ACETYLENE REACTION WITH THE SI(111) SURFACE - A SEMIEMPIRICAL QUANTUM CHEMICAL STUDY

The interaction between the acetylene molecule and the Si(111) surface was modeled using the geometry optimization pathway of the Zerner intermediate neglect of differential overlap semiempirical quantum chemical program. The surface was represented by a 49-atom cluster containing four layers of silicon atoms. To determine the effect of the interaction upon the silicon surface, 12 central atoms from the top two layers were allowed to move to stable positions. The geometry of the silicon surface was initially optimized without acetylene, resulting in a significant rearrangement of the mobile atoms. Nine separate calculations were then performed, differing in the initial position and orientation of the acetylene molecule above the surface. The geometry of the resulting surface structures was found to be highly dependent upon the initial placement and orientation of the acetylene. In each case, the acetylene was found to react with the silicon surface by the formation of Si-C bonds. An analysis of the Wiberg bond indices revealed that the initial triple bond between carbon atoms was reduced to approximately a single bond, the exact bond order varying slightly from case to case. It was also found that Si-Si bonds surrounding the reaction site were weakened, and in some cases broken, due to the strain induced by the Si-C bond formation. The degree to which the surfaces were rearranged was found to correlate with the final energies, indicating that the most distorted surfaces were the most energetically favorable.