ION-BEAM DEPOSITION OF HARD, AMORPHOUS-CARBON FROM A FULLERENE DISCHARGE

The design and operation of an ion source that uses fullerene molecular solid as a feedstock of pure carbon is described. Fullerenes were vacuum sublimed and directed into a discharge plasma, which resulted in dissociation of the fullerenes into molecular carbon fragments. Molecular carbon ions extracted from the discharge impinged energetically upon a substrate and condensed into a thin film of hard, amorphous carbon at growth rates of approximately 1 μm/h. For fixed ion energies of 300 eV, substrate temperature was an important process parameter, affording a means for tuning the physical properties of the films. For deposition temperatures ranging from 250 to 575 K, room-temperature dc resistivity ranged from 105 to 10-2 Ωcm. The more resistive films, deposited at lower temperatures, were infrared transparent and showed no evidence of discrete absorption due to either C-H stretching modes or fullerenes. Broad band tails were observed in the resistive films, with optical gaps of 1.1 eV. The optical gaps of the conductive films were below 0.1 eV. Activation energies of conduction were extracted from the temperature dependance of the film resistivities. Both the resistive and conductive films displayed two activation energies in the range 77 to 650 K, with room temperature the demarcation between low-temperature values of tens of meV and high-temperature values of tenths of an eV. The resistivity and infrared transmission data were interpreted as evidence for the growth of sp2-bonded regions with increased deposition temperature. © 1995 American Institute of Physics.