NOx sensitivity of In2O3 thin film layers with and without promoter layers at high temperatures

With tighter emission controls, there is a need to develop high temperature gas sensors able to operate at high temperatures (>500 °C) in diesel exhaust or coal based power plants. Indium oxide (In2O3) layers were found to be sensitive to NOx (5–50 ppm) when tested at operating temperatures of 500–650 °C enabling their use in high temperature applications. In2O3 thin films were RF sputter deposited on microhotplate structures with different thickness (40–300 nm) in pure Ar as well as Ar and O2 (10% and 25%) mixture. In2O3 films without promoter layers and with gold or TiOx promoter layers (∼3 nm) were investigated for NOx sensing. Some of the films (without promoters, Au and TiOx promoter films) were also annealed at 900 °C in ambient of either N2, O2, 2% H2 in Ar, or 50% O2 in N2 mixture to stabilize the gas sensing properties of the In2O3 films and understand the effect of anneal conditions on the gas sensor response. Atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), four point probe measurements and gas sensor characteristics were employed to study the surface morphology, microstructure, composition, electrical and gas sensing properties of thin film In2O3. Thin In2O3 films (∼40 nm) exhibit higher sensor response (S ∼ 14) to 5 ppm NOx compared to thicker films (S ∼ 1) at 500 °C. 150 nm thick In2O3 films deposited in Ar and O2 presented the highest response (S ∼ 50) to 25 ppm NOx at 500 °C when compared to films (S ∼ 5) deposited in Ar. Au and TiOx promoter layers increased the sensor response and generated faster time constants (τ1rise ∼ 10 s) when compared to sensors without promoter layers (τ1rise ∼ 60 s). Annealed films exhibited less sensor response to NOx (S ∼ 1) when compared to as-deposited films. Sensor response and response time constants are presented from both as-deposited and annealed films to investigate the influence of annealing.