Optical properties of amorphous Ge1-xSex and Ge1-x-ySexAsy thin films - optical gap bowing and phonon modes

Abstract

We investigated the optical properties of Ge1−xSex and Ge1−x−ySexAsy amorphous films by usingspectroscopic ellipsometry and Raman spectroscopy. The dielectric functions and absorptioncoefficients () of the amorphous films were determined from the measured ellipsometric angles( ,). We obtained the optical gap energies and the Urbach energies from the absorption coefficientsand found a strong bowing effect in the optical gap energy of Ge1−x−ySexAsy, where theendpoint binaries were Ge0.50Se0.50 and Ge0.31As0.69. Based on the correlation between the opticalgap energies and the Urbach energies, we attributed the large bowing parameter to electronic disorder. Using Raman spectroscopy, we measured the phonon modes and discussed the compositiondependence of the phonon peak frequencies and lineshapes in terms of structural units. Based onthe composition dependence of the phonons in Ge1−x−ySexAsy, we identified the phonon modes ofGe0.31As0.69. A resonant Raman phenomenon was observed in Ge0.40Se0.60 at a laser excitationof 514 nm (2.41 eV). We verified that this laser energy corresponded to the transition energy ofGe0.40Se0.60 by using the second derivative of the dielectric function of Ge0.40Se0.60.