Chemistry Reference
In-Depth Information
836
associated with water was determined by Dreiner to be at 25 °C in
water and ethylene glycol. According to Dreiner,
water enters only into the outermost layer, then dissociates. Further diffusion is in the
form of moving hydroxyl groups. The first step (water entry) is rapid whereas the dif-
fusion is a slower process which can be increased by anodic bias.
The different results obtained from a number of studies indicate that the mobile
species during anodic oxidation of silicon may depend on the anodization conditions.
At relatively low field in water-containing electrolytes, anion inward movement is the
dominant process, whereas at high field in water-free electrolytes, cation outward move-
ment may be important. Based on some recent studies, the chemical nature of the mobile
species may also be different under different conditions. According to Bardwell
et al.,
449
the transport follows different mechanisms at low, <3.5 V, and high potentials, >3.5 V.
At low potentials, the growth may be due to lattice diffusion of or whereas
at high potentials, it may be by short-circuit diffusion of The potential for the latter
mechanism to be possible is the potential at which evolution on the electrode surface
becomes important and therefore molecular oxygen is available for transport through
the oxide. This change of transport mechanism is accompanied by a larger oxide growth
per volt.
139
A change of the voltage growth rate from 7 Å/V in the potential range of
1-3.5 V to 22 Å/V at potentials larger than 3.5 V is observed in the anodization of silicon
in
in
solution.
3.4.3.
Growth on n
-Si
Anodic reaction typically involves holes which are the majority carrier in
p
-type
silicon but are the minority carrier in
n
-type silicon. The anodization of
n
-type sub-
strate thus requires generation of carriers either by extra field strength or by illumina-
tion. According to Schmidt and Michel,
117
for
n
-Si an initial high voltage, with an excess
voltage, above that of
p-
Si or strongly illuminated
n
-Si is needed for electrons to
tunnel through the barrier at the silicon surface. The formation of the first layers of the
oxide decreases the height of the barrier inside the
n
-Si and the anodization can proceed
further at lower voltages. can be viewed as the potential drop in the space charge
layer which is required for generation of charge carriers to sustain the anodization
reaction.
1038
According to Hasengawa
et al
.,
370
the excess voltage, can be defined as
and is a function of doping concentration
light inten-
sity
-Si the holes required for anodization may be generated by
either avalanche breakdown or illumination. In the dark, anodization of
and time
t.
For
n
type involves
avalanche breakdown in the semiconductor depletion layer and depends on the
doping level of the semiconductor as shown in Fig. 3.17. Under a sufficient illumina-
tion intensity the excess voltage is zero and the anodization curve of
n
-Si becomes
essentially the same as that of
p-
Si as shown in Fig. 3.6 due to the switching of the
rate-limiting process from hole supply to ionic transport. On the basis of the avalanche
multiplication mechanism, the photocurrent
n
through a semiconductor depletion
region is given by
