Competing local orders in liquid and amorphous structures of Ge2Sb2Te5: Influence of exchange-correlation functional (original) (raw)
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Physical Review B, 2011
The as-deposited (AD) amorphous structure of the prototype phase change material Ge 2 Sb 2 Te 5 (GST-225) has been studied by density functional calculations for a 648-atom sample generated by computer-aided deposition at 300 K. The AD sample differs from a melt-quenched (MQ) sample in essential ways: (1) Ge atoms are predominantly tetrahedrally coordinated, and (2) homopolar and Ge-Sb bonds are more common and reduce the number of ABAB squares (A = Ge, Sb; B = Te), the characteristic building blocks of the material. The first observation resolves the contradiction between measured (EXAFS) and calculated Ge-Te bond lengths, and the latter explains the very different crystallization speeds. Sb and Te have higher chemical coordination than suggested by the "8-N rule" of covalent networks (N is the number of valence electrons). The EXAFS signal calculated for AD agrees much better with experiment than that calculated for MQ.
Polymorphism in amorphous Ge2Sb2Te5: comparison of melt-quenched and as-deposited structures
Atomistic simulations on phase-change materials have focused on melt-quenched (MQ) samples, and both system size and quench time have posed challenges. We present here results of massively-parallel density functional (DF) simulations of the as-deposited (AD) amorphous structure of the prototype phase change material Ge 2 Sb 2 Te 5 . We have studied a 648-atom sample generated by computer-aided deposition at 300 K and compare the results with those for a 460-atom MQ sample we reported previously. The AD structure differs from MQ in essential ways: (1) Ge atoms are predominantly tetrahedrally coordinated, (2) homopolar and Ge-Sb bonds are more common and reduce the number of ABAB squares (A=Ge,Sb; B=Te), the characteristic building blocks of the material. The first observation resolves the contradiction between measured and calculated Ge-Te bond lengths, and the latter explains the large differences in crystallization speeds. Sb and Te are more highly coordinated than expected from the "8-N rule" (N is the number of valence electrons), and a-GST cannot be regarded as a covalent network glass.
Crystallization of amorphous Ge2Sb2Te5: Order from disorder
2012
The rate-limiting process in phase change (PC) optical memories is the extremely rapid (nanosecond time scale) crystallization of nanosized amorphous “marks” in a polycrystalline layer. Our knowledge of the amorphous and ordered structures of Ge/Sb/Te and Ag/In/Sb/Te alloys has improved significantly in recent years and has led to plausible pictures for the transition between them, but the simulation of the actual crystallization process is complicated by the need to study large numbers of atoms over time scales that are difficult to attain, even with modern supercomputers. We have performed density functional/ molecular dynamics (DF/MD) simulations on a sample of Ge2Sb2Te5 (GST-225) with 460 atoms at 500 K, 600 K, and 700 K for up to 600 picoseconds. Crystallization has been promoted by fixing the structure of a crystalline "seed" (4x4x4 sites, 58 atoms, 10% vacancies). We present a progress report on the results and on a simulation of 648 atoms with a similar “seed” at 6...
A concerted rational crystallization/amorphization mechanism of Ge2Sb2Te5
Journal of Non-Crystalline Solids, 2009
Reverse Monte Carlo refinements using electron diffraction data and density functional theory calculations of the local atomic structure of amorphous Ge 2 Sb 2 Te 5 confirm presence of a noticeable number of four-membered rings with the general Ge(Sb)TeGe(Sb)Te composition similar to the building blocks of its cubic crystalline phase. The persistence of these rings, as well as the presence of the medium range order at the scale of about 1 nm, suggests that the amorphization/crystallization transition in Ge 2 Sb 2 Te 5 can be modelled with a concerted rotation of the sheets of atom-squares in {1 0 0} faces of cubic subcells of the cubic crystalline phase, similar to Rubik's cube rotation. This mechanism can produce large models of material that agree with a range of the previous experimental and theoretical studies and also with the experimental electron diffraction data.
Applied Physics Letters, 2006
The three-dimensional atomic configuration of amorphous Ge 2 Sb 2 Te 5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge 2 Sb 2 Te 5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four-and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd-and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge 2 Sb 2 Te 5 is the key for the fast crystallization speed of the material.
Advanced Functional Materials, 2012
We revealed for the first time, by modelling the amorphous structure of Ge 2 Sb 2 Te 5 based on synchrotron radiation anomalous X-ray scattering data, that germanium and tellurium atoms form a "core" Ge-Te network with ring formation. It is also suggested that the Ge-Te network can stabilize the amorphous phase at room temperature and can persist in the crystalline phase. On the other hand, antimony does not contribute to ring formation but constitutes a "pseudo" network with tellurium, in which the characteristic Sb-Te distance is somewhat longer than the covalent Sb-Te bond distance. This suggests that the Sb-Te pseudo network may act as a precursor to forming critical nuclei during the crystallization process. The findings conclude that the Ge-Te core network is responsible for the outstanding stability and rapid phase change of the amorphous phase while the Sb-Te pseudo network is responsible for triggering critical nucleation.
Local bonding arrangements in amorphous Ge2Sb2Te5: the importance of Ge and Te bonding
Journal of Materials Science: Materials in Electronics, 2007
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EXAFS study of local order in the amorphous chalcogenide semiconductor Ge2Sb2Te5
Journal of Physics and Chemistry of Solids, 2007
Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently switching of the achalcogenide Ge 2 Sb 2 Te 5 has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching provides researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory -or BCT -provides a powerful framework for understanding the structure and properties of a-materials. Applications of BCT to switching in Ge 2 Sb 2 Te 5 holds the promise of finding the best composition suited for switching applications. This work presents EXAFS data that describe local bonding configurations in as-deposited Ge 2 Sb 2 Te 5 . The data show that Ge 2 Sb 2 Te 5 may best be viewed as a random array of Ge 2 Te 3 and Sb 2 Te 3 structural units imbedded in a tissue of a-Te, 17% of which is over-coordinated. In addition, a valence alternation pair defect is introduced to the model to satisfy charge conservation constraints.
Applied Physics Letters, 2006
The three-dimensional atomic configuration of amorphous Ge 2 Sb 2 Te 5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge 2 Sb 2 Te 5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four-and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd-and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge 2 Sb 2 Te 5 is the key for the fast crystallization speed of the material.