Temperature Dependent Evolution Of Local Structure In Chalcogenide-Based Superlattices

Interfacial phase change memory utilizing chalcogenide-based superlattices (CSLs) offers outstanding device performance and is an emerging contender to replace conventional phase change memory based on single layer materials. In this work, evolution of local structure in epitaxial GeTe/Sb2Te3 based superlattices grown on Si (111) substrates at different growing temperatures by pulsed laser deposition (PLD) is studied by a combination of atomic-resolution scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (EDX). Experimental results show the formation of single Ge-rich Ge-Sb-Te units intercalated with Sb2Te3 building units at low deposition temperatures (100-120 degrees C). However, the growth of thermodynamically stable layered Ge-Sb-Te crystal structures is observed at high growing temperatures (185-200 degrees C), while layered GeSb2Te4 thin film is formed at a deposition temperature of 220 degrees C. Atomic-resolution EDX analysis revealed disordered Ge/Sb cation layers within of Ge-Sb-Te building units. Overall, this work demonstrates that the formation of layered Ge-Sb-Te structures is thermodynamically driven process. It cannot be suppressed at high growing temperatures, regardless of deposition technique. However, diffusivity of Ge and Sb atoms can be largely restricted at low deposition temperatures. This opens a way to further optimize the microstructure of CSLs.