Chemistry Reference
In-Depth Information
at relatively large bias but curved at small bias. The potential intersection of the
Mott-Schottky plot, considering the effect of the Helmholtz layer resulting from high
doping concentration, is given by
where a voltage shift from
by an amount of
is due to the potential
drop in the Helmholtz layer.
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When there is a high density of surface states, the Fermi level can become pinned
within the potential range in which the surface states lie. For a monoenergetic surface
state, the intersect of the Mott-Schottky plot is shifted from its true flatband potential
value as illustrated in Fig. 1.13.
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In this case the applied potential is dropped in the
Helmholtz layer and the capacitance does not change with applied potential. When the
surface states are fully occupied or fully empty, the capacitance starts to change again
with applied potential according to the Mott-Schottky relation. The slope of the central
region in Fig. 1.13 is related closely to the density of surface states. The higher the
slope is in the central region, the lower the density of states. An infinite density of states
will lead to a zero slope.
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Assume is the positive charge when the donor surface
states are fully empty and is the negative charge when the acceptor surface states
are fully occupied. The effect of the donor surface states on the flatband potential can
be described by
and that of the acceptor surface states by
The presence of an oxide layer on the silicon surface can also cause the flatband
potential to shift. The potential drop across an oxide is on the order of
where
and
are the dielectric constants of silicon and silicon oxide, respectively,
d
