  Diamond nucleation under very low pressure (0.1-1.0 torr) was obtained at
very high nucleation densities and very rapid rates using hot-filament chemical
vapor deposition (HFCVD). The density on mirror-polished silicon was as high as
10^10 - 10^11 cm^{-2}, equivalent to the highest density in a microwave-plasma
CVD system. That on scratched silicon substrates was up to 10^9 cm^{-2}, 1-2
orders of magnitude higher than that obtained under conventionally low pressure
(tens of torr, 10^7 - 10^8 cm^{-2}). Also, the density on scratched titanium
substrates was as high as 10^10 cm^{-2}. The samples were characterized using
scanning electron microscopy (SEM) and Raman spectroscopy. The mechanism is
investigated in detail, revealing that, under very low pressure, very long mean
free path of the gas species, strong electron emission from the hot filament,
and high efficiency of decomposition of hydrocarbon species by the filament
greatly increase the concentration of reactive hydrocarbon radicals and atomic
hydrogen on the substrate surface, and therefore, dramatically enhance the
nucleation eventually. This work has great practical applications and
theoretical significance.
