Chemistry Reference
In-Depth Information
where is the extra current mostly due to the anisotropic effect and is responsible for
the formation of the side pores. The current density at different sites on the bottom
depends on It is the largest at the tip where and is the smallest on the bound-
ary of the bottom where Such a distribution of current density is provided by
the distribution of radius of curvature along the pore bottom, which determines the field
at the silicon surface and the nature of the reactions. For different HF concentrations
and potentials the current density distribution on the pore bottom is different and so is
the shape of the pore bottom. When the pore bottom is curved to such an extent that
the current density distribution on the pore bottom for a given HF concentration and a
given potential satisfies Eq. (8.16), pores will propagate stably.
For macropores in a stable growth condition, the distribution of current density on
the surface of an individual pore bottom is bell-like and is constant with increasing depth
as shown in Fig. 8.70(a). For micro PS, the pores tend to grow in a randomly fashion.
The distribution of the current at the dissolution front of the PS is highly modulated
across the surface as illustrated in Fig. 8.70(b). The distribution changes with time but
at any given time there are areas on which the current is near zero (e.g. on the walls).
For two-layer PS, in which macropores can be filled or partially filled with micro
PS, the current profiles are illustrated in Fig. 8.70(c,d). The bell-shaped distribution of
the current responsible for the growth of the macropores consists of fine modulations
of the current responsible for the growth of the micro PS. When the current densities
are such that no oxide occurs on any area of the pore bottom, the macropore is fully
filled with micro PS. When the current densities are such that oxide forms at the tip
area, and no micro current modulation is possible on the oxide-covered surface, the for-
mation of partially micro PS-filled macropores occurs. These different levels of current
modulation are governed by the effects of curvature, orientation, and illumination.
