On the origin of photo-induced and thermally induced irreversible bleaching of amorphous Ge-Se films

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Abstract
Irreversible photo-induced and thermally induced changes in the optical properties of amorphous Ge31.4S68.6 thin films were studied using visible and far-infrared transmission and using Raman spectroscopy. It was found that the blue shift of the optical gap induced by illumination (+ 0.07 eV) and by annealing (up to + 0.19 eV) is accompanied by an increase in the density of heteropolar Ge-Se bonds. Simultaneously the medium-range order increases as inferred from an increase in the relative intensity of the ‘companion’ line (at about 220 cm−1) in the Raman spectra. Hence a simultaneous increase in the short- and medium-range order accompanies the blue shift of the gap. It is, however, supposed that a key role in the blue shift of the gap is played by changes in the short-range order.