Direct Evidence for a Systematic Evolution of Optical Band Gap and Local Disorder in Ag, In Doped Sb2Te Phase Change Material

Abstract

Rapid and reversible switching properties of Ag, In doped Sb2Te (AIST) phase change material is widely used in re-writable optical data storage applications. We report here a systematic evolution of optical band gap (Eg), local disorder (Tauc parameter, β), and Urbach energy (EU) of AIST material during amorphous to crystalline transition using in situ UV–Vis–NIR spectroscopy. Unlike GeTe-Sb2Te3 (GST) family, AIST material is found to show unique characteristics as evidenced by the presence of direct forbidden transitions. Crystallization is accompanied by a systematic reduction in Eg from 0.50 eV (as-deposited amorphous at 300 K) to 0.18 eV (crystalline at 300 K). Moreover, decrease in EU (from 272 to 212 meV) and β is also observed during increasing the temperature in the amorphous phase, revealing direct observation of enhancement of the medium-range order and distortion in short range order, respectively. These findings of optical transition would be helpful for distinguishing the unique behavior of AIST material from GST family. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim