Growth of diamond at conditions where it is the metastable phase can be achieved by various chemical vapour deposition methods. Atomic hydrogen plays a major role in mediating rates and in maintaining a proper surface for growth. Low molecular weight hydrocarbon species (e.g. CH
3
and C
2
H
x
are believed to be responsible for extension of the diamond lattice, but complete understanding of attachment mechanisms has not yet been achieved. The nucleation of diamond crystals directly from the gas phase can proceed through a graphitic intermediate. Once formed, the growth rate of diamond crystals is enhanced by the influence of stacking errors. Many of the commonly observed morphologies, e.g. hexagonal platelets and (apparent) decahedral and icosahedral crystals, can be explained by the influence of simple stacking errors on growth rates.
In situ
measurements of growth rates as a function of hydrocarbon concentration show that the mechanism for diamond growth is complex and may involve surface adsorption processes in rate limiting steps. The transport régime in diamond deposition reactors varies widely. In the hot-filament and microwave reactors, which operate from 20 to 100 Torr (1 Torr ≈ 133 Pa), the transport of mass and energy is dominated by molecular diffusion. In the atmospheric pressure combustion and plasma methods, transport is dominated by convection.
In situ
measurements of H atom recombination rates in hot-filament reactors show that, under many commonly used process conditions, transport of atomic hydrogen to the growing surface is diffusion limited and H atom recombination is a major contributor to energy transport.