Chemistry Reference
In-Depth Information
behavior is very similar to that of
-Si under illumination. On the other hand, for
p-
Si
under a cathodic polarization, a much larger voltage, which decreases with oxide thick-
ness, is required in the dark than under illumination.
The difference in electrical properties in electrolyte between anodic and thermal
oxides can also be revealed with impedance spectra as shown for example in Fig.
3.32.
139
In solution the thermal oxide, with a capacitance of and a
charge transfer resistance of shows a near-ideal capacitor behavior. On
the other hand, for the anodic oxide the phase angle does not reach 90° and the charge
transfer resistance is which is several orders of magnitude below that of thermal
oxides. The low charge transfer resistance of the anodic oxide films indicates that the
transport of ions in the film is relatively large.
Transient current and transient capacitance measurements on removal of the
anodic polarization, which provide information on the transient flatband potentials,
indicate that both positive and negative charges are present in the anodic oxide film
during its growth.
601
n
601
According to Chazalveil,
and the negative charges are associated with
groups and/or terminal groups. The anodic oxide is composed of an inner
layer, a mostly dry, substoichiometric layer, and an outer hydrated layer. On removing
the anodic polarization, the positive charge decays by backward migration and injec-
tion of holes into the electrode and the negative charges stay until the oxide layer is
partly dissolved.
Kirah
et
the positive charges are related to
charged oxygen “semivacancies”,
.
294
calculated the transient distribution of the ionic species in anodic
oxide based on the experimental data of Chazalveil.
601
For the oxide formed at 5 V in
a BHF solution, Fig. 3.33 shows the density of ionic species inside the oxide layer are
al
positive and its density decreases from the electrolyte side to the Si side. It was attrib-
interface to the
electrolyte side waiting for reaction with the oxidant species. By further assuming the
total charges in the oxide to be ionic charges distributed in the bulk and trapped charges
at the interface states, the interface trapped states were found to be positive on
the order of which decreases with thinning of the oxide film.
Exposed to humid air or water after forming, the electrical properties of anodic
oxides tend to decay due to the action of water incorporating significant amount of
hydrogen into the oxide structure.
793
For the oxides formed in diethylene glycol con-
taining after several days of immersion in water, the permittivity was found to
decrease from 3.85 to 6.2 and the resistivity decreased from
uted to accumulation of the positive ions transported from the
to
for a field
of 1 mV/cm.
Depending on the anodization system, certain conditions can result in better
electrical properties of the anodized oxides. Annealing of anodized oxides generally
decreases the surface state density.
139,228,244
Laser-enhanced oxidation may lead to
better electrical properties than in the dark.
605
Under a constant voltage, the surface
300
state density tends to decrease with increasing final oxidation rate.
In dry electrolytes,
it was found that constant current anodization at a current density of less than
117,427
produces oxide films of poor quality.
This is attributed to the oxide growth
at low currents occurring by the inward motion of hydroxyl ions which results
in the hydrated and poor-quality films. Breakdown field, flatband voltage, fixed
oxide charge density, surface state density, and mobile ionic charge density, for
