Structure and Photo-Induced Structural Changes in Amorphous Arsenic Sulfide by X-Ray Absorption Spectroscopy and Differential Anomalous X-Ray Scattering.

The goal in this study is to investigate the structure of amorphous arsenic sulfide and the mechanism behind the photodarkening phenomenon in both the short and intermediate structural ranges using x-ray absorption spectroscopy and x-ray differential scattering techniques. X-ray absorption spectroscopies indicate that the photodarkening in a-As _{rm x}S _{rm 1-x} (0.4 <= x <= 0.5) is related to changes in the amorphous As-S network rather than to any changes in As_4S_4 molecular units. The photo-irradiation on annealed a-As _{rm x}S _{rm 1-x} films at x = 0.4, 0.42, and 0.45 results in a more disordered state as well as the creation of As-As homopolar bonds and modifications in the network structure, such as the increase of the As -As radial distances in the second shell. In non-stoichiometric a-As_{rm x}S _{rm 1-x} (0.4 <= x <= 0.5), the presence of As_4S_4 molecules embedded in an As_2S_3 network tends to break up this network. Decoupling of the As_2S_3 network may lead to increased steric freedom which appears to be associated with photoinduced structural changes in amorphous arsenic sulfide. Differential anomalous x-ray scattering results indicate that variation in structure induced by photosoaking is strongly correlated with changing As-As atomic pair correlations. The first sharp diffraction peak is related to the intermediate range correlations which extend as far as 7.0 A. A stacked-layer structure is proposed in a-As_2S_3 . The intralayer structure is described in terms of a two dimensional random covalent network. Several layers could be stacked, most likely in a crimped and disordered fashion. And also, a mechanism for reversible photo-induced structural changes in a-As_2S _3 is proposed. Photoinduced structural changes are related to modifications in both the short range order and intermediate range order. Energy released from non-radiative recombination could lead the formation of "cross-linked" As-As homopolar bonds. Because of this, the structural configuration changes to a more disordered state and distorted lone pair electron wave correlations result in a red-shift of the optical absorption gap, or photodarkening, as well as the other photoinduced structural changes.