Infrared studies of transition layers at SiO2/Si interface

We investigated transition layers at the interface of the thin ${\mathrm{SiO}}_{\mathrm{2}}$ film successively etched back by diluted HF, using infrared reflection-absorption spectroscopy. The etching rate of the oxide film reveals that 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 transition layer is assumed to be Si-rich suboxide layers caused by the compositional roughness of the ${\mathrm{SiO}}_{\mathrm{2}}\mathrm{/Si}$ interface. Through estimating the phonon frequencies which depend on the composition of the suboxide structure in this model, we found that the phonon frequency apparently starts to shift at around 1.5 nm from the interface, even if there are suboxide-rich layers within 0.6 nm, which can be caused by 1–2 monolayers of roughness.