PHOTOINDUCED STRUCTURAL-CHANGES IN AMORPHOUS AS2S3 AS MEASURED BY DIFFERENTIAL ANOMALOUS X-RAY-SCATTERING

Differential anomalous x-ray scattering measurements were made on a we ll-annealed and a photodarkened amorphous As2S3 film of 4 mu m in thic kness as well as on a well-annealed 30 mu m sample, all of which were supported on Si (100) wafers. The thicker sample was used to evaluate the influence of x-ray scattering from the Si wafer. From the data mea sured, no structural change was found in the first coordination shell after light exposure. There is also no significant change in the regio n around 3.5 Angstrom as far as the interatomic distances and coordina tion numbers are concerned, but a large increase in the disorder is se en, after illumination. The variation of the S apex angle for the As-S -As correlation is increased by about 2.4 degrees. It is proposed that this is the fundamental cause for the reversible photodarkening obser ved in alpha-As2S3 films. The model developed based on these results e xplains many of the observations regarding the reversible photodarkeni ng phenomenon, such as the dependence on light exposure, pressure, and the light intensity, and on the chalcogen concentration. It may also apply to other light-sensitive disordered materials. The present model establishes a clear relation between a local event, viz., the increas ed spread in the bond angle, and the matrix reorganization that affect s a longer-range order, such as that responsible for the first sharp d iffraction peak.