Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 180

involved in the reactions with fluoride species in the solution. Presence of mettalic con-
taminants can strongly affect rate of recombination. 1016 An increase of 100 folds in the
rate of recombination is found with the presence of 100 ppb Cu in the solution.
The occurrence of the recombination peak shown in Fig. 5.18 can be qualitatively
explained by the relative concentrations of the majority and minority carriers. 158 Recom-
bination requires availability of both majority and minority carriers. With small band
bending the number of minority carriers at the surface is small, resulting in low surface
recombination. On the other hand, with large band bending the minority carriers created
by light or injection cannot recombine at the surface because the majority density is
low. At a certain bending condition, when the density of both types of carriers is large,
the recombination reaches a maximum. The width of the recombination peak depends
on the relative distribution of the potential drop in the space charge layer to that in the
rest of the interface region. 278 The recombination peak for silicon electrodes in various
electrolytes can be widened with increasing dopant concentration or increasing oxide
film thickness, which causes an increase in the relative amount of the applied potential
dropped across the Helmholtz layer and the oxide film.
Figure 5.19 shows that for

n -Si in
density of surface recombination centers increases with increasing light intensity. 94,808
These recombination centers are suggested to be physically associated with the oxida-
tion intermediates in the oxide formed under illumination.

at pH 9, the

5.4. EFFECTIVE DISSOLUTION VALENCE

The effective dissolution valence is defined as the average number of electrons
flowing through the external circuit per dissolved silicon atom. It reflects the nature of
the reactions during the dissolution processes, e.g., the extent of electrochemical reac-
tions relative to the chemical reactions. For silicon, the effective dissolution valence of