By chemi-STEM experiments, the structural and chemical features of the major defects were observed in hexagonal GeSb2Te4, the underlying reason of their abundance and effects were also explored with DFT simulation. This work was accepted by Chemistry of Materials on 2nd July 2018.

Phase-change materials GeSbTe (GST) are one of the most promising candidates for next generation storage and computing technology. In particular, the hexagonal phase, which extends three orders of magnitude of resistance window, is claimed to achieve multi-level storage. Recently, by transmission electron microscopy experiments, many defects in hexagonal GeSbTe compounds has been observed. However, the underlying reasons why these defects can form and their effects on the electronic and transport properties still remain unclear.

In this work, by combining spherical aberration corrected (Cs-corrected) high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and the energy-dispersive X-ray mapping (EDX) experiments, the atomic structure and chemistry of hexagonal GeSbTe was investigated. The existence of these defects and their chemical composition were determined for the first time. To study their effects on transport properties, the supercells up to 1701 and 1638 atoms were built and performed large-scale DFT calculations. The results show that no localization below Fermi level will be induced by any of these defects. All these defects may act as possible sources for electron scattering, affecting the quantitative value of electrical resistance in hex-GST, but cannot lead to any qualitative change in transport character.

This work is supported by National Natural Science Foundation of China (61774123 and 51621063), the Youth Thousand Talents Program of China and the Young Talent Support Plan of Xi'an Jiaotong University.

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