Chemistry Reference
In-Depth Information
At a given anodic current density, the potential, which determines the width of
the space charge layer, is different for different doping type and concentration due to
different current conducting mechanisms. The combination of doping and potential thus
provides a wide range of space charge layer widths and thus a wide range of pore diam-
eters. Figure 8.73 schematically illustrates the space charge layer width for differently
doped materials. For
n
-Si the space charge layer and thus the dimension of the pores
increases with decreasing doping concentration and increasing potential. For
p-
Si
,
on
the other hand, the application of an anodic potential reduces the width of the space
charge layer which results in smaller pores. However, when the doping concentration
is lower than a certain level, the effect of resistivity of the substrate becomes impor-
tant and formation of macropores occurs. A micro PS layer can form on the surface of
the macropores when the width of the space charge layer is also in effect. The band
structure for such a case is illustrated in Fig. 8.73(a). This mechanism should also
operate for lowly doped
n
-Si as illustrated in Fig. 8.73(d).
Under illumination, photocarriers are generated in the semiconductor from the
surface to the depth of penetration determined by the wavelength of the light. The thick-
ness of the space charge layer which has a field effect on the photocarriers then varies
from almost zero to the full width of the space charge layer depending on where those
carriers are generated. Correspondingly, the size of pores and wall thickness for the PS
formed under illumination may vary from zero (i.e., corrosion of PS) to that compara-
ble to the width of the space charge layer, resulting in a fractal-like structure.
