Chemistry Reference
In-Depth Information
Thus, in general, the dissolution of silicon at an anodic current results in three
regions of the silicon substrate which is exposed to the electrolyte as shown in Fig.
8.41. Such an etched layer prior to the initiation of pores is involved in all types of PS
because the etching causes roughening of the surface, which is required for pore initi-
ation. The etching phase is associated with the formation of macropores on both
n
and
p
types of silicon and with formation of micropores on
-Si.
794,1126
For example, for
n-
Si under back illumination initiation of macropores is preceded by an etching phase.
As shown in Fig. 8.42, the thickness of the etched layer increases with increasing sub-
strate resistivity.
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The pores are larger on the substrates of higher resistivity and a
thicker etched layer is required to generate a higher degree of roughening for pores to
initiate. On the other hand, the etched layer depends only very weakly on potential.
Two-Layer PS.
Two-layer PS, with a micro PS layer on top of a macro PS layer,
can form under certain conditions. For
p
, formation of a two-layer PS is associated
with front illumination, although it can also be formed with back illumination.
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For
p-
Si, two-layer PS is found to form on lowly doped substrates. For moderately or
highly doped
p-
Si or for
n
-Si
n
-Si
in the dark, formation of two-layer PS has not been
observed.
