Chemical Synthesis, Characterization and Gas-Sensing Properties of Thin Films in the In₂O₃-SnO₂ System

Abstract

Thin films in the SnO₂-In₂O₃ system, with relative concentrations of the two oxides ranging from 2% to 98% (molar percentage of the oxide), were deposited by sol-gel and solution processes. The films for the physical characterizations were deposited onto oxidized silicon substrates, while the films for the gas-sensing tests were deposited onto alumina. The starting solutions were characterized by FTIR spectroscopy, while the films on silicon, heated at various temperatures, were characterized by X-ray diffraction and SEM observations. The interaction between the two systems is particularly evident in the case of the system described by a 50% In₂O₃-50% SnO₂ nominal composition. The crystallization on In₂O₃ during the film heat-treatment hinders the crystallization of SnO₂. thus Sn is dispersed as an n-dopant in the In₂O₃ lattice, indeed showing a current signal, in the gas-sensing test, two orders of magnitude higher than the pure film. The response of the mixed-oxide based device to NO₂ is better than pure In₂O₃. On the other hand, the response (relative resistance change) of pure SnO₂ to low (from 0.1 to 1 ppm) NO₂ concentrations ranges from 150 to 300, a result that can be correlated with the nanostructure of the film, which, from SEM and XRD results, seems constituted by very small grains.