Enhancement of NO₂ sensing properties of In₂O₃-based thin films using an Au or Ti surface modification

Metal oxide gas sensors using In₂O₃ are able to detect low concentrations of NO₂. The gas sensitive properties of In₂O₃ can be improved by adding catalysts or promoters. In the present work, the surface modification of reactively RF-sputtered In₂O₃ films (cubic phase) using Ti or Au is presented. The surface modified In₂O₃ films are investigated with respect to physical and gas sensitive properties. Fine distributed Au particles can be formed on top of the In₂O₃ surface. This enhances the NO₂ sensitivity significantly by a factor of 9 at 350°C and by a factor of 3.6 at 400°C. A low cross-sensitivity toward several gases, such as SO₂, NH₃, CO and H₂, is found. The response time τ₅₀ at 400°C is decreased from 4.0 to 2.2 min. On the other hand, particles of amorphous TiO₂ can be produced on top of the In₂O₃ surface. The Ti additive improves the signal shape of the response toward traces of NO₂. The response time τ₅₀ at 400°C is decreased from 4.0 to 1.8 min. For the Au surface modification a possible sensing mechanism is discussed.