Polymorphism of GeSbTe Superlattice Nanowires

Scaling-down of phase change materials to a nanowire (NW) geometry is critical to a fast switching speed of nonvolatile memory devices. Herein, we report novel composition-phase-tuned GeSbTe NWs, synthesized by a chemical vapor transport method, which guarantees promising applications in the field of nanoscale electric devices. As the Sb content increased, they showed a distinctive rhombohedral–cubic–rhombohedral phase evolution. Remarkable superlattice structures were identified for the Ge₈Sb₂Te₁₁, Ge₃Sb₂Te₆, Ge₃Sb₈Te₆, and Ge₂Sb₇Te₄ NWs. The coexisting cubic–rhombohedral phase Ge₃Sb₂Te₆ NWs exhibited an exclusively uniform superlattice structure consisting of 2.2 nm period slabs. The rhombohedral phase Ge₃Sb₈Te₆ and Ge₂Sb₇Te₄ NWs adopted an innovative structure; 3Sb₂ layers intercalated the Ge₃Sb₂Te₆ and Ge₂Sb₁Te₄ domains, respectively, producing 3.4 and 2.7 nm period slabs. The current–voltage measurement of the individual NW revealed that the vacancy layers of Ge₈Sb₂Te₁₁ and Ge₃Sb₂Te₆ decreased the electrical conductivity.