Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 259

reacts chemically with silicon resulting in the etching of silicon (etch rate of p- Si
is large and is independent of the cathodic potential). On the other hand, on -Si at

cathodic potentials the reduction of can proceed more favorably by capturing the
electrons from the conduction band than by injecting holes into the valence band. Thus,
the etch rate of n -Si at cathodic potentials is almost zero due to the lack of holes to ini-
tiate the dissolution process. The anodic current on n -Si in the presence of

n

in the

dark is due to electron injection from the silicon reaction intermediates.
According to Bressers et al. , 14 because the ratio of bromine to silicon is 1:1.6 for
the etching reaction at the OCP, one bromine atom is involved in the dissolution of
more than one silicon atom and the attack is probably at the Si-Si back bonds to form
two Si-Br bonds. They proposed the reaction scheme shown in Fig. 6.20 for the dis-
solution of silicon in the presence of in HF. The first step involves a chemical attack
of the silicon back bond producing silicon and bromine reaction intermediates which
are surface states with energy levels in the midband gap. There are four possible paths
for the reaction of intermediates to proceed depending on the type of silicon and poten-
tial range. The first is by further chemical reaction causing the formation of two Si-Br
bonds (path a, which involves no carriers).
The other three paths are electrochemical reactions involving carriers in the
bonds. At sufficiently negative potentials on n -Si or on illuminated p- Si, electrons from
the conduction band can be captured by the surface states causing the repair of the Si-Si
bond (path b) which constitutes the first step of the bromine reduction. In the dark on
p- Si when conduction band electrons are not available, this reduction process can also
proceed by hole injection (path c). The dark anodic current on n -Si is due to electron
injection into the conduction band causing oxidation of a silicon atom (path d).