In Situ Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary mGeTe·Bi₂Te₃ (m = 3-8) Nanowires and First-Principles Calculations.
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.
†Department of Chemistry, Korea University, Jochiwon 339-700, Korea.
Chan Su
CS
Jung
†Department of Chemistry, Korea University, Jochiwon 339-700, Korea.|‡Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea.
Han Sung
HS
Kim
†Department of Chemistry, Korea University, Jochiwon 339-700, Korea.
Hyung Soon
HS
Im
†Department of Chemistry, Korea University, Jochiwon 339-700, Korea.
Kidong
K
Park
†Department of Chemistry, Korea University, Jochiwon 339-700, Korea.
Jeunghee
J
Park
‡Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea.
Jae-Pyoung
JP
Ahn
§Nano-Bio Electron Microscopy Research Group, Korea Basic Science Institute, Daejeon 305-806, Korea.
Seung Jo
SJ
Yoo
§Nano-Bio Electron Microscopy Research Group, Korea Basic Science Institute, Daejeon 305-806, Korea.
Jin-Gyu
JG
Kim
⊥Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea.
Jae Nyeong
JN
Kim
⊥Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea.|#Department of Physics and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea.
Ji Hoon
JH
Shim
GeTe·Bi2Te3; Phase-change materials; electrical conductivity; nanowires; superlattice