Chemistry Reference
In-Depth Information
When the current is larger than a certain value at which oxide starts to form at
the tip of pores, increasing current density will increase the coverage of the oxide film
on the pore bottom. In such a case the relative change in the current at the pore tip
with an increase in the overall current, is less than that at the side of the pore bottom,
that is, As a result, the pore bottom is flattened, which results in
larger pores and thinner walls. On the other hand, when the current is relatively small
so that the current density at the pore tips is still much less than that required for the
formation of oxide, an increase in applied current density will cause a relatively larger
change at the tip than on the side of the pore bottom (the slope of an
i-V
curve in this
current range increases with increasing current density), that is,
As a result,
the pore bottom becomes sharper with increasing current density.
On the other hand, increasing the concentration of HF increases the dissolution
rate of oxide, which in turn increases the sharpness of the pore bottom. As a result, the
pores become smaller and the walls thicker.
Variation of Morphology from Surface to Bulk. Initiation of Pores.
Forma-
tion of pores is a result of preferential dissolution of the silicon surface due to the sen-
sitivity of the electrochemical reactions to the surface curvature. A silicon surface, no
matter how well it is prepared, is not perfectly flat at the atomic scale, but has surface
defects such as surface vacancies, steps, kink sites, and dopant atoms that constitute
the intrinsic microroughness of the surface (see Chapter 7). The dissolution of the
surface is thus not uniform but modulated at the atomic scale with higher rates at the
defects and depressed sites. The microroughness of the surface will increase with the
amount of dissolution due to the sensitivity of the reactions to surface curvature asso-
ciated with the micro depressed sites. These sites, due to the higher dissolution rates,
will evolve into pits and eventually into pores. It is generally observed that for two-
layer PS on
p
-Si and illuminated
n
-Si, a certain thickness of micro PS or etching, which
is required to generate the right radius of curvature, occurs prior to the initiation of the
macropores.
The pores so initiated are very small in size but large in number due to the nature
of surface defects. However, for PS formed under a steady state at a given anodization
condition, the morphology has certain characteristics in terms of pore size, density,
branching, and so on. Thus, the tiny pores initiated on the surface are not stable but
tend to grow to the size required for a stable pore propagation. As the pores propagate
into the bulk, some vanish and some grow gradually in size. The thickness of the ini-
tiation phase is comparable to the diameter of the pores grown at the steady state as
shown for example in Fig. 8.37.
8
Thus, the initiation layer is very thin for micro PS
and relatively thick for macro PS.
It is important to note that although surface defect sites are associated with the
initiation of pores, they do not determine the density and dimension of the pores in the
bulk PS. The bulk morphology of PS is determined by the property of semiconductors
and anodization conditions. However, under certain conditions such as those for the
formation of macropores on lowly doped materials, control of the initiation sites by
surface patterning can to some extent change the PS morphology.
Flatness of the Growth Front of PS.
The growth front of all types of PS is gen-
erally flat and parallel to the initial surface of the substrate, meaning that the pores,
which may have a range of diameters and shapes, propagate at essentially the same rate.
