STM STUDY OF C2H2 ADSORPTION ON SI(001)

We present here a scanning tunneling microscope study of the initial b onding structure and subsequent reaction mechanism of C2H2 With the Si (001) surface. Upon exposure of the sample at room temperature to 0.2 L of C2H2 (approximately 20% coverage) adsorption of the molecule on a lternate dimer pairs is observed, leading to either a local 2x2 or c(2 x4) structure. In the filled-state image, a local minimum is observed in the center of the reacted dimer pairs, while the unreacted dimer pa irs maintain thr normal bean-shaped contour of the clean surface. The molecule forms an overlayer with either local 2x2 or c(4x2) order, lea ding to a saturation coverage of 0.5 monolayers. Upon annealing the su bstrate at 775 K the surface becomes disordered and the steps are no l onger visible. After further annealing at 875 K, SiC clusters are form ed and the 2x1 structure is again seen between the clusters. For a sta rting coverage of 20%, annealing to higher temperatures around 1100 K leads to pinning of the step movement by the SiC clusters. For a start ing coverage of 0.5 monolayer, annealing at 1100 K results in faceting of the surface. Further annealing at 1275 K creates anisotropic facet s that are oriented along the [(1) over bar 10] direction with a typic al aspect ratio of approximately 4 to 5. These facets act as nucleatio n sites for subsequent carbonization and SiC growth.