Structure of ultrathin silicon dioxide films

Differential infrared spectroscopy has been used to study the silicon-oxygen stretching band of thin silicon dioxide films thermally grown on single-crystal silicon. We consistently observe an asymmetry in the spectra of films thicker than about 100 Å, of about 9 cm^-1. The peak position, width, and degree of asymmetry are also found to be sensitively dependent upon film thickness below 100-150 Å, while above this level these features are only very weakly dependent upon film thickness, indicating the presence of a thin layer of different structural or bonding properties. Our interpretation suggests that the infrared spectra of layers up to 100 Å thick are significantly affected by strain originating at the silicon-oxide interface, in agreement with recent observations.