Atomistic observation of photo-expansion and photo-contraction in chalcogenide films by in situ EXAFS

In situ EXAFS measurements have been performed to study the changes in the local structure of a-As2S3 (around As and S K-edges) and a-GeSe2 (around Ge and Se K-edges) thin films. To study the effect of illumination on the local structure and environment, the structural parameters (nearest neighbor distances, Debye–Waller factor) are estimated for as-deposited films, during illumination and after illumination for both these compositions. For As2S3 films, there is an expansion in the sulfur nearest neighbor distances and a small contraction in the Arsenic nearest neighbor distance during illumination. For GeSe2 films, both Ge and Se show a contraction in the nearest neighbor distances by illumination. Ordering of the structure (a decrease in the Debye Waller factor) is seen with illumination in both the compositions. a-As2S3 films show larger photo-induced changes and also a larger transient part in the changes as compared to a-GeSe2 films, which is suggested to be due to a more 3d rigid structure in GeSe2. Larger changes are seen around the chalcogen atom as compared to As and Ge atoms. From the results, we are able to confirm photo-expansion in As2S3 films and photo-contraction in GeSe2 at an atomistic level.