Conduction model of SnO2 thin films based on conductance and Hall effect measurements

Thin and porous SnO2 films (70nm thick with grain size between 10 and 30nm) have been prepared by e-beam evaporation onto alumina substrate provided with platinum electrodes. The Ohmic character of the contacts was preserved in all measurement conditions utilized for investigations. The dependence of electrical conduction on the composition of the ambient atmosphere has been studied by means of Hall and four point conductance measurements. The experiments were performed in different gas atmospheres containing N2, O2, and CO and at different operation temperatures (between room temperature and 420°C). A relatively low effective mobility (5–30cm2V−1s−1) and a high charge carrier effective concentration (1018–1019cm−3) were deduced when using the single crystals recipe, as required by the established models for granular materials. The analysis of these experimental data showed the inadequacy of the geometrical models and effective medium theories to correctly extract the electrokinetic parameters from conductance and Hall measurements in the case of gas sensitive layers and to predict their temperature and gas composition dependences. The conventional approach fails because it considers the samples at different temperatures as one physical system while, in fact, the surface chemistry in oxygen atmosphere leads to new trap generation, which is equivalent to the doping level modification. The use of a nonconventional approach, taking into account the film interaction with the ambient through quasichemical equations, and associated mass action laws together with the surface scattering influence on the carrier mobility allowed for the understanding of the involved mechanisms and good fits for the experimental data.