Phase Change via Intermediary Metastable Local Structure of Ge Atoms in Ge₂Sb₂Te₅Nanowires during Electrical Switching

In this study, local structural changes induced by electrical SET/RESET operations in single-crystalline Ge2Sb2Te5 (GST) nanowires (NWs) fabricated via the vapor-liquid-solid method have been investigated using nanoscale extended X-ray absorption fine structure experiments. Under the aforementioned situation, GST NWs were observed to exhibit significant alterations in morphology and composition induced by atomic migration without undergoing any structural phase transition. As the characteristics of atomic migration during memory switching operation is very different from those corresponding to thin GST film-based device switching, the modulation of the local structure during the former process has not yet been clearly understood. In order to analyze the local structure of GST NWs, special nanoscale X-ray absorption fine structure (XAFS) analysis configurations were prepared, which enabled XAFS measurements over the scale of a few hundred nanometers. The results obtained from the nanoscale XAFS experiment suggested the simultaneous formation of metastable amorphous-like tetrahedral and FCC-like structures in GST NWs during RESET operations as a consequence of atomic migration, which enabled repeating SET/RESET operations in NWs. Finally, the results obtained from the nanoscale XAFS experiments were confirmed via Raman and transmission electron microscopy analysis, thus verifying the possible phase-change process between the FCC-like and hexagonal closed packed structure through an intermediary metastable local structure in asymmetrical NW structures with low thermal conduction.