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 me and the bare band mass of 0.247 me 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 Froehlich coupling constant with respect to the bare band mass in polycrystalline ZnO films.