Chemistry Reference
In-Depth Information
to improve the reproducibility of the flatband potential in acetonitrile.
615
In another
study, exposing a freshly HF rinsed sample in methanol vapor as a surface preparation
procedure caused a shift of the flatband potential of
n
-Si in acetonitrile
solution from
to
and it reduced the change of flatband potential
686
with time possibly due to the formation of groups.
Similarly, time of immersion of a freshly prepared silicon sample has a signifi-
cant effect on the flatband potential due to the active nature of the silicon surface. A
HF-cleaned silicon surface is generally terminated by hydrogen, which, although rela-
tively stable, tends to be replaced by a thin oxide film. The flatband potential of such
a surface may change with time in the electrolyte.
83,716,1108
As an example, for an
n
-type
silicon in acetonitrile
for a freshly prepared sample to
solution the flatband potential changes from -0.02
after 24h immersion.
686,935
The shift is
due to the gradual generation of surface states as a result of the slow oxidation of the
silicon surface caused by traces of water in the electrolyte.
Considering the change of surface condition with time, it is difficult to obtain
reproducible flatband potentials because of the high reactivity of the silicon surface
in HF solutions.
716,841
The high reactivity tends to cause roughening of the surface in
general and formation of porous silicon in particular. Roughening of the silicon surface
in HF solution may cause a change of capacitance by more than a factor of 5.
475,1153
As
a result, the flatband potential measured in HF solutions tends to vary with many factors
related to conditions and procedures. For example, the flatband potential of
n-
Si in HF
solution has been found to change with the initial potential at the beginning of poten-
tial sweep.
39
The difference between the flatband potentials of
p-
Si and
n
-Si plus the differ-
ences between the bulk Fermi level to the corresponding band edges equal the band
gap,
Such situations have been observed.
21,716
However, in many situations the measurement
when the band edges of the two materials are the same in the solution.
of flatband potentials of
p-
Si and
n
-Si does not yield the band gap. There are two pos-
sible explanations. In one the band edges of
p
-Si and
n
-Si may not have the same energy
