Chemistry Reference
In-Depth Information
of an amorphous oxide film masks the crystallographic nature of the substrate as shown
in
Fig. 9.3c.
9.6. RELATIVITY
An important mechanistic aspect is the relative nature of the dimensions and
events in space and in time. The electrochemical reaction processes can vary in quality
and quantity in temporal and spatial scales of many orders of magnitude, which is
responsible for the diverse phenomena observed on silicon electrodes. The under-
standing of the relative nature of dimensions and events are essential in mechanistic
descriptions of this complex system. The following are the relative dimensions and
events that are important in determining the electrode phenomena of silicon.
1. The amount of reaction with fluoride based species relative to that with water
based species determines the relative surface coverage by hydride, hydroxide,
and oxide, the reactivity of the surface and the crystallographic character of
the surface.
2. The amount of chemical reactions relative to electrochemical reactions
determines the amount of anodic hydrogen evolution and the value of effec-
tive dissolution valence.
3. The rate of oxide formation relative to dissolution of the oxide determines the
surface coverage, thickness, and properties of oxide, occurrence of passiva-
tion and current oscillation as well as uniformity of anodic dissolution.
4. Rate of removal of atoms from kinks and steps relative to that from terraces
determines the anisotropicity of reactions.
5. Atomic scale roughness in terms of the density of kinks sites and steps rela-
tive to surface atomic density determines effect of surface lattice structure
on the rate of reactions; when the roughness is low relative to atomic density
the reactions show a high degree of anisotropicity and when it is high the
reactions tend to be isotropic.
6. The radius of surface curvature relative to the width of space charge layer
determines the sensitivity of reactions to surface roughness, the distribution
of reactions on the surface. It is the principal factor in the formation of pores
in semiconductors and the porous morphology.
7. The wavelength of light relative to the width of the space charge layer affects
the quantum efficiency of photocurrent, the vector of photocarrier flux on the
surface, and is responsible for the fractal-like morphology of the micro PS in
two-layer PS.
8. The relative distribution of potential change in different physical phases along
the current path determines the rate determining processes and the role of each
physical phase in the overall electrode phenomenon.
In addition to the ones listed above, there are other relative dimensions and events
that are associated with specific phenomena, for example, relative carrier depletion
in the directional growth of pores on a semiconductor electrode. Fundamentally,
the relativity of these dimensions and events are associated with the relativity of
