Transition between Efros-Shklovskii and Mott variable-range hopping conduction in polycrystalline germanium thin films

We report on the electrical transport properties of polycrystalline germanium thin films which are grown by the DC magnetron sputtering method. The temperature dependent resistance of seven devices are measured from 290 K down to 10 K. The thermal excitation model dominating the transport properties at the high temperature regime (above ∼60 K) is demonstrated and the low temperature electron transport is governed by the variable-range hopping (VRH) mechanism. Moreover, we observed a transition from Efros-Shklovskii to Mott VRH at ∼25 K over the entire VRH conduction regime, which is well described by a universal scaling law.