Bonding Configurations and Collective Patterns of Ge Atoms Adsorbed on $\mathbf{Si}(111)\mathrm{\text{−}}(7\times 7)$

We report scanning tunneling microscopy observations of Ge deposited on the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface for a sequence of submonolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, until small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.

Figure 1

Schematic top view of the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ reconstruction. The outlined is the $7\times 7$ unit cell with the faulted and unfaulted half unit cells located on the left and right sides, respectively.

Figure 2

Filled state STM images of the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface with Ge coverages of (a) 0.02 ML, (b) 0.08 ML, and (c) 0.10 ML. A $7\times 7$ unit cell is marked by two triangles in (a), where F and U represent the FHUC and UHUC, respectively. Sample bias: $-2.2\mathrm{V}$ in (a) and $-1.5\mathrm{V}$ in (b) and (c); tunneling current: 0.5 nA in (a) and 0.2 nA in (b) and (c). The scanning area is $20\mathrm{nm}\times 20\mathrm{nm}$. Three different configurations of Ge protrusions distributions are denoted in (b) and (c) by the dotted-line triangle, the solid-line triangle, and the dashed-line triangle, respectively. The schematics for the three typical Ge protrusions, named type A (d), type B (e), and type C (f), are also shown.

Figure 3

The profile lines corresponding to the dashed-arrow lines in the STM images of Figs. 2b (a) and 2c (b), respectively.

Figure 4

Schematics of the minimum-energy configurations for a Ge atom on the Si(111) surface: (a) Ge at a $B_2$ site and the nearby Si adatom at the position off its original site, (b) Ge at a substitutional $S_4$ site and the Si adatom at a $B_2$ site, and (c) Ge at an $S_4$ site with the substituted Si adatom diffused away. The bond lengths are shown in $\AA$.