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.