Fig. 7. Bachmann diagram for CVD diamond film growth (adapted from [80].)

An understanding of the mechanism of CVD diamond growth requires a knowledge of the thermodynamics and kinetics of a complex sequence of homogeneous gas phase reactions and surface reactions. A detailed kinetic treatment of diamond deposition has been developed by Frenklach and Wang [82]. Clearly, the processing window in which CVD diamond grows favours formation of sp3 carbon rather than the more stable sp2 carbon. The role of hydrogen is crucial and it appears that it both promotes growth and stabilises sp3 carbon at the reaction interface and gasifies or selectively etches sp2 carbon.

A new, low-pressure, plasma-assisted process for synthesising diamonds has been found by Roy et al [83,84], An intimate mixture of various forms of carbon with one of many metals (e.g., Au, Ag, Fe, Cu, Ni) is exposed to a microwave plasma derived from pure hydrogen at temperatures ranging from 600-1000 °C. Roy et al postulate a mechanism in which a solid solution of atomic hydrogen and the metal, Me, facilitates dissolution of carbon to form molten droplets of Mex-Cy-Hz. Diamonds nucleate at the surface of the droplets as the temperature is reduced.

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