STRUCTURAL DISORDER IN ION-IRRADIATED CARBON MATERIALS

The effects of ion irradiation on carbon based materials are reviewed laying emphasis on the well known ability of carbon to have different kinds of bonding configuration with the surrounding atoms. It was found that two kinds of bonding configuration of the carbon atoms are allowed in solid amorphous carbon phases. These rearrange the four valence electrons of carbon into sp(2) (trigonal bond) and sp(3) (tetrahedral bond) hybridizations. Driving the trigonal carbon fraction (x), the physical and chemical nature of solid carbon materials can change in a dramatic way ranging from metallic (x approximate to 100%) to insulating (x approximate to 0%) through semiconductor properties. The amount of the tetrahedral (or trigonal) carbon atoms can be controlled by ion beam irradiation, using suitable conditions and/or introducing foreign species such as hydrogen or silicon by the implantation technique. In hydrogenated amorphous carbon (a-C:H) and hydrogenated amorphous silicon-carbon alloys (a-Si1-xCx:H), the ion beam effects are able to produce stable and reproducible compounds, achieved by tuning the hydrogen (silicon) concentration with well defined equilibrium curves between the trigonal carbon fraction and hydrogen (silicon) content. Raman spectroscopy and temperature dependent conductivity experiments performed on these alloys suggest clustering effects in samples with high carbon content (x approximate to 0.5) due to the strong binding energy of the C-C double bond with respect to C-Si and Si-Si. Several models and theoretical studies such as the ''random covalent network'' (RCN) and molecular dynamics calculations have been used to fit the experimental results. It is shown that, while RCN models are highly inaccurate because of the clustering effects, molecular dynamics calculation data are very close to the experimental measured physical properties and confirm the ability of the trigonal carbon to cluster in graphite-like aggregate.