Chemistry Reference
In-Depth Information
The direct dissolution is sensitive to surface geometrical factors such as surface cur-
vature and orientation, while the indirect dissolution through the formation and disso-
lution of oxide is insensitive to the surface geometrical factors. Formation of an oxide
film, which generally shifts the rate limiting process to inside the oxide, masks the semi-
conductor properties of silicon.
9.2. SURFACE CONDITION
Silicon is a rather active element and unless in a vacuum its surface is never “clean”
because of the adsorption by foreign species. In water and aqueous solutions, the
surface of silicon can be terminated by various species including hydrogen, hydroxyl,
fluorine, and oxide. The specific type of termination, in terms of structure and compo-
sition, depends on how the surface is prepared and cleaned. In non-HF aqueous solu-
tions, the silicon surface is generally covered by an oxide film and in HF solutions the
silicon surface tends to be terminated by hydrogen (in the form of hydrides). The for-
mation of a surface hydride layer or oxide layer is responsible for the stability of silicon
in aqueous solutions.
In HF solutions the surface coverage of hydrogen and oxide is a function of poten-
tial. As schematically illustrated in Fig. 6.7, the surface is fully covered by an oxide
film at anodic potentials higher than the passivation potential, while it is fully covered
by hydrogen at potentials more negative of the open circuit potential. From the open
circuit potential to the passivation potential, the surface coverage of hydrogen gradu-
ally decreases as the coverage of oxide increases. Also, while at potentials near the open
circuit potential and below the passivation potential the hydride or oxide layer is on the
order of a monolayer, the hydride layer at the cathodic potentials and the oxide layer
at potentials higher than the passivation potential is relatively thick. The difference in
the kinetics for formation of a hydride layer and an oxide layer and in the stability
of these two layers plays a critical role in the diverse phenomena observed on silicon
electrodes.
The silicon surface may be hydrophobic or hydrophilic depending on whether it
is terminated by hydroxyl or hydrogen groups. Also, surface impurities, in the form of
metals, ceramic, and organic species can reach a surface atomic concentration higher
than 0.1% at 1 ppm impurity concentration in the electrolytes. In addition, a silicon
surface, even under the best prepared conditions, is not completely flat but has a certain
degree of roughness. The roughness can be associated with vacancies, kinks, adatoms,
steps, and terraces at the atomic scales and with pits, hillocks, scratches, deposits, etc.,
at a macro scale. Once in an electrolyte, the hydrophobicity and the roughness tend to
change over time as electrode processes occur, which is the basis for the processes,
such as cleaning, for surface preparation and control.
9.3. OXIDE FILM
Silicon oxide plays a particularly important role in the properties of silicon elec-
trodes. In air the surface of silicon is always covered with a very thin oxide film, the
thickness of which may vary from 5 to 20
Å
depending on the preparation conditions.
