Scaling of Efros–Shklovskii variable range hopping conduction in ZnO:Cd thin films by sol–gel

Electrical transport properties of sol–gel derived ZnO:Cd thin films were investigated. It was observed that for the thin films with relatively high resistivity, a crossover from the Mott variable range hopping (VRH) conduction to the Efros–Shklovskii (ES) VRH conduction was observed at low temperatures. The temperature dependent resistivity within the whole VRH conduction range from the Mott type to the ES type could be well described by a universal scaling law. The observation of ES VRH conduction in ZnO:Cd thin films suggests that the many-body electron–electron interactions should be considered for the ZnO:Cd thin films with high resistivity