Optical properties of amorphous Ge_1-xSe_x and Ge_1-x-ySe_xAs_y thin films - optical gap bowing and phonon modes

We investigated the optical properties of Ge_1-xSe_x and Ge_1-x-ySe_xAs_y amorphous films by using spectroscopic ellipsometry and Raman spectroscopy. The dielectric functions and absorption coefficients (α) of the amorphous films were determined from the measured ellipsometric angles (Ψ,Δ). We obtained the optical gap energies and the Urbach energies from the absorption coefficients and found a strong bowing effect in the optical gap energy of Ge_1-x-ySe_xAs_y, where the endpoint binaries were Ge_0.50Se_0.50 and Ge_0.31As_0.69. Based on the correlation between the optical gap 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 composition dependence of the phonon peak frequencies and lineshapes in terms of structural units. Based on the composition dependence of the phonons in Ge_1-x-ySe_xAs_y, we identified the phonon modes of Ge_0.31As_0.69. A resonant Raman phenomenon was observed in Ge_0.40Se_0.60 at a laser excitation of 514 nm (2.41 eV). We verified that this laser energy corresponded to the transition energy of Ge_0.40Se_0.60 by using the second derivative of the dielectric function of Ge_0.40Se_0.60.