Infrared absorption in SiO2-Ge composite films: Influences of Ge microcrystals on the longitudinal-optical phonons in SiO2.

Abstract

Infrared absorption in composite films of ${\mathrm{SiO}}_{2}$ and microcrystalline Ge has been investigated with p- and s-polarized light under 45\ifmmode^\circ\else\textdegree\fi{} oblique incidence. In the case of p-polarized incident light, pure ${\mathrm{SiO}}_{2}$ films clearly exhibit an absorption peak of the LO-phonon mode around 1240 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in addition to that of the TO mode at 1080 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, 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.