Chemistry Reference
In-Depth Information
tial of silicon is much more negative than that of water. OCP of silicon in water is gen-
erally a corrosion potential. The rate of corrosion, that is, the dissolution rate at OCP,
can be measured in terms of corrosion current. It can be estimated by determining
the polarization resistance at OCP, which was originally formulated by Stern and
1148,1149
based on the mixed potential concept for an electrode under an activa-
tion-controlled condition. In the case where the corrosion rate of the semiconductor is
limited by the minority carriers, the corrosion current simply equals the limiting current
as illustrated in Fig. 1.25.
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As can be seen in Fig. 1.25, the corrosion potential may vary not only with the
value of but also with the slopes of the anodic and cathodic curves as well as the
limiting current values. Since the limiting current is sensitive to the lighting condition,
OCP or corrosion potential varies with the background photonflux. As stated above,
the corrosion potential is determined by the anodic current and cathodic current. Any
changes, such as surface preparation, solution composition and concentration, pH,
temperature, time, convection, aeration, background lighting, and so on, that affect the
anodic and the cathodic reactions will affect the value of the corrosion potential. Thus,
the corrosion potential as well as the corrosion current can vary greatly, depending on
the specific conditions.
The change of the corrosion potential in either the anodic or the cathodic direc-
tion may correspond to a decrease or increase in the corrosion current. The variation
of the corrosion potential and corrosion currents under various conditions can be gen-
eralized using schematic polarization curves in Fig. 1.26.
970
The corrosion potential of
an active electrode in a solution is
Geary
and
are the corrosion
potentials under changed conditions.
