Chemistry Reference
In-Depth Information
8
Porous Silicon
8.1. INTRODUCTION
Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF
solutions. The formation of PS was first reported in the late 1950s in studies on elec-
tropolishing of silicon.
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Since then, particularly after 1990 when luminescence of
PS was discovered, numerous investigations have been undertaken. These investiga-
tions have revealed that PS has extremely rich morphological features with properties
that are very different from those of silicon and the formation process of PS is a very
complex function of many factors such as HF concentration, type of silicon, current
density, and illumination intensity.
The amount of information on PS in the published literature is enormous and it
is not possible to cover all aspects of PS in one chapter. Thus, the focus in this chapter
is on the phenomena related to the properties of silicon, such as the formation of PS
and the resulting morphology. The phenomena associated with the properties of PS and
the applications of PS are only briefly mentioned at the end of the chapter. In particu-
lar, there is so much information related to the luminescence of PS that it would require
a separate topic to organize this body of information.
In this chapter, the conditions for the formation of PS, the relation between the
formation conditions and PS morphology, and the mechanisms for the formation of PS
and morphology are discussed. The various aspects of surface condition, nature of reac-
tions, and reaction kinetics that are fundamentally involved in the anodic dissolution
of silicon are discussed in Chapters 2-5.
8.2. FORMATION OF POROUS SILICON
8.2.1. Characteristics of i-V Curves
PS can either be formed by anodization in HF-containing solution under an anodic
bias or by an electroless process. The formation condition can best be characterized by
i-V
curves. Figure 5.3 shows a typical plot of an
i-V
curve of silicon in HF solutions.
At small anodic overpotentials the current increases exponentially with the electrode
potential. As the potential is increased, the current exhibits a peak and then remains at
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