OXYGEN GAS-SENSING PROPERTIES OF UNDOPED AND LI-DOPED SNO2 THIN-FILMS

Responses towards oxygen for SnO2 thin films, either pure or Li doped, are obtained at different working temperatures. SnO2 and SnO2(Li) thin films are deposited by reactive sputtering with the same growth parameters. XRD analysis shows that both films are polycrystalline and present a preferential (110) orientation; the presence of lithium causes a reduction of the crystalline average size, which passed from 140 angstrom (undoped film) to 90 angstrom (doped film). The electrical conductivity of doped and undoped films at temperatures between 350 and 500-degrees-C depends on the partial pressure of the oxygen according to the power law: G approximately G0(P(O2))-n where the exponent n changes from 0.25 (pure films) to 0.40 (lithium-doped films). Lithium improves the sensor selectivity to oxygen, since it increases the film sensitivity to the oxygen and leaves unchanged the sensitivity to reducing gases like hydrogen, carbon monoxide and ethyl alcohol. The mechanisms of surface and bulk detection of oxygen in SnO2 thin films are also outlined.