Probing the electronic structure of fine and large-grained SnO₂ layers by spectroscopic ellipsometry and current-voltage measurements

Small- and large-grained undoped SnO₂ films were deposited by atmospheric pressure chemical vapor deposition. The electronic structure of films was studied using spectroscopic ellipsometry measurements at temperatures up to 400 °C and at two thermal cycles, which were analyzed using the Tauc-Lorentz (T-L) and the Drude physical models. The parameters of the physical models have revealed information about details of the electronic structure of these films. It was found that the average energy of optical transitions E_n and the broadening C, related to the energy distance between valence and conduction band (VB and CB) and to the width of bands respectively, are the crucial parameters that influence the light absorption within the visible range. Small values of C and of the amplitude factor A which is related to the extent of the overlapping of wave-functions, observed for small-grained SnO₂ samples, indicate the strong localization of wave-functions in the CB and high effective mass of electrons therein, which lead to a linear-quadratic shape of the I-V curves. Gap states, the extent of which was expressed by the difference between the Tauc and the T-L gaps, were found to act as electron donors similarly to oxygen vacancies. Heating in air caused the decrease of gap states and the oxidation of samples, which in turn caused shifts of the plasma frequency, ω_p, whose magnitude and direction also depend on the effective mass of carriers, i.e., on the values of A and C.