Electrical conductivity of ion-irradiated carbon

Amorphous carbon films have been irradiated with Cl-ions with energies between 1 and 40 MeV, and the electrical conductivity of the material has been measured as a function of the ion dose. The room temperature conductivity is increased by nearly three orders of magnitude and saturates at a dose of about 10^(15)cm^(−2). The rate of conductivity change vs the ion energy can be explained by an ion track model. The temperature dependence of the conductivity between 100 and 300 K at low doses is in accordance with variable range hopping with a temperature exponent of 1/2. Hopping sites are assumed to be graphite rings or microcrystallites. At higher doses the variable range hopping is replaced by a metallic conductivity with a linear dependence on the temperature similar to that of polycrystalline graphite. This is probably due to the formation of complete percolation paths through the material by interconnection of the graphite regions. Estimates of the size of the crystallites, the hopping activation energy, and the cross section for producing crystallites in the material have been extracted