Chemistry Reference
In-Depth Information
potential due to a change of current density in the current path is the sum of the poten-
tial drops in these phases:
As described above, the factors that cause change of the current distribution on the
bottom of pores will affect the morphology of PS. Thus, the phases in the current
path that have relatively large resistance will take large portions of the change in the
applied potential and thus change the current distribution along the pore bottoms,
affecting the morphology of PS. For a given condition, i.e., doping type and concen-
tration, HF concentration, current density, potential, illumination intensity and direc-
tion, there is a certain distribution of the applied potential in the different phases of the
current path.
The last term, in Eq. (8.17) is the resistance of the electrolyte. It causes a
potential drop that is linearly distributed in the electrolyte inside the pores and thus
does not have an effect on the current distribution on the pore bottom although it takes
a significant amount of in the applied potential. However, as will be discussed later, the
potential drop in the electrolyte has an important effect in maintaining the flat growth
front of the PS layer.
In the case when the substrate is moderately doped and the surface is free of
oxide, the rates of reactions are determined by the resistance in the space charge layer
and in the Helmholtz double layer. The reactions under these conditions have a great
tendency to localize because the rates of charge transfer in both layers are sensitive to
geometric factors. The reaction that is kinetically limited by the space charge layer is
sensitive to the radius of curvature and that by the Helmholtz layer to the orientation
of the surface. Depending on the relative effect of each layer, the curvature effect versus
anisotropic effect can vary.
When the pore bottom is covered with an oxide, partially or fully, the change of
applied potential occurs almost entirely in the oxide due to the very high resistance of
the oxide. The rate of reactions is now limited by the chemical dissolution of the oxide
on the oxide-covered area and when the entire pore bottom is covered with an oxide
the rate of reaction is the same on the entire surface of the pore bottom. As a result,
the bottom flattens and no PS forms. The change of oxide coverage on the pore bottom
can also occur when diffusion of the electrolyte inside deep pores becomes the rate-
limiting process. A decreased HF concentration at the pore bottom due to the diffusion
effect can result in the formation of an oxide on the bottom of deep pores under con-
ditions in which it does not occur in shallow pores.
In the case when the resistance of the substrate is high and a significant amount
of potential is dropped in the substrate, the potential drop may not be uniform along a
curved pore bottom due to the nonlinear potential distribution in the material sur-
rounding the bottom. Formation of macro PS on lowly doped materials due to resistive
effect becomes possible under such conditions.
Relativity of the Dimensions and Events.
Formation of PS is due to preferen-
tial dissolution of a silicon surface; the rate is larger at some areas of the surface rela-
tive to others. Such relative rates with respect to the spatial position of the areas, on
which these processes occur, are determined by the relative nature of the physical and
