Chemistry Reference
In-Depth Information
energetic characteristic to that in the solution are found
0.38eV below the conduction band in They are independent of solu-
tion pH from pH 3 to 11, indicating that the surface states are physically the same for
both hydrogen-terminated (at low pH) and oxide-passivated surfaces (at high pH),
although the density is different.
In some organic solvents, the surface states are associated with the Si-O-H bonds
and are independent of solvent type.
395
In acetonitrile solutions the surface states are
found to increase from the initial value of to after 1 day in
solution and to when the surface is oxidized corresponding to an increase
in the amount of Si-O-H bonds.
929
The surface states are found to be absent initially
when the electrode is first immersed into the electrolyte but develop gradually.
935
The
rate of change depends on the type of redox couple in the electrolyte. The surface states
are attributed to the slow oxidation of the surface due to the traces of water present in
the organic electrolyte. They have a broad energy distribution extending through the
band gap with a maximum density near the band edge.
942
In acetonitrile solution con-
taining a redox couple of 0.01 M ferrocene/0.001 M ferricinium, the density of surface
states is found to be Because of the presence of surface states, the
Fermi level can be pinned leading to a constant Schottky barrier height for different
redox couples.
935
The results of the large number of studies on surface states indicate that those on
the surface of silicon tend to be associated with oxide when the surface is covered
with an oxide or to reaction intermediates when the surface is not covered with an
oxide. A high density of surface states is generally observed when the electrode is
illuminated or a current is passed through. Reaction intermediates can behave like
surface states.
2.7. FLATBAND POTENTIALS
The flatband potential of silicon electrodes is an important parameter that has
been determined in many solutions. Table 2.14 is a collection of the flatband potentials
for various silicon/electrolyte interfaces. The values of determined in real systems
depend on a range of factors including doping concentration, type of solvent, type of
electrolyte, pH, illumination, redox couple, and surface condition. Mechanistically,
according to Eq. 15 in Chapter 1, the value of the flatband potential is determined by
The first is subject
to the variation in dopant concentration because the Fermi level with respect to the con-
duction band edge,
two parameters: the bulk Fermi level and the Helmholtz potential
is equal to
for a nondegenerated semiconductor.
The second parameter,
is subject to the variation of surface condition and the nature
of the electrolyte.
2.7.1. Effect of pH
It can be expected that the Helmholtz potential of a silicon electrode in an aqueous
electrolyte is a strong function of pH since hydrogen adsorption is a dominant process
on the surface of silicon. Table 2.15 shows the dependence of flatband potential on pH
