Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 402

phous PS of the very fine PS appears to be associated with oxidation. The as-grown PS
on lowly doped p- Si , which may have various amounts of amorphous structure, can be
oxidized to a great extent from 10 to 50%. 35,176,566
8.3.10. Summary
The morphology of PS has extremely rich details as shown in Fig. 8.19, determined by
the numerous factors involved in the anodization. Despite the enormous amount of
research effort, the quantitative and, to some extent, qualitative correlation of the for-
mation condition and morphology are still not clear. Generally, p -Si and n -Si have
distinct differences in the correlation between the formation conditions and PS mor-
phology. Among all formation conditions, doping concentration appears to show the
most clear functional effect on morphology. For example, Fig. 8.57 shows the growth
rate, dissolution valence, and porosity of the PS on p- Si and n -Si as a function of doping
concentration and current density. 1084 Still, the correlations such those shown in Fig.
8.57, although covering a wide range of conditions, involve only a very limited region
of the large multidimensional space of the conditions. For example, the correlations
between doping concentration and morphology shown in Fig. 8.57 can be rather dif-
ferent in a different solution. They may also be different under illumination of differ-
ent intensities and directions (i.e., front or back) and at different potentials.
Nevertheless, as a rough generalization the various morphological aspects can be qual-
itatively correlated to the pore diameter of PS as shown in Fig. 8.56.
Some generalizations can be drawn about the structure and morphology of PS
based on the information presented in the previous sections:
Pore size ranges in orders of magnitude from about 1nm to several
micrometers.
The distribution of pores is uniform laterally across the exposed silicon surface.