Chemistry Reference
In-Depth Information
When a semiconductor is brought into contact with an electrolyte, equilibrium
is attained when the Fermi levels in the two phases become equal, that is,
In the case shown in Fig. 1.2 for an
n
-type semiconductor, where of the semi-
conductor is higher than that in solution, electrons will flow from the semiconductor
to the solution phase. The resulting excess charge in the solid semiconductor does not
reside at the surface as it would in a metal, but instead is distributed in a region near
the surface called the space charge region. The resulting electric field in the space
charge region is shown by a bending of the bands. In the case of Fig. 1.2 where the
semiconductor is positively charged with respect to the solution, the bands are bent
upward (with respect to the level in the bulk semiconductor). On the other hand, an
ionic layer on the solution side establishes an electric double layer between the solid
surface and solution.
Thus, several charged layers exist at the interface of a semiconductor and an elec-
trolyte. Figure 1.3 illustrates the different charged layers in the semiconductor/
electrolyte region. On the semiconductor side there is the space charge layer associated
The ionic layer on the solution side can be further divided.
The charged layer lies between the solid and the plane at the position of the closest
approach of mobile ions, called the outer Helmholtz plane (ohp). The Helmholtz layer
with a band bending of
is formed by ions attracted to the electrode surface by the excess charge in the space
charge layer and also by the polar water molecules. In particular, the adsorption of
and
is an important process that determines the potential drop in the Helmholtz
layer of many semiconductors. Between the outer Helmholtz plane and the solid surface
there is also a layer (not illustrated in the diagram) containing solvent molecules and
sometimes other specifically adsorbed species.
44
The locus of the specifically adsorbed
ions is called the inner Helmholtz plane. The charge layer that extends from the outer
Helmholtz layer into the bulk, called the Gouy-Chapman layer, is a region of solution
with excess ions of one sign and its thickness depends on the electrolyte concentration.
In concentrated electrolytes (>0.1 M) the contribution of the Gouy-Chapman layer is
negligible and the potential drop on the solution side of the double layer can be
expressed by the potential drop in the Helmholtz layer,
