Silicon reconstruction

Topographic image of a reconstructed silicon surface showing outermost atomic positions, photograph of Dr. James Batteas. National Institute of Standards and Technology. Reproduced by permission p. 

In-situ spectroscopic measurements have shown that water is dissociatively chemisorbed onto reconstructed silicon surfaces in ultrahigh vacuum at room temperature [52-54] ... 

Figure 2,13. The reconstructed silicon (100) crystal face as obtained by LEUD surface crys-lailogoaphy. Note ihal surface relaxation CMcnds to three atomic layers mto the bulk ll6fi. ...

Surgical implants, female breast reconstruction, silicone (Vol. 74 1999)... 

Gold, platinum and iridium (100) crystal faces all show reconstruction [1]. Figure 10 shows the stucture of the reconstructed silicon (100) crystal face. In this surface, the silicon atoms form a dimer-like surface structure and there is a relaxation or contraction at... 

Figure 11 shows the beautiful diffraction patterns exhibited by one of the more stable structures on the reconstructed silicon (111) surface. This is the 7x7 structure which has a complex unit cell which is still not resolved by surface crystallography. It is hoped that a resolution in this surface structure will be forthcoming within a year. It should be noted that the silicon (100) and (111) surfaces are frequently used as substrates for electronic circuitries. As a result, the atomic surface structure of these surfaces is of utmost importance in the integrated circuitry technology, since the electronic transport properties are clearly dependent on the location of atoms at the surface. 

Fig. 6.6 Symmetries of the reconstructed silicon (2x1) and (7x7) surfeces, observed via rotation of the polarization vector of the exciting laser (field strength E w)) within the crystal plane and measurement of the second...

For laser irradiation along the surface normal one expects SH signals by the symmetry classes 1, Im and 3, 3m. For Im-symmetry the nonlinear susceptibilities vanish in the directions xyx, yyy and yxx, and thus in the y-direction the observed intensity /y is proportional to sin 20. In Fig. 6.5 the measured SH intensities along the y-direction (upper part) and x-direction (bottom part) are shown for two differently reconstructed silicon crystals the (2x1) reconstruction, which results from cleaving of the single crystal in vacuum, and the (7 X 7) equilibrium reconstruction, which can be obtained by annealing to surface temperatures of, usually, above 1000 °C. The solid lines in the y-direction are fits to the measured points assuming a Im-symmetry. 

Therefore it is reasonable to prepare already the data acquisition for a three dimensional evaluation in cone-beam-technique by means of two-dimensional detectors. The system is already prepared to integrate a second detector- system for this purpose. An array of up to four flat panel detectors is foreseen. The detector- elements are based on amorphous silicon. Because of the high photon energy and the high dose rates special attention was necessary to protect the read-out electronics. Details of the detector arrangement and the software for reconstruction, visualisation and comparison between the CT results and CAD data are part of a separate paper during this conference [2]. 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

In 1985 Car and Parrinello invented a method [111-113] in which molecular dynamics (MD) methods are combined with first-principles computations such that the interatomic forces due to the electronic degrees of freedom are computed by density functional theory [114-116] and the statistical properties by the MD method. This method and related ab initio simulations have been successfully applied to carbon [117], silicon [118-120], copper [121], surface reconstruction [122-128], atomic clusters [129-133], molecular crystals [134], the epitaxial growth of metals [135-140], and many other systems for a review see Ref. 113. 

FIG. 1 Freeze-etching image of a bacterial cell of (a) Desulfotomaculum nigrificans (bar, 100 nm). Atomic force micrographs of the S-layer proteins of (b) Bacillus sphaericus CCM 2177 and (c) Bacillus stearothermophilus PV72/p2 recrystallized in monolayers on silicon wafers. Bars, 50 nm. The insets in (b) and (c) show the corresponding computer-image reconstructions. 

The various experiments above have spawned four models that could explain the neutralization of boron. These four models are shown in Fig. 11. According to my model (Fig. 11a), Hj is tied to one of the four silicon atoms surrounding the substitutional boron atom, thus leaving all the valence bonds satisfied. Sah et al. (1984) proposed that H is tied to B, thus requiring the reconstruction of dangling bonds between adjacent Si atoms (Fig. 11(b)). An alternative model by Sah (1984) is a bridging bond between B and Si (Fig. 11(c)). The model proposed by Hansen et al. (1984) has a hydroxyl group attached to boron (Fig. 11(d)). 

The surface condition of a silicon crystal depends on the way the surface was prepared. Only a silicon crystal that is cleaved in ultra high vacuum (UHV) exhibits a surface free of other elements. However, on an atomistic scale this surface does not look like the surface of a diamond lattice as we might expect from macroscopic models. If such simple surfaces existed, each surface silicon atom would carry one or two free bonds. This high density of free bonds corresponds to a high surface energy and the surface relaxes to a thermodynamically more favorable state. Therefore, the surface of a real silicon crystal is either free of other elements but reconstructed, or a perfect crystal plane but passivated with other elements. The first case can be studied for silicon crystals cleaved in UHV [Sc4], while unreconstructed silicon (100) [Pi2, Ar5, Th9] or (111) [Hi9, Ha2, Bi5] surfaces have so far only been reported for a termination of surface bonds by hydrogen. 

Reactions that occur on the surface of covalent solids have a complexity that is not as prevalent in metals. Many metal surfaces, especially the close-packed faces, retain the same geometry and bonding arrangement as would be present in the bulk phase. Most semi(x>nductor surfaces, however, undergo reconstructions in which the surface atoms move significant distances from the bulk terminated positions. For example, the Si(001) surface, if bulk terminated, would have each atom bonded to two other silicon atoms in the second layer (Fig. 7). There would be two dangling bonds each with one electron, and the... 

For the deposition of silicon on Si(001) and Si(l 11) surfaces, Gossmann and Feldman determined that epitaxial growth occurs for substrate temperatures maintained over 570 K and 640 K, respectively . Above the epitaxial temperature the growth is single crystal, while below this temperature the growth is amorphous. This difference in epitaxial growth temperature between the two faces was ascribed by Gossmann and Feldman to be due to the different surface reconstructions, where the (111) surface presumably... 

In a different approach to this problem, Brenner and Garrison used molecular dynamics to examine the chemical mechanisms which lead to reordering of the atom-pairing reconstruction during atom deposition . This simulation incorporated a dissociative valence-force field potentiaF and consisted essentially of a high-temperature anneal of monolayers of silicon atoms which had been deposited on a silicon (001) reconstructed surface. 

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