Tin Oxide Thin-Film Sensors for Aromatic Hydrocarbons Detection: Effect of Aging Time on Film Microstructure

SnO₂ thin films for gas-sensor devices were deposited by pulsed laser ablation on < h 00> silicon wafers and on substrate heater elements with 200 nm thick Si₃N₄ membranes as physical supports of the sensor stack. Microscopic analysis showed that the thin films were polycrystalline, with ∼40-60 nm equiaxial grains, high porosity, and high specific surface area. The sensors were exposed to gas mixtures containing benzene, wet synthetic air, and CO, with 0.5 ppm of benzene added to 30% relative humidity synthetic air and 20 ppm CO. A 100% relative conductance variation, Delta I / I ₀, of the sensing layer was observed. However, this Delta I / I ₀ value consistently decreased after prolonged testing (up to 2 weeks) in a gas-sensor chamber. The sensing layer was studied using several analytical techniques to understand the aging time effects on sensor performance under working conditions. X-ray diffractometry showed a marked decrease of the residual microstrain after prolonged heat treatments, suggesting a substantial reduction of lattice (point and line) defects. Furthermore, photothermal deflection spectroscopy highlighted a consistent decrease of the sensing layer absorption coefficient near 1 eV that could be related to a reduction of carrier concentration. Because structural defects in SnO₂, such as oxygen vacancies or dislocations, are electrically active and variations in the carrier concentration are, at least in part, related with changes in the defect density, the annealing of these defects might have accounted for the observed worsening of sensor performance with the aging time.