"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Radical Chemistry on Silicon Surfaces 205
actions determine the reactivity and selectivity of the reaction [46]. On the other
hand, in radical chemistry consequences should be attenuated.
8.5.1 OXIDATION OF HYDROGEN-TERMINATED SILICON
SURFACES
The reaction of hydrogen-terminated Si(111), H w Si(111), with molecular
oxygen under photochemical conditions has been investigated in some detail
[48]. Figure 8.4 highlights the photoinduced reactivity of H w Si(111) in air
followed by FTIR. Spectrum (a) shows that the exposure of material to air in
the dark does not affect its stability, whereas after 30 min of photolysis with a
mercury lamp, the area of the original peak decreases to 14 % (spectrum d). Xray
photoelectron spectroscopy (XPS) of the irradiated surface shows a strong
O(1s) signal and a shifted Si(2p) signal indicating the formation of SiO2.
Ellipsometry studies reveal the presence of an 8 A ˚ film. When the surface is
exposed to 350 nm light for the same total energy, the loss of Si w H bonds is
decreased appreciably (spectrum c). With 450 nm light no detectable loss of
Si w H is observed (spectrum b).
It is worth mentioning that the photooxidation of porous silicon behaves
differently [49]. Indeed, FTIR spectra show that there is a tremendous increase
in nSi O, without a correspondingly large loss of nSi H peak intensity. The
decrease of the nSi H band is offset by an increase in the nOSi H band, resulting
in no net loss of hydride species on the surface during the course of the photooxidation
reaction. These data apparently suggest that oxidation does not result
in the removal of H atoms, implying that Si w Si bonds are attacked directly.
Arb. Units
dark
99�1%
0
98�2%
0
91�2%
0
(a)
450 nm
(b) (c) (d)
350 nm
250 nm
14%, 8
2150
2100 (cm−1 )
2050
Figure 8.4 ATR-FTIR spectra of H w Si(111) after exposure to air (a) in the dark, (b) with
450 nm (broadband) illumination, (c) with 350 nm (broadband) illumination, and (d) with Hg
lamp illumination. Reprinted with permission from Reference [48]. Copyright 2000 American
Chemical Society.