Surface dynamics, hydrogen/silicon

Despite the many differences in HF and KOH solutions as shown in Table 5.8, the overall reactions are similar in two important aspects the silicon surface is dynamically terminated by hydrogen and breaking of the silicon-silicon back bond is facilitated by the adsorption of electronegative ligands such as F" or OH". More specifically this means (1) the initial surface is hydrogen terminated (2) the Si-Si back bond requires that the hydrogen termination is first replaced by F" or OH" and (3) the silicon atoms on the newly exposed layer are terminated by hydrogen so that the surface after the dissolution of one silicon layer is identical to that before the dissolution. 

The surface of silicon during anodic dissolution is dynamically terminated by hydrogen the dissolution of silicon atoms proceeds by first forming a Si-H bond. 

Brenner and Garrison introduced a potential which was derived by rewriting a valence force expression so that proper dissociation behavior is attained . Because the equations were extended from a set of terms which provided an excellent fit to the vibrational properties of silicon, this potential is well suited for studying processes which depend on dynamic properties of crystalline silicon. For example, Agrawal et al. have studied energy transfer from adsorbed hydrogen atoms into the surface using this potential . 

As already mentioned, in the case of semiconductor surfaces there is often a strong surface rearrangement upon adsorption due to the covalent bonding of the semiconductor substrate. The benchmark system for the study of the adsorption and desorption dynamics at semiconductor surfaces is the interaction of hydrogen with silicon surfaces [2, 61]. Apart from the fundamental interest, this system is also of strong technological relevance for the growth and passivation of semiconductor devices. 

According to Palik et al. for p-Si at potentials negative of Vp the reaction is dominated by reaction (5.22). For n-Si at cathodic potential it is a mix of reaction (5.22) and (5.23) while reaction (5.24) is increasingly involved with decreasing potential. However, the reaction scheme described by equation (5.22) to (5.23) results in a OH" terminated surface which is not in agreement with the later experimental findings that silicon surface is also terminated, dynamically, by hydrogen in KOH solutions similar to that in HF solutions. 

Hofer, U. (1996) Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces. Appl. Phys. A, 63, 533-547. 

In the 1970s and 1980s both the clean and H-covered Si surfaces were characterized by diffraction and spectroscopic methods, but only in the last decade have there been reproducible studies of chemical kinetics and dynamics on well-characterized silicon surfaces. Despite the conceptual simplicity of hydrogen as an adsorbate, this system has turned out to be rich and complex, revealing new principles of surface chemistry that are not typical of reactions on metal surfaces. For example, the desorption of hydrogen, in which two adsorbed H atoms recombine to form H2, is approximately first order in H coverage on the Si(lOO) surface. This result is unexpected for an elementary reaction between two atoms, and recombi-native desorption on metals is typically second order. The fact that first-order desorption kinetics has now been observed on a number of covalent surfaces demonstrates its broader significance. 

In a similar way, chemically induced dimmer configuration prepared on the silicon Si(l 0 0) surface is essentially untitled and differs, both electronically and structurally, from the dynamically tilting dimers normally found on this surface [71]. The dimer units that compose the bare Si(l 0 0) surface tilt back and forth in a low-frequency ( 5 THz) seesaw mode. In contrast, dimers that have reacted with H2 have their Si—Si dimer bonds elongated and locked in the horizontal plane of the surface. They are more reactive than normal dimers. For molecular hydrogen (H2) adsorption, the enhancement is even 10 at room temperature. In a similar way, boundaries between crystaUites and amorphous regions seem to be active sites of chain adsorption on CB surface. CB nanoparticles can be understood as open quantum systems, and the uncompensated forces can be analyzed in terms of quantum decoherence effects [70]. The dynamic approach to reinforcement proposed in this chapter becomes an additional support in epistemology of it, and with data from sub-nanolevel. 

However Montanari et al. [17] observed polymerization of D3 to silicone during adsorption on SG in dynamic conditions which could explain the partial regeneration. A combination of both mechanisms can match all these results mainly hydrogen bonds form at low siloxane uptake when SG load increases surface polymerization (stronger bonds) occurs and makes siloxanes difficult to desorb. 

Doren, D. J. (1996a). Kinetics and dynamics of hydrogen adsorption and desorption on silicon surfaces. 4<7v. Chem. Phys. 95, 1. 

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