  It is known that a higher concentration of free carriers leads to a higher
oxide growth rate in the thermal oxidation of silicon. However, the role of
electrons and holes in oxidation chemistry is not clear. Here, we report
real-time second-harmonic-generation data on the oxidation of H-terminated
(111)Si that reveal that high concentrations of electrons increase the chemical
reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds.
However, the thicknesses of the natural oxides of all samples stabilize near 1
nm at room temperature, regardless of the chemical kinetics of the different
bonds.
