Diamond was synthesized by microwave plasma CVD from CH
4-H
2 based mixed gases containing varying proportions of either O
2, CO
2 or CO as an additive. H
2 flow rate and CH
4 concentration were fixed at 1.67cm
3/s and 3.0% respectively and microwave power was 220W. Addition of minute amounts of all of the additives resulted in increasing deposition rate. O
2 and CO
2 were effective in improving the morphology, while CO had no appreciable effect. CO
2 was found most effective in both increasing the deposition rate and improving the morphology.
An increased concentration of O
2 or CO
2 resulted in more idiomorphic diamond crystals with less secondary growth on the surface.
The effectiveness of CO
2 addition in improving both deposition rate and morphology of diamond is thought to be due to the increase in both carbon and oxygen concentration in the reaction gas system. Amorphous carbon would be eliminated by active oxygen in the plasma, and methyl radical, an intermediate product on the way to diamond, would be formed.
Raman spectroscopy is especially effective in evaluating amorphous carbon.
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