Photoinduced changes in the Urbach tail in Ge- and As-based chalcogenide glasses

The Urbach tail features of normal and obliquely deposited Ge- and As-based chalcogenide films have been investigated. A correlation of these features with composition, angle of deposition, photoinduced edge shift, average coordination number, and photoinduced volume contraction has been established. The far infrared (IR) vibrational spectra reveal that bond angle variations are the dominant features of the photoinduced structural transformation, rather than the bond stretching variations. The far IR and Urbach tail studies suggest that the strength of electron-phonon coupling is enhanced in Ge-based chalcogenide films having a very porous, columnar microstructure. Further, the coupling is maximum at the best glass forming composition of the chalcogenide