The role of methyl radicals and acetylene in [100] vs. [111] diamond growth

The interaction of mechanistic experiments and detailed models are greatly improving our understanding of the mechanism of diamond growth by chemical vapor deposition. Methyl-radical models typically predict growth rates on (111) planes that are much smaller than experiments, unless contributions from acetylene in nucleating new layers are included. These models predict rather different contributions of methyl radicals and acetylene to growth on (100) vs. (111) planes. On the other hand, other models predict rapid inter-conversion of adsorbed hydrocarbons and surface migration, and equivalence of the behavior of methyl radicals and acetylene (apart from a sticking coefficient) might be expected. We have nucleated and grown mum-sized diamond particles at 800 degreesC in a flow-tube apparatus that permits growth from only methyl radicals or acetylene in atomic hydrogen, in contrast to the complex mixture of species found in a normal reactor. Growth from methyl radicals only produced cubo-octahedral crystals with an a value (root3 x the ratio of growth rates in the [100] and [111] directions) near 1.8, indicating that the absence of acetylene is not a significant impediment in nucleating new (111) planes. Diamond growth from pure acetylene produced octahedra (alpha = 3), indicating that (100) growth is much more facile than (111) growth in the absence of methyl radicals, and the (111) facets had a high concentration of contact twins. The implications of these results for the mechanism of diamond growth are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.