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 (pO 2 ) on the structural, compositional, and NO 2 gas-sensing properties of sputter-deposited nonstoichiometric indium oxide (In 2 O 3– 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 In 2 O 3 . Notably, the film grown at 3.0% pO 2 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% pO 2 -deposited film. The analysis of the NO 2 gas-sensing characteristics demonstrates that the structural and defect optimization achieved at 3.0% pO 2 directly translated to superior performance. This optimized In 2 O 3– x thin film sensor exhibits an exceptional response of 75.5 toward a 10 ppm of NO 2 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 pO 2 is a key factor for engineering the In 2 O 3– x microstructure and defect chemistry, resulting in high-performance, real-time semiconductor NO 2 sensors.