Efficiency of methane and acetylene in forming diamond by microwave plasma assisted chemical vapor deposition

Diamond films were grown by microwave plasma assisted chemical vapor deposition using mixtures of 1% ¹³CH₄ and 0.5% ¹²C₂H₂ in H₂, 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% ¹³C. 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.