Real-space imaging of atomic arrangement and vacancy layers ordering in laser crystallised Ge2Sb2Te5 phase change thin films

Lotnyk, Andriy; Bernütz, Sabine; Sun, Xun; Roß, Ulrich; Ehrhardt, Martin; Rauschenbach, B.

doi:10.1016/j.actamat.2015.12.010

Cited by 88 publications

(54 citation statements)

References 49 publications

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“…Such tetrahedral bonding of Ge surrounded by Vcs has been conjectured some time ago also for the bulk of the GST-225 rocksalt structure by using electron microscopy and diffraction data in combination with DFT results [40]. However, this interpretation has not been confirmed in subsequent works using TEM [31,41,42], XRD [43,44], extended x-ray absorption fine structure (EXAFS) [45,46], and more refined DFT calculations, partially combined with molecular dynamics (MD) simulations [47,48]. Interestingly, a nuclear magnetic resonance (NMR) study provided some hints for the presence of tetrahedral Ge within a so-called nanocrystalline GST-225 phase, but not in its microcrystalline counterpart.…”

Section: Introductionmentioning

confidence: 57%

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

et al. 2017

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Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase-change material Ge 2 Sb 2 Te 5 as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless permits the detection of the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable Ge 2 Sb 2 Te 5 . First, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of 10 −4 nm −2 are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the buildup of vacancy layers.

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Section: Introductionmentioning

confidence: 57%

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

et al. 2017

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“…However, there is controversies with respect to the vacancy distribution, which was reported as a random distribution based on the X-ray diffraction (XRD) data1516, but some theoretical studies predicted that the vacancies could be highly ordered and even form layers on the (111) planes814 or be more complicated18. Recently, it was proposed that vacancy ordering in cubic GST may be controlled by thermal annealing319 or electronic beam radiation20, and a vacancy semi-ordered21 cubic GST has been observed. The vacancy distribution affects not only the structural stability and the phase transition behavior814, but also the physical properties, such as the electronic3 and thermal19 transports.…”

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confidence: 99%

Vacancy Structures and Melting Behavior in Rock-Salt GeSbTe

Zhang

Wang

Shen

et al. 2016

Sci Rep

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Ge-Sb-Te alloys have been widely used in optical/electrical memory storage. Because of the extremely fast crystalline-amorphous transition, they are also expected to play a vital role in next generation nonvolatile microelectronic memory devices. However, the distribution and structural properties of vacancies have been one of the key issues in determining the speed of melting (or amorphization), phase-stability, and heat-dissipation of rock-salt GeSbTe, which is crucial for its technological breakthrough in memory devices. Using spherical aberration-aberration corrected scanning transmission electron microscopy and atomic scale energy-dispersive X-ray mapping, we observe a new rock-salt structure with high-degree vacancy ordering (or layered-like ordering) at an elevated temperature, which is a result of phase transition from the rock-salt phase with randomly distributed vacancies. First-principles calculations reveal that the phase transition is an energetically favored process. Moreover, molecular dynamics studies suggest that the melting of the cubic rock-salt phases is initiated at the vacancies, which propagate to nearby regions. The observation of multi-rock-salt phases suggests another route for multi-level data storage using GeSbTe.

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“…It is known that the metastable (cubic) GST225 phase is formed before the formation of stable (trigonal) GST225 phase. In the cubic GST225 crystal structure, the Ge, Sb and vacancies are randomly distributed with different Ge/Sb ratios per cation sites24. The transition from cubic to trigonal phase begins by ordering of vacancies into vacancy layers separated by GST building blocks containing 4 and 5 Te layers25.…”

Section: Resultsmentioning

confidence: 99%

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

Lotnyk

Roß

Bernütz

et al. 2016

Sci Rep

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Insights into the local atomic arrangements of layered Ge-Sb-Te compounds are of particular importance from a fundamental point of view and for data storage applications. In this view, a detailed knowledge of the atomic structure in such alloys is central to understanding the functional properties both in the more commonly utilized amorphous–crystalline transition and in recently proposed interfacial phase change memory based on the transition between two crystalline structures. Aberration-corrected scanning transmission electron microscopy allows direct imaging of local arrangement in the crystalline lattice with atomic resolution. However, due to the non-trivial influence of thermal diffuse scattering on the high-angle scattering signal, a detailed examination of the image contrast requires comparison with theoretical image simulations. This work reveals the local atomic structure of trigonal Ge-Sb-Te thin films by using a combination of direct imaging of the atomic columns and theoretical image simulation approaches. The results show that the thin films are prone to the formation of stacking disorder with individual building blocks of the Ge2Sb2Te5, Ge1Sb2Te4 and Ge3Sb2Te6 crystal structures intercalated within randomly oriented grains. The comparison with image simulations based on various theoretical models reveals intermixed cation layers with pronounced local lattice distortions, exceeding those reported in literature.

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Real-space imaging of atomic arrangement and vacancy layers ordering in laser crystallised Ge2Sb2Te5 phase change thin films

Cited by 88 publications

References 49 publications

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

Vacancy Structures and Melting Behavior in Rock-Salt GeSbTe

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

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