Suboxides at the Si/SiO₂ interface: A Si2p core level study with synchrotron radiation

Synchrotron radiation X-ray photoemission spectroscopy (SRXPS), used with an optimized surface and interface sensitivity, is a unique tool to determine the chemical composition and spatial extension of the suboxide layer present at the Si/SiO₂ interface. The bonding at 'thermal interfaces' appear to be essentially dependent on the Si crystal orientation. For Si(001)/SiO₂ a detailed photon energy-dependent and angle-dependent SRXPS study (in the 120 ÷ 175 eV photon energy range) has been performed. Previous results on higher-oxidation states cross-section resonances around hv = 130 eV are re-examined. The various oxide states do not present photoelectron diffraction peaks, when polar scans are performed in the (110) azimuthal plane. On the other hand, there is evidence that substrate silicon atoms close to the interface retain orientational order. Limits in the vertical distribution of the suboxides are given. The compatibility of our experimental findings with models of the current literature - in particular the so-called 'dimerized interface' model - is examined.