The thermal conduction characteristics of GeTe and Ge2Sb2Te5(GST) nanowires were investigated using an optical method to determine the local temperature by Raman spectroscopy. Since the localization of surface charge in a single-crystalline nanostructure can enhance charge-phonon scattering, the thermal conductivity value (κ) of single crystalline GeTe and GST nanowires was decreased significantly to 1.44 Wm(-1) K(-1) for GeTe and 1.13 Wm(-1) K(-1) for GST, compared to reported values for polycrystalline structures. The SET-to-RESET state in single-crystalline GeTe and GST nanowires are characteristic of a memory device. Unlike previous reports using GeTe and GST nanowires, the SET-to-RESET characteristics showed a bipolar switching shape and no unipolar switching. In addition, after multiple cycles of operation, a significant change in morphology and composition was observed without any structural phase transition, indicating that atoms migrate toward the cathode or anode, depending on their electronegativities. This change caused by a field effect indicates that the structural phase transition does not occur in the case of GeTe and GST nanowires with a significantly lowered thermal conductivity and stable crystalline structure. Finally, the formation of voids and hillocks as the result of the electromigration critically degrades device reliability.

Modified amorphous GeTe, formed by the pulsed laser irradiation of as-grown GeTe, was analyzed in terms of variations in the local bonding structure using Raman spectroscopy and X-ray absorption fine structure in tandem with first-principles density functional theory. Amorphized GeTe (acquired from the crystalline phase) was compared with the modified amorphous GeTe to investigate the similarities and discrepancies between these two amorphous phases. Raman spectroscopy showed that these materials have a similar distribution of Ge-centered local structure in both phases, which is mainly composed of an octahedral-like structure. However, extended X-ray absorption fine structure results show the presence of a unique second type of Ge-Te bonding in the amorphized GeTe, which can effectively reduce the energy required for recrystallization. A computational study based on molecular dynamics simulations verified our experimental observations, including the existence of a second type of Ge-Te bonding in the amorphized phase. Moreover we distinguished the structural characteristics underlying the different amorphous phases, such as local atomic configurations and structural symmetries.

Ag-Incorporated Ge2Sb2Te5 (AGST) crystallizes faster and at a lower temperature than Ge2Sb2Te5 (GST) owing to the changes in local structure and chemical bonding.

Although Sb 2 Te 3 , as a candidate material for nextgeneration memory devices, has attractive properties such as higher operation speed and lower power consumption than Ge 2 Sb 2 Te 5 , its poor stability prevents its application to commercial memory devices. Transition metal dopants provide enhancements in its phase change characteristics, improving both thermal stability and operation energy. However, the enhancement mechanism remains to be sufficiently investigated, and standard properties need to be achieved. Herein, the phase change properties of Sb 2 Te 3 are confirmed to be enhanced by the incorporation of a heavy transition metal element such as Ag. The crystallization temperature increases by nearly 40%, and the operation energy is reduced by approximately 60%. These enhancements are associated with the changes in the local Sb 2 Te 3 structure caused by Ag incorporation. As the incorporated Ag atoms substitute Sb in the Sb−Te octahedron, this turns into a Ag−Te defective tetrahedron with a strong Ag−Te bond that induces distortion in the crystal lattice. The formation of this bond is attributed to the electron configuration of Ag and its fully filled d orbital. Thus, Ag-doped Sb 2 Te 3 is a promising candidate for practical phase change memory devices with high stability and high operation speed.