INFRARED-ABSORPTION IN SIO2-GE COMPOSITE FILMS - INFLUENCES OF GE MICROCRYSTALS ON THE LONGITUDINAL-OPTICAL PHONONS IN SIO2

Infrared absorption in composite films of SiO2 and microcrystalline Ge has been investigated with p- and s-polarized light under 45-degrees oblique incidence. In the case of p-polarized incident light, pure SiO2 films clearly exhibit an absorption peak of the LO-phonon mode around 1240 cm-1 in addition to that of the TO mode at 1080 cm-1, due to the so-called Berreman effect. It was found that the intensity of the LO absorption relative to that of the TO absorption decreases as the volume fraction of Ge microcrystals in the film increases. Theoretical calculations based on the Bruggeman and Maxwell-Garnett effective-medium theories have been performed. The calculations made by the Bruggeman theory could successfully reproduce the observed spectra and well explain the decrease in the intensity of the LO absorption, while those made by the Maxwell-Garnett theory disagreed with the experiments.