Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 174

potentials more positive than is called the passivation current, which, at a steady

state, reflects the dissolution rate of silicon in the passive region.
For p- Si , the i-V curves are generally independent of illumination and there
are two relatively steep increases in current at 3.7 and For n -Si, the current
in the passive region increases steadily from 0 to 10 V in the dark but it is similar to
that for p- Si under illumination. Also, the i-V curves are essentially identical for
samples with different doping levels. 925 The data show, similar to the fluoride solutions,
that the basic features of the anodic behavior in alkaline solutions are determined by the
chemical nature of the silicon material but not by the electronic properties. The i-V
curves in other alkaline solutions are in general similar to those shown in Fig. 5.9 (see
Chapter 7).

5.3. PHOTOEFFECT

Illumination with light having a wavelength larger than the band gap of silicon
generates a photocurrent under an anodic potential on an n -Si electrode but has essen-
tially no effect on p- Si , as would be expected from the basic theories of semiconduc-
tor electrochemistry. 69,962 However, the photocurrent may not be sustained because of
the formation of an oxide film, which passivates the silicon surface to various degrees
depending on the electrolyte composition. In solutions without fluoride species, the
photocurrent is only a transient phenomenon before the formation of the oxide film. In
fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be
obtained.
The anodic photocurrent of n -Si in aqueous solutions in the absence of fluoride
decays very rapidly due to the formation of an oxide film. 77 It decays less rapidly
in the presence of a reducing agent which can compete efficiently for the holes from
the valence band and slow down the rate of oxidation. Ferrate ions and iodine ions have
been found to compete favorably with the oxidation of the silicon surface. 77,600 On
the other hand, in solutions containing a small amount of fluoride ions insufficient to
completely remove the oxide film, the photocurrent exhibits a fast decay. According
to Matsumura and Morrison, 77 the rapid decay of the photocurrent in solutions with a
small amount of fluoride is due to the catalytic effect of fluoride ions at the
interface.
The photocurrent in nonfluoride solutions is affected by the amount of preanodic
current passed through the sample as shown in Fig. 5.10. 873 It is also seen that the pho-
tocurrent onset potential is shifted to more anodic values with formation of an oxide
film and the amount of shift is related to the thickness of the film. This shift is due to
the potential drop across a growing oxide layer and is one of the reasons for the dif-
ference between the photocurrent onset potential and the flatband potential. 77,695
When the concentration of fluoride is sufficiently high to prevent the formation
of an oxide film, sustained photocurrent can be obtained. The photocurrent value
increases with increasing light intensity until it reaches a value which is double that
in the absence of fluoride in the solution as shown in Fig. 5.10 due to electron
injection into the conduction band resulting in current multiplication. 873 At certain
higher light intensities, depending on fluoride concentration, photocurrent will again