Optical characterization of aluminum-doped zinc oxide films by advanced dispersion theories

ZnO:Al films were fabricated by transition mode sputtering at various working points, and their optical properties for the visible, near-infrared and far-infrared wavelength regime was modeled. Especially, the behavior of free carriers was investigated by various models and the resulting sheet resistance was compared with measurements obtained by four probe method. For optical characterization in visible and near-infrared range we used a combination of reflectance and transmittance spectra, spectroscopic ellipsometry at variable angles as well as Fourier transform infrared spectroscopy. For free carriers the classical Drude theory is compared with the more sophisticated models by Hamberg based on ionized impurity scattering and by Sernelius, where the polar character of ZnO is taken into account. We show that the model of Hamberg leads to a significant improvement in the description of ellipsometry, reflectance and transmittance spectra in the visible and the near-infrared regime. Further, a proper description of the reflectance spectra in the IR regime requires to take into account the polar character of ZnO as in the work of Sernelius.