Chemistry Reference
In-Depth Information
function of potential. Flatband potential can also be estimated by measurement of the
onset potential for photocurrent.
In an idealized case when the effect of the Helmholtz layer can be neglected, i.e.,
when there is a negligible amount of surface states, that is, The
total capacitance of the semiconductor/electrolyte interface described by Eq. (1.45)
becomes The interface capacitance as a function of the electrode potential then
follows the Mott-Schottky equation:
A plot of versus potential is linear and thus the potential where the line
intersects the potential axis yields the value of and the slope yields the value of
doping level The resulting flatband potential can be used to determine the potential
of the conduction band edge
V
at the surface
where
N
c
is the effective density of states in the conduction band. The Schottky barrier
height is
In practice, the determination of flatband potential according to the Mott-
Schottky equation can be affected by a number of factors, e.g., high doping concen-
tration (i.e., is not valid), the presence of a high density of surface states (i.e.,
is not valid), and the influence of diffusion-controlled processes.
44,274,935
For
highly doped semiconductors, the effect of potential drop in the Helmholtz layer, even
without the influence of surface states, can be significant.
274
Figure 1.12 indicates that
a significant fraction of the external potential variation can be dropped in the Helmholtz
layer for moderately and highly doped semiconductors especially near the flatband
potential at which the capacitance of the space charge layer is the largest. Because of
the contribution from the Helmholtz layer, the Mott-Schottky plot tends to be linear
