Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 218

or above the conduction band edge and can inject electrons into the conduction band
without significant band banding. On the other hand, the electron injection in the dark
is from the surface silicon atoms which are not oxidized and are located energetically
much lower than the conduction band edge and can only inject electrons into the con-
duction band at large band bending. The energy levels and density of the surface species
are related to the fluoride concentration in the solution since the potential needed for a
large current decreases with increasing HF concentration. 9
The results discussed in the preceding sections indicate that the rate-limiting
process in the anodic electrode reactions can be involved in the bulk of silicon, in the
space charge layer, in the oxide, or in the electrolyte depending on the potential range,
silicon type, illumination condition, and solution composition. As schematically illus-
trated in Fig. 5.60, each of the possible processes in the multilayer silicon/electrolyte
interface region can be the rate-limiting process under certain conditions. For example,
the anodic reaction processes on n -Si in the dark are limited by the minority hole trans-
port in the bulk of silicon, that is,

and illuminated n -Si in fluoride solutions
at potentials negative of positive of the first current peak, the reaction rate is deter-
mined by the charge transfer process across the electrode/electrolyte interface, that is,
and At potentials positive of i.e., the electropolishing region, the rate-
determining step in the anodic reaction is the dissolution of the anodic oxide film, that
is, The dissolution of the oxide film formed in the electropolishing region at low
fluoride concentrations is mainly kinetically controlled, that is,

For p- Si

whereas for high

fluoride concentrations the process is mainly diffusion controlled,
In KOH solutions the rate-limiting process at OCP is of chemical nature, i.e., only
is involved. Electrochemical processes, that is, and are increasingly involved