Adatom-Rest atom Model

Figure Al.7.6. Schematic diagrams of the DAS model of the Si(l 11)-(7 x 7) surface structure. There are 12 adatoms per unit cell in the outennost layer, which each have one dangling bond perpendicular to the surface. The second layer, called the rest layer, also has six rest atoms per unit cell, each with a perpendicular dangling bond. The comer holes at the edges of the nnit cells also contain one atom with a dangling bond.

The DAS model has the least number of dangling bonds (19) among all the models ever proposed. There are 12 dangling bonds at the adatoms, 6 at the rest atoms, and 1 at the center atom deep in the corner hole. The 19 dangling bonds are at different energy levels. 

Fig. 9a, b. Structural model of (a) the c(2x8) Gieflll) reconstruction and (b) the (7x7) Si(lll) reconstruction. The small, dark shaded balls represent second layer atoms and the larger shaded balls, first or rest layer atoms the fighter shaded large balls represent rest atoms that have a dangling bond. The partially transparent circles represent the Si or Ge adatoms. The heavy solid fines fiighfigfit the surface unit cells. 

Fig. 5.2-16 Top view of Ge( 111 )c(2 x 8) simple adatom model. Large and small empty circles represent adatoms and rest atoms respectively. Both ideal shaded) and c(2 X 8) unit cells are shown [2.28]...

Figure 9.27 Dimer-adatom-stacking fault (DAS) model of the Si(l 11 )-(7x 7) reconstruction. Each of the 12 adatoms with almost empty dangling bonds in the first layer saturate three top atoms in the second layer. The top atoms are arranged in a local (2x2) subunit. The dangling bond orbits of the adatoms are the dominant features observed in scanning tunneling microscopy. The remaining six rest atoms in the second layer...

Besides the (7x7) structure, the DAS model can easily be extended to (2n+1) X (2 +1) reconstructions with n= 1,2,3,4,... The corresponding unit cells possess n(n +1) adatoms and (n — 1) rest atoms. The domain walls separating the (2x2) adatom islands exhibit 3x dimers. Vanderbilt [56] suggested a relatively simple model for the energetics of the Si(lll) and Ge(lll) surfaces. For Si, he estimated the formation energy for the various stractural elements ... 

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