Efficiency of methane and acetylene in forming diamond by microwave plasma assisted chemical vapor deposition
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Diamond films were grown by microwave plasma assisted chemical vapor deposition using mixtures of 1% 13 CH 4 and 0.5% 12 C 2 H 2 in H 2 , and stable gaseous products were analyzed by mass spectrometry. The major gaseous products are methane and acetylene, and scrambling of the 13C label can be controlled at relatively high gas flow rates. At the highest flow rate studied a diamond film was grown with 77% 13C incorporation, while the methane in the reactor exhaust gas at this flow rate contained 83% 13 C. By comparing gaseous 13C compositions with that of the films, the efficiency of diamond growth from methane (possibly via the methyl radical) is estimated to be about ten times higher than that for acetylene.
I. INTRODUCTION The search to identify gas phase species responsible for diamond growth by chemical vapor deposition (CVD) has focused recently on the methyl radical and acetylene as likely growth species, based on experimental data and modeling studies.1^1 Strong experimental evidence for the methyl radical (or possibly methane) as the major diamond growth precursor has been reported for both hot filament2'5'6 and microwave plasma1'7 CVD reactors. In these experiments acetylene also appears to contribute to diamond growth, but with less effectiveness. Experimental techniques found useful in differentiating the roles of methane/methyl radical and acetylene include carbon-13 labeling of reagents2 and use of high gas flow rates7 to limit potentially complex hydrocarbon chemistry, thereby allowing nearly independent study of the methane/methyl radical system and the acetylene system. In the present work we have combined these two approaches to study the efficiency of forming diamond from methane and acetylene in a microwave plasma reactor using a mixture of 0.5% 12 C 2 H 2 and 1.0% 13 CH 4 in H2 as the feed gas. As in closely related work by Chu et al.2 using a hot filament system, the approach involves measurement of the 13C composition in films (by Raman spectroscopy) and in stable gas phase species (by mass spectrometry). II. EXPERIMENTAL The diamond deposition flow tube reactor and substrate preparation procedures have been described in detail elsewhere.8'9 The reactor consists of a 12.5 mm diameter quartz tube that passes through an Evenson resonant microwave cavity. Films were deposited on single crystal 7 x 13 mm 2 Si(100) substrates that were seeded by abrading the surface with
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