Spectroscopic determination of carbon dimer densities in Ar-H2-CH4 and Ar-H2-C60 plasmas

In contrast to conventional methods of diamond chemical vapour deposition (CVD), nanocrystalline diamond CVD takes place with only a small fraction of feed gas hydrogen. Minimal amounts of CH3, believed critical in hydrogen-rich CVD, are expected to be produced in hydrogen-deficient systems and alternative mechanisms for diamond growth must be considered. The carbon dimer, C-2, is believed to be an important species in these growth environments. We have experimentally determined the density of gas phase C-2 in Ar-H-2-CH4 and Ar-H-2-C-60 microwave plasmas used to deposit nanocrystalline diamond. The C-2 density is monitored using high-sensitivity absorption spectroscopy of the d(3)Pi <-- a (3)Pi (0, 0) band as chamber pressure, microwave power, substrate temperature and feed gas mixtures are varied for these two chemical systems. The absolute density of C-2 is most sensitive to the total chamber pressure and fraction of carbon in all molecular species in the feed gas in Ar-H-2-C-60 discharges and to the total chamber pressure and substrate temperature in Ar-H-2-CH4 plasmas. We discuss possible C-2 production channels in both chemical systems. The efficiency of C-2 production from fullerene precursors is over an order of magnitude greater than that from hydrocarbon precursors.