Buckled dimer

The Si (100 surface reconstructs by forming dimers of (2x1) symmetry, which arrange themselves into parallel rows. It is now well established that these dimers buckle to form the higher-order p(2x2) and c(2x4) reconstructions. This buckling is due to a transfer of an electron from the lower to the upper atom of the dimer, which opens up a gap between the occupied and unoccupied states. At room temperatures or above, the buckled dimers oscillate in time, and therefore appear symmetric under... 

Figure 5.3. Models of the Si(100) and Ge(100) surface (Left) (2 x 1) dimer reconstruction involving symmetric dimers (Middle) c(4 x 2) dimer reconstruction with buckled dimers These two structures are observed for silicon at room temperature and lower temperature, respectively. For germanium, the structure at (Right), the p(2 x 2) dimer reconstruction with buckled dimers, is also observed at lower temperatures. In the top view model, the open circles represent the top layer atoms, with the larger and smaller circles designating the up and down atoms of the dimer, respectively. The filled circles represent the next layer of atoms.

Yoshinobu, J. Physical properties and chemical reactivity of the buckled dimer on Si(100). Progress in Surface Science 77, 37 (2004). 

Wolkow, R. A. Direct observation of an increase in buckled dimers on Si(001) at low temperature. Physical Review Letters 68, 2636 (1992). 

K. Hata, Y. Shibata and H. Shigekawa, Fine electronic structure of the buckled dimers of Si(100) eludicated by atomically resolved scanning tunneling spectroscopy and bias-dependent imaging, Phys. Rev. B 64, 235310 (2001). 

Figure 1 Top view of the unreconstructed (ideal) surface and the reconstructions discussed in the text. Smaller and darker circles represent deeper atoms the small black circles are second layer atoms and the larger gray and white circles are surface atoms. In the buckled dimer c(4 x 2) and p(2 x 2) reconstructions the larger white circles protrude further out of the surface than the gray circles. The dashed lines and shaded areas represent the surface unit cells. The symmetric p(2 x 1) reconstruction is (1.8 0.1) eV/dimer lower in energy than the ideal structure, as calculated by first principles DFT calculations [35]. The c(4 x 2) reconstruction is (0.17 0.03) eV/dimer lower in energy than the p(2 x 1) structure. The difference between the c(4 x 2) andp(2 x 2) structures of 3 meV/dimer in favor of the former is within the calculation error bar.

Si (100) (di- (2x1) Buckled dimer model is best fit to data. MEIS/17/... 

Experimentally the situation was unclear because of STM experiments showing apparently symmetric dimers on an almost defect-free terrace of Si(001)(2xl), with tilting only near steps (Wiesendanger et al., 1990). However, recent temperature-dependent STM work has shown that on cooling to 120 K, the number of buckled dimers increases (Wolkow, 1992). It seems likely that the bistability of the asymmetric dimer results in flipping between... 

Fig.5.2-7a-c Dimer models for the 2x1 reconstruction of Si and Ge(lOO) faces, with (a) symmetric and (b) buckled dimers (side views), (c) Top view of the symmetric case. Ideal shaded) and reconstructed unit cells are shown... 

Fig. 5.2-10 Ordered arrangement of buckled dimers that gives rise to c(4x2) reconsttuction of the (100) face of Si and Ge (top views). Ideal (shaded) and reconstmcted unit cells are shown...

Si p(2x2) Same as above Ordered arrangement of buckled dimers (Fig. 5.2-8) Present locally ( 5 %)... 

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