Page 97 - Visions of the Future Chemistry and Life Science

86 P. W. MAY

It is apparent that many of the problems with expansion mismatch and
carbon solubility could be eliminated if the deposition were to occur at
much lower temperatures. Many groups world-wide are focusing their
research efforts in this direction, and the answer may lie in different gas
chemistries, such as use of carbon dioxide or halogen containing gas mix-
tures. Until then, the difficulties associated with diamond growth on prob-
lematic materials have ensured the continuing popularity of silicon as a
substrate material. It has a sufficiently high melting point (1410°C), it
forms only a localised carbide layer (a few atoms thick), and it expands rel-
atively little upon heating. Molybdenum and tungsten display similar
qualities, and so are also widely used as substrate materials. They can also
be used as barrier layers – thin coatings deposited upon certain of the more
problematic substrate materials to allow subsequent diamond CVD.

5.6 Nucleation

Nucleation is the process whereby gas phase carbon atoms join together on
a surface to make the beginnings of a new crystal structure. Growth of
diamond begins when individual carbon atoms nucleate onto the surface
in the specific diamond-like tetrahedral structure. When using natural
diamond substrates (a process called homoepitaxial growth), the template
for the required tetrahedral structure is already present, and the diamond
structure is just extended atom-by-atom as deposition proceeds. But for
non-diamond substrates (heteroepitaxial growth), there is no such tem-
plate for the carbon atoms to follow, and those carbon atoms that deposit
in non-diamond forms are immediately etched back into the gas phase by
reaction with atomic hydrogen. As a result, the initial induction period
before which diamond starts to grow can be prohibitively long (hours or
even days). To combat this problem, the substrate surface often undergoes
a pre-treatment prior to deposition in order to reduce the induction time
for nucleation and to increase the density of nucleation sites. This pre-
treatment can involve a number of different processes. The simplest is
abrasion of the substrate surface by mechanical polishing using diamond
grit ranging in size from 10nm to 10 m. It is believed that such polishing
aids nucleation by either (a) creating appropriately-shaped scratches in the
surface which act as growth templates, or (b) embedding nanometre-sized
fragments of diamond into the surface which then act as seed crystals, or
(c) a combination of both. An example is given in Figure 5.5(a), which