Chemistry Reference
In-Depth Information
alkaline solutions, the active surface species are the atoms on the (111) steps which are
likely to be bonded by two or three foreign species such as H, F, and OH and are thus
most easily removed from the lattice in a reaction with the electrolyte. The fact that
(100) is a preferentially attacked direction during PS formation indicates that the same
mechanism as that involved in alkaline solutions may be involved in the anisotropic
growth of pores. The difference between this case and that in alkaline solutions is that
in HF solutions the removal of silicon atoms is an electrochemical reaction involving
holes such that the rate of reaction depends on whereas in alkaline
solutions it depends only on (see Fig. 7.40). The rate of the electrochem-
ical reaction is a function of the potential across the Helmholtz layer.
Simplistically, the effect of anisotropic dissolution on the growth of pores can be
described by the difference in the current densities between the (100) and (111) orien-
tations,
and the current density on pore bottoms as shown in Fig.
8.67. Thus, at the tips of the main pores, the total current density is that due to carrier
effect plus that due to lattice effect
