TEMPERATURE DEPENDENCIES OF SENSITIVITY AND SURFACE CHEMICAL-COMPOSITION OF SNOX GAS SENSORS

The electrical resistivity response to CO gas exposure versus temperat ure has been measured for different types of SnOx-based gas sensors. T he chemical composition of the sensor surfaces and the electronic stru cture of the valence band are investigated by X-ray photoelectron spec troscopy and scanning Auger microscopy techniques, with the aim of exp laining resistivity changes in terms of the surface oxidation/reductio n mechanism. The samples are treated by Ar+ sputtering, thermal anneal ing in UHV and oxygen at various temperatures up to 400 degrees C. An ultrathin Pt overlayer, which enhances the gas sensitivity in a low op erating temperature range, is found to be very porous. The band-gap st ates induced by oxygen vacancies and adsorbed hydroxyl groups are reve aled by valence-band spectra. The resistivity changes of the sensors d ue to exposure to reducing or oxidizing gases are caused more by the c hanges of the surface-defect density than by the variation of excess s urface charge due to oxygen adsorption.