The nucleation behavior of diamond during gas phase synthesis has been investigated. Firstly, the dependence of the nucleation density and growth rate on the deposition conditions have been observed systematically in hot filament CVD method. Secondly, the driving force for diamond deposition has been calculated on a quasi-equilibrium model, and the experimental results have been explained with the calculated driving force. Thirdly, the annihilation of nucleation sites during the deposition has been examined because the saturation density of diamond particles is determined by the competition between the nucleation rate and the annihilation rate of nucleation sites. Finally, the nucleation and growth behavior on the various substrate materials and the catalytic effect of Ni substrate have been investigated.
To observe the effects of the deposition variables on the nucleation and growth of diamond particles, depositions have been performed under the conditions of 0.3-1.5% methane in hydrogen, 2000-2200℃ of filament temperature, 840-1040℃ of substrate temperature, 50-300 mbar of reactor pressure, and 50-150 sccm of flow rate. The nucleation density increases with increasing methane concentration, filament temperature, pressure, and flow rate. By increasing the substrate temperature, the nucleation density increases first and shows maximum around 950℃. The free energy change, calculated under the assumption that diamond is deposited from the supercooled gas mixture with no solid to the equilibrium gas and solid mixture at the substrate temperature, agrees well with the experimental results. The growth rate shows a similar dependence on the driving force to that of the nucleation density when well defined crystalline diamond particles are deposited.
The effects of thermal annealing and erosion with atomic hydrogen on the defect healing have been observed to investigate the annihilation behavior of nucleation sites. Nucleation sites disappear by thermal annealing and by erosion with atomic hydrogen. The annihilation rate of nucleation sites is more rapid at higher substrate temperature and depends strongly on the concentration of atomic hydrogen. In real diamond deposition process, nucleation sites are annihilated dominantly by erosion with atomic hydrogen during the deposition with 0.3% methane in hydrogen, while only the effect of thermal annealing exists during the deposition with 1% methane in hydrogen.
The nucleation density depends more strongly on the surface treatment condition rather than the chemical properties of the substrate materials. Only a low nucleation density is obtained on the mirror polished substrate surface regardless of the type of the substrate material. But the growth behavior seems to depend on the chemical properties of the substrate.
On Ni substrate, the deposition behavior varies according to the substrate temperature and methane concentration in hydrogen. Diamond is deposited around 860℃ of substrate temperature and graphite appears at the lower substrate temperature at 0.5% methane in hydrogen. The catalytic effect of Ni substrate is discussed to explain the dependence of the deposition behavior on the deposition conditions.