Chemistry Reference
In-Depth Information
chemical dimensions and events. Specifically, the relative nature of the following
aspects are important in determining the morphology of PS:
1.
The phases that produce a
relatively large contribution
to the change of applied
potential affect the current distribution along the pore bottom and thus affect
the morphology of PS.
2.
The effect of radius of curvature only becomes significant in affecting the
width of the space charge layer when its value is on the same order or less
relative
to the width of the space charge layer.
3.
It is the
relative depletion,
not the absolute depletion, that is important in deter-
mining the directional growth of pores (the walls of many types of PS are not
depleted). The relative depletion is due to the
relatively fast advance
of the
pore tips which allows very little time for reactions on the walls before the
wall region becomes depleted.
4. The
relative magnitude of the current
in the direction of the carrier source to
that in the direction of <100> affects the tendency to branch. The larger the
former compared to the latter, the smaller is the tendency to branch.
5. The
relative dissolution rate
on the (100) surface to that on the (111) surface
is a determining factor for the orientation of pores.
6. The
relative growth rate
of the side pores to that of the main pores affects the
length of the side pores. The length of the side pores is short when their rate
of growth is much smaller than that of the main pores because the relatively
fast-growing main pores quickly deplete the carriers at the tips of the side
pores.
7. The
relative change of the profile of
the pore bottom due to a given amount
of dissolution caused by local current perturbation determines the straightness
of pores. Assume that the amount of dissolution due to a current perturbation,
is
A
is a constant. This amount of dissolution may cause
only a small change of the profile of the bottom of a large pore but can sig-
nificantly change that of a small pore and thus alter the growth direction of
the small pore as shown in Fig. 8.72. This is why the large pores are gener-
ally straighter than the small pores.
8.
Related to point 7 is the scale of the regular morphological features in a PS
relative
to the size of the perturbation caused by the reaction processes which
are stochastic near the atomic scale. The stochastic nature of these reactions
affects the phenomena of larger scales through different factors in the solu-
tion, interface, and substrate, seen as inhomogeneities in the crystalline struc-
ture, temperature, concentration, and so on. This aspect of relative regularity
at different scales is reflected in the dimensional distribution of the morpho-
logical features such as the size of pores and smoothness of the pore walls.
As a result, the larger scale features of PS generally have more regular pat-
terns than the smaller ones.
Pore Diameter and Interpore Spacing.
Pore diameter and interpore spacing are
the prominent morphological parameters which as discussed above are determined by
two large groups of factors: those that affect the dimension of the space charge layer and
those that affect the distribution of the reactions on the pore bottoms. The first group of
where
