Majority-carrier mobilities in undoped and n-type doped ZnO epitaxial layers

Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about n_s ∼ 10¹⁶ cm^-3 and 440 cm²/Vs, respectively. In the experimental 'mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at n_s ∼ 5× 10¹⁸ cm^-3 are significantly smaller than those at higher densities above ∼ 1020 cm^-3. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.