Chemistry Reference
In-Depth Information
Under typical anodization conditions, holes from the valence band are responsi-
ble for the oxidation reaction for
p-
Si whereas injection of electrons into the conduc-
tion band is responsible for that for
n
-Si (requiring a high voltage to narrow the surface
barrier). The oxidation reaction occurs at the silicon/oxide interface through several
intermediate steps forming partially oxidized species which can act as interface states.
These partially oxidized silicon species, due to their different energy levels, are further
oxidized by electron tunneling into the conduction band at much lower voltages. This
is responsible for the oxidation of
n
-Si after it is covered with an oxide film of a certain
thickness.
The oxygen required to form the oxide structure is from the water, either resid-
ual or generated during anodization, in the electrolyte. The water molecules enter into
the first layers of the oxide and dissociate into ionic species, such as and/or
which then migrate toward the silicon/oxide interface under the effect of the electrical
field in the oxide.
The oxide, which is capable of conducting a large current under anodizing con-
ditions, behaves as a doped semiconductor, and is responsible for the side reactions and
low ionic current efficiency. Depending on the nature of the solvent, whether aqueous
or nonaqueous, the side reaction can be predominantly due to the oxidation of water
in aqueous solutions, and due to the oxidation of solvent molecules, R, in nonaqueous
solutions. It is not clear whether the electronic current is by hole or electron, but the
band structure of the silicon/oxide interface indicates that electron conduction is more
likely. In either case the charged carriers going through the energy steps at the
