Dielectric modelling of optical spectra of thin In2O3 : Sn films

Optical transmittance spectra of In2O3 : Sri (ITO) films were simulated with a Computer program based on dielectric modelling. The films were prepared by radiofrequency sputtering under various oxygen fluxes such that the carrier density varies from 3 x 10(19) to 1.5 x 10(21) cm(-3). The dielectric function used is the sum of three types of electronic excitations: intraband transitions of free electrons (Drude model), band gap transitions. and interband transitions into the upper half of the conduction band. The parameters of these excitations are evaluated as a function of the carrier density. The damping in the Drude term was modelled frequency-dependent to account for the low extinction coefficient observed in the visible spectral range. The parameters resulting from the optical measurements were compared with those from the electrical measurements. Both the optical mobility and carrier density are found to be higher than those of the respective electric parameters. These discrepancies are attributed to a pronounced microstructure with badly conducting grain boundaries. The refractive index at 550 nm decreases linearly with increasing electron concentration. This is due both to the shift of the plasma edge and the Burstein-Moss shift of the band edge. All band gap transitions go up to the Fermi level.