In Situ EXAFS Study of the Photoexcited State and Defects in Chalcogenide Glasses
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oinduced changes in optical absorption are caused by changes in structure. The first direct proof of réversible structural changes was given by K. Tanaka,' 9 who demonstrated that the first peak ("first sharp diffraction peak" or FSDP) in the x-ray diffraction pattern underwent réversible changes on illumination and subséquent annealing.
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Figure 1. Réversible photoinduced shift of the absorption edge in AS2S3. On irradiation, the edge shifts from state 1 (well-annealed As2S3 film at ~180°C) to 2 (irradiated; exposed to 4880 A light); upon annealing, the absorption edge returns to state 1. Température is 14 K52
The phenomenology of the effect is well-described within a configurationcoordinate model assuming a doublewell potential for the ground state.20"22 The nanoscale mechanism however has long remained unclear despite numerous efforts to relate expérimental observations to defect models, since the nature of changes is local and dynamical and thus cannot be probed by spatially averaged and time-averaged techniques. Another difficulty was détermination of the structure of an amorphous solid with the needed précision. The reader is referred to récent reviews 23-25 for further détails. A slight réversible change in the Raman spectra of a n n e a l e d and illuminated As 2 S 3 films was attributed to création of a small number (about 6%) of "wrong" As-As bonds after illumination. 26 Studies using extended x-ray absorption fine structure (EXAFS), which is a local probe technique, performed by various groups, confirmed the formation of like-atom bonds on illumination of'As 2 S.i glass (with a somewhat smaller c h a n g e of about 1%).27'28 This could not however account for the photodarkening, which has also been observed in pure sélénium, where "wrong" bonds cannot exist. A change in the bond angle subtended at sulfur atoms was also detected by EXAFS, accompanied by a simultaneous change in the dihedral angle between the adjacent AsSe 3 pyramids. 27 The interprétation of previous EXAFS data was not straightforward. Firstly, the chalcogenides studied were mostly binary glasses. Elemental chalcogens, such as sulfur or sélénium, which also exhibit réversible photodarkening, 6 " 8 hâve basically not been studied. The only reported EXAFS work on amorphous sélénium (a-Se)29 evidenced that disorder in the material increases upon light irradiation. Secondly, ail previous measurements were performed ex situ. A film kept under light irradiation during the measurement has never been studied. As a resuit, information about the excited state and, hence, about the mechanism of the structural change was missing. The dynamical nature of synchrotron radiation probes has enabled study of such metastable local structure during photoexcitation and s u b s é q u e n t decay of the excited state. In this report, we review in situ EXAFS investigation of photostructural changes in a-Se which elucidated the nanoscale mechanism.
In Situ EXAFS of the Photoexcited State EXAFS is a dynamical
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