Chemistry Reference
In-Depth Information
reaction and cathodic reaction at OCP. Increasing concentration leads to a higher
degree of hydrogen termination of the surface, which passivates the surface and thus
decreases the anodic dissolution. This effect tends to shift the potential in the positive
direction. On the other hand, fluoride promotes the anodic dissolution and thus tends
to shift the potential in the negative direction. The combined effect of these two species
therefore determines the final OCP value.
OCP of silicon in HF solutions tends to vary with time for 0.1-0.2 V within
several hours depending on the type of silicon material.
1153
The change of OCP of a
p
-Si in HF solutions is related to the change occurring on the surface; a colored film
(a porous silicon layer) gradually develops on the surface. The change of OCP can be
drastic when the surface is significantly altered due to the reactions at OCP as for
example shown in Fig. 2.40.
254
In that case, the variation of OCP with time is due to
the copper microdeposits acting as cathodic sites for hydrogen reduction. According to
Bertagna
et al
.,
254
after an incubation period, which varies for
p
and
n
materials, OCP
increases linearly with the density of copper nuclei which is proportional to its con-
centration in the solution. A plateau is reached as a saturation level of nuclei is reached
and the further increase of copper coverage comes only from the growth of the nuclei
which is much slower. The plateau at longer time is attributed to the weaker catalytic
effect of the larger copper clusters which balances the continuing increase in the size
of the clusters.
2.8.2. Corrosion Current
Corrosion current is a measure of the rate of the anodic reactions at OCP. The
corrosion current may or may not be equivalent to the dissolution rate of silicon in a
solution since silicon may dissolve chemically and/or electrochemically, and the cor-
rosion current is a measure of only the electrochemical part of the dissolution. It is a
useful parameter as a measure of the electrochemical reactions in processes such as
