ON THE MECHANISM OF PHOTOSTRUCTURAL CHANGES IN AS-BASED VITREOUS CHALCOGENIDES - MICROSCOPIC, DYNAMIC AND ELECTRONIC ASPECTS

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 importa nt role of mechanical strain accumulated in chalcogenides for the phot ostructural changes is demonstrated. A unified model is presented whic h shows how As-As bond creation in the strained regions can lead to op ening up of the valence angle subtended at the bridging chalcogen atom accompanied by a simultaneous change in the dihedral angle between As S3 pyramids, and why these changes result in photodarkening. The effec t of mechanical strain is discussed within the configurational approac h comparing photostructural changes in bulk glass and thin films as we ll as stoichiometric and As-rich films. Electronic processes reponsibl e for the photostructural changes are also considered.