Initial stages of oxidation of Si(111) with condensed O2 and N2O at 20 K.

Abstract

The oxidation induced by soft-x-ray illumination of condensed multilayers of ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O on cleaved Si(111) at 20 K has been investigated with high-resolution core-level spectroscopy. The results for ${\mathrm{O}}_{2}$/Si(111) show that a ${\mathrm{SiO}}_{2}$ phase is produced by illumination with as few as 4.3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ photons ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ of energy 130 eV. This ${\mathrm{Si}}^{4+}$ oxide is characterized by a Si 2p binding energy that is shifted 3.4 eV relative to bulk Si, and its effective thickness increases with illumination. Intermediate oxides having ${\mathrm{Si}}^{1+}$, ${\mathrm{Si}}^{2+}$, and ${\mathrm{Si}}^{3+}$ bonding configurations are also observed, and their total thickness also increases. The enhanced formation of these intermediate oxides is due primarily to the growth of ${\mathrm{Si}}^{3+}$ bonding configurations throughout the ${\mathrm{SiO}}_{2}$ layer. Exposure of the oxidized Si surface to white light from the synchrotron-radiation source or annealing to 300 K produces structural changes as the ${\mathrm{SiO}}_{2}$ layer thickens at the expense of the intermediate oxides. Hence, the defectlike ${\mathrm{Si}}^{3+}$ configuration converts to a ${\mathrm{SiO}}_{2}$ configuration and a sharper interface develops. Studies of ${\mathrm{N}}_{2}$O/Si(111) interactions during photon irradiation show that the initial oxidation rate is slower than with ${\mathrm{O}}_{2}$. After more extended illumination, the oxidation rate with ${\mathrm{N}}_{2}$O approximates that with ${\mathrm{O}}_{2}$ as ${\mathrm{N}}_{2}$O molecules are dissociated. The intermediate oxide formed with ${\mathrm{N}}_{2}$O is thicker than with ${\mathrm{O}}_{2}$, and extended growth of the intermediate oxide reflects the formation of both ${\mathrm{Si}}^{2+}$ and ${\mathrm{Si}}^{3+}$. From these ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O results, we conclude that the oxidation rate for the submonolayer oxide is determined mainly by the availability of surface reaction sites and the cross section for electron capture by the condensed species. Further oxide growth is also limited by the existing oxide layer through which atomic oxygen must diffuse in order to react at the interface.