Optical Absorption and Structural Transformations in Arsenic Selenide Films

Abstract

Optical absorption has been studied as a function of composition and temperature for poorly investigated Se-rich As _x Se_1 − x noncrystalline alloys. The samples were thin amorphous films and bulk glasses. It was shown for the first time that the absorption above the fundamental (Urbach) edge follows the Tauc law. The only exception to this law is pure selenium, for which a linear dependence of absorption coefficient on photon energy holds. The Tauc law is valid over a substantial range of absorption.

Although several reviews have appeared on various properties and applications of chalcogenide glasses, there is no thorough study of local atomic structure and its modification for Se-rich amorphous As _x Se_1 − x (a-As _x Se_1 − x ). This chapter is concerned with this problem. Structural transformations are examined by Raman scattering measurements of amorphous Se-rich As _x Se_1 − x (0 ≤ x ≤ 0. 2) alloys. It is found that the molecular structure of amorphous Se (a-Se) on the scale of medium-range order differs from the structure of most inorganic glasses and may be placed between three-dimensional (3D) network glasses and polymeric ones. Further experiments show the existence of successive phases in laser-induced glass–crystalline transition with pronounced threshold behaviour. By comparing peak width, peak location and Raman intensity in the range of bond modes, it is derived that the changes occur non-monotonically with increasing As content. The composition-induced changes of the spectra are explained by crosslinking of Se chains. Under laser irradiation, the changes in the optical transmission, holographic recording properties and Raman spectra of a-As _x Se_1 − x films with 0 < x ≤ 0. 2 have been examined. The dependence of the transmissivity and diffraction efficiency on the irradiation energy density shows two qualitatively different regions. Below the energy density threshold E _th only small changes in the local structure of the system can be detected. In the low-energy region, transient changes in transmissivity are observed. This behaviour may be explained qualitatively by associating such changes with alternation of deep defect states. Above E _th, the changes were attributed to crystallisation transformation. The corresponding Raman spectra reveal transformation of the system from amorphous phase to the crystalline phase under laser irradiation.