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

208 Silyl Radicals in Polymers and Materials
oxide growth rate. Indeed, infrared studies have shown that the half-life of the
Si H stretch in air of an H Si(111) surface is humidity dependent. It was
w w
suggested that the oxidation of Si HtoSiOH by H2O is the rate-limiting
w w
step in the native oxide formation [51]. On the other hand, dissolved oxygen in
40 % aqueous ammonium fluoride solution initiates the formation of etch pits in
the terraces of the otherwise ideal H Si(111) surface. Removal of oxygen from
w
the fluoride solution by sparging with argon substantially reduces the initiation of
etch pits. It was suggested that oxygen molecules are reduced to superoxide
radicals (O :
2 ) at the negative open-circuit potential of the silicon surface and
that O :
2 abstracts hydrogen atoms from the H Si(111) terraces to form silicon
w
radicals [52].
Whatever the initial step of formation of surface silyl radicals, the mechanism
for the oxidation of silicon surfaces by O2 is expected to be similar to the
proposed Scheme 8.10. This proposal is also in agreement with the various
spectroscopic measurements that provided evidence for a peroxyl radical
species on the surface of silicon [53] during thermal oxidation (see also references
cited in [50]). The reaction being a surface radical chain oxidation, it is
obvious that temperature, efficiency of radical initiation, surface precursor and
oxygen concentration will play important roles in the acceleration of the surface
oxidation and outcome of oxidation.
8.5.2 HALOGENATION OF H w Si(111)
The analogy drawn between radical reactions of organosilanes and those of
H w Si(111) led to the exploration of halogenation reactions. Chlorine-terminated
Si(111) surfaces, Cl w Si(111), were prepared by treating H w Si(111)
with PCl5 in chlorobenzene under free-radical conditions using either thermal
decomposition of dibenzoyl peroxide [54] or UV illumination under reflux [55].
Cl w Si(111) was also obtained by reaction of Cl2 gas with H w Si(111) activated
either by heating at 80 8 C for 10 min or by irradiating with a 300 W tungsten
lamp for 2 min at room temperature [56]. Analogously, H w Si(111) surfaces were
brominated by reaction with N-bromosuccinimide or BrCCl3 to give Br w Si(111)
using radical initiating conditions [57]. Halogen-terminated Si(111), X w Si(111),
were characterized by several spectroscopic methods (XPS; AES; ATR-FTIR,
HREELS, and NEXAFS) and have been further used in reactions with organolithium
reagents (RLi) to obtain monolayer films R w Si(111) [54,55,57].
8.5.3 ADDITION OF UNSATURATED COMPOUNDS ON H w Si(111)
Wet chemical procedures for forming covalently bonded monolayer films on
H w Si(111) are well developed. Among various methods, the radical approach
is the most common one.