Chemistry Reference
In-Depth Information
Hole injection from the oxidizing species in the solution into the valence band is postu-
lated to be responsible for the chemiluminescence observed on porous silicon in these
solutions at the OCP.
However, for and despite their very positive redox poten-
tials, the reduction proceeds primarily via the conduction band with minimal hole
injection into the valence band.
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The weak hole injection with strong oxidizing
agents and is attributed to the two-electron reduction process in which the
first step requires a conduction band electron to create reaction intermediates, and
which can then inject a hole into the valence band. The reduction reaction
can be more complicated when more than one redox couple is present. It has been
found that hole injection by is increased when is also present in the solution.
The ions react with to generate hydroxyl radicals in the solution. This
implies that hole injection during the reduction of occurs from the intermediate
which is in the solution rather than on the surface as surface states. Hole injection from
the redox species in the solution into the valence band can also be affected by other
factors. As shown in Fig. 6.22, hole injection is affected by the formation of surface
oxide. For
hole injection involves the hydrogen reaction that also occurs in
this region.
The redox couples, which inject holes into the conduction band, induce anodic
current on
n-
Si in the dark. According to Gerischer and Lubke,
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anodic current is due
to electron injection by silicon dissolution intermediates for redox couples with one
electron transfer per molecule. Depending on the number of electrons involved in the
reduction process of the redox couple, the photocurrent may have a quantum yield
varying from 1 to 2. The quantum yield is 2 without redox couples. It remains the same
when a redox couple with only one oxidation step per molecule such as is
present. However, for the oxidizing agents and the reduction of which
involves two electrons, the quantum yield is smaller than 2 as shown in Fig. 6.25. This
is attributed to the adsorption of the reduction intermediates on the surface, which
inhibits the electron injection from silicon dissolution intermediates responsible for the
quantum yield of 2.
In general, it is difficult to oxidize a redox species on silicon in aqueous elec-
trolytes because of the highly reactive nature of silicon; oxidation of silicon results
in a passive oxide film in nonfluoride solutions, whereas dissolution of silicon is the
