Infrared spectroscopic analysis of the Si/SiO2 interface structure of thermally oxidized silicon

The nature of the silicon oxide transition region in the vicinity of the Si /SiO2 interface is probed by infrared and x-ray photoelectron spectroscopie s. The layer-by-layer composition of the interface is evaluated by uniforml y thinning thermal oxide films from 31 Angstrom down to 6 Angstrom. We find that the thickness dependence of the frequencies of the transverse optical and longitudinal optical phonons of the oxide film cannot be reconciled by consideration of simple homogeneous processes such as image charge effects or stress near the interface. Rather, by applying the Bruggeman effective medium approximation, we show that film inhomogeneity in the form of substo ichiometric silicon oxide species accounts for the observed spectral change s as the interface is approached. The presence of such substoichiometric ox ide species is supported by the thickness dependence of the integrated Si s uboxide signal in companion x-ray photoelectron spectra. (C) 2000 American Institute of Physics. [S0021-8979(00)00903-8].