Chemistry Reference
In-Depth Information
substrates than on (100) because for substrates of non-(l00) orientations, no <100> direc-
tion coincides with the direction of the hole source. Due to the large tendency to branch,
it is difficult to produce straight pores with smooth walls on (110) and (111) samples, even
on the surface-patterned samples under back side illumination.
768
8.3.5. Interface between PS and Silicon
The growth front of a PS layer (not individual pores) is always perpendicular to the
surface of the substrate with back electrical contact. The growth front is planar, inde-
pendent of the orientation of the silicon samples, because the growth of pores depends
on the supply of carriers which are transported from the back of the sample. The inter-
face between the PS layer and silicon substrate is essentially flat with a difference
between high and low points within a few micrometers.
8.3.6. Depth Variation
The morphology of PS generally varies in the depth direction from the surface to the
bulk. There are two types of depth variations: (1) The change of pore diameter near the
original surface is gradual from the surface to the bulk where pore diameter is constant
as shown in Fig. 8.19(6b); and (2) the change of pore diameter is abrupt from a surface
layer to the bulk layer with a difference in diameter as large as three orders of magni-
tude as shown in Fig. 8.19(6c). The surface layer for the first type is formed due to the
initiation of pores and their transition to steady-state growth. The second type is the
two-layer PS with a micro PS on top of a macro PS. While two-layer PS forms only
under certain conditions, the transition PS layer of varying thickness exists on the
surface of all PS layers. Also, for two-layer PS the macropores may be filled to various
extents by a micro PS layer as shown in Fig. 8.19(4). Layered PS structure due to the
fluctuations in the substrate has also been observed.
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Transitional Layer.
The transitional layer is formed only on the original sample
surface and is associated with the nucleation of pores. It is present on top of the PS of
8,36,40,763,1084,1133
The pores in the transitional layer are generally much
smaller than those in the bulk. There is no clearly definable boundary separating the
surface layer and the bulk, the pore sizes gradually changing to those of the bulk. The
thickness of the surface layer is related to the size of pores; the smaller the pores, the
thinner the surface layer.
For
all types of silicon.
large the transition layer with smaller pores can be clearly seen as for
example shown in Figs. 8.26 and 8.37.
8
The distributions of the pores in the transition
layer and in the bulk stable growth layer are also different as shown in Fig. 8.38.
1133
On the other hand, for
n
-Si
this surface layer is very thin (near zero) for some PS with
extremely small pores. Figure 8.39 shows that the surface layer is a function of
current.
40
Also, the thickness of this surface layer depends on dopant concentration as
shown in Fig. 8.40. The thin transition layer on
p-
Si is denser than the bulk PS.
36,40
The morphology of the transition layer, unlike the bulk morphology, depends
sensitively on surface conditions, particularly surface roughness. For example, pores
tend to form along a scratch.
60,154
For
n-
Si, which usually requires a large potential to
generate current in the dark, the formation of PS can occur at much lower potentials if
p
-Si
