The reversible photostructural changes, unique to amorphous chalcogenides, are of interest since the 1970’s. The photodarkening is a shift of the optical absorption edge to lower energies and has been considered as a completely reversible photostructural change after thermal annealing to the glass-transition temperature. Accompanied by refractive index changes, the photodarkening is explored primarily for its applications in optical recordings, and many attempts to clarify its nature have been undertaken. In general, the photodarkening is explained by bond changes in chalcogenide atoms (accompanied with changes of their lone-pair electrons) which lead to increase of network disorder. However, as reviewed here in spite of the improvement of the models of the microscopic mechanism of the changes on the basis of studies utilizing modern techniques, many questions concerning the details of the photostructural process remain to be answered. Moreover, in our recent studies in ternary chalcogenide films we have found a new light annealing effect (LAE) with decreasing the film thickness to nano-sizes – after reaching a saturated value the photodarkening begins to decrease and completely disappears on prolonged enough illumination times. Further, in studies of the attendant changes of parameters of disorder we have confirmed that under such conditions the photodarkening is in competition with a photobleaching process, which orders the photodarkening-disordered structure. This finding leads to a new insight into the fundamentals of photodarkening based on the dual action of light in the photostructural process – light not only increases network disorder, but also decreases it in the disordered state. This new concept accounts for many unusual photostructural changes which up to now have been unexplained.

Keywords: chalcogenides, photoinduced changes, optical properties

   

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GEORGI NADJAKOV INSTITUTE OF SOLID STATE PHYSICS
Bulgarian Academy of Sciences
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Created: 30 August 2011
Created: 21 October 2015