Iridium oxide as low temperature NO2-sensitive material for work function-based gas sensors

This paper presents the results on work function-based NO₂-sensing properties of iridium-oxide thin films at 130°C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO₂, H₂, CO, Cl₂, and NH₃ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by ∼100 mV on exposure to 5-ppm NO₂ (German safety limit). Cross sensitivity to other gases (except NH₃) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO₂ sensor.