Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 217

34,73,108,109,207 For n-

and KOH solutions proceed predominantly over the valence band.
anodic reactions in the dark at current densities higher than the limiting current are a
conduction band process but can involve both the conduction band and the valence
band under illumination. The relative participation of conduction band versus valence
band is a function of light intensity; the conduction band dominates at low light inten-
sity and the valence band at high light intensity. 74,695,873 Table 5.6 summarizes the par-
ticipation of the two carrier bands under different conditions.
The charge transfer mechanism across the interface barrier layer is different for
lowly doped and heavily doped p -type silicon. For lowly doped p -type silicon the
process is by thermal emission of holes to go over the barrier layer whereas it is by
Zener tunneling for heavily doped materials. 34,964 For n -Si the conduction band
processes depend on doping density and on illumination intensity. For heavily doped
n -Si it is by Zener tunneling and the i-V curve is identical to that for p -Si. For moder-
ately or lowly doped n -Si in the dark the reaction is limited by the minority holes, which
are required to initiate the dissolution process. Significant dissolution of n -Si can
proceed when a large number of holes are generated by illumination.
Large current can also be generated on n -Si in HF solutions in the dark at a rel-
atively high anodic potential due to an interface tunneling process. 8,38 When a large
anodic polarization is applied to the interface, a large band bending occurs at certain
potential values. The space charge layer becomes very thin so that electrons can tunnel
through directly from the silicon atoms on the surface into the conduction band in the
semiconductor. 1151 Interface tunneling is in essence the same process as current multi-
plication under illumination in that both proceed by electron injection from the surface
into the conduction band. The difference is in the energy levels of the surface species
from which the electron injection occurs. The electron injection in current multiplica-
tion occurs from the reaction intermediates, the energy levels of which are located near

Si,