Role of Oxygen Partial Pressure on NO ₂ Sensing Properties of Sputter-Grown In ₂ O 3– x Thin Films

This study investigates the influence of oxygen partial pressure (pO2) on the structural, compositional, and NO2 gas-sensing properties of sputter-deposited nonstoichiometric indium oxide (In2O3–x) thin films grown at a substrate temperature of 200 °C. Structural analysis confirms that all the films are polycrystalline and exhibit the cubic phase of In2O3. Notably, the film grown at 3.0% pO2 exhibited the highest crystallinity, larger grain size, granular morphology, and increased surface roughness. Significantly, the compositional analysis revealed the existence of higher oxygen vacancy concentrations in the 3.0% pO2-deposited film. The analysis of the NO2 gas-sensing characteristics demonstrates that the structural and defect optimization achieved at 3.0% pO2 directly translated to superior performance. This optimized In2O3–x thin film sensor exhibits an exceptional response of 75.5 toward a 10 ppm of NO2 concentration at an optimal operating temperature of 140 °C, along with rapid response and recovery times of 11 and 14 s, respectively. Overall, this work demonstrates that precisely tuning pO2 is a key factor for engineering the In2O3–x microstructure and defect chemistry, resulting in high-performance, real-time semiconductor NO2 sensors.