Microscopic structure of the SiO₂/Si interface

The bonding of Si atoms at the SiO₂/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 O₂, the density of Si atoms in intermediate oxidation states is (1.5±0.5)×10¹⁵ cm^-2. This value is obtained by measuring the core-level intensity, the escape depth in Si and SiO₂, 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 SiO₂ and Si. The electrical properties of the interface (band lineup, Fermi, and vacuum level) are determined. Annealing in H₂ 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.