Hopping conduction in boron doped amorphous carbon films

The electrical transport properties of boron doped graphitelike amorphous carbon films have been studied. Variable range hopping (VRH) conduction is found to be the conduction mechanism in these carbon films. The conduction in high temperature regime follows the Mott VRH [J. Non-Cryst. Solids 1, 1 (1968)], whereas a crossover from the Mott [Electronic Process in Non-crystalline Materials (Clarendon, Oxford, 1971)] to the Efros-Shklovskii [J. Phys. C 8, L49 (1975)] VRH is observed at low temperatures. Additional support to this transition is evident from negative magnetoresistance in VRH regime when the sample is deep inside the insulating side of the metal insulator transition. Various calculated parameters such as localization length, density of states at the Fermi level, and Coulomb gap for insulating samples have been estimated from the experimental data. The density of states at the Fermi level shows a gradual shift with a corresponding variation in boron doping level, indicating a change in the density of conducting \pi electrons due to substitutional doping of boron in the carbon network