Anodic Oxide - Electrochemistry of Silicon and Its Oxide - page 121

and A second region extends about 30 Å into the film. The in this
layer is compressed because the density of Si atoms is higher. For oxides grown in pure
the density of Si atoms in the intermediate oxidation states is (about
two monolayers of Si) near the interface region. The interface is not abrupt because of
this high density of intermediate oxidation states. The interface, in addition to
partially oxidized silicon species, may have defects such as Si and O vacancies and
interstitial elements. 165,338,479,702
The Si intermediate complexes generated from the oxidation of a part of the
excess Si bonds in this transition region are responsible for fixed charge and fast surface
states. 338 They may be formed according to the reaction

where the dangling bonds, are identified with fast surface states and the
are identified with fixed positive charges.
The partially oxidized silicon species are responsible for the anodic current tran-
sient measured at the end of etching of an anodic oxide film-covered n -Si electrode in
the dark as shown in Fig. 3.25. 74 For a clean n -Si surface, the anodic current is very
small. This dark current during the etch-back experiment, whose peak position depends
on the thickness, occurs on anodic oxide as well as on thermal oxide. The data shown
in Fig. 3.25 indicate that the anodic reaction proceeds by injection of electrons from
the partially oxidized silicon species at the silicon/oxide interface. The amount of

charge associated with the current transient, which is similar for anodically and ther-

mally oxidized surface, is about
Si 3+

corresponding to two monolayers of

on a (100) surface. The partially oxidized species may extend to a number of atomic

layers, fewer for thermal oxide than anodic oxide as shown in Fig. 3.25.
The interface property of silicon oxide can also be described in terms of dipoles
which are formed due to partial charge transfer that takes place on formation of inter-
face bonds. 76 The charge transfer occurs because of the difference in electronegativity