Basic Theories of Semiconductor Electrochemistry - Electrochemistry of Silicon and Its Oxide - page 21

717

the oxide thickness, and
and

the surface field. For a thin oxide film of about 20 Å,
(the dielectric constant of native oxides is much larger than
that of thermal oxide due to inclusion of water molecules), Thus, the
presence of a thin oxide film such as the native oxide has in general a weak effect on
the determination of flatband potential when the oxide is not associated with a high
density of surface states.

1.4. KINETICS OF CHARGE TRANSFER

1.4.1. Basic Theories
Electrochemical reactions on a semiconductor electrode involve charge transfer
between the species in the solution and charge carriers in the semiconductor. The basic
assumption in the theories of the kinetics of charge transfer reactions is that the elec-
tron transfer is most probable when the energy levels of the initial and final states of
the system coincide. 44,79 This requires that the vibration of molecules and their sur-
rounding solvation shell be slow compared with the actual electron transfer between
the electrode and redox system in the solution. Thus, the efficiency in the redox reac-
tion processes is primarily controlled by the energy overlap between the quantum states
in the energy bands of the semiconductor and the donor or acceptor levels in the reac-
tants in the electrolyte. Often, the overlap between the electronic states in the semi-
conductor and the levels in the electrolyte is unfavorable, and radical surface states that
located within the band gap become the most important for the charge transfer.
In an ideal case when surface states are absent and charge transfer proceeds
directly between the energy levels in the bands and in the solution, according to
Gerischer, 79 an anodic current involving an electron transfer from a molecule in the
electrolyte to the electrode and a cathodic current involving an electron transfer from
the electrode to a molecule in the electrolyte are given by