Density-of-state effective mass and non-parabolicity parameter of impurity doped ZnO thin films

The density-of-state effective masses of impurity doped polycrystalline ZnO thin films were measured by the method of four coefficients technique. By applying the first-order non-parabolicity approximation, the polaron effective mass and the bare band mass at the conduction band minimum, together with the corresponding non-parabolicity parameters, were analysed successfully. The determined perpendicular polaron mass of 0.29 m_e and the bare band mass of 0.247 m_e at the conduction band minimum corresponded very well to the previous results obtained for ZnO single crystals. The non-parabolicity parameter of 0.457 eV^-1 derived for the polaron effective mass was larger than 0.33 eV^-1 which was obtained for the bare band mass due to the increasing function of the Fröhlich coupling constant with respect to the bare band mass in polycrystalline ZnO films.