Carbon nanostructures - diamond nucleation centers formed during the bias pretreatment

Current models of diamond nucleation during the so-called bias pretreatment in chemical vapour deposition (CVD) consider the possibility that nuclei form in or on top of a carbonaceous layer that is deposited initially on the substrate. In this context we have investigated the stability of different carbon modifications under plasma conditions. Microcrystalline graphite, hydrogenated amorphous carbon, and turbostratic carbon films were etched within a few minutes under both bias and growth conditions without enhancing the nucleation density appreciably. In contrast, fibrous carbon is observed in scanning electron micrographs to grow during the bias pretreatment. The growth of these fibers appears to be initiated by defects and metal contamination on the silicon substrate. Even on already existing diamonds, prolonged biasing yields fibrous structures. On the fibrous material a high nucleation density was achieved without further pretreatment. A detailed TEM characterization reveals that the fibers are some tens of nanometers thick, and that they are hollow carbon tubes with a layered wall structure. The layer distance was determined to be 0.335 nm which is the distance of the basal graphite layers observed also in carbon nanotubes. We thus conclude that the formation of carbon nanostructures is one possible mechanism of diamond nucleation during the bias pretreatment. This conclusion is supported by other authors, who observed the formation of small diamond particles in fullerene onions during electron bombardment. (C) 1998 Elsevier Science S.A.