Chemistry Reference
In-Depth Information
In water after a sufficient time lapse the surface is always covered with a thin oxide
film and the steady state thickness depends on the initial surface condition. The steady
state thickness of the native oxide films formed on the silicon surface in water is similar
to that formed in air. Water is essential for the formation of oxide on the silicon surface
in different solutions, organic or inorganic. Oxide film does not form on the surface in
water when the concentration of HF is higher than
0 ppm.
The native oxide can be thickened with application of an anodic potential. The
rate of growth and final thickness at a given potential depends, among other factors, on
solution composition. Holes from the valence band are responsible for the oxidation
reaction for p-Si while injection of electrons into the conduction band is for n-Si. The
oxidation reaction occurs at the silicon/oxide interface through several intermediate
steps forming partially oxidized species which can act as interface states. The oxygen
required to form the oxide structure is from the water, either residual or generated
1
during anodization, in the electrolyte. The water molecules enter into the first layers of
and/or which then migrate
toward the silicon/oxide interface under the effect of the electrical field in the oxide.
As illustrated in Fig. 3.19, anodic oxide behaves like a doped semiconductor and
is capable of conducting a large electronic current under anodizing conditions, which
are responsible for the low ionic current efficiency due to side reactions. The side reac-
tion is the oxidation of water in aqueous solution and is the oxidation of the solvent
molecules in non-aqueous solutions. Photo emission may result from the charged car-
riers going through the energy steps at the oxide/electrolyte interfaces, silicon oxide
interfaces, and localized states in the oxide during the oxidation process.
The oxide films formed by anodization have generally a loose structure contain-
ing a significant amount of water, hydroxyl ions and other species, which are present
in the electrolyte. As-formed anodic oxides may contain SiO, SiOH and SiH groups,
the oxide and dissociate into ionic species, such as
absorbed water, oxidation products of the solvent and ionic impurities to levels as high
as
Due to the incorporation of OH and water, anodic oxides are generally
non-stoichiometric with silicon deficient structures. Also, anodic oxide is not uniform
but changes with distance from the
interface to interface and
tends to change with time, which plays a critical role in current oscillation.
The physical, chemical, and electrical properties of anodic silicon oxides are
determined by this loose structure and the incorporation of foreign species. The elec-
trical properties of anodically formed oxides is very poor in comparison with those of
thermal oxides due to the high concentration of charges and states associated with the
loose structure and high levels of impurities. Most notably, the etch rates of anodic
oxides are generally many times higher than thermal oxide. Also, the silicon/oxide inter-
face region has a high density of partially oxidized silicon atoms, which significantly
contribute to interface charge. The reaction of these partially oxidized silicon atoms
with hydrogen is also responsible for the small amount of hydrogen evolution and an
oxidation valence of less than four in the passive region.
The role of oxide in the major phenomena of silicon electrodes in terms of surface
coverage and rates of formation and dissolution can be qualitatively described by Fig.
9.2. Depending on the condition, the surface coverage of oxide may vary from zero to
one. Under a condition when the formation rate for the first layer of oxide is larger than
the dissolution rate the oxide will grow in thickness and full oxide coverage of the
