Optical Gap Bowing and Phonon Modes of Amorphous Ge1-x-ySexAsy Thin Films

We investigated the optical properties of Ge1-xSex and Ge1-x-ySexAsy amorphous semiconductor films using spectroscopic ellipsometry and Raman spectroscopy. The dielectric functions and absorption coefficients of the amorphous films were determined from the measured ellipsometric angles. We obtained the optical gap energies and Urbach energies from the absorption coefficients, and found a strong bowing effect in the optical gap energy of Ge1-x-ySexAsy where the endpoint binaries were Ge0.50Se0.50 and Ge0.31As0.69. Based on the correlation between optical gap energies and Urbach energies, the large bowing parameter was attributed to the electronic disorder. We found the composition dependence of several phonon modes using Raman spectroscopy. For Ge1-x-ySexAsy, the D mode (232-267 cm-1) changed from As-As (or As3 pyramid), to As(Se1/2)3 pyramid, and finally to Se clusters, as the Se composition increased. Resonant Raman phenomenon was observed in Ge0.38Se0.62 at a laser excitation of 514 nm (2.41 eV). We verified that this laser energy corresponds to the transition energy of Ge0.38Se0.62 using the second derivative of the dielectric function of Ge0.38Se0.62.