Optical and Electrical Properties of Sn-doped ZnO Thin Films Studied via Spectroscopic Ellipsometry and Hall Effect Measurements

We investigated the optical and the electrical properties of Sn-doped ZnO thin films grown via RF co-sputtering deposition methods at room temperature. Through annealing, the carrier concentrations and mobilities were improved. The ellipsometric angles, ${\Psi}$ and ${\Delta}$, of the ZnO:Sn thin films were measured via spectroscopic ellipsometry. Dielectric functions were obtained from the ellipsometric angles by using the Drude and the parametric optical constant models. With an increase in the Sn doping concentration, the Drude model amplitude increased substantially. The Urbach and the optical gap energies of the ZnO:Sn films were determined using the dielectric functions. The carrier concentrations and the mobilities of the ZnO:Sn thin films were measured using Hall-effect measurements. The effective mass of ZnO:Sn was estimated to be $0.274m_0$, assuming that the carrier concentrations measured via ellipsometry and Hall-effect measurements were the same. A shift in the optical gap energy of the Sn-doped ZnO was found to be due to a combination of the Burstein-Moss effect, electron-electron interactions, and electron-impurity scattering. The discrepancy between the measured and the calculated shifts in the optical gap energy is attributed to Sn-alloying effects.