Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 261

On n -Si the cathodic current shows similar dependence on potential with posi-
tive (initially active surface) and negative (initially passive surface) potential scan direc-
tions. However, on p- Si cathodic current is only observed on an active surface in the
dark (with a positive scan rate). Also, the cathodic current on illuminated p- Si on a pas-
sivated surface is much smaller than that on an active surface. Furthermore, the cathodic
photocurrent on p- Si at low light intensity is double that without At high light
intensities the photocurrent becomes the same on active and passive surfaces and the
current multiplication factor is less than 2.
According to Meerakker, 595 on a passivated surface the reduction of is by
a conduction band electron forming a reaction intermediate which then injects a hole
into the valence band:

The reduction current on p- Si is small in the dark because it is limited by reaction
(6.14), which requires electrons. On the active surface the reaction scheme is more
complex due to the interaction between silicon radical and hydrogen peroxide gener-
ating a

radical:

The then injects a hole into the valence band, which is responsible for the reduc-
tion current observed on an active surface of p- Si in the dark. At anodic potentials, reac-
tion (6.16) competes favorably with the electron injection into the conduction band
from the silicon radical because when is present the anodic current peak on n -Si

(due to the electron injection) disappears.
When an active p -Si surface is illuminated, both reactions (6.14) and (6.15) take

place resulting in a current which is larger than that on the passivated surface. At high
instead of via a valence band by reaction (6.15), may
also proceed via the conduction band:

light intensities, reduction of

Reaction (6.17) accounts for the reduced multiplication factor at high light intensities.
Other Redox Species. Reduction of ferricyanide in KOH solution takes place
via hole injection into the valence band. 541 The reaction path depends on whether an
oxide film is present on the surface. On an oxide-free p- Si the reduction proceeds by
hole injection as shown in Fig. 6.22. On an oxide-covered electrode, which is anodized
at 0 V prior to the transient, the drop of current at about 3.5 min is due to the complete
dissolution of the oxide film, resulting in the same current as that on the oxide-free
surface. The lower current on the oxide-free surface is attributed by Bressers et al . 541
to the reaction of silicon, which consumes a part of the injected holes by the reduction
of ferricyanide. On the oxide-covered surface, silicon dissolution does not occur and
all of the injected holes flow into the semiconductor. Figure 6.23 shows the dependence