Spatially-resolved X-ray Absorption Near-edge Spectroscopy of Silicon in Thin Silicon-oxide Films

The electronic structure of oxide thin films on silicon single crystals has been measured with high spatial resolution by x-ray absorption near edge spectroscopy (XANES) using a photoelectron microscope and monochromatic synchrotron radiation. Native oxide overlayers on Si(100) and Si(111) crystals were studied at various stages during thermal desorption of the oxide. The spatial resolution capability of the microscope is used to perform selected-area XANES measurements of silicon and silicon oxides, which reveal for the first time the absorption characteristics of intermediate oxidation states of silicon (Si 2+ and Si 3+ ), and identify the spin-orbit splitting of the silicon 2 p core-exciton structure in Si 2+ , Si 3+ , and SiO 2 . Our results confirm photoemission measurements which find Si 2+ and Si 3+ states existing at vertical interfaces between Si and SiO 2 . Unlike the photoemission spectra, however, selected-area XANES can resolve the spectroscopic signatures of these intermediate oxidation states in laterally inhomogeneous interfaces.