Alkali metal-induced Si(111) √21 x √21 structure:: The Na case

We report the first scanning tunneling microscopy (STM) and core-level photoemission (CL-PES) studies of a root 21 x root 21 structure induced by sub-monolayer Na deposition on the Si(1 11) root 3 x root 3-Ag surface. In the filled-state STM images, five bright protrusions in the root 21 x root 21 unit cell are located on the Ag trimers of the root 3 x root 3-Ag structure. The Si 2p core-level photoemission decomposition shows that the surface-shifted component originated from the Si-trimer atoms in the root 3 x root 3-Ag surface splits into two for the root 21 x root 21 structure, which are attributed to the Si-trimer atoms in the root 21 x root 21 unit cell which are affected and unaffected by the Na adsorption. These results show strong similarities between the Na-induced root 21 x root 21 superstructure and the noble metal-induced ones, indicating a common atomic structure and formation mechanism. (c) 2005 Elsevier B.V. All rights reserved.