Optical band-gap evolution and local structural change in Ge2Sb2Te5 phase change material

Abstract

The amorphous to crystalline phase transition in Ge2Sb2Te5 (GST) phase change material is investigated using XRD and the systematic variation in the optical band-gap (Eg ) and structural disorder (B 1/2) employing UV-Vis-NIR spectroscopy. The amorphous phase is observed to have Eg value of 0.708 eV while crystalline phase (200 °C) shows 0.442 eV. Variation in B 1/2 slope of 13.4 % is noted around the crystallization temperature (150 °C), depicting structural disorder reduction and hence structural ordering in the material forming the cubic phase. The change in the optical band-gap and local structural disorder upon crystallization occurs due to alternation in the atomic bonding configurations, which is explored using XPS technique. The findings reveal Ge-Te (~1218.35 eV binding energy) and Sb-Te (~528.8 eV) bonds in the amorphous phases. However, their bond lengths increase (hence binding energy reduces) as the annealing temperature rises, demonstrating phase switching occurs upon reaching the crystallization temperature. Insight into the optical band-gap, local structural disorder, and atomic arrangement governs many vital features of phase change material, such as fast crystallization speed, better thermal stability, high endurance, and large resistance contrast, which provide the path for non-volatile memory and nanophotonic applications.