Band gap narrowing and the band structure of tin-doped indium oxide films

Abstract

Films of tin-doped In 2O 3 were prepared by r.f. reactive sputtering using a metallic alloy target (InSn, 80-20). The annealing of films in different ambients of N 2, O 2 and cracked ammonia as well as different thicknesses of the films yielded carrier densities in the range 1×10 20 to 1×10 21 cm −3. The reflectance and transmittance were measured in the energy range 1.38–4.14 eV. The band gap was found to increase with increasing carrier density. The present data, along with the reported results for indium tin oxide (ITO) films, have been analysed by taking into consideration the band gap narrowing due to electron-electron and electron-impurity scatterings within the random phase approximation. Addition of band gap narrowing at different carrier densities to the measured band gaps at respective concentrations reported by different workers, when equated to the Burstein-Moss widening, shows that the intrinsic band gap of the films depends on the preparation conditions. Almost all the reported data of ITO films can be fitted to an average-reduced effective mass m vc ∗=0.22 m e . The band structure of ITO can be represented by a parabolic band with the conduction band curving upwards and the valence band downwards rather than the previously reported structure where both bands curve upwards.