Chemistry Reference
In-Depth Information
The data presented in the following section concern only the hydrogen reaction
at cathodic potentials. Those on hydrogen termination are presented in Chapter 2
and on silicon dissolution in Chapter 5. It is to be noted that as a reduction reaction,
hydrogen evolution has not been well investigated at cathodic potentials although it
has been the subject of numerous studies on the phenomena at anodic or open-circuit
potentials.
6.2.1. Kinetics
Hydrogen evolution is the dominant cathodic reaction on silicon electrodes in
aqueous solution lacking other redox couples. In HF solutions, evolution is the only
cathodic reaction and the charge transfer process proceeds almost entirely over the con-
duction band.
34,73
Figure 6.1 shows the i-V curves measured on
n-
Si in 1% HF.
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The
current increases exponentially with decreasing potential with the apparent Tafel slope
being 140-160mV/decade. The exponential
i-V
relationship is also seen on illuminated
p-
Si at currents that are significantly lower than the saturation value as shown in Fig.
6.2.
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The Tafel slope for the hydrogen reaction for all types of silicon materials in the
dark or under illumination appears to be between 140-200mV/decade.
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This is much
higher than the 60mV/decade required for the processes that are limited by the supply
of electrons from the semiconductor, which suggests that the charge transfer reaction
involved in the hydrogen evolution at cathodic potentials is mainly controlled by the
processes in the Helmholtz layer.
A relatively large overpotential is required for hydrogen evolution to occur at
appreciable rates. Figure 6.2 shows that the potential required for the onset of hydro-
gen evolution on silicon is about 300mV more than that on platinized Si.
812
An over-
potential as high as 1.1V was required for a reduction current of hydrogen at a current
of about on an
n
-Si in a solution.
272
The evolution of
hydrogen on silicon electrodes, both for
n
-Si in the dark and for illuminated
p-
Si
,
is
