Gas-sensing properties of SnO2 pyrolytic films subjected to ultrviolet radiation

Chemisorption of different species changes the electrical resistivity of many semiconductor films. In this work we have studied the behavior of thermally treated SnO2 films. At room temperature thermal desorption of many species is negligible in this material, so the electrical resistivity of the SnO2 films remains invariant with various gas compositions in the surrounding. To operate a gas sensor at room temperature with this material, we have used UV-radiation to produce photodesorption. At the same time this radiation produces photodissociation of the chemisorbed molecules. Using nitrogen as a carrier gas, small amounts of acetone and trichloroethylene vapors are injected into the system, measuring the current through the films with a constant applied voltage. Selectivity is achieved in this case by using filters that modify the IV excitation spectrum. The short-wavelength cut-off of this spectrum should be no longer than about 3200 Å, corresponding to the minimum photon energy to excit electron-hole pairs in the SnO2 films. Nitrogen/oxygen gas transitions have been investigated.