\theta) \tag{2}\]  
\[\delta=n/L^{2} \tag{3}\]  
where \(L\) has the usual meaning as indicated in above and \(n\) is a factor that is very close to the unity for lowest possible dislocation density.