Infrared external reflection spectroscopy of self-assembled monolayer films on Si substrate with a buried metal layer (BML) structure

This paper reports a study of the signal behavior of IR reflection spectra on the flat Si substrate with a buried metal layer (BML) and MBE-grown Si overlayer. A self-assembled monolayer of octadecyltrichlorosilane on the surface was used as a typical sample of a well-oriented layer. The signal intensities strongly depend on the orientation of surface vibration, the polarization of IR radiation and the angle of incidence. The dependence is quite different from that seen on metal and normal (without BML) semiconductor surfaces and can be explained by local electric field intensity based on classical electromagnetic treatment. On a relatively thin Si overlayer (typically 700–1200Å), vibration modes only with the transition moment in the vertical direction were observed for p-polarization and modes only in the lateral direction were observed for s-polarization. On a relatively thick Si overlayer (typically 2000A˚), modes with lateral as well as vertical components show significant signals for p-polarization, and only lateral modes could be detected at significant intensity for s-polarization. These results show that the orientation of surface species can be determined by analyzing the spectral dependence on polarization, angle of incidence and the thickness of the Si overlayer.