STM observations of Ag adsorption on the Si(111)– [formula omitted] surface at low temperatures

Abstract

We have systematically studied the structural evolutions during adsorption of additional Ag atoms on the Si(111)– 3 × 3 -Ag surface at 70 K by scanning tunneling microscopy. In the coverages less than 0.02 ML (monolayer), the Ag adatoms distribute randomly as monomers on the 3 × 3 -Ag surface. With coverage increase up to 0.1 ML, two-dimensional (2D) nuclei consisting of four Ag adatoms appear, the density of which is much higher at surface steps than on terraces. With being deposited further, the 21 × 21 -Ag domains appear by coalescing the 2D nuclei with each other. In the coverages from 0.14 to 0.20 ML, a well-ordered 21 × 21 -Ag superstructure is formed with ±10.89° orientations with respect to [11 2 ̄ ] directions. The out-of-phase domain boundaries of the 21 × 21 phase are usually straight and along the direction of [11 2 ̄ ] ±10.89°. An atomic structural model for the 21 × 21 phase has been proposed in which its unit cell contains four Ag adatoms adsorbed on the Ag trimers of the unaltered 3 × 3 -Ag framework. This model seems to be consistent with the 2D nuclei created at the initial stage of adsorption and also with the domain boundary structure. This model also seems to be applicable to the Au-induced 21 × 21 phase on the 3 × 3 -Ag surface.