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Diamond thin films 77

formed, cannot spontaneously convert to the more stable graphite phase.
Diamonds are, indeed, forever!
To overcome these problems, researchers realised that in order to form
diamond, they had to choose conditions where diamond, and not graphite,
is the more stable phase. The knowledge of the conditions under which
natural diamond is formed deep underground, suggested that diamond
could be formed by heating carbon under extreme pressure. This process
forms the basis of the so-called high-pressure high-temperature growth
technique, first marketed by General Electric, and which has been used to
produce ‘industrial diamond’ for several decades. In this process, graphite
is compressed in a hydraulic press to tens of thousands of atmospheres,
heated to over 2000°C in the presence of a suitable metal catalyst, and left
until diamond crystallises. The diamond crystals this produces are used for
a wide range of industrial processes which utilise the hardness and wear
resistance properties of diamond, such as cutting and machining mechan-
ical components, and for polishing and grinding of optics. However, the
drawback of this method is that it still produces diamond in the form of
single crystals ranging in size from nanometres to millimetres, and this
limits the range of applications for which it can be used. What is required
is a method to produce diamond in a form which can allow many more of
its superlative properties to be exploited – in other words, diamond in the
form of a thin film.

5.2 Chemical vapour deposition

Rather than try to duplicate Nature’s method, diamond could conceivably
be produced if carbon atoms could be added one-at-a-time to an initial tem-
plate, in such a way that a tetrahedrally bonded carbon network results (see
Figure 5.1). These ideas led to experiments in which carbon-containing
gases were heated under reduced pressure until the molecules broke apart,
and then these fragments were condensed onto a nearby surface. Analysis
showed that the thin film that resulted from this did, indeed, contain
diamond. However, the rate of growth in these early experiments was low,
and the films were impure, containing a large proportion of unwanted
graphite. The breakthrough came in the late 1960s, when researchers in the
USA discovered that the presence of atomic hydrogen during the deposi-
tion process would remove graphite from a surface much faster than
diamond. This meant that the impure components were removed from the