A study of undoped and molybdenum doped, polycrystalline, tin oxide thin films produced by a simple reactive evaporation technique

Abstract

Uniformly thick, conductive, tin oxide thin films ranging in thickness from 3 to 85 nm were deposited by reactive evaporation of tin, in the presence of oxygen, onto heated glass substrates. The conductivity of the films could be increased by doping with molybdenum using a simple, novel technique which is described. Film resistivity was found to increase with the time of exposure to laboratory ambients subsequent to deposition. Mobility and carrier density data for the more conductive films are presented. An approximately constant sheet resistance was found for the undoped films from which it is inferred that conduction is probably dominated by a highly conductive surface layer. The sheet resistance of the doped films increased with decreasing thickness. The films produced consisted entirely of tin oxide phases as indicated by X-ray diffraction results and were highly transparent (T>85%). There was no significant difference between the optical absorption edge (3.6-3.9 eV) for doped and undoped films. A direct optical transition was found to be the dominant absorption process. The direct bandgap was found to lie in the region 4.1+or-0.1 eV.