Humidity-Sensing Characteristics of Divalent-Metal-Doped Indium Oxide Thin Films

Metal-doped indium oxide thin films that have been prepared on glass substrates via a dip-coating method from an aqueous sol show a rapid decrease in electrical conductivity at room temperature (by approximately three orders of magnitude) when brought into contact with moist air. This observation is in contrast to the conductivities of nondoped indium oxide films that are independent of moisture. Thus, the doped films can be applied as a humidity sensor. The film thickness has no substantial effect on the sensing properties, probably because of the porous nature of the films that are prepared by a sol-gel process. To examine the effects of film morphology, a comparison between the humidity-sensing properties of the films that have been prepared from organic and aqueous sols also has been conducted. The roles of the doped metal ions are also discussed.