Microscopic structure of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">SiO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>/Si interface

The bonding of Si atoms at the ${\mathrm{SiO}}_{2}$/Si interface is determined via high-resolution core-level spectroscopy with use of synchrotron radiation. All four oxidation states of Si are resolved, and their distribution is measured for Si(100) and Si(111) substrates. For oxides grown in pure ${\mathrm{O}}_{2}$, the density of Si atoms in intermediate oxidation states is (1.5\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. This value is obtained by measuring the core-level intensity, the escape depth in Si and ${\mathrm{SiO}}_{2}$, and the relative Si 2p photoionization cross section for different oxidation states. From the density and distribution of intermediate-oxidation states, models of the interface structure are obtained. The interface is not abrupt, as evidenced by the high density of intermediate-oxidation states (about two monolayers of Si) and by their nonideal distribution. The finite width of the interface is explained by the bond-density mismatch between ${\mathrm{SiO}}_{2}$ and Si. The electrical properties of the interface (band lineup, Fermi, and vacuum level) are determined. Annealing in ${\mathrm{H}}_{2}$ is found to influence the electrical parameters by removing the ${P}_{b}$ centers that pin the Fermi level. The distribution of oxidation states is not affected.