High sensitivity chlorine gas sensors using multicomponent transparent conducting oxide thin films

Newly developed semiconductor thin-film gas sensors with a high sensitivity for chlorine (Cl2) gas using multicomponent transparent conducting oxide (TCO) thin films such as MgO–In2O3, ZnO–In2O3 and Zn2In2O5–MgIn2O4 are described. The multicomponent oxide thin-film gas sensors used in this work exhibited an increase in resistance with exposure to Cl2 gas. The sensitivity of multicomponent TCO thin-film gas sensors could be controlled by altering the chemical composition of the thin-films. The highest sensitivity for Cl2 gas was obtained in sensors using a Zn2In2O5–MgIn2O4 thin film prepared with Zn2In2O5 contents of about 60 mol%; when operated at 300°C in air, they were able to detect Cl2 gas at a minimum concentration of 0.01 ppm. The sensitivity or resistance of these (Zn2In2O5)0.6–(MgIn2O4)0.4 thin-film gas sensors was increased by a factor of about 100 when exposed to Cl2 gas with a concentration of 7 ppm. An increase of resistivity in multicomponent TCO thin-film sensors with exposure to Cl2 gas resulted from a simultaneous decrease of both carrier concentration and Hall mobility. The increase in resistivity is attributed to the trapping of free electrons resulting from Cl2 being adsorbed on grain boundaries and/or the thin film surface, the same as that produced by adsorption of oxygen. The sensitivity and resistance of Zn2In2O5–MgIn2O4 thin-film gas sensors exhibited very stable long-term operation in air containing a high concentration of Cl2 gas.