STM study of C2H2 adsorption on Si(001)

We present here a scanning tunneling microscope study of the initial bonding 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 alternate dimer pairs is observed, leading to either a local 2x2 or c(2x4) structure. In the filled-state image, a local minimum is observed in the center of the reacted dimer pairs, while the unreacted dimer pairs maintain thr normal bean-shaped contour of the clean surface. The molecule forms an overlayer with either local 2x2 or c(4x2) order, leading to a saturation coverage of 0.5 monolayers. Upon annealing the substrate at 775 K the surface becomes disordered and the steps are no longer visible. After further annealing at 875 K, SiC clusters are formed and the 2x1 structure is again seen between the clusters. For a starting coverage of 20%, annealing to higher temperatures around 1100 K leads to pinning of the step movement by the SiC clusters. For a starting coverage of 0.5 monolayer, annealing at 1100 K results in faceting of the surface. Further annealing at 1275 K creates anisotropic facets that are oriented along the [(1) over bar 10] direction with a typical aspect ratio of approximately 4 to 5. These facets act as nucleation sites for subsequent carbonization and SiC growth.