Diffusion anisotropy of Ag and In on Si(1 1 1) surface studied by UHV-SEM

Anisotropic features of Ag and In electromigration on clean and Au-precovered Si(1 1 1) surfaces were studied by in situ scanning electron microscopy in ultrahigh vacuum. It was noted that the migration direction of Ag was determined by both applied direct-current direction and step orientation on the substrate surface; on an Si(1 1 1) surface with steps inclined with respect to the current direction, the electromigration direction shows an apparent deviation from the accurate current direction. On clean and Au-precovered Si(1 1 1) surfaces with various coverages of Au (within submonolayer range), the migration behaviors of Ag and In drastically changed with Au coverages and showed different diffusion anisotropy (either thermal diffusion and electromigration) depending on the adsorbate surface structures. Particularly, on a beta-square root of 3 x square root of 3-Au surface of one monolayer Au coverage, In migrated with the highest mobility across the step bands, whereas In showed only a slow movement on the 7 x 7 clean surface due to a migration barrier at step edges. This result implied that the beta-square root of 3 x square root of 3-Au surface phase served as an intermediate layer for In adatoms migration. On the contrary, Ag showed negligible migration on the beta-square root of 3 x square root of 3-Au surface, while the 7 x 7 surface was the substrate for appreciable migration of Ag atoms. The results are discussed in terms of step-edge barriers in migration and on-terrace migration.