Chemistry Reference
In-Depth Information
8.3.2. Diameter and Interpore Spacing
Among the morphological features, the average pore diameter of a PS, as a quan-
tifiable and easily measurable parameter, is most commonly determined. Table 8.2
shows the pore diameter and interpore spacing reported for the PS formed under various
conditions. Pores can be classified as micropore if the pore diameter is less than
2 nm, as mesopore if it is between 2 and 50 nm, and as macropore if it is larger than
50nm.
12
For simplicity, the pores discussed in this chapter are categorized as only
micropores (less than l0nm) and macropores (larger than 50nm). Pores between 10
and 50 nm may be grouped into either micro- or macropores depending on the specific
situation.
Effect of Doping.
The diameter of pores and interpore spacing, under the
same anodization condition, vary with doping type and concentration in a wide
range, from about 1 nm to about about 4 orders of magnitude.
60,952,1084,1162
The
PS formed on different substrates can be roughly grouped into four main cate-
gories according to doping concentration: (1) moderately doped
p-
Si
(2) highly doped
and
and (4) lowly doped
p
-Si
i has extremely small pores typically
from 1 to l0nm. For heavily doped
p
and
n
types, the pores have diameters typically
ranging from 10 to
The PS formed on moderately doped
p
-
S
, the pores have a wide range of possible diame-
ters from l0nm to For lowly doped
p
-Si, the PS can have two distinct dis-
tributions of pore diameters: large pores with a distribution of diameters on the order
of micrometers and small pores on the order of nanometers. The effect of doping on
pore diameter strongly depends on solution composition, potential, and illumination
conditions.
For a given type of pores, pore diameter varies continuously with doping con-
centration in a continuum. Figure 8.20 shows that for
n-
Si the diameter of the pores
increases with the sample resistivity at different current densities.
1084
In contrast, the
pore diameter of
100nm
. For
n
-Si
i of moderate or high doping concentrations decreases with increas-
ing doping concentration. Macropores can also form on highly doped
p
-
S
n
-Si
and lowly
i under certain conditions.
247,952,1139,1165
Effect of Potential, Current, and HF Concentration.
Pore diameter generally
increases with increasing potential and current density.
8,47,50,1084
Figure 8.20 shows that
the diameter of pores formed on both
p
-Si and
doped
p
-
S
-Si increases with current density over
a wide range of doping concentrations.
47,1084
Figure 8.21 shows the cross sectional mor-
phology for lowly and highly doped
p
-Si.
35
Figures 8.22 and 8.23 show that the diam-
eter of the pores formed on nisi increases with potential.
8
Also, the PS formed in or
near the transition region according to the
i-V
curve tends to have relatively larger
pores; macropores can form on highly doped
p
-
n
i.
2
They are also found on highly doped
S
n
-Si.
1139
For a given substrate, the sensitivity of pore diameter to HF concentration
strongly depends on the solvent.
247,1027
Figure 8.24 shows that pore diameter decreases
with increasing HF concentration.
1086,1140
A wider range of variations in pore diameters
can be obtained in organic solvents than in aqueous solutions. For example, it has been
found that under the same conditions, macropores can form in organic solutions but
may not in aqueous solutions.
1139
