Chemistry Reference
In-Depth Information
the surface defects will vary from point to point. In some cases electrode reaction can
result in the formation of bulk phases, for example, oxide films. The states and charges
that exist in the oxide and/or at the semiconductor/oxide interface are another form of
surface state (or interface state).
There are two kinds of surface states with respect to the energy levels of states
under filled or empty conditions: one kind has different energy levels when the surface
states are filled or emptied due to a reorganization effect, and the other, showing no
reorganization effect, has negligible change in their energy levels when the states are
filled or emptied. For example, according to Chazalviel and Rao,
83
on a silicon surface
the surface states induced by deposited gold atoms belong to the second kind whereas
those associated with a surface oxide formed by the reaction between silicon and water
belong to the first kind.
It is generally difficult to quantitatively describe the surface states of a semicon-
ductor electrode considering the great diversity of surface states.
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In general, surface
states are energetically distributed in the band gap of the semiconductor with a density
expressed as and the associated charge density is a function of Under some
idealized conditions such as when the surface states are uniformly distributed in energy
or are localized at a single energy level, the charge density associated with surface states
can be quantitatively related to band bending For example, for a uniform dis-
tribution of acceptor surface states centering around an energy of
(when the states
can be expressed as
270
are filled to an energy
there is no net surface charge),
where
is the total number of surface states per square centimeter. Note that when
In the case of acceptor states at a single energy level, we have
270
where
is the degeneracy of the energy level.
The capacitance of surface states, corresponding to the variation of the charge
stored in the surface states with
is
The surface state capacitance is different from the space charge layer capacitance
and the Helmholtz layer capacitance
in that there is in general no distance asso-
ciated with the surface state capacity.
Measurement of capacitance as a function of potential is most commonly used
for evaluation of the nature of surface states. For example, when the surface states are
energetically close to the conduction band they can be filled or emptied by electron
transfer with the conduction band:
