Methane Detection by MIM Sensor Devices Based on Nano ZnO Thin Films Obtained by Sol-Gel and by Anodization: A Comparative Study

Amongst various gas sensor structures developed so far probably the least investigated one is the Metal- (Active) Insulator-Metal or (MIM) structure. It has been reported that the vertical electron transport mechanism of this structure offers high response with fast response and recovery for gases like H₂ and methane compared to the conventional planar configuration. In this paper we report on the comparative performances of two kinds of MIM sensors based on nanocrystalline-nanoporous ZnO as the active sensing layer derived by sol-gel and by the UV assisted electrochemical anodization method respectively. The sensor structures based on Pd-Ag (26%)/ZnO/Zn were investigated in presence of five methane concentrations (0.01, 0.05, 0.1, 0.5 and 1.0%) and in the temperature range 150°C to 300°C. The electrochemically developed ZnO showed superior performance in terms of operating temperature, response magnitude, response time and recovery time. Moreover, it showed response to much lower concentrations (like 0.01% and 0.05 %) of methane that could not be detected by the sol-gel derived sensor. The possible reason for the superiority of the electrochemically grown ZnO compared to the sol-gel derived one was shown to be the significantly lower grain size and appreciably high porosity. But, in spite of its promising performance the electrochemically developed MIM sensor has no selection of the substrate and so it is not compatible to the standard IC technology. It leads to its restricted use to develop only the stand-alone sensor devices. On the other hand, sol-gel derived MIM device is based on the Si substrate. So, it is IC compatible and is suitable for the integrated sensor platform.