Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 184

For n -Si the efficiency of hydrogen evolution depends on illumination intensity
as shown in Fig. 5.13. 599,695 The efficiency of hydrogen evolution is close to zero at low
light intensity but increases with increasing light intensity to 100% at high light inten-
sities. The increased efficiency of hydrogen evolution corresponds to a decrease of the
quantum efficiency of the photocurrent from near 4 at low light intensities to about 2
at high light intensities.
The efficiency of hydrogen evolution and effective dissolution valence are
directly correlated, and their relation varies with potential, illumination, and doping of
the silicon. The overall relation among these two parameters and the factors are sum-
marized in Fig. 5.23.

5.6. LIMITING CURRENT

The intrinsic limiting current for n -Si depends on the diffusion of the minority
carrier as follows:

For an -Si with doping of of of

is on the order of The dark limiting currents measured on silicon elec-
trodes in electrolytes are usually several orders of magnitude larger than these values,
as shown in Table 5.2. The large dark limiting current may be due to several causes
such as surface states, electron injection from silicon oxidation intermediates, and inter-
face tunneling by the redox species in the solution.
Due to the role of surface states, the dark limiting current of silicon electrode is
extremely sensitive to surface defects and thus surface preparation. Any scratch even
barely visible on the mirror like surface can result in a significant increase of the anodic
limiting current. According to Chazalviel, 717 defects associated with surface treatment
are primarily responsible for the large limiting current values reported in the literature.
The effect caused by surface states may, however, be reduced by the formation of a

n

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