An investigation of the bandgap and Urbach tail of vacuum-evaporated SnO2 thin films

Tin oxide (SnO2) thin films with thickness in the range 100-600 nm were prepared by vacuum evaporation on glass substrates at ambient temperature. X-ray diffractograms showed that the films have poor crystallinity. The transmittance of the films was measured at room temperature in the wavelength range 290-1100 nm. The absorption coefficient was plotted as a function of photon's energy for films of different thickness and oscillatory behaviour was observed in the high energy side. This oscillatory behaviour which was found to increase with film thickness is evidence on the presence of nanoparticles. Direct and indirect optical bandgap energies were estimated and found to be decreasing with film thickness. Urbach tailing in the bandgap was observed and the width of the tail which is related with disorder and localized states was estimted and found to be decreasing with film thickness. (C) 2012 Elsevier Ltd. All rights reserved.