Carbon dimer, C2, as a growth species for diamond films from methane/hydrogen/argon microwave plasmas

Abstract

It was shown recently that nanocrystalline diamond films can be grown using fullerenes as precursors in an argon microwave plasma without the addition of hydrogen or oxygen. Extensive fragmentation of C60 in the microwave discharge leads to a copious production of the carbon dimer molecule, C2, as evidenced by intense Swan-band emission. Here we have investigated hydrogen–methane–argon plasmas and found that high argon fractions (≳50%) lead to intense C2 emission, indicating significant production of C2 in the plasma. In situ measurements of the substrate reflectivity were used to determine the growth rate. A correlation between the C2 emission intensity and growth rate was observed. These results prompted us to propose a scheme for diamond film growth on the (100)–(2×1): H reconstructed diamond surface with C2 as the growth species. Each surface carbon atom (bonded twice to carbons in the bulk, once to a surface carbon, creating a ‘‘dimer’’ and then forming a five-membered ring) is terminated with hydrogen. With C2 as the growth species, no hydrogen abstraction reactions are required because its very high energy of adsorption (815 kJ/mol C) allows the C2 molecule to insert directly into the dimer bonds. A second C2 can form across the adjoining trough. The added carbons then dimerize, forming a new (2×1) surface on a new layer with dimer rows orthogonal to the original rows.