A theoretical study of growth mechanisms of the (110) surface of diamond from acetylene and hydrogen mixtures

Possible mechanisms for the growth of the (110) surface of diamond by chemical vapor deposition (CVD) from an acetylene carbon source are examined by MNDO and PM3 semiempirical quantum mechanical calculations. A large model compound is shown to be necessary to obtain reasonable convergence. Growth is assumed to proceed via addition of neutral acetylene molecules to radical sites on the surface which are made available by H atom abstraction by gas-phase hydrogen atoms. A multistep reaction pathway is investigated whereby the added C₂H₂ forms either an ethylene-like or an ethyl radical-like species attached to the diamond surface by two single carbon-carbon bonds. The various ways in which any two of these species, adjacent to one another, may associate to produce net sp³ growth are also examined. The association of a third two-carbon species in this manner makes the growth essentially irreversible. The relationship of the present study to previously proposed mechanisms for CVD growth of (110) diamond by acetylene addition is discussed. All such mechanisms are shown to produce locally oriented growth which implies much higher defect densities in the resulting films than are observed experimentally. This inconsistency may be reconciled if some one-carbon species are also incorporated during growth and/or if any of the two-carbon species have a high surface mobility. A critical evaluation of the semiempirical methods is made by comparing theoretical and experimental energetics for a number of known carbon-based radical reactions.