Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 222

5.12.2. Later Modifications

Model to Account for Electron Injection into the Conduction Band. Later
studies showed that the quantum efficiency of illuminated n -Si in HF solutions varies
from 400% (current quadrupling) at low light intensities to 200% (current doubling) at
high light intensities as shown in Fig. 5.15. 74,177 The efficiencies of 200% and 400%
suggest that the value in the following reaction varies from 1 to 2 corresponding to
current quadrupling and current doubling:

For current quadrupling this means, according to Matsumara and Morrison, 74 that
during the oxidation of Si to all steps except perhaps the first occur via electron
injection into the conduction band. Whether current doubling or quadrupling occurs is
determined by the competing processes depending on the availability of holes as illus-
trated in Fig. 5.63. At low illumination, reaction step III is more favorable and current
quadrupling occurs. At high illumination intensities, which generate a large quantity of
holes, reaction via steps IV and V is more favorable than step III, resulting in current
doubling. This model still assumed a fluoride-terminated initial surface. Also, it did not
consider the effect of hydrogen evolution on quantum efficiency.

Modification for Hydrogen Termination. In the late 1980s and early 1990s an
extensive amount of research established that the surface of silicon in HF solution is
terminated by hydrogen. The models proposed afterwards generally take into account
71,700,904,921

this aspect. Figure 5.64, for example, shows a model, proposed by Lehmann

and Gosele, 71 for dissolution at anodic potentials negative of the potential for the first
current peak in Fig. 5.3). In this potential region, the adsorbed hydrogen atoms are
first replaced by fluoride atoms. The fluoride-terminated silicon atoms then react with
HF molecules to break the silicon atoms in the lattice and dissolve into the solution.