A series of silica gel films were spin-coated on single crystal silicon
(c-Si) substrates and their structure was characterized by vibrational spectroscopy.
The films were either dried at room temperature or partially densified
at 450 0C. Fourier transform infrared absorption spectra have been obtained
for each film and they are compared to the spectrum of thermal SiO2
films. The gel films (ca. 150 nm thick) show the presence of residual OH
groups, but very little molecular water or organic species and the fundamental
Si-O-Si vibrations exhibit shifts toward lower frequencies, compared to the
thermal oxide. The Si-O-Si antisymmetric stretch near 1070 cm was narrower
for the gels and the shoulder on the high frequency side was stronger. The nature
of this feature is discussed based also on oblique incidence transmission
and reflection-absorption spectra taken with polarized infrared light.