Structural fluctuation of SiO2 network at the interface with Si

We analyzed the density of thermally grownSiO2/Siby X-ray reflectivity measurements and local vibration properties by infrared spectroscopy. The macroscopic density varied with growth conditions. A lower growth temperature caused higher film density. We also found a thin (1 nm) and dense (2.35–2.4 g/cm3) transient layer at theSiO2/Siinterface. The film density was constant to the thickness direction without the transient layer. On the other hand, IR properties showed characteristic film thickness dependencies. At a film thickness greater than 10 nm, the frequency of the transverse optic (TO) mode of Si-O stretching shifted to the red direction, decreasing with the film thickness; while the frequency of the longitudinal optic (LO) mode is unchanged. This red shift of TO mode has no relation to the film density. At a film thickness of less than 6 nm, we found that both the TO and LO mode shift to the red direction simultaneously. The red shifts gradually increased with decreasing film thickness. This indicates that the SiO2 network was densified by compressive stress. We assumed that the macroscopic and microscopic fluctuation were related to the oxide growth and formation of a hetero-junction.