A systematic evolution of optical band gap and local ordering in Ge1Sb2Te4 and Ge2Sb2Te5 materials revealed by in situ optical spectroscopy

Abstract

A unique amalgam of fascinating potentials enabled the usage of phase change materials in rewritable optical and electrical storage technology. Furthermore, the identification of stoichiometric tuning of disorder in these materials opened up new pathways for the selection of materials for multi-bit data storage. Despite a reasonable understanding of structure and properties, an in situ study could aid in unravelling vital insights. Hence, we report on an analogy between the degree of disorder in Ge1Sb2Te4 and Ge2Sb2Te5 materials by employing novel temperature-dependent correlations of optical band gap (E g), Tauc parameter (measure of disorder, B 1/2 slope) and the change in band tail. E g decreases from 0.8 eV (90 K) to 0.34 eV (480 K) for Ge1Sb2Te4 and from 0.82 eV (90 K) to 0.36 eV (480 K) for Ge2Sb2Te5. This trend reveals the semiconducting nature of both amorphous and crystalline phases. The change in disorder as exemplified by an increase in B 1/2 of 2.7% and 14.9% during the phase transition of Ge1Sb2Te4 and Ge2Sb2Te5 respectively reveals a stronger degree of disorder in the cubic phase of Ge1Sb2Te4 as compared to the cubic phase of Ge2Sb2Te5. These findings are strongly supported by an unambiguous change in the width of the band tail. This confirms the origin of a pronounced disorder-induced localization in Ge1Sb2Te4 compared to the Ge2Sb2Te5 material. © 2018 IOP Publishing Ltd.