Chemistry Reference
In-Depth Information
PS in nonoxidizing electrolyte such as acetonitrile (MeCN), whereas they are not filled
in oxidizing electrolytes such as DMF. However, it has been reported that the PS on
p-
Si formed in anhydrous HF-MeCN solution has only the macropore layer
with no nanometer PS covering the surface of the macropore walls and the sample
surface.
248
For the two-layer PS formed on front-illuminated
n
-Si, the degree of filling of
the macropores depends on light intensity, on the amount of charge passed, as well as
on the magnitude of the current relative to the saturation photocurrent.
10,16,247,767
The
micro PS thickness increases with the amount of charge passed reaching a relative
constant value as shown in Fig. 8.49 and it decreases with increasing light intensity.
247
The macropores formed on lowly doped
n
-Si under back illumination can also be filled
by micro PS and the degree of filling decreases with increasing potential.
12
8.3.7. Density and Specific Surface Area
The density of PS can be expressed as absolute density in units of grams per cubic cen-
timeter or relative density (versus single-crystalline silicon) in units of percent. It can
also be expressed as porosity which measures the amount of the open space with PS.
Thus, the relative density plus porosity equals 1. In many studies the density of PS is
determined by a simple gravimetric method of measuring the weight difference before
and after anodization and the actual volume of PS.
32,36,48,50
It may also be determined
by measuring the volume of liquid capillary absorbed in the pores.
50
Porosity and density are most often used for characterization of PS in the litera-
ture due to their conveniently measurable nature. However, porosity is not a morpho-
logical parameter but a quantity, determined by the morphology. A given morphology
has a definite density, but a given density may have different morphologies.
Thus, porosity or density of PS alone does not specify the morphology of PS. Because
