TIN DIOXIDE THIN-FILM GAS SENSOR PREPARED BY CHEMICAL-VAPOR-DEPOSITION - INFLUENCE OF GRAIN-SIZE AND THICKNESS ON THE ELECTRICAL-PROPERTIES

Tin dioxide films are elaborated by a chemical vapour deposition (CVD) method. An accurate control of deposition parameters (temperature, to tal pressure, duration) so that appropriate annealing conditions (dura tion, temperature) can be used to modify the structural properties of the films: grain size, thickness, and stoichiometry. Important modific ations of electrical performances in tin dioxide films for gas-sensing applications are observed. A correlation between structural propertie s of CVD films and their electrical behaviour is proposed. The main re sults are: (i) a sharp increase in the electrical conductance under pu re air G0 from a critical value of the grain size D = 2L, due to the a pparition of a conduction channel between adjacent grains; the depleti on layer L is evaluated to 35 angstrom; (ii) a dependence of the elect rical conductance G0 with stoichiometry observed for various depositio n temperatures and various annealing conditions; the predominant effec t of stoichiometry variations for films deposited at high temperature (100-300 angstrom grain size range) is responsible for the decrease of G0, and (iii) a strong influence of film thickness e, with a maximum of sensitivity for the thinnest films, in which tin dioxide is more di scontinuous and disordered, and an increase in G0 with e due to the in crease of the number of percolation paths up to 3000 angstrom correspo nding to a percolation threshold.