Basic Theories of Semiconductor Electrochemistry - Electrochemistry of Silicon and Its Oxide - page 18

Using Eq. (1.42), we obtain

Thus, acts as a capacitor in series with the parallel combination of and In
many situations because of the presence of an oxide film on the surface, particularly
for silicon in non-HF electrolytes, an extra RC component is involved due to the charges
and states in the oxide and at the semiconductor/oxide interface. The characteristics of
these charges and states are discussed in Chapter 3 on silicon oxide.

In the absence of an interfacial phase such as an oxide, any potential drop is across
The poten-
tial distribution across the space charge layer and in the Helmholtz layer, in the absence
of surface states and interfacial layer when is determined by the doping level,
band bending, and the capacitance of the Helmholtz layer. The partitioning of the poten-
tial can be measured in terms of the potential in the space charge layer relative to the
total potential drop across the interface region by a partitioning coefficient,

the space charge layer or/and the Helmholtz layer, that is,

848

where may vary from 0 to 1. The potential partitioning can also be measured by
the differential form in terms of the fraction of the applied potential change dV across
the space charge layer:

The two coefficients are different in that quantifies the potential partitioning of
the total potential drop while reflects the partitioning as a result of potential change.
Thus, characterizes the steady condition of the interface or the dc condition and
characterizes the transient condition or the ac condition. Figure 1.11, as calculated
by Oskam et al ., 848 shows the change of these two coefficients as a function of applied
potential. It can be noted that under a negative potential corresponding to an accumu-
lation band bending, the coefficients become rather small indicating that becomes
comparable to C H and there is a significant drop of applied potential across the
Helmholtz layer. Because of the shift in potential drop from the space charge region to

the Helmholtz layer, there is a maximum band bending above which all the applied
the maximum band bending potential drops in the Helmholtz layer. For

in the accumulation regime is about 300 mV.

1.3.8. Flatband Potentials

Flatband potential is a very important parameter for characterization of a semi-
conductor/electrolyte interface as it correlates the band edges to the redox potentials
in the electrolyte. It is most commonly determined by measuring the capacitance as a