Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 182

values with increasing HF concentration because
tion. 2,40,939 Figure 5.21 also shows that the dissolution valence is less than 2 at relatively

increases with HF concentra-

low current densities for a given solution. This is due to the chemical dissolution of the
porous silicon formed on the surface (see Chapter 8).
The value of dissolution valence in the electropolishing region is somewhat
lower than 4 as shown in Figs. 5.20 and 5.21. This indicates that hydrogen reaction,
which is a chemical reaction, still occurs at such high anodic potentials, where
the reactions are characterized by the formation and dissolution of an anodic oxide
film.

In alkaline solutions, the effective dissolution valence at OCP, as shown in Table
5.1, is zero and changes only very slightly with anodic polarization before the passi-
vation peak; it is less than 0.4 at . The dissolution reaction below the passivation
potential is almost completely chemical. 22,108,109 The dissolution valence in the passive
region in alkaline solutions, which is not found in the literature, is likely close to 4
since the growth of anodic oxide films should be identical to that in HF solutions (see
Chapter 3).
The data in Table 5.1 show that for -Si at cathodic potentials there is no

participation of carriers and the reaction is almost 100% chemical such that the etching
of silicon is characterized by the dissolution of one silicon atom and the evolution
of two hydrogen molecules. For

p

-Si at cathodic potentials only a part of the evolved
hydrogen is the result of directly reacting silicon and the rest is due to capturing
the electrons from the semiconductor, which leads to a much reduced etch rate
compared to that at OCP. Palik et al. 108 noted that the charges are not balanced for
n

n

-Si at cathodic potentials, meaning that some of the hydrogen gas is not collected
during the experiment. One possibility is that some hydrogen may enter the silicon
substrate, as has been reported to occur in HF solutions at cathodic potentials. 241 At