Electrical behaviour and optical properties of screen-printed ITO thin films

Abstract

Tin-doped indium oxide (ITO) films have been prepared by a screen printing (SP) method. The obtained SP ITO films are granular and porous. Sheet resistance of as-made layers, measured by the four-probe method, varies from 5 to 10 k Omega depending on firing time. The electrical behaviour of ITO films under different storage conditions was studied. The authors show qualitatively that the sheet resistance of ITO films depends on oxygen and water vapour pressures. The effect of heat treatment on sheet resistance in air and vacuum was also studied. The interpretation of the observed behaviours is based on the adsorption-desorption and diffusion phenomena. The refractive index n of the films has been determined from experimental R( lambda ) spectra. The dielectric constant epsilon infinity of the films is deduced from an n2=f( lambda 2) plot by using Drude theory. The degree of porosity of the films and the refractive index of the ITO crystallites were evaluated by simulating SP ITO as an effective medium. Carrier concentration and mobility were estimated from optical parameters by taking the electron reduced mass m*=0.3me. Finally, the first allowed direct transitions between the valence band and the conduction band, estimated from an alpha 2(h nu ) plot, were found to be in the range 4-4.12 eV, depending on fabrication parameters.