2X1 Reconstruction

Figure 1, Area of rough gold (110) 2x1 reconstruction, with a nurnerical calculation inset - see Atomic columns are...

Fig. 5. Schematic diagrams of (a) bulk silicon, (b) the ideal (100) surface, (c) the (100)-2x1 reconstructed surface, (d) the 2 dangling bonds per atom on the ideal surface, and (e) the pairing of the dangling bonds to form dimers on the 2x1 surface. The schematics neglect dimer buckling. Figure courtesy of S. R. Schofield.

Figure 2(b) shows this (2x1) reconstruction of the surface Si along the (Oil) direction... 

Figure 14-2. The 2x1 reconstructed Si(001) surface. The three silicon atom top layers are shown...

At an atomic level (100)-H surfaces are 2x1 reconstructed [115] (see above). (Ill) faces have the ideal (Ixl)-H arrangement after treatment in buffered NH4F [105, 116, 117] or in hot water [118]. The 1 x 1-H/Si(lll) surface has been imaged in dilute H2SO4 [17] and in NaOH [20] (although etching proceeds in the latter case). Etching in 1% HF apparently leaves (111) terraces covered by Si trihydrides... 

Fig. 20. Left hand panel 23 A x 23 A STM image of the (2x1) reconstruction on Nao.67W03(100) taken at +0.4 V sample bias and 1 nA tunnel current. Top right hand panel unrelaxed Nao.sO surface plane. The oxygen ions (large spheres) and sodium ions (small spheres) are assigned their conventional Shannon-Prewitt radii. Bottom right panel schematic of the relaxed (2x1) reconstruction with peroxide-like oxygen ion dimers. Adapted from ref. 298.

Eigure 8 shows the environment of the O atoms at the surface of the (2x1) reconstruction. This result was obtained using the projection method to get the approximate atomic geometry without a starting model. The vertical and horizontal cuts near the surface establish the position of O atoms to be at the long bridge sites. 

Fig. 9.10. Representation of ideal and 2x1 reconstructed geometries for Si (001) (courtesy of Lloyd Whitman).

This surface displays a missing row (2x1) reconstruction the structural details of which resemble Pt(110)(l x2). 

Both the Si(100) and Ge(100) surfaces have stable (2x1) reconstructions which involve the saturation of dangling bonds by the formation of" dimers between the atoms in the top layer of the bulk termination of the solid. In the case of Si(100)(2xl), although the existence of surface dimers is no longer controversial, there have been contradictory reports of dimers oriented parallel to the surface (the symmetric dimer model) or tilted in the plane perpendicular to the surface (the asymmetric dimer model), see fig. 10. 

Fig. 12. Schematic illustration of the (2x1) reconstruction of the Si(111) surface. The numbers label the atoms in the bulk termination (see fig. 11). The bold lines show the surface unit cell.

The adsorption of 0.4 ML of the transition metal Co on Si(100) results in the removal of the (2x1) reconstruction with the adatom occupying the coplanar four-fold hollow site formed by four top layer Si atoms (Meyerheim et al., 1991). The Si Co bond length is found to be 2.35 A. 

Fig. 14. Schematic illustration of the four high-coordination adsorption sites on the Si/ Ge 10Q)-(2x 1) reconstructed surface which preserve the (2x1) reconstruction.

Fig. 25. Si(001)(2x1) reconstruction (Roberts and Needs, 1990). Plan view, white and grey circles representing atoms in the top layer (white circles atoms moved outwards grey circles atoms moved inwards) and black circles atoms in the second layer.

These results make the idea that the 100 2x1 reconstruction results from a high concentration of surface vacancies [78, 202] or a canted ridged structure [79] rather improbable, since in both models significant atom migration would be required for the change to 100 1 x 1 to occur. 

The results of Pandey et al. [249] applied toa lll 7x7 surface, but the more detailed results of Schliiter et al. [247] and Larsen et al. [250] were almost certainly obtained from a 111 2x1 reconstructed surface, although simultaneous LEED measurements were not reported. The precise form of reconstruction was, however shown to be important by Rowe et al. [248], who studied Cl2 adsorption on 111 7x7, 111 2x1 and 100 2xl surfaces by photoemission polarization effects with synchrotron radiation. The first point is an apparent difference between their LEED results and those of Florio and Robertson [246] and Pandey et al. [249]. Rowe et al. [248] found that the saturation coverage of chlorine on Ill 7x7 and lll 2xl surfaces produced a primitive lxl pattern, whereas the other workers observed that the 7x7 periodicity... 

At the (100) surface, the presence of highly unstable dangling bonds at the top of the fully unrelaxed surface (indicated in dark in Figure 38a) is partly reduced by the formation of new bonds leading to a (2x1) reconstruction (Figure 38c). A 2-D cell of double size is necessary to model such a reconstruction. 

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