Chemistry Reference
In-Depth Information
materials in the dark or under illumination. In fluoride-containing or alkaline solutions,
in which SiO
2
has a high solubility, passivation requires the application of an anodic
potential.
The phenomenon of passivation can be simply characterized by an
curve as
shown in Fig. 5.1. In the active state, the electrode dissolves and the dissolution current
increases sharply with increasing potential. At the passivation potential the current starts
to decrease rapidly to much lower values and marks the onset of passivity. The current
on the passivated surface, called the passivation current,
magnitude smaller than that on an active surface at the same potential. The passivation
i-V
can be several orders of
current indicates the dissolution rate in the passive state, that is, the dissolution rate of
anodic oxide. The passivation potential tends to change with potential scanning rate.
969
A certain amount of oxidation, measured by the total charge under the current peak, is
required for passivation, so that the faster the scanning rate the more positive is the
passivation potential. Table 5.5 lists the passivation potentials and passivation currents
observed in various silicon/electrolyte systems. The passivation current can vary in a
wide range from a few microamperes per square centimeter in 1 M KOH solution to
in 1 M HF solution. In non-aqueous solution containing HF,
passivation does not occur at anodic current as high as 1 A/cm
2
due to the lack of water
in the solution.
136,1015
The silicon surface in nonfluoride and nonalkaline solutions is spontaneously pas-
sivated due to the formation of a thin native oxide film at a rate depending on many
factors as discussed in Chapter 2. For
the order of
-Si samples in aqueous solutions under illumi-
nation the occurrence of passivation causes a decrease of the photocurrent as shown in
Fig. 5.11.
74,873
In the absence of HF, photocurrent rapidly reduces to near zero due to
the formation of an oxide film. The stationary photocurrent increases with increasing
HF concentration. For a given light intensity, there is a HF concentration above which
the photocurrent does not decrease from the initial value. The surface is free of oxide
film at this HF concentration.
The reactions on passivated silicon surface are characterized by two essential
processes, oxide formation and dissolution. In practice, the low current measured on
silicon electrode at anodic potentials in alkaline solutions is generally attributed to pas-
sivation. On the other hand, the occurrence of current peak and the subsequent current
plateau in HF solutions are generally not regarded as due to passivation. However, in
essence, the processes that produce current at potentials higher than
KOH solutions are largely identical in chemical nature, that is, oxidation of silicon
n
in both HF and
atoms at the silicon/oxide interface and ionic transport in the oxide. Kinetically, the
passivation in HF solutions, in which the silicon oxide dissolves rapidly yielding a large
passivation current, is different from that occurring in the alkaline solutions where
the passivation current is very small.
5.9.2. Passivation in Alkaline Solutions
The passivation of silicon surface in KOH at anodic potentials has been exten-
sively investigated in relation to anisotropic etching. Figures 5.36 and 5.37 show the
typical
curves of
n-
and
p
-type silicon in KOH.
109,378
Substrate conditions, particu-
larly orientation and type of doping, have significant effects on the characteristics of
i-V
