CH2 Group Migration between H-Terminated 2 × 1 Reconstructed {100} and {111} Surfaces of Diamond

Successful growth of diamond by chemical vapor deposition requires that chemisorbed hydrocarbon species, most notably CH2 groups, are able to migrate on the growing surface. Quantum mechanical and hybrid quantum mechanical/molecular mechanical (QM/MM) cluster models are here used to investigate the energetics of CH2 migration on the C{111}:H surface and between C{100}:H 2 × 1 terraces separated by a region of C{111}:H surface. Many migration pathways of this type proceeding via structures involving 3-, 4-, and 5-membered rings are found to have relatively low barriers, so that migration should be relatively facile at typical diamond growth temperatures. In contrast, CH2 migration via one particular C{111}:H/C{100}:H 2 × 1 step-edge geometry results in the formation of a very stable 6-membered ring intermediate. The energetics suggest that this process will be irreversible and should thus result in incorporation. This type of step-edge also occurs in the limiting case of two C{100}:H 2 × 1 terraces separated by a monolayer step, and migration of CH2 species along the lower C{100}:H 2 × 1 terrace toward such step edges is predicted to favor incorporation. These findings offer a rationale for the deduced propensity for step-flow growth and the observation of stepped {100} terraces in CVD diamond samples.