The chemical vapour deposition (CVD) of diamond by a hot filament method was studied in two ways : one is the effect of gas pressures ranging from 1 to 500 Torr, and the other is the effect of substrate materials.
In the first part of the experiments, between 30 and 100 Torr, faceted diamond is deposited by the gas mixture of 2% $CH_4$ - 98% $H_2$, As the pressure increases from 100 to 500 Torr, the morphology of the deposit changes from the well-faceted diamond to the non-diamond cauliflower structure and at the same time the deposition rate decreases. The optimum ratio of methane to hydrogen for well-faceted diamond decreases with increasing the pressure ; for example, the optimum is 0.2% $CH_4$ at 500 Torr and 2.0% $CH_4$ at 20 Torr. The deposition rate increases with decreasing the distance between the filament and the substrate at pressures higher than 100 Torr. In the lower pressure range from 1 to 5 Torr, the morphology changes from the cauliflower structure to the well faceted one with increasing the distance at 2.0% $CH_4$. This means that the quality of diamond improves in the processing condition the concentration of the atomic hydrogen decreases.
In the second part of the experiments, diamond was deposited on the metal or the oxide substrate at 20 Torr. When the substrate material is oxide such as $Al_2O_3$ or $SiO_2$, the well-faceted diamond is deposited on the substrate. On the other hand, for Fe, Ni, Pt, Pd, and Rh substrates, the deposits is graphite. We found the good correlation between the substrate material resulting in the graphitic soot and the charge transfer rate at the surface in the electrocatalysis. It is found that the substrate materials which generate soot formation are electrocatalysis materials. Theses results show which the stability between diamond and graphite depends on the charge transfer rate of the substrate materials.
