Influence of heat treatment on structural, electrical, impedance and optical properties of nanocrystalline ITO films grown on glass at room temperature prepared by electron beam evaporation

Abstract

Indium tin oxide (ITO) thin films with composition of 9.42 wt% SnO2 and 89.75 wt% In2O3, and impurities balanced on glass substrates at room temperature were prepared by electron beam evaporation technique and then were annealed in air atmosphere at different temperatures from 350 to 550 °C for 1 h. XRD pattern showed that increasing annealing temperature increased the crystallinity of thin films and at 550 °C, high-quality crystalline thin films with grain size of about 37 nm were obtained. Increasing annealing temperature also increased conductivity of ITO thin films. Carrier densities (ND) and the flat-band potentials (EFB) were calculated by using electrochemical impedance spectroscopy (EIS). EIS results showed an enhancement in carrier densities and a shift of the flat-band potential to more positive values as the annealing temperature was increased. The UV-visible transmittance spectra were also confirmed that the annealing temperature has significant effect on the transparency of thin films. The highest transparency over the visible wavelength region of spectrum (93%) obtained at 550 °C on annealing temperature. This result was equivalent with those values that have already been reported but with higher level (20 wt%) tin-doped indium oxide thin films and also at 350 °C substrate temperature. The allowed direct band gap at the annealing temperature range 350-550 °C was estimated to be in the range 3.85-3.97 eV. Band gap widening with an increase in annealing temperature was observed and is explained on the basis of Burstein-Moss shift. A comparison between the electron beam evaporation and other deposition techniques showed that the better figure of merit value can be obtained by the former technique. © 2007 Elsevier B.V. All rights reserved.