Chemistry Reference
In-Depth Information
Similarly, pore density (number of pores per unit area) can be estimated as
As examples, according to the above equations for a PS with a density of 50% and for
a porediameterof
thespecificareais
andtheporedensity is 2.5 ×
and the poredensity
canbeformed
in a wide range of conditions.
35,47,772,1118
Specific area as high as can be
associated with the PS formed on
p
-Si.
35,647,1103
This means that an increase on the order
of 10
6
in surface area can result from the formation of PS on a solid silicon cube of
1cmoneachside(6cm
2
intotal surfacearea).Thus,PSexhibits arangeofproperties
characteristic of the large surface area such as high chemical reactivity.
. For a pore diameter of 2 nm the specificarea is
12
/cm
2
is 4
×
10
.PS withaspecificsurfaceareaontheorderof
8.3.8. Composition
The composition of PS may deviate greatly from that of pure silicon due to the large
surface area, which can be terminated by H, OH, O, or F (see Chapter 2). For macro
PS, the ratio of surface silicon atoms to those in the bulk is small and the PS is largely
composed of pure silicon with a small amount of adsorbed foreign species. On the other
hand, for micro PS, the composition of the PS can be significantly different from the
silicon substrate due to the huge surface area adsorbed by foreign species. As an
example, assuming that the first layer of atoms on the surface has a thickness of
a PS with a specific area of has 0.03 cm
3
of silicon atoms in the form of
surface compounds. This represents 5% of the silicon atoms in the PS with a porosity
of 40%. It accounts for as much as 50% of the silicon atoms for the PS with a specific
and a porosity of 70%. Such a PS is essentially a surface of silicon
compounds. This simple analysis is in general agreement with experimental observa-
tions. On the other hand, the PS formed on
n
-Si with doping levels from to
which has large pores and relatively smaller specific area, is almost pure
silicon with a small amount of fluorine, hydrogen, and oxygen.
42
The atomic bonds in as-grown PS in HF solutions for both
p
and
n
types of silicon
samples are found to consist of Si-H, Si-Si, and Si-O with a few percent fluoride
area of
