Chemistry Reference
In-Depth Information
prominence at potentials more positive than as shown in Fig. 5.32b, The
capacitance associated with this loop is potential dependent and increases from
about at low potentials to about above This impedance
is clearly separated from that of the space charge layer dominating at more negative
potentials. Since it is the only capacitive loop seen in the potential region (Fig. 5.32c)
where the Tafel relation between current density and potential is observed, it is asso-
ciated with the Helmholtz double layer.
A third capacitive loop, loop III, is seen at more positive potentials (Fig. 5.32d).
It starts to appear in the transition region between PS formation and electropolishing
and dominates in the electropolishing region at low frequencies. The capacitance asso-
ciated with this loop is estimated to be about and is essentially independent
of potential within the experimental range (between 0.2 and Loop III is clearly
separated from loops I and II, and thus is associated with neither the space charge layer
nor the double layer. It is attributed to the oxide film formed in the electropolishing
region. This agrees with the fact that the impedance response in the electropolishing
region, characterized by loop III, is virtually independent of doping type and concen-
tration as shown in Fig. 5.34. Due to the presence of oxide film in the electropolishing
region, the difference in the substrate materials is masked.
At potential corresponding to the Tafel region and in the transition region, an
inductive impedance also occurs as shown in Fig. 5.32c,d. This may be indicative of
the relaxation of coverage by intermediate species to the dissolution of silicon.
