We present here a scanning tunneling microscope study of the initial bonding structure and subsequent reaction mechanism of ${\mathrm{C}}_2{\mathrm{H}}_2$ with the Si(001) surface. Upon exposure of the sample at room temperature to 0.2 L of ${\mathrm{C}}_2{\mathrm{H}}_2$ (approximately 20% coverage) adsorption of the molecule on alternate dimer pairs is observed, leading to either a local $2\times 2$ or $c(2\mathrm{}\times 4)$ 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 the normal bean-shaped contour of the clean surface. The molecule forms an overlayer with either local $2\times 2$ or $c(4\mathrm{}\times 2)$ 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 $2\times 1$ 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¯10] direction with a typical aspect ratio of approximately 4 to 5. These facets act as nucleation sites for subsequent carbonization and SiC growth.

• Received 11 November 1996

DOI:https://doi.org/10.1103/PhysRevB.56.4648