# Study on the Ozone Gas Sensing Properties of rf-Sputtered Al-Doped NiO Films
## 3 Results and Discussion  
### Characterization  
The XRD patterns of rf-sputtered Al-doped NiO films deposited on Si substrates at room temperature with different O\({}_{2}\) content in plasma that underwent thermal annealing at 400 \({}^{\circ}\)C for 24 h are presented in Figure 2. It can be seen that all films had a polycrystalline structure with a peak of high intensity at 20 = 43.3\({}^{\circ}\) corresponding to a reflection from the (200) planes and two low intensity peaks at 20 = 36.5\({}^{\circ}\) and 62.9\({}^{\circ}\) corresponding to reflections from the (111) and (220) planes, respectively, according to JCPDS Card No. 047-1049. No peaks corresponding to other materials were found, indicating that all Al atoms were introduced in the NiO structure. Using Equation (1) for the diffraction peak with the highest intensity (200), the crystallite size of the films was calculated (Table 1) and found to be between 8.68 nm for films prepared with 2% and 2.8% O\({}_{2}\) in plasma and 7.89 nm for those prepared with 4% O\({}_{2}\) in plasma. In the case of the films prepared with 4% O\({}_{2}\), the decrease of the crystallite size may lead to a further increase of defect states inside the crystal structure, which can also be observed through the intensity of the main peak. In addition, the lattice constant (a\({}_{0}\)) of the films was calculated from Equation (2) and was varied between 0.417 nm and 0.421 nm (Table 1). Both the crystallite size and lattice constant are in agreement with the values reported in the literature for Al-doped NiO films prepared by various techniques [25; 31].  
The surface morphology of the NiO:Al films is presented in Figure 3a-c. It can be seen that all films had a smooth and uniform surface consisting of grains with a diameter of around 40 nm The increased defects on the surface of the film prepared with 4% O\({}_{2}\) can be also be observed.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **\% O\({}_{2}\)** & **\%at.** & **t** & **D** & **a\({}_{0}\)** & **E\({}_{g}\)** \\
**No.** & **in Plasma** & **Al** & **(nm)** & **(nm)** & **(nm)** & **(eV)** \\ \hline S1749 & 2.0 & 5.3 & 160.3 & 8.68 & 0.417 & 3.66 \\ S1746 & 2.8 & 6.5 & 168.7 & 8.68 & 0.421 & 3.65 \\ S1743 & 4.0 & 6.7 & 167.5 & 7.89 & 0.420 & 3.66 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Deposition parameters, thickness (t), crystallite size (D), lattice constant (a\({}_{0}\)), and optical energy bandgap (E\({}_{g}\)) of rf-sputtered Al-doped NiO films.  
Figure 2: X-ray Diffraction (XRD) patterns of rf-sputtered NiO:Al films grown at room temperature with different O\({}_{2}\) content in plasma that underwent thermal annealing at 400 \({}^{\circ}\)C for 24 h.  
The transmittance spectra of the Al-doped NiO films are shown in Figure 4. All films were highly transparent (>70%) in the visible region as well as in the near infrared one. Using Equation (3) and the Tauc plot (inset of Figure 4), the optical energy bandgap was calculated and found to be around 3.66 eV (Table 1), comparable with the values reported by [PERSON] et al. for Al-doped NiO films grown by the rf sputtering technique [25].  
Figure 4: Transmittance spectra of rf-sputtered NiO:Al films grown at room temperature with different O\({}_{2}\) content in plasma that underwent thermal annealing at 400 \({}^{\circ}\)C for 24 h. Inset: Tauc plot to determine the optical energy band gap of the films.  
Figure 3: SEM images of rf-sputtered NiO:Al films grown at room temperature with (**a**) 2.0%, (**b**) 2.8%, and (**c**) 4.0% O\({}_{2}\) content in plasma that underwent thermal annealing at 400 \({}^{\circ}\)C for 24 h.