Studies on spray deposited SnO2, Pd:SnO2 and F:SnO2 thin films for gas sensor applications

Undoped tin oxide (TO), palladium-doped tin oxide (PTO) and fluorine-doped tin oxide (F TO) thin films have been prepared by spray pyrolysis method, under optimized conditions. Transmission measurement of the TO, PTO and FTO films have been studied in the UV-Visible region. Band gap energy evaluated from the spectral data lies between 3.15 eV and 3.8 eV. The X-ray diffraction analysis of the films confirms a polycrystalline, tetragonal rutile structure of tin oxide, with preferred orientation along the (101) plane. The lattice constants, a and c, were evaluated as 4.7256 angstrom and 3.1768 angstrom. The average grain size lies between 470nm and 850nm and its variation as a function of annealing temperature have been studied and reported. The AFM micrographs of the films reveal smooth and uniform surface pattern without any dark pits, pinholes and microcracks. The sheet resistance (R) and electrical resistivity (rho) of the films under ambient condition and CO gas atmosphere have been studied at different operating temperatures, in a specially designed sensor test rig setup. From the resistance measurement, response and recovery characteristics, detection sensitivity and optimum operating temperature of the as-coated TO, FTO and PTO films towards CO gas have been investigated and the results are discussed. Studies show a maximum detection sensitivity of 13.6% with a minimum response time of 110 sec. for the PTO thin films, at the operating temperature of 350 degrees C.