Diamond growth was studied by injecting thermally decomposed Cl atoms into CH4/H2 or C2H2TH2. Owing to the extremely short residence time (25 μs) and low gas temperature (< 1000 °C) in the decomposition system, the gas reaction is insignificant. Therefore, the carbon species near the substrate surface can be nearly identical to the input carbon source. With 0.3% CH4 being the input carbon source, CH4 remained the dominant carbon species near the surface (only 2.5% C2H2 was formed), and an almost continuous diamond film was deposited after 2 h growth. Raman spectra confirmed the formation of diamond. With 0.15% C2H2 being the input carbon source, C2H2 remained the dominant carbon species near the surface (10% CH4 was formed), but only a few very small particles were deposited. Therefore, we conclude that CH3 radicals seem the only diamond growth precursor under the Cl-rich conditions, whereas C2H2 is not efficient to grow diamond.
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