Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 377

morphology decreases with increasing HF concentration. 768 Low HF concentration, low
temperature, and low growth rate favors formation of deep uniform pores.
Interpore Spacing. The variation of interpore spacing or wall thickness is more
complex than pore diameter and there is much less information on the correlation
between wall thickness and formation conditions. Interpore spacing depends on the for-
mation conditions such as applied potential and HF concentration rather differently than
does pore diameter. 8,1084 In general, the walls are on the same order as or smaller than
the pore diameter. The wall is thin in the PS of small pores but may also be thin in the
PS of large pores. In particular, the interpore spacing depends on potential; it increases
with potential at small currents but at a certain current it starts to decrease with increas-
ing potential. When the spacing is reduced to zero, pores no longer form, and instead,
shallow pits form as shown in Fig. 8.22(d).
Distribution of Pore Diameter. Pore diameters of the PS layer formed under a
given set of conditions have a distinct distribution. Normal, log-normal, bimodal,
fractal, and nonuniform distributions have been found for the PS formed under differ-
ent conditions. 16,47,133,772,767,780,856
Figure 8.28 shows the pore size distributions for the PS formed on highly doped
p- Si at different current densities. 47 For the PS formed on heavily doped silicon, the
pores at a given HF concentration have a narrower distribution at lower current; at
a given current density, the distribution is narrower at lower HF concentrations. A
bimodal distribution of pore diameter is generally associated with two-layer PS on
lowly doped p- Si and illuminated

. The PS formed on p- Si has also been found to
have a bimodal distribution of particles sizes: small spherically shaped particles with
a diameter of a few nanometers and large cylindrically shaped particles oriented with
their axis perpendicular to the surface. 771,772 The distribution of pore size with multiple
peaks may be attributed to the fact that PS may have a surface micropore layer and
smaller branched pores. 133,780 Due to the hierarchical structure of the branched pores,
the distribution of pore diameters for highly branched PS is found to be fractal-like. 60,575

n -Si