Structural Origin of Optical Band Edge Shift in Se70Ge30-xBix (0≤x≤10) Glassy Alloys

Abstract

Bulk glasses of Se−Ge-Bi system have been prepared by quenching the melt. The XRD exhibits very weak peaks riding over halos. DSC scans show a single stage to a multistage crystallization with an incremental substitution of Ge by Bi, indicating evolution of structurally inhomogeneous phases at the molecular level with the increasing content of Bi. The Raman spectra (50-500 cm-1) reveal the formation of the corner and edge-sharing GeSe4/2terahedral and Bi2Se3 pyramidal units. Transmission electron microscope (TEM) Images depicts the formation of submicron to nano-sized spherical shaped structural units in a chain-like configuration. Fragmentation of chains and refinement effect of Bi are noticed in TEM and SEM Images. The optical absorption edge found to be red-shifted along with a sharp decrease in optical band gap (1.74-0.82), with the increasing fraction of Bi in alloys. Attempts have been made to elucidate the results on the structural considerations (bond and band diagram) and Burstein-Moss effect. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim