# Study on the Ozone Gas Sensing Properties of rf-Sputtered Al-Doped NiO Films
## 3 Results and Discussion
### Gas Sensing  
The gas sensing performance of rf-sputtered Al-doped NiO films was investigated by exposing them to ozone gas with a concentration varying from 10 ppb to 3000 ppb, at operating temperatures between RT and 150 \({}^{\circ}\)C. The duration in ozone exposure was 5 min for films prepared with 4% O\({}_{2}\) in plasma and 10 min for the films prepared with 2% and 2.8% O\({}_{2}\) in plasma, while the recovery of the sensors was carried out by exposing them to synthetic air for 10 min and 15 min, respectively. The applied voltage was 10 V when the operating temperature equals to RT, while for the rest of the operating temperatures the applied voltage was 1 V.  
A typical measurement of electrical current variation for different O\({}_{3}\) concentrations is presented in Figure 5. In the presence of ozone the electrical current of the film is increased reaching a maximum value, while in the absence of the target gas the electrical current of the film is decreased, as a result of its interaction with synthetic air, reaching its initial value. Moreover, it is noted that the sensor cannot be fully recovered to its initial electrical current value, especially at high concentrations (>600 ppb), probably due to residual oxidation. The gas sensing mechanism for Al-doped NiO films is the typical one for a p-type semiconductor. Thus, upon the exposure to synthetic air, Oxygen species (O, O\({}^{2-}\), etc.) are formed as a result of the O\({}_{2}\) adsorption on the films' surface. This is described through the following Equations (5)-(7). \[\mathrm{O}_{2(\mathrm{gas})}+\mathrm{e}^{-}\rightarrow\mathrm{O}_{2(\mathrm{ adsorbed})}^{-} \tag{5}\]  
\[\mathrm{O}_{2(\mathrm{gas})}+2\mathrm{e}^{-}\rightarrow 2\mathrm{O}_{(\mathrm{ adsorbed})}^{-} \tag{6}\]  
\[\mathrm{O}_{2(\mathrm{adsorbed})}^{-}+\mathrm{e}^{-}\rightarrow\mathrm{O}_{ (\mathrm{adsorbed})}^{2-} \tag{7}\]  
where the kind of oxygen ions is dependent on the operating temperature (T\({}_{\mathrm{oper}}\)), being O\({}_{2}^{-}\) for T\({}_{\mathrm{oper}}\) < 100 \({}^{\circ}\)C, O\({}^{-}\) for 100 \({}^{\circ}\)C < T\({}_{\mathrm{oper}}\) < 300 \({}^{\circ}\)C, and O\({}^{2-}\) for T\({}_{\mathrm{oper}}\) > 300 \({}^{\circ}\)C. The Oxygen species take out electrons from NiO, increasing the number of holes, leading to an increase of electrical conductivity, thus a depletion layer is formed. As a result, the height of the potential barrier at the grain boundaries is decreased and the electrical current is slightly increased. When NiO:Al films interact with ozone, which is a strongly oxidizing gas, the ozone molecules are adsorbed on the films' surface trapping the free electrons, resulting in an increase of the hole concentration as well as in a decrease of the thickness of the depletion layer. Thus, an extra increase of the electrical current is occurred [32; 33]. Figure 5: Electrical current variation of a NiO:Al film prepared with 4% O\({}_{2}\) in plasma, as a function of time for different O\({}_{3}\) concentrations, at 110 \({}^{\circ}\)C.  
In Figure 6a-c the variation in sensitivity of the NiO:Al films grown with different O\({}_{2}\) content in plasma are shown. This was calculated by Equation (4) as a function of ozone concentration for various operating temperatures and plotted in logarithmic scale. In addition, a linear fitting has been applied confirming that the sensitivity is increased with O\({}_{3}\) concentration, independent of the Oxygen content in the plasma or the operating temperature. From Figure 6, it becomes clear that all films can detect the ozone gas even at an ultra-low concentration of 10 ppb. Specifically, the films prepared with 2% O\({}_{2}\) in plasma had a lowest detection limit of around 200 ppb with a sensitivity 1.88%, 11.31% and 38.66% at RT, 110 \({}^{\circ}\)C and 150 \({}^{\circ}\)C, respectively, while at 80 \({}^{\circ}\)C the lowest detection limit was 400 ppb with a sensitivity of 5.17%.