Page 93 - Visions of the Future Chemistry and Life Science

82 P. W. MAY

bond’, which needs to be terminated in some way. If too many dangling
bonds are left unterminated, they will tend to join together (cross-
link), and the surface structure will begin to resemble that of graphite.
The vital surface termination is normally performed by hydrogen
which attaches to the dangling bond and thereby keeps the tetrahedral
diamond structure stable. During diamond growth, some of these
surface hydrogen atoms need to be removed and replaced by carbon-
containing species. A large number of reactive hydrogen atoms close
to the surface can quickly bond to any excess dangling bonds, so pre-
venting surface graphitisation.
(ii) Atomic hydrogen is known to etch graphite-like carbon many times
faster than diamond-like carbon. Thus, the hydrogen atoms serve to
remove back to the gas phase any graphite-like clusters that may form
on the surface, while leaving the diamond clusters behind. Diamond
growth could thus be considered as ‘five steps forward, but four steps
back’, with the net result being a (slow) build up of diamond.
(iii) Hydrogen atoms are efficient scavengers of long chained hydrocarbons,
breaking them up into smaller pieces. This prevents the build up of
polymers or large ring structures in the gas phase, which might ulti-
mately deposit onto the growing surface and inhibit diamond growth.
(iv) Hydrogen atoms react with hydrocarbons such as methane (CH ) to
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create reactive radicals such as methyl (CH ) which can then attach to
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suitable surface sites.
There have been many suggestions for the identity of the diamond
growth species, however, the general consensus is now that the bulk of the
evidence supports CH as being the important radical. The basic picture
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which emerges for CVD diamond growth is believed to be as follows.
During growth, the diamond surface is nearly fully saturated with hydro-
gen. This coverage limits the number of sites where hydrocarbon species
(probably CH ) may stick. A schematic illustration of the resulting pro-
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cesses is shown in Figure 5.4, suggesting that diamond growth can be con-
sidered to be a one-by-one addition of carbon atoms to the existing
diamond structure, driven by the presence of reactive atomic hydrogen. In
oxygen-containing gas mixtures, it is believed that the hydroxyl (OH)
radical plays a similar role to atomic hydrogen, except that it is more effec-
tive at removing graphitic carbon, leading to higher growth rates and better
quality films at lower temperatures.