Chemistry Reference
In-Depth Information
One problem in meaningful application of the basic electrochemical theories is
related to surface states that may be associated with surface defects, interface states at
silicon/oxide interface, adsorbed species, or reaction intermediates. They are often con-
veniently considered to be responsible for the results that are inconsistent with the basic
theories. However, the understanding on the nature of surface states at the silicon/elec-
trolyte interface is still poor; one is aware of some consequences of surface states but
knows little of their origins and their specific roles in various electrode processes.
A closely related matter is the measurement and use of the flatband potential. The
existing data show that for a silicon/electrolyte interface the flatband potential is spe-
cific to the given surface condition. Also, the flatband potential generally drifts due to
the fact that the surface of silicon in electrolytes changes constantly with time. Also, it
changes with application of potentials which is generally required for the determina-
tion of flatband potential. Therefore, any theory which assumes a fixed value of flat-
band potential will be limited in its scope of validity.
Another problem in application of the basic theories is associated with surface
geometry. Most theories are developed to describe the relationships among the area-
averaged quantities such as charge density, current density, and potentials assuming a
uniform electrode surface. In fact, the silicon surface may not be uniform at the microm-
eter, nanometer, or atomic scales. There can be great variations in the distribution of
reactions from extremely uniform, for example, in electropolishing, to extremely
nonuniform, for example, in the formation of porous silicon.
There are two principal aspects with respect to the nature of an electrode surface:
1) the chemical nature in terms of the specific of events at the atomic scale and the
kinetic quantities measured at a macroscopic scale which are the global average of the
atomistic events; and 2) the physical nature in terms of surface geometry and its effect
on the kinetics and its evolution during the reaction processes.
Surface lattice structure at the atomic scale and surface curvature at the nanome-
ter to micrometer scale are two major geometric factors that affect the uniformity of
the reactions on the surface and are responsible for the occurrence of anisotropic etching
and formation of porous silicon. The difference in the reactivity of the atoms at differ-
ent surface lattice structures is responsible for the anisotropic nature of reactions, while
the electrochemical reactions that depend on the supply of charge carriers is sensitive
to surface curvature. The carrier density at depressed sites which have smaller radii of
curvature are larger than those of the surrounding area. Such sites may preexist due to
the roughness of the surface or may be generated as a result of interaction between the
electrode and electrolyte. The spatial distribution of electrochemical processes is deter-
mined by geometric factors such as surface lattice structure and curvature, the study of
which can be called as geometric electrochemistry.
The overall electrode process consists of carrier transport in the semiconductor,
electrochemical reactions at the interface, and mass transport of the reactants and reac-
tion products in the electrolyte. There are a number of physical phases associated in
the current path and the change of potential in each phase has a specific effect in rela-
tion to surface geometry. Also, a number of different reactions can occur simultane-
ously on the surface and compete in surface coverage and in reaction rate. Particularly,
the anodic reactions of silicon in HF solutions have two parallel paths: silicon may
react with fluoride species and dissolve directly or may react with water to form oxide.
