Chemistry Reference
In-Depth Information
result in the dissolution of PS without the assistance of an applied anodic potential and
are responsible for the etching of the PS layer, On the other hand, the photocarriers
generated at different depths within the space charge layer are collected at the bottom
surface of the macropores, resulting in the formation and growth of the micro PS. The
holes that are generated beyond the space charge layer are mostly collected and react
at the bottom of the macropores. Also, depending on the current density relative to that
for the formation of oxide, the tip area may or may not be covered with an oxide film.
When the entire pore bottom is not covered with any oxide, the macropores will be
fully filled with micro PS. On the other hand, when the tip area of pores is covered
with an oxide film, local micromodulation of current density due to photocarriers gen-
erated at different depths is not possible so that the macropores are only partially filled
with micro PS as illustrated in Fig. 8.74.
8.6.3. Summary
PS displays a wide range of morphological variations. However, under a given
condition, except for the transition layer near the surface, the pores are spatially
uniformly distributed and the reactions occurring anywhere within PS during its
growth can be represented by what occurs on the surface of a single pore-wall-
pore unit. Thus, the formation of PS having specific morphological features is basi-
cally determined by the distribution of reactions and their rates along the surface of
this unit.
Kinetically, the overall dissolution process consists of carrier transport in the
semiconductor, electrochemical reactions at the interface, and mass transport of the
reactants and reaction products in the electrolyte. Also, there are a number of reactions
involved at the interface and these reactions consist of several steps and subreactions.
At any given time the dissolution kinetics can be controlled by any one or several of
these steps. The distribution of reactions along a pore bottom under a steady-state con-
dition during pore propagation must be such that pore walls are relatively less active
than the pore tip. Then, the dissolution reactions are concentrated at the pore tip result-
ing in the preferential dissolution and formation of pores. The formation of pores is the
consequence of spatially and temporally distributed reactions.
The distribution of the reactions and their rates on the surface of a silicon
electrode in HF solution are determined by at least the following processes: (1)
Electrochemical reactions on a semiconductor surface are sensitive to surface curva-
ture. The reaction rates at depressed sites which have smaller radii of coverture are
larger than those of the surrounding area. Such sites may preexist due to the in-
trinsic random roughness of the surface or may be generated after a certain amount of
dissolution which roughens the surface. (2) The reactivities of the atoms on the surface
of different crystal orientations are intrinsically different (anisotropic nature). (3) In
the entire current path the phases that cause significant potential changes due to
current perturbation will affect the distribution of the current and reactions on a curved
surface. (4) The reactions involved in silicon dissolution have two paths: Silicon
may react with fluoride species and dissolve directly into the solution or may react
with water to form oxide and dissolve indirectly. The direct dissolution is sensitive to
surface geometrical factors such as surface curvature and orientation, but the indirect
