Comparative study on nanocrystalline diamond growth from acetylene and methane

The amount and composition of amorphous non-diamond phases in nanocrystalline diamond (NCD) films deposited from microwave C2H2/H2/Ar-rich and CH4/H2/Ar-rich plasmas have been compared to obtain a clue to phase-pure NCD films. The emission intensity of C2 radicals for C2H2 was a few times higher than that for CH4, indicating efficient production of C2 as important growth precursors by using C2H2. The threshold Ar concentration for a transition from microcrystalline to nanocrystalline diamond was found to be around 70 and 90 vol.% for C2H2 and CH4 systems, respectively. Visible Raman spectroscopy for NCD films with average crystallite sizes of 15–25 nm revealed that the amount of amorphous carbon corresponding to the D peak intensity was reduced and consequently the diamond peak was clearly observed when C2H2 was used. The reduction of amorphous carbon was likely accompanied by an increase in the fraction of polymer-like hydrogenated carbon including transpolyacetylene, suggesting the crucial role of hydrogen in phase control.