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In Situ Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary mGeTe·Bi2Te3 (m = 3–8) Nanowires and First-Principles Calculations

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journal contribution
posted on 10.06.2015, 00:00 by Chan Su Jung, Han Sung Kim, Hyung Soon Im, Kidong Park, Jeunghee Park, Jae-Pyoung Ahn, Seung Jo Yoo, Jin-Gyu Kim, Jae Nyeong Kim, Ji Hoon Shim
Phase-change nanowires (NWs) have emerged as critical materials for fast-switching nonvolatile memory devices. In this study, we synthesized a series of mGeTe·Bi2Te3 (GBT) pseudobinary alloy NWsGe3Bi2Te6 (m = 3), Ge4Bi2Te7 (m = 4), Ge5Bi2Te8 (m = 5), Ge6Bi2Te9 (m = 6), and Ge8Bi2Te11 (m = 8)and investigated their composition-dependent thermal stabilities and electrical properties. As m decreases, the phase of the NWs evolves from the cubic (C) to the hexagonal (H) phase, which produces unique superlattice structures that consist of periodic 2.2–3.8 nm slabs for m = 3–8. In situ temperature-dependent transmission electron microscopy reveals the higher thermal stability of the compositions with lower m values, and a phase transition from the H phase into the single-crystalline C phase at high temperatures (400 °C). First-principles calculations, performed for the superlattice structures (m = 1–8) of GBT and mGeTe·Sb2Te3 (GST), show an increasing stability of the H phase (versus the C phase) with decreasing m; the difference in stability being more marked for GBT than for GST. The calculations explain remarkably the phase evolution of the GBT and GST NWs as well as the composition-dependent thermal stabilities. Measurement of the current–voltage curves for individual GBT NWs shows that the resistivity is in the range 3–25 mΩ·cm, and the resistivity of the H phase is lower than that of the C phase, which has been supported by the calculations.