Fabrication parameters and NO2 sensitivity of reactively RF-sputtered In2O3 thin films

The strong sensitivity of the electrical properties of In2O3 thin films toward O3 and NO2 makes In2O3 a promising material for gas sensors in the air quality control and for the safety at work. The microstructure and the related gas-sensitive behavior of reactively sputtered In2O3 thin films can be altered by varying the fabrication parameters like the sputtering power and the composition of the process atmosphere. In particular, the sensitivity can be increased. The layers usually show a [111] texture. However, with specific process conditions (sputtering power: 100 W (power density: 0.32 W/cm2), process pressure: 9.0×10−3 mbar, 80% Ar/20% O2), a preferential [211] texture is obtained. The sensitivity (S=(ΔR/R)/c (gas), ΔR/R is the relative change of the electrical dc-resistance, c (gas) is the gas concentration (ppm)) of the films toward NO2 increases due to the [211] texture by one order of magnitude. At an operating temperature of 400°C, the sensitivity toward 10 ppm NO2 is between 1.6 and 2.1 ppm−1 and the response time τ50 is 3.9 min. At 450°C, τ50≤1.0 min and a very good sensor linearity is found. The sensitivity at 450°C is half as high than at 400°C. These layers exhibit a one order of magnitude smaller cross-sensitivity toward 10 ppm SO2 and a two orders of magnitude smaller cross-sensitivity toward 60 ppm CO, 100 ppm NH3 and 1000 ppm H2 than toward 10 ppm NO2.