Chemistry Reference
In-Depth Information
Illumination during formation of PS on
p-
Si has been found to affect the distribution
of pore diameter; it increases the amount of the smaller nanocrystals, while reducing
the amount of larger crystals.
1124
For the PS formed under an illuminated substrate,
the relative amount of small crystals is found to increase with reduction of light
wavelength.
774
Pore Density.
The density of pores is determined by the diameter and pore
spacing, and depends on any factors that have an effect on the diameter and spacing.
Figure 8.29 is a summary by Lehmann
et al
.
1084
of pore density as a function of doping
concentration. It shows that except for micro PS (less than 2nm in size) the density of
pores increases with doping concentration. Generally, the density of pores increases
with decreasing pore size.
The density of pores is generally higher near the surface than in the bulk.
8,1133
At
the growing front of PS in the bulk, the pore density is independent of the initial surface
condition. For example, for the bulk pores formed on
with patterned initiation sites
under back illumination the pores will merge or branch from the initially patterned
pores depending on the specific conditions.
763
Figure 8.24 shows that for the PS formed on
n
-Si
in the dark pore density
increases with increasing HF. Pore density also increases with increasing tempera-
ture.
1133
Figure 8.30 shows that for the PS formed on back-illuminated
n
-Si, the pore
density in the transition phase increases with increasing potential for different doping
levels; high potential favors nucleation of pores. Figure 8.31 shows that the stable
growth-phase pore density increases with potential for highly doped material but
decreases with potential for lowly doped. Such a dependence of pore density on doping
can be explained using the data in Fig. 8.23, namely, the pore density decreases with
increasing potential at low potentials due to increase of the space charge layer thick-
n
-Si
