Chemistry Reference
In-Depth Information
an example of the effect of light intensity, HF concentration, and amount of charge
passed on the thickness of the three layers.
The micro PS on top of the macro PS formed on
-Si under illumination can have
a very complex structure. For example, it has been reported that the PS formed on
n-
Si has an irregular and square-shaped framework of pores with interconnected columns
of 100-200 nm for lowly doped materials and 100-400 nm for highly doped mate-
rials.
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Bounded to this rigid square-shaped framework and contained within is a finer
structure with wirelike structures of sizes varying from 3 to 12 nm. Fractal morphol-
ogy has been suggested to be associated with the micro PS.
The macropores formed on illuminated
n
-Si, once initiated, can increase in diam-
eter with time without reaching a constant value as shown in Fig. 8.47.
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Also, the
diameter of the macropores formed under front illumination increases with increasing
HF concentration as shown in Fig. 8.48, in contrast to the case of macropores formed
in the dark (Fig. 8.24). The diameter of the macropores increases linearly with increas-
n
