Er-induced 2 root 3x2 root 3R30 degrees reconstruction on Si(111): influence on the very low Er coverage silicide growth

Interaction of Er with Si(111) is investigated by scanning tunneling microscopy. Er reaction at 500 degrees C with Si(111)7 x 7 in the submonolayer range is characterized by the formation of numerous islands and holes exposing a 2 root 3 x 2 root 3R30 degrees surface reconstruction. This is the first report on the formation of such superstructure upon reaction of a rare-earth element on Si(111). The 2 root 3 x 2 root R30 degrees unit cell is composed of two inequivalent halves and exhibits protrusions with a shape dependent on the bias voltage. Taken with a negative bias voltage, each half-cell exhibits three dimers aligned with the [10 (1) over bar] direction of the substrate. By using a positive sample bias voltage, the surface reconstruction is atomically resolved. The protrusions observed in this mode are accommodated in a hexagonal ring of T-4 sites, with sizeable atomic displacements with respect to the perfect T-4 sites. The size and number of such reconstructed holes and islands are found to strongly depend on the growth conditions. They are mainly observed below 0.5 monolayer coverage, coexisting with small hexagonal-shaped one-monolayer height islands. Upon increasing the annealing temperature up to 700 degrees C, the 2 root 3 x 2 root R30 degrees reconstructed areas disappear. At this stage, the one monolayer height silicide islands are still observed along with additional two-monolayer height silicide islands. This contrasts with what occurs in the 0.5-1 monolayer coverage range, i.e. the formation of one-monolayer silicide only. The most striking feature here is that the two-monolayer silicide islands, induced by the formation of 2 root 3 x 2 root R30 degrees reconstructed islands, grow on the Si(111) terraces along one preferential orientation only. They grow along the close-packed [10 (1) over bar] direction of the substrate, giving rise to rod-shaped islands, as opposed to those formed in the higher coverage regime (above one monolayer) which exhibit an hexagonal shape, consistent with the threefold symmetry of the two-layer silicide. These rod-shaped islands exhibit a 2 x 1 surface periodicity. (C) 1997 Elsevier Science B.V.