Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 223

An important feature of the scheme is that after the dissolution of each layer of silicon
atoms the new surface is terminated with hydrogen.
Consideration of Chemical versus Electrochemical Reaction. Based on a

study of the effect of pH on the relative contribution of the chemical versus electro-
solutions, Allongue et al . 904 proposed a reaction scheme
which consists of a chemical path in competition with an electrochemical path. In the
chemical path the first step is The Si-OH bond is then
replaced by Si-F followed by attacking of the Si-Si back bond by water, leading to the
detachment of the silicon atom. In the electrochemical path the first step is
in which the oxidation of the hydrogen in the surface Si-H bond
is considered to require catalytic involvement of The proceeding steps are the same
as those in the chemical path. As pH increases, the chemical reaction, which is rela-
tively low at low pH, becomes dominant. The model, however, lacks the detail to
account for the participation of holes and electrons and the different reactions under
various conditions.
Individual Steps in the Transfer of Valence Electrons. A number of studies,
for example, by Lewerenz et al . 626,908 and Stumper and Peter, 599,695,921 have shown that
the current multiplication factor varies with light intensity from about 4 at low
intensities to about 2 at high intensities while the efficiency of hydrogen evolution
changes from 0 to near 1 as shown in Fig. 5.14. They proposed a reaction scheme that
involves hole capturing as the first step followed by electron injection as the second
step. The oxidized silicon intermediate, having a valence of 2, is then further oxidized
by injection of two electrons or reaction with the hydrogen ions depending on light
intensity:

chemical reactions in