Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 231

is reduced by an electron from the Si-Si bond. The breaking of the Si-Si back bond
by any groups other than HF and does not preserve the local charge neutrality (the
two specific atoms involved in the bonding) and has to involve carriers from the bulk.

This would explain why hydrogen adsorption onto silicon does not occur easily by the
or HF molecules through dissociated hydrogen ions in the solution but rather by

the attacking of the Si-Si back bonds.
It is experimentally established that even though the surface is predominantly ter-
minated by hydrogen, there is still a small portion of the surface silicon atoms termi-
nated by fluoride. These fluoride atoms may be adsorbed at the surface defects such as
kink sites. It can be assumed that the surface fluoride atoms are at equilibrium with the
fluoride and hydrogen ions in the solution at OCP:

Note that this reaction results in the oxidation of the adsorbed hydrogen from valence
of 0 to +1 and requires the participation of carriers. Thus, reaction (5.29), which results
in the replacement of hydrogen by fluoride, is different from reaction (II) in Fig. 5.68,
which does not oxidize the replaced hydrogen atom.
Therefore, and are the reactants involved in KOH solutions, and
HF, and are those involved in HF solutions. and are responsible for initi-
ating the attack by replacing the adsorbed hydrogen atoms, and and HF are respon-
sible for attacking the Si-Si back bonds. The elemental steps involving these reactions
shown in Figs. 5.68 and 5.69 account for the essential features, that is, dynamic hydro-
gen termination of the silicon surface and weakening and breaking of the silicon back
bond due to adsorption of fluoride or hydroxyl ions. The relative contributions of these
processes in the reactions can then account for the variations in the effective dissolu-
tion valence and in the quantum efficiency under different conditions. An important
aspect is that electronic carriers in the silicon semiconductor do not affect the chemi-
cal nature of the reactions so that the reactions shown above are the same on p- and n-
type materials. The electronic carriers, however, affect the rate of the reactions and the
path of the reactions as will be illustrated in the following.
Reactions Paths. Figures 5.70-5.73 show the reaction schemes for the various
situations occurring on silicon electrodes in HF and KOH solutions. The elemental reac-
tion steps (I) to (IV), and reaction (5.29) described above involve different combina-
tions in these reaction paths depending on the solution, potential range, and illumination
condition. These different paths can account for the many details experimentally
observed in the dissolution or passivation of silicon in HF and KOH. Table 5.9 sum-
marizes the reaction paths involved in different potential ranges in HF and KOH solu-
tions. Variations of the reaction paths from those shown in Figs. 5.70-5.73 are also
possible based on the elemental steps outlined in the previous section.
Reaction paths (I) and (II) in Fig. 5.70 account for the anodic reactions on p- Si
and illuminated n -Si in HF solutions at high light intensities. Path (I) is involved in the
exponential region at an anodic potential much lower than responsible for direct dis-
solution of silicon and dissolution valence of 2, while path (II) is involved at a poten-
tial above responsible for the indirect dissolution of silicon through formation and
dissolution of oxide and for the dissolution valence of 4. At a potential that is lower