Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 354

a relatively constant value. PS forms in the exponential region but not at potentials
higher than that corresponding to the current peak, Electrochemical polishing occurs
in this region. At potentials above the exponential region and below the potential of the
current peak, PS formation also occurs, but the PS layer does not completely cover
the sample surface. 2 The surface coverage of PS decreases as the potential approaches
the peak value. Hydrogen evolution simultaneously occurs in the exponential region
and its rate decreases with potential and almost ceases above the peak value. The
exisistance of these different regions are similar for other fluoride-containing solutions
of various compositions and pH values. 34,700,775,939
In the exponential region where PS forms, the slope of the logarithmic current
density versus potential plot, i.e., the Tafel plot, is typically about 60mV/decade for p
type and heavily doped n types of silicon samples as shown in Table 5.3.
Successive i-V curves, measured on the same specimen (Fig. 8.1), are virtually
the same except for the first one when the surface is transformed from smooth to
2
porous. Because each curve is subsequently obtained on the electrode with a thicker
PS layer, it means that the reactions are independent of PS thickness, and are not con-
trolled by mass transport. The small difference between the first and subsequent ones
in Fig. 8.1 suggests that the active surface area is not drastically changed by the for-
mation of PS, although the total surface area is much increased due to the growth of
PS. Also, the interface capacitance is not modified by the presence of a PS layer with
increasing PS thicknesses. 5 This means that the pore walls in PS are depleted of carri-
ers and are not active so that it is only conductive at the pore tips where active reac-
tions take place. However, the formation of PS transforms the surface morphology so
that the electrode behavior is changed from that of the original condition as shown in
Fig. 8.2. 1153 After 5min of polarization at at which PS forms, the capacitive
loop of the n + silicon in 1% HF is greatly reduced. Formation of PS, as will be dis-
cussed later, results in an increased effective area as well as in an increased kinetics
due to the enhancement of the electric field by the curvature at the pore tips.
The typical characteristics of the i-V curves of p -Si of different doping levels and
orientations are essentially identical except for a slightly higher current density of the