Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 80

in the solution, for example, when the Fermi level is pinned by a large density of surface
states. In the other, the actual band edges of the two materials are the same but the
values obtained from the flatband potentials of the two materials are different due to
difficulty in obtaining definite and reproducible flatband potential values for the two
materials. Usually, it is more difficult to determine a reproducible flatband potential of
p- Si due to its more active nature. 475,901 In the presence of HF, significant dissolution
occurs under an anodic polarization, resulting in a rapid structural transformation of
the surface. For example, in 5% HF the flatband potential of n -type silicon is indepen-
dent of the methods of surface preparation whereas that of p type strongly depends on
the methods of preparation. 901 In another case, the flatband potential of p type in 35%
HF solution is found to strongly depend on frequency; a shift of as much as 1300 mV
is observed by changing the frequency from 10 to 100 kHz. On the other hand, that of
n type is independent of frequency from 1 to 500 kHz. 5
2.7.3. Effect of Surface States
A large density of surface states may be associated with certain surface condi-
tions. When an electrode surface has a large density of states, significant amounts of
potential variation may drop across the Helmholtz layer instead of the space charge
layer. This results in Fermi level pinning or unpinning of the band edges. Specifically,
without the influence of surface states the flatband potential is independent of the pres-
ence of the redox couple in the solution. When Fermi level pinning occurs, the flatband
potential tends to change with the potential of the redox couple in the solution as shown
in Fig. 2.31. 21,389,683 The more negative the redox couple is, the larger the shift of the
flatband potential. However, the slope in Fig. 2.31 is less than unity, indicating that the
Fermi level is not completely pinned by the surface states. It has been found that due
to Fermi level pinning, the flatband potential of p type may show no difference from
that of n type in an acetonitrile solution. 372