Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 195

For n -Si the i-V behavior, impedance response, and near-perfect Mott-Schottky
show that the applied potential is predominantly dropped across the space charge
layer in the semiconductor. It is not so obvious for p -type and heavily doped n -type
silicon samples. All of these samples display, besides a shift along the potential axis,
similar curves which show a Tafel region at lower potentials and a current plateau

at higher potentials. The origin of the 60 mV/decade Tafel slope of the silicon electrode
in HF solutions has been attributed to either a rate-limiting process involving charge
transfer in the Helmholtz layer or one involving carrier supply in the space charge
layer. 2,5,194,724
For heavily doped materials, either n or p type, the surface is degenerated and
the material behaves like a metal electrode, meaning that the charge transfer reaction
in the Helmholtz double layer is the rate-determining step. This is supported by the lack
of an impedance loop associated with the space charge for the heavily doped materi-
als. Also, for heavily doped

i-V

-Si large current in the dark is due to electron injection,
which is not characterized by a slope of 60 mV/decade. For

n

-Si, electron injection into
the conduction band may also occur during the anodic dissolution.
For non-heavily doped p -Si the potential is mostly dropped within the space
charge layer before the onset of current. At potentials higher than that at which current
becomes measurable, the potential is likely to drop also in the Helmholtz layer in addi-
tion to the space charge layer. This would mean a Tafel slope larger than 60 mV/decade.
Also, for very lowly doped material, potential drop in the bulk semiconductor can be
significant. 700
Thus, as a summary, Tafel slopes of the

p

curves observed on differently doped
materials, although they have similar values, are determined by different factors. The
experimentally observed values of about 60 mV/decade in the exponential region are
due to several factors which have different effects on the current-potential relationship:
(1) relative potential drops in the space charge layer and the Helmholtz layer; (2)
increase in surface area during the course of an

i-V

i-V curve measurement due to for-
mation of PS which tends to reduce the slope; (3) change of the dissolution valence
with potential which has an effect of reducing the slope if a significant part of the poten-
tial is dropped in the Helmholtz layer; (4) electron injection into the conduction band
which reduces the slope; and (5) potential drops in the bulk semiconductor and elec-
trolyte, which increases the slope.

5.9. PASSIVATION

5.9.1. Occurrence
Passivation is a process in which the electrode surface changes from an active
state to a relatively inactive state owing to the formation of a surface barrier layer. The
processes involved in passivation play a critical role in many anodic electrode phe-
nomena on silicon such as electropolishing, cleaning, etching, porous silicon forma-
tion, and current oscillation. In nonfluoride and nonalkaline solutions the surface silicon
electrode is, in general, passivated due to the formation of a thin layer of oxide film.
Very small current can pass through the passivated silicon surface of either n- or p- type