Fabrication parameters and NO 2 sensitivity of reactively RF-sputtered In 2O 3 thin films

Abstract

The strong sensitivity of the electrical properties of In 2O 3 thin films toward O 3 and NO 2 makes In 2O 3 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 In 2O 3 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/cm 2), process pressure: 9.0×10 −3 mbar, 80% Ar/20% O 2), a preferential [211] texture is obtained. The sensitivity ( S=(Δ R/ R)/ c (gas), Δ R/ R is the relative change of the electrical dc-resistance, c (gas) is the gas concentration (ppm)) of the films toward NO 2 increases due to the [211] texture by one order of magnitude. At an operating temperature of 400°C, the sensitivity toward 10 ppm NO 2 is between 1.6 and 2.1 ppm −1 and the response time τ 50 is 3.9 min. At 450°C, τ 50≤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 SO 2 and a two orders of magnitude smaller cross-sensitivity toward 60 ppm CO, 100 ppm NH 3 and 1000 ppm H 2 than toward 10 ppm NO 2.