In situ thermal behavior of resistance drift in GeTe and Ge2Sb2Te5 nanowires via Raman thermometry†
Time-dependent resistance drift in chalcogenide phase change materials is known to cause instabilities in phase-change random access memories (PRAMs). In this study, the resistance drift of the RESET state of GeTe and Ge2Sb2Te5 (GST) nanowires during memory switching was investigated using an optical method to determine the relationship between variation in resistance and time-dependent thermal properties. Changes in morphology and crystal structure were caused by the formation of voids and hillocks in nanowires in the RESET state, which affected the phonon scattering in GeTe and GST nanowires. Consequently, the thermal conduction of the GeTe and GST nanowires in the RESET state was observed to be significantly lower than that in the initial single-crystalline state. Further, it was observed that as the annealing duration of the retention test was increased, the resistance decreased and thermal conduction increased, even though the morphologies of the nanowires were restored to the initial state. These changes were induced by thermally assisted atomic migration, indicating the occurrence of a local structural phase transition in the GeTe and GST nanowires via the formation of voids and hillocks. Our results explain the cause behind the resistance drift observed in GeTe and GST nanowires in the RESET state with bi-polar memory switching characteristics.