Chemistry Reference
In-Depth Information
potential than the applied value. As the surface area gets smaller, the effective poten-
tial approaches the applied value.
It has been observed that in an NMA solution, if anodization is continued after
the voltage is increased to 350 V, the oxide breaks down; the voltage starts to oscillate
without rising further.
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But if the sample is held at a constant voltage of 300 V for
some time, the forming voltage can be raised to 560 V at which bright sparks appear
in the solution. Also, in the NMA solution, if an as-grown film is cathodically polar-
ized at a voltage between 12 and 15 V for a certain time, the current increases in
magnitude. The anodic oxide is “deformed” by the cathodic polarization because on
switching the current back to the anodic direction, the voltage necessary to drive a given
current is much smaller than before cathodic polarization.
3.3.5. Effect of Illumination
V
curves of both
p-
Si and
n
-Si is shown in
Fig. 3.13.
370
The anodic reaction kinetics of
The effect of illumination on the
I
-
-type silicon are not affected by illumi-
nation because the reaction consumes holes which are the majority carriers and their
concentration is little affected by illumination. For
n-
Si during anodization, the inter-
face is reversely biased and in order to sustain the reaction, either holes have to be gen-
erated or electrons have to be injected from the electrolyte. Thus, in the dark, an extra
voltage, above that which is required for anodization of
p-
Si
,
is needed to drive the
current.
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For example, about 100 V is required for initially in NMA. The
extra voltage needed to anodize
n-
Si diminishes with increasing illumination intensity
and at sufficient light intensity the anodic current becomes identical to that for
p-
Si.
The quantum yield of illuminated anodization is low; a value as low as 1% has been
found for the anodization of silicon under illumination.
605
Figure 3.13 also shows that the sensitivity to illumination decreases with increas-
ing oxide thickness, and at a thickness corresponding to about 380 Å further growth is
no longer sensitive to light.
117,370
The sensitivity can be measured by the excess voltage
p
