Room temperature humidity sensors based on co-evaporated TeO2 and Sn

Thin layers prepared by vacuum co-deposition of TeO2 and Sn are studied as humidity sensors working at room temperature. They are amorphous and, due to chemical interaction between the two substances, are built up of a mixed Sn- and Te-oxide matrix and a dispersed phase that at RSn/Te < 2.0 consists of elemental Te. The optimal composition of the layers is obtained by varying the atomic ratio RSn/Te. Auger analysis is used to follow the in-depth distribution of the components. It is shown that the layers have a homogeneous composition over their entire depth. For RSn/Te = 0.8-0.9, they exhibit very high sensitivity to humidity -the electrical resistance changes by more than 4 decades in the range of 20 - 90% RH. The sensor response is very fast and the recovery period, very short. A very high long-term stability is observed as well.