Silicon backbond

In the frame of this model the anisotropy of alkaline etchants, the low etch rate observed for (111) oriented silicon surfaces, can be interpreted as an insufficient polarization of three silicon backbonds by only one Si-OH surface bond that can... 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

It has been speculated that there is a common origin of the reduced chemical etch rate for (111) oriented silicon substrates and for highly p-type doped substrates. But the electrochemical investigations discussed above indicate that the passivation of highly doped p-type Si can be ascribed to an oxide film already present at OCP, while no such oxide film is observed on (111) silicon below PP. This supports models that ascribe the reduced chemical etch rate on (111) planes to a retarded kinetic for Si surface atoms with three backbonds, present at (111) interfaces [Gil, A12], as discussed in Section 4.1. 

Fig. 6B). The methyl group affords no possibility for backbonding or reversed orientation as shown by the carbon and silicon profiles in Table 1, so the only effect aside from partial surface passivation is the chemisorptive acidity shown in Fig. 5B. 

Some of the most dramatic STM images have been recorded for the Si(l 11) 7X7 reconstruction, as depicted in Figure 3.14.138 These images have been recorded at several different biases (see Spectroscopy and Chemical Selectivity, below) and provide one of the best examples of how STM can be used to better understand the chemistry of such surfaces ( specifically, the electrophilicity and nucleophilicity of the individual surface atoms). Clearly depicted in these results are the orbitals associated with surface atoms, rest atoms, and backbonds. Such studies have continued and been greatly extended into the exploration of a variety of chemical reactions that occur on silicon surfaces. These studies are described in detail in a recent review.135... 

It appears that di-coordinated oxygen configurations such as backbond and dimer-bridge are the most stable forms and play a significant role in the initial growth of silicon oxide film. 

Fig. 29. Structural model for two-electron dissolution of silicon by electrochemical ligand exchange followed by Insertion of HF into the Si—Si backbonds. Insertion of HjO into the backbonds is also possible. Reaction of the unstable products results in the formation of SiFg. The reaction can proceed through the formation of Si-F (a) or Si-OH (b).

Assuming that the single Si-F ligand at the kink site is sufficiently polar to allow successive reaction of the two Si-Si backbonds with HF, then the overall reaction is consistent with pore formation in n- and p-type silicon where two charges are transferred per silicon atom. For the case of photoanodic dissolution of n-type silicon in NH4F [120-124] the characteristic photomultiplication effect at low light intensities is also consistent with the hole capture/electron injection sequence shown in Fig. 29 a. 

Fig. 31. Structural model for two-electron dissolution of silicon by electrochemical reaction of SiHj to SiFj and subsequent insertion of HF (or H2O) into the Si-Si backbonds.

Some insight into possible reaction mechanisms can be gained from examination of the Si/KOH system. Chemical etching of silicon in KOH involves reaction with water and is characterized by the formation of two H2 molecules per silicon atom dissolved [129, 130]. The reaction product is thought to be Si(0H)2(0 )2 [130]. Palik et al. [129] have suggested that the reaction mechanism proceeds through attack of Si-Si backbonds at a Si-OH-terminated surface by H2O ... 

For an OH-terminated silicon atom, corresponding to a site on a (100) surface or a kink site on a (111) facet, reactions (27) and (28) are repeated for each backbond, resulting in the formation of the reaction product Si(0H)2(0 )2. The overall reaction is given by ... 

Reactions (27) and (28) are similar to the proposed mechanism for the smoothening process that occurs during open-circuit etching of silicon in buffered HF solutions, given in reactions (2) and (3). In this case, electrochemical ligand exchange involves the transformation of Si-H - Si-OH followed by HF attack of the Si-Si backbonds. 

Y. Nagasawa and H. Ishida, Backbond oxidation of hydrogen passivated silicon surface, Solid-State... 

The hydrogen adsorption onto a silicon atom is a reduction process since the valence of the hydrogen atom is changed from +1 to 0. It occurs when the backbond of Si—SiF or Si—SiOH is broken by... 

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