On the mechanism of photostructural changes in As-based vitreous chalcogenides microscopic, dynamic and electronic aspects

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Abstract
Experimental evidence for microscopic changes of the structure of As-based vitreous chalcogenides on illumination (Raman and extended X-ray absorption fine structure spectroscopies) are summarized. The important role of mechanical strain accumulated in chalcogenides for the photostructural changes is demonstrated. A unified model is presented which shows how As-As bond creation in the strained regions can lead to opening up of the valence angle subtended at the bridging chalcogen atom accompanied by a simultaneous change in the dihedral angle between AsS3 pyramids, and why these changes result in photodarkening. The effect of mechanical strain is discussed within the configurational approach comparing photostructural changes in bulk glass and thin films as well as stoichiometric and As-rich films. Electronic processes reponsible for the photostructural changes are also considered.