Publisher's Note: Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change material<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Ge</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>Sb</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>Te</mml:mtext></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>[Phys

Akola, Jaakko; Jones, R.; Kohara, Shinji; Kimura, S.; Kobayashi, K.; Takata, Masaki; Matsunaga, Toshiyuki; Kojima, Rie; Yamada, Naofumi

doi:10.1103/physrevb.80.029902

Cited by 23 publications

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“…Without angular constraints, RMC simulations can produce configurations with a broad scatter of bond angles and incorrect electronic properties, such as a vanishing band gap in a semiconductor. 22 The angular distributions show several distinctive features: Te has a maximum at 95 • and a smaller one near 180 • corresponding to a defective octahedral environment, whereas the helical chains in c-Te have angles of 103.2 • (2 + 4 coordination). Ga is peaked strongly at the tetrahedral value (109.5 • ), and Ge shows a broader maximum at 105 • with extended tails on both sides.…”

Section: A Xrd Nd and Rmc Analysismentioning

confidence: 99%

Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained densi

et al. 2012

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The atomic structure and electronic and vibrational properties of glassy Ga 11 Ge 11 Te 78 have been studied by combining density functional (DF) simulations with x-ray (XRD) and neutron diffraction (ND), extended x-ray absorption fine structure (EXAFS), and Raman spectroscopies. The final DF structure (540 atoms) was refined using reverse Monte Carlo methods to reproduce the XRD and ND data as well as Ge and Ga K-edge EXAFS spectra, while maintaining a semiconducting band gap and a total energy close to the DF minimum. The local coordination of Ga is tetrahedral, while Ge has twice as many tetrahedral as defective octahedral configurations. The average coordination numbers are Ga, 4.1, Ge, 3.8, and Te, 2.6. The chemical bonding around Ga involves Ga 4s, Ga 4p, Te 5s, and Te 5p orbitals, and the bond strengths show bonding close to covalent, as in Ge. There are fewer Te chains and cavities than in amorphous Te, and a prepeak in the structure factor at 1.0Å −1 indicates medium-range order of the Ga/Ge network. Density functional calculations show that contributions of Te-Te, Ga-Te, and Ge-Te bonds dominate the experimental Raman spectra in the 110-150 cm −1 range.

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Section: A Xrd Nd and Rmc Analysismentioning

confidence: 99%

Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained densi

et al. 2012

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“…The total coordination numbers for AD are Ge: 4.2, Sb: 3.7, and Te: 2.8. Earlier work on MQ samples [16,24,27] indicated that the total coordination numbers of Sb and Te in a-GST do not satisfy the ''8-N rule'' of covalently bonded networks, and additional support for this conclusion is found by calculating the bond orders (number of chemical bonds) for each interatomic connection [26].…”

Section: As-deposited Vs Melt-quenched Gst-225mentioning

confidence: 92%

“…The AD structure of a-GST (648 atoms, over 200 ps) differs from MQ (460 atoms, [27]) in essential ways: (1) The environment of Ge atoms is predominantly tetrahedral in AD and disordered octahedral in MQ, and our results for the Ge-Te bond lengths agree with EXAFS measurements [8,29,30]. This resolves the contradiction between measured and bond lengths and those found in earlier DF calculations, (2) homopolar and Ge-Sb bonds are more common and reduce the number of ABAB squares, which are needed for rapid crystallization.…”

Section: As-deposited Vs Melt-quenched Gst-225mentioning

confidence: 99%

“…Experimental samples are often deposited on a substrate, and the structure is likely to differ. We have developed a method to generate an ''as deposited'' (AD) sample with 648 atoms at 300 K [26] and have compared the results with those for a 460-atom MQ sample, refined as described above with respect to XRD and XPS data [27].…”

Section: As-deposited Vs Melt-quenched Gst-225mentioning

confidence: 99%

“…13(c,d)]. The structure of the MQ sample [27] does not correspond to a DF energy minimum, so we performed a simulation at 300 K using the same functional (PBEsol [28]) and atomic density as for AD. Initialization (5 ps, 300 K) was followed by data collection (35 ps).…”

Section: As-deposited Vs Melt-quenched Gst-225mentioning

confidence: 99%

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Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

Akola

Jones

2012

Physica Status Solidi (b)

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Since their first development in the 1960s, phase change (PC) memory materials have become essential components of optical memories (DVD‐RW, Blu‐ray Disc, …) used in countless households worldwide. They are now poised to play a decisive role in future non‐volatile computer memories. PC memory materials are restricted to those with an extremely rapid and reversible transition between the amorphous and crystalline phases of an appropriate recording medium, and their development and optimization has been hindered by the inherent difficulties in determining amorphous structures. Alloys of germanium, antimony, and tellurium (“GST” alloys) are among the most widely used in the above contexts, and there has been much speculation concerning their amorphous structures. Theoreticians were slow to appreciate the rich array of problems awaiting their attention, but the past few years have seen a dramatic change in this situation. Density functional (DF) calculations, which are generally free of adjustable parameters, have now been reported on numerous systems. We review here the information that has resulted about the amorphous structures of GST‐alloys.

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Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

Skelton

Pallipurath

Lee

et al. 2014

Adv Funct Materials

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Phase‐change alloys are the functional materials at the heart of an emerging digital‐storage technology. The GeTe‐Sb2Te3 pseudo‐binary systems, in particular the composition Ge2Sb2Te5 (GST), are one of a handful of materials which meet the unique requirements of a stable amorphous phase, rapid amorphous‐to‐crystalline phase transition, and significant contrasts in optical and electrical properties between material states. The properties of GST can be optimized by doping with p‐block elements, of which Bi has interesting effects on the crystallization kinetics and electrical properties. A comprehensive simulational study of Bi‐doped GST is carried out, looking at trends in behavior and properties as a function of dopant concentration. The results reveal how Bi integrates into the host matrix, and provide insight into its enhancement of the crystallization speed. A straightforward explanation is proposed for the reversal of the charge‐carrier sign beyond a critical doping threshold. The effect of Bi on the optical properties of GST is also investigated. The microscopic insight from this study may assist in the future selection of dopants to optimize the phase‐change properties of GST, and also of other PCMs, and the general methods employed in this work should be applicable to the study of related materials, for example, doped chalcogenide glasses.

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Publisher's Note: Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change materialGe2Sb2Te5[Phys

Cited by 23 publications

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Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained densi

Structure, electronic, and vibrational properties of glassy Ga11Ge11Te78: Experimentally constrained densi

Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

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