ABSTRACT
The recently developed technique of ellipsometry in the infrared region of the electromagnetic spectrum provides information about amplitude, phase and polarization state for low-energy radiation reflected off a surface. We have built and tested an infrared ellipsometer for the study of ultrathin layers of organic/inorganic materials. Measurements have so far been performed on doped GaAs films, electro-polymerized polythiophene and polypyrrole, aerogels, various superlattices and bulk inorganic crystals. The actual build-up is based on a Fourier-transform spectrometer which has been equipped with a computerized ellipsometric attachment. Positioning of the polarizers and calibration of their azimuthal settings are done automatically by a computer. Since oriented polymer films generally behave as biaxial optical systems evaluation of the data requires some special considerations. Furthermore, the sharp variation in the refractive index near resonances may 'distort' the band shape of vibrational reflection spectra. In addition, spectra from thicker films exhibit the usual interference features. Comparison between measured and calculated amplitudes/phases allows valuable informations to be extracted. For example, in the case of electropolymerized polythiophene in benzonitrile at 15 V (J. Mardalen et al., (1990)) the reflected s- and p-amplitudes are found to behave strikingly different. In the p-case, where the electric field vector has a non-vanishing component in a direction perpendicularly to the surface of the Pt substrate the aromatic CH-mode near 3000 cm-1 is found to be absent. The vanishing mode indicates that an ordering mechanism is present during film-growth, thereby favouring a structure where the thiophene rings are parallel to the substrate surface. This conclusion is in qualitative agreement with the x-ray recordings of Ito et al. (Journal of Polymer Science, C 24, 147-151 (1986)). Infrared ellipsometry combines the multiplex advantage of Fourier-transform spectroscopy and the phase sensitivity of ellipsometry. A full polarometric analysis is necessary when interpreting data from a sample with a film-like structure.
