Variable-range-hopping conduction processes in oxygen deficient polycrystalline ZnO films

TL;DR

This study investigates charge transport in oxygen-deficient polycrystalline ZnO films, revealing a temperature-dependent crossover between Mott and Efros-Shklovskii VRH conduction processes, described by a universal scaling law.

Contribution

It demonstrates the dominance of Mott VRH at higher temperatures and ES VRH at lower temperatures in ZnO films, with a universal scaling law describing the entire VRH regime.

Findings

VRH conduction dominates below 100 K

Crossover from Mott to ES VRH at tens of Kelvin

Resistivity behavior follows a universal scaling law

Abstract

We have fabricated oxygen deficient polycrystalline ZnO films by the rf sputtering deposition method. To systematically investigate the charge transport mechanisms in these samples, the electrical resistivities have been measured over a wide range of temperature from 300 K down to liquid-helium temperatures. We found that below about 100 K, the variable-range-hopping (VRH) conduction processes govern the charge transport properties. In particular, the Mott VRH conduction process dominates at higher temperatures, while crossing over to the Efros-Shklovskii (ES) VRH conduction process at lower temperatures. The crossover occurred at temperatures as high as a few tens degrees Kelvin. Moreover, the temperature behavior of resistivity over the entire VRH conduction regime from the Mott-type to the ES-type process can be well described by a universal scaling law.