On the Dependence of H2 Gas Sensitivity of ZnO thin Films on Film Thickness

There is a real need for a device which is capable of detecting low levels of H2 ranging from 0.1 ppm to 100 ppm for applications involving leak detection in hydrogen fuel storage systems. Due to its versatile properties, ZnO thin films are good candidate for hydrogen gas detection. This research addresses the H2 gas sensitivity of ZnO thin films as a function of film thickness. Zn thin films of different thicknesses (100, 150 and 200 nm) were deposited, using e-beam evaporation technique on SiO2/Si substrates and then post-annealed at 500 °C temperature with a constant oxygen gas flow. Crystallographic structure of ZnO thin films was studied by X-Ray Diffraction (XRD) method while a Field Emission Scanning Electron Microscope (FESEM) was used for surface morphology and chemical composition studies. Electrical resistivity and carrier concentration of samples were measured by a four point probe instrument and Hall effect investigation system, respectively. The electrical response of ZnO thin films was tested to 40 ppm of H2 gas in the temperature range of 200-450 °C. Results showed that the best sensitivity was attributed to the ZnO film of 100 nm thickness at the operating temperature of 400 °C while an increase in film thickness and operating temperature caused the reducing of H2 gas sensitivity of the samples.