Photostructural change in the Urbach tail in chalcogenide glasses

Absorption properties in the Urbach tail range were studied in relation to the photostructural change (PSC) or the photodarkening effect in amorphous-chalcogenide semiconductors. A reversible absorption change in the Urbach tail of amorphous-chalcogenide films due to photoirradiation and heat treatment is first reported. The Urbach tail is relaxed by photoirradiation and recovered by heat treatment, an effect which is associated with the effect of the photoenhanced far-infrared band broadening in amorphous-chalcogenide films. Temperature dependence of the absorption is found to be closely associated with the frequency of the dominant far-infrared absorption of the vibration mode. It is indicated that optically excited randomness, due to the photostructural change, contributes to the photoenhanced relaxation of the exponential tail slope. Analysis of the tail slope parameter reveals strong electron-phonon couplings at the Ge-Se (S) or As-Se (S) bond. The photostructural change physical mechanism is discussed, and the frozen-in fictive temperature concept is found adequate.