Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 170

which are anodized at different potentials in this region, indicates that porous silicon
forms in the exponential region but not at potentials more positive than the peak poten-
tial. The potential corresponding to the maximum slope of the i-V curve is about the
upper limit for formation of a uniform porous silicon layer. 2 At potentials between the
maximum slope and the current peak, the porous layer may still form but its surface
coverage is not uniform. The anodic behavior, in terms of the morphology of the sur-
faces, is characterized by the transition from porous silicon formation to electropol-
ishing with increasing potential from OCP to more positive than that of the current
peak. 2,33
The potentials of the two current peaks, and extend to more positive values
for lowly doped p -Si . A difference of as much as 1 V for the first current peak and about
1.5 V for the second is found between lowly and highly doped p -type materials as shown
in Fig. 5.4. 700 This is attributed to a larger potential drop in the space charge layer of
the lowly doped sample.
For n -Si, the onset potential at which current rises sharply in the dark marks
the breakdown of the barrier layer and initiation of interface tunneling on the silicon
surface. 8 The breakdown potential depends on the doping level of the material: the
lower the doping, the higher the breakdown potential as shown in Fig. 5.5. 12,38,42,858 For
n -Si at potentials negative of the breakdown potential, the anodic reactions require
illumination to supply holes. When the current is lower than independent of the
potential, the dissolution results in the formation of porous silicon. At a current higher
than the reactions associated with electropolishing become dominant. Also, the peak
current for n -Si tends to increase with decreasing scanning rate due to the increased
surface area as a result of surface roughening related to PS formation. 8
Effect of Solution Composition. Figure 5.6 shows that the i-V curves
greatly vary with solution composition, particularly the fluoride concentration and pH
value. 939 The characteristic current values,

to as a function of pH and fluoride