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 template 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 shows the initial stages of nucleation, with individual diamond crystallites growing in scratches on the surface. Another, better-controlled version of this is to use ultrasonic agitation to abrade the substrate immersed in a slurry of diamond grit in water. Whatever the abrasion method, however, the need to damage the surface in such a poorly-defined manner prior to deposition may severely inhibit the use of diamond for applications in, say, the electronics industry, where circuit geometries are frequently on a submicron scale.
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