MOLECULAR-DYNAMICS STUDIES OF ELEMENTARY SURFACE-REACTIONS OF C2H2 AND C2H IN LOW-PRESSURE DIAMOND-FILM FORMATION

Molecular dynamics studies of some of the important elementary reactions involved in the low-pressure synthesis of diamond films are reported. The C(111) surface is modeled with an ensemble of 127 atoms and a velocity reset procedure to incorporate the thermal effects of the bulk. The hydrocarbon potential developed by Brenner [Phys. Rev. B 1990, 42, 9458] is employed in all calculations for both the surface and the incident gas-phase molecules. The principal results are as follows: (1) The sticking coefficients for acetylene on a clean C(111) surface lie in the range 0.17-0.45 for incident translational energies between 1.5-2.0 eV with surface temperatures in the range 1000-1500 K. (2) Chemisorption of acetylene most frequently involves the formation of two C-C(s) single bonds to adjacent adsorption sites on the C(111) surface (3) Surface of acetylene via the formation of one C-C(s) single bond to yield an ethenyl radical is observed and the subsequent of this species from a clean C(111) surface does not appear to be a high probability process. (4) The addition of a second acetylene molecule to form an ethenyl radical is a very low probability process for all surface structures investigated. When such chemisorption does occur, the probability of subsequent desorption is large unless the ethenyl radical is able to subsequently form a second C-C bond. (5) Addition of a .C=CH radical to a chemisorbed acetylene group proceeds with a much higher probability than is the case for C2H2. The ethynyl radical is also chemisorbed readily to other surface structure with a low probability of subsequent desorption. It therefore appears likely that C2H is an important diamond-growth species even in experiments where its concentration is 1 or 2 orders of magnitude less than that of acetylene.