Chemistry Reference
In-Depth Information
chemical pathway, attack of a Si-Si bond is initiated by water, whereas in the electro-
chemical pathway, the oxidation reaction is initiated by hole and ligand attachment onto
a Si-Si bond.
5.12.4. An Overall Reaction Scheme
The results presented in the previous sections show that the anodic reactions on
a silicon electrode may proceed via different paths depending on the conditions and
that those in HF solutions and those in KOH solutions are rather different. They also
show that the mechanistic models proposed for the reactions in HF and KOH solutions
from the many studies in the literature are largely separated. However, in both HF and
KOH solutions, the silicon/electrolyte interface is fundamentally similar differing only
in the concentrations of hydroxyl and fluoride ions. Thus, a reaction scheme must be
coherent with respect to the experimental observations in both HF and KOH solutions.
For comparison, Table 5.8 summarizes the characteristic features of the reactions occur-
ring on silicon in HF and KOH, in terms of nature of the reaction, rate, effective dis-
solution valence, photoeffect, and uniformity of the surface.
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 or
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 or
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.
A key aspect of the reaction scheme is the valence state of the silicon and hydro-
gen atoms at different stages of the various reaction paths. The nonpolarized nature of
the Si-H bond indicates that the hydrogen atom bonded to the silicon atoms on the
