Chemistry Reference
In-Depth Information
As discussed in the section on the distribution of the applied potential in the different
phases in the current path, the electrolyte inside a pore does not affect the current dis-
tribution on the pore bottom due to the even distribution of the potential along the length
of the pores. However, it is the potential drop in the electrolyte phase that maintains
the same growth rate for the different pores. For a pore of length
l
, the potential drop in
the electrolyte with resistivity
inside the pore at an average current density
i
is
which is independent of pore diameter and shape and depends only on one geo-
metric factor, that is, the length of the pores. Assuming that one pore is significantly
ahead of the rest of the pores, the current
i
in this pore will increase due to the reactions
occurring on the side wall of the pore. This will then increase the potential drop,
which in turn will result in a reduction of the potential available for other phases in
the current path. The reduction of the potential in other phases will then reduce the
current density on the pore bottom and slow the growth of the advanced pore. Thus,
due to such a process individual pores can only grow at the same rate and as a result,
the growth front of a PS layer is flat at a scale larger than the diameter of pores.
Two-Layer PS.
Two-layer PS with a micro PS on top of a macro PS layer is
formed on lowly doped
p-
Si or illuminated
n-
Si
.
For lowly doped
p
-Si, two-layer PS
can form when the conditions are such that the space charge layer and the resistive
layer differ in dimension by several orders of magnitude and both are significantly
involved in the rate-limiting process due to the effect of surface curvature on the current
flow near the surface and in the substrate. For
n
-Si, two-layer PS can form on a front-
illuminated substrate as long as the conditions exist for the formation of macropores.
It may also form in the dark under conditions similar to those for the formation of two-
layer PS on lowly doped
p-
Si
.
Figure 8.74 illustrates the two-layer PS formed on illuminated
n
-Si. The photo-
generated holes are located near the surface and flow in various directions depending
on the direction of the field inside the crystallites of the PS. These photocarriers can
