Computational studies of elementary steps relating to boron doping during diamond chemical vapour deposition

Density functional theory-based electronic structure computations on small models of the diamond {100} surface have enabled prediction of the energetics and activation parameters of a number of plausible mechanistic steps for boron incorporation into, and boron loss from, the growing diamond surface. Initial proving calculations for the carbon-only case show, as in previous work, that the rate-limiting step for diamond growth involves opening of a five-membered ring species, and subsequent closure to form six-membered rings as in bulk diamond. The five-membered ring intermediate arises following 2 x 1 reconstruction of the {100} surface, or at steps on the {111} surface. Diamond growth arises as a result of successful competition between the ring-opening step and a two-carbon loss step, both of which involve significant activation barriers. In the boron case, we find that BH(x) (x = 0-3) species can all bind to radical sites on the diamond {100} surface to form stable adducts. Interconversion between the surface bound BH, species is facile at the H and H2 number densities and temperatures typical for diamond CVD conditions. B incorporation can occur by a ring expansion mechanism, as in the all-carbon case, and by direct insertion of surface bound BH (and B) species into the C-C bond on the diamond {100} surface. BH(x) loss processes identified include release of surface bound BH3 and/or CH2BH species into the gas phase. Both B incorporation into, and B loss from, the diamond {100} surface are deduced to be significantly less energy demanding than the corresponding carbon addition and loss processes.