UNDOPED AND PD-DOPED SNO2 THIN-FILMS FOR GAS SENSORS

The aim of this work is the study of the structure and temperature dependence on the conductance and sensitivity of undoped and Pd-doped SnO2 thin films (0.5-2 mum). Polycrystalline SnO2 thin films have been synthesized by pyrolysis, on a heated substrate (oxidized silicon [100]), of an aerosol produced by ultra-high frequency spraying of a volatile precursor solution. A mixture of two precursors is used to obtain Pd-doped SnO2. Growth rate and microstructure of the films are particularly well controlled by the deposition temperature (460-560-degrees-C). Conductivity measurements were performed between 50 and 500-degrees-C, alternately under pure air (G0) and polluted air (G) (ethanol or CO), using gold layers as electrodes. The pure SnO2 sensitivity (G - G0)/G0 to ethanol increases when the elaboration temperature decreases, in connection with the specific area increase. Palladium incorporation significantly promotes the sensitivity. CO sensitivity is increased by 30 when palladium is incorporated into the SnO2 thin films and exhibits a marked peak at low temperature. The sensitivity stability is better when the deposition temperature is higher, according with a better stability of the microstructure. A variable frequency electrical study was carried out using complex impedance spectroscopy (1 mHz-20 MHz). Ethanol and CO reactions were observed and the role of grain boundaries studied.