Spectroscopic determination of carbon dimer densities in and plasmas

Abstract

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, C2, is believed to be an important species in these growth environments. We have experimentally determined the density of gas phase C2 in Ar-H2-CH4 and Ar-H2-C60 microwave plasmas used to deposit nanocrystalline diamond. The C2 density is monitored using high-sensitivity absorption spectroscopy of the d3Πn ← a3Π (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 C2 is most sensitive to the total chamber pressure and fraction of carbon in all molecular species in the feed gas in Ar-H2-C60 discharges and to the total chamber pressure and substrate temperature in Ar-H2-CH4 plasmas. We discuss possible C2 production channels in both chemical systems. The efficiency of C2 production from fullerene precursors is over an order of magnitude greater than that from hydrocarbon precursors.