Chemistry Reference
In-Depth Information
5
Anodic Behavior
5.1. INTRODUCTION
The anodic behavior of silicon electrodes has been extensively studied due to
its importance in a number of applications involving passivation, oxide film, etching,
cleaning, photoelectrochemical cells, and porous silicon. A wide range of phenomena
have been characterized, such as current multiplication and the change of dissolution
valence, the occurrence of passivation and current oscillation, and the drastic variation
in the distribution of reactions on the silicon surface from extremely uniform, which
results in electropolishing, to extremely nonuniform, which results in the formation of
porous silicon. These phenomena can be categorized according to two principal aspects:
(1) the chemical nature of reactions on the atomic scale and the experimentally observed
kinetic quantities (such as current density), which are the average of the atomistic
events, and (2) the physical nature of the silicon surface (such as orientation, geome-
try, and homogeneity), and its evolution during the anodic reactions. The materials
described in this chapter are limited to the first aspect. The physical nature of the surface
in terms of morphology, geometry, and transformation is dealt with in Chapter 7 on
Etching of Silicon and Chapter 8 on Porous Silicon. The anodic oxide films formed
under large potentials are discussed separately in Chapter 4. Those formed at relatively
low potentials of less than a few volts, are discussed in the chapter.
5.2. CURRENT-POTENTIAL RELATIONSHIP
Anodic behavior of silicon can best be characterized by
i-V
curves. Neglecting
the details associated with a silicon substrate such as doping, the current-potential rela-
tionship of silicon in aqueous solutions can be considered to be principally determined
by the pH and HF concentration as illustrated in Fig. 5.1. In nonalkaline and nonfluo-
ride aqueous solutions, silicon as an electrode is essentially inert, showing a very small
current at anodic potential due to the presence of a thin oxide film. In alkaline solu-
tions, silicon is also passivated by an oxide film at anodic potentials but is active below
In fluoride solutions, the silicon electrode is active in the
whole anodic region as shown by the large anodic current.
the passivation potential,
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