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, total pressure, duration) so that appropriate annealing conditions (duration, temperature) can be used to modify the structural properties of the films: grain size, thickness, and stoichiometry. Important modifications of electrical performances in tin dioxide films for gas-sensing applications are observed. A correlation between structural properties of CVD films and their electrical behaviour is proposed. The main results are: (i) a sharp increase in the electrical conductance under pure air G0 from a critical value of the grain size D = 2L, due to the apparition of a conduction channel between adjacent grains; the depletion layer L is evaluated to 35 angstrom; (ii) a dependence of the electrical conductance G0 with stoichiometry observed for various deposition temperatures and various annealing conditions; the predominant effect 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 discontinuous and disordered, and an increase in G0 with e due to the increase of the number of percolation paths up to 3000 angstrom corresponding to a percolation threshold.