Anodic Oxide - Electrochemistry of Silicon and Its Oxide - page 110

whose mean free path is about 10 nm, with the light emission centers in the oxide then
results in the light emission corresponding to the LE peak. The HE emission is attrib-
uted to the transition due to band discontinuity at the oxide/silicon interface. The light
intensity increases with the oxide thickness because the emission region increases. The
emission center, located about 2 eV below the conduction band, is considered to have
the same nature as that described by Hasengawa et al. 370
Light emission also occurs on thermal oxide film under anodization conditions,
indicating that the light emission is not associated with defects particular to anodic
oxide but rather with a common phenomenon associated with the system. 370
Also, that light is emitted only above a certain thickness indicates that the lumines-
cence is not due to the chemical reaction but to the mode of charge transfer involved
in the formation of No light emission occurs for thicknesses of less than 15 nm
because up to this thickness the electron can tunnel through the oxide layer directly
into Si. The fact that the energy and intensity of emission varies with thickness indi-
cates that the process depends on the structure of the oxide. The enhancement of the
low-energy peak by the addition of KCl in the solution can be explained as due to the
incorporation of chloride ions in the oxide film. 1033
3.4.5. An Overall Growth Model
The general processes involved in the anodic oxidation of silicon can be sum-
marized by the band diagram of interfaces illustrated in Fig. 3.19
considering the information on the possible reactions, on the species responsible for
ionic transport inside the oxide, and on the differences between p- Si and n- Si .