Chemistry Reference
In-Depth Information
while at the tips of the side pores the current density is
That the growth rate of the side pores is generally much slower than that of the main pores
may reflect the fact that carrier current of the side pores is much smaller than
due to the unfavorable geometric location. Also, for the side pores the component due to
carrier effect and that due to lattice effect do not overlap. Thus, side pores grow in an
angle, from the growth direction of the main pores and only become
normal to the main pores when Also, when the side
pores do not grow very long before they cease growing because the growing bottom of
the main pore quickly moves the source of holes farther away from the side pore. There
is little branching under such a condition.
Reactions on the Surfaces of Silicon and Silicon Oxide.
As discussed in
Chapter 5, there are two basic reactions, that is, direct dissolution of silicon and indi-
rect dissolution through the formation and dissolution of silicon dioxide. The rates of
both reactions increase with potential and proceed simultaneously on the surface of
silicon. They compete with each other in rate and coverage of the surface area, and the
relative surface coverage of the two reactions depends on the potential. At a low poten-
tial the surface coverage of oxide is zero when the rate of oxide formation is low com-
pared with the dissolution rate of the oxide. On the other hand, the coverage is 1 and
the entire surface is covered with an oxide film when the rate of oxide formation is
much higher than the dissolution rate of oxide. Between the two extremes the surface
is partially covered with oxide films at local places where the rate of oxide formation
equals the oxide dissolution rate. At low potentials the oxide formation rate is low com-
pared with the dissolution rate and the surface is not covered with an oxide film. Thus,
depending on the radius of curvature the bottom of a pore may be covered with an
oxide film of varying thickness as shown in Fig. 8.68.
