Chemistry Reference
In-Depth Information
the surface may be hydrolyzed. With increasing anodic potential the rate of hydrolyza-
tion of Si-H bonds increases and at a certain potential the H termination of the silicon
surface is no longer preserved. Passivation corresponds to the condensation of neigh-
boring surface Si-OH bonds forming Si-O-Si bridges.
The characteristics of passivation have been found to strongly depend on sub-
strate orientation. The current peak on the
curves, shown in Figs. 5.36 and 5.37,
109
is about six times smaller on the {111} samples than on the {100} samples. Also, the
i-V
i-V
curve repeated after the first curve is similar to the first curve on the (111) surface,
but not on the (100) where the current of the second curve is much smaller than
the first curve.
22
The shape of the current peak also depends on the silicon substrate;
{100} samples have a single peak whereas {111} samples have two peaks as shown
in Figs. 5.36 and 5.37.
109
The double peaks for (111) samples are not found in 40%
KOH solution at 60 °C.
192
The behavior of (111) and (100) samples also differs according to current
transients.
22,183,291
A smaller amount of charge is required to reach a steady-state condi-
tion for the (111) than the (100) surface when the potential is stepped from OCP to an
anodic potential.
22
This suggests that the passivation of a (111) surface requires less
material to be oxidized than a (100) surface. In addition, current transients at various
potentials positive of OCP on the (111) samples exhibit a current decrease within the
initial few seconds followed by a current peak while the current transient on (100)
samples shows only a monotonic decrease.
291
For the (111) material the transients
consist of only one maximum at potentials between OCP and V
p
. On the other hand,
they have two maxima at potentials positive of the passivation potential. According to
Smith
et al
.
291
the first current maximum is due to the dissolution of a film preexistent
at OCP while the second is associated with the formation of an anodic oxide film on
the (111) surface. The amount of charge associated with this transient increases with
applied potential and is higher on
p
-Si than on
n
-Si. Smith
et al.
192
also found that the
passivation potential in KOH is independent of the potential sweeping rate for (100)
samples whereas it changes with sweeping rate for (111) samples as shown in Fig. 5.38.
The behavior of (111) silicon is attributed to the presence of a prepassive layer with a
charge density of 2.4 mC/cm
2
which is converted to oxide at a potential positive of the
passivation potential.
For non-heavily doped materials, the
characteristics near the passivation
potential are essentially independent of carrier density.
109,378
However, for heavily doped
materials, the current peak, marking the occurrence of passivation, is much lower as
shown in Fig. 5.39; the ratio of the peak current on lowly doped sample to that on
heavily doped sample is about 6.
269
For lowly doped materials, long-duration immer-
sion in the solution causes very little change of the
i-V
characteristics, whereas it causes
the current peak to disappear for highly doped materials. Also, the dependence of pas-
sivation potential and passivation overpotential, V
P
-OCP, on temperature appears to be
opposite for lowly and highly doped materials as shown in Fig. 5.40.
269
Figure 5.41 shows that the passivation potential decreases with doping concen-
tration and is largely independent of orientation. The change in the values of passiva-
tion potential is more than 1 V from low to high. The distribution of this extra potential
associated passivation in the Helmholtz layer, in the space charge layer, in a preexis-
tent oxide, or in the substrate has not been determined. The passivation overpotential,
i-V
