Structure and electronic properties of a nongraphitic disordered carbon system and its heat-treatment effects

The heat-treatment effect on electronic properties is investigated in relation to structural change for pulsed-laser-deposited amorphous carbon thin films having sp(2)/sp(3) ratio approximate to9. The heat treatment at temperatures 200-400 degreesC increases conductivity and modifies the hopping conduction mechanism at low temperatures, resulting in the generation of a Coulomb gap at E-F. This is attributed to the heat-treatment-induced modification of the disorder nature of the structure from atomic-scale sp(2)/sp(3) disorder to a disordered graphitic sp(2)-domain network by the migration of sp(3) defects. In the heat-treatment temperature region above 600 degreesC, where the structure is featured with graphitic sp(2) domains, considerably small positive thermoelectric power is suggestive of carrier compensation by the competition of hole and electron carriers that originate from the inhomogeneous charge distribution caused by the difference of Fermi levels among graphitic sp(2) domains. In the high-heat-treatment-temperature region 800-1100 degreesC, the formation of an infinite percolation path network of the graphitic sp(2) domains induces an insulator-to-metal transition, where the electron transport in the sp(2)-rich metallic state is featured by weakly temperature-dependent conductivity with majority hole and minority electron carriers.