INFRARED STUDIES OF TRANSITION LAYERS AT SIO2 SI INTERFACE/

We investigated transition layers at the interface of the thin SiO2 fi lm successively etched back by diluted HF, using infrared reflection-a bsorption spectroscopy. The etching rate of the oxide film reveals tha t there is a Si-rich transition layer within 0.6 nm of the interface. However, frequency shift in the longitudinal optical phonon due to Si- O-Si asymmetric stretching toward lower wave numbers takes place less than 1.5 nm from the interface. We propose a model in which the transi tion layer is assumed to be Si-rich suboxide layers caused by the comp ositional roughness of the SiO2/Si interface. Through estimating the p honon frequencies which depend on the composition of the suboxide stru cture in this model, we found that the phonon frequency apparently sta rts to shift at around 1.5 nm from the interface, even if there are su boxide-rich layers within 0.6 nm, which can be caused by 1-2 monolayer s of roughness. (C) 1998 American Institute of Physics. [S0021-8979(98 )07723-8].