STM STUDY OF C2H2 ADSORPTION ON SI(100)

The adsorption of C2H2 on Si(100) 2x1 has been studied for the first t ime using STM. Upon exposure of the sample at room temperature to 0.2 L C2H2 (approximately 20% coverage) adsorption of the molecule on alte rnate 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 the normal bean shaped contour of the clean surface. Upon an nealing the substrate at 775 K, the surface becomes disordered and the steps are no longer visible. After further annealing at 875 K, SiC du sters are formed and the 2x1 structure is again seen between the clust ers. Annealing to higher temperatures around 1100 K leads to pinning o f the step movement by the SiC clusters. We find that at saturation co verage, the molecule forms a well-ordered 2x2 overlayer on the surface . Annealing to intermediate temperatures below 900 K leads to results similar to that seen for lower coverages. However, annealing at 1100 K results in facetting of the surface. Annealing at 1275 K creates anis otropic facets which are oriented along the [110] direction with a typ ical aspect ratio of approximately four to five. These facets act as n ucleation sites for subsequent carbonization and SIC growth.