Electrical conduction in amorphous carbon

We report measurements of electrical conduction in thin films of amorphous carbon. The magnitude of the conductivity varies greatly from sample to sample. At low fields the current is always linear in voltage and shows the temperature dependence which is characteristic of the Mott hopping mechanism. At high fields conduction appears to be by the Poole-Frenkel mechanism with a range of trap depths and a large effective dielectric constant. In the case of conduction through thin carbon films the resistivity appears to become independent of temperature at very low temperatures, and for this we have no adequate explanation.