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See detailEffect of concentration in Ge-Te liquids: A combined density functional and neutron scattering study
Micoulaut, Matthieu; Coulet, M.V.; Piarristeguy, A. et al

in Physical Review. B : Solid State (2014), 89(17), 174205

The structural properties of three compositions of Ge-Te liquids (Ge10Te90, Ge15Te85, Ge20Te80) are studied from a combination of density functional based molecular dynamics simulations and neutron ... [more ▼]

The structural properties of three compositions of Ge-Te liquids (Ge10Te90, Ge15Te85, Ge20Te80) are studied from a combination of density functional based molecular dynamics simulations and neutron scattering experiments. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions and compare our results with experimental findings. Most noticeable is the good agreement found in the reproduction of the structure in real and reciprocal space, resulting from the incorporation of dispersion forces in the simulation. This leads to Ge and Te coordination numbers which are lower than in previous studies and which can now be followed with temperature, while also strongly depending on the chosen cutoff distance. Results show a gradual conversion of higher coordinated species (TeIV, GeV) into lower coordinated ones at lower temperature, while leaving anticipated coordinations from the octet rule (TeII and GeIV) nearly unchanged. Structural correlations are characterized as a function of temperature and composition. The vibrational density of states is also measured from inelastic neutron scattering for different compositions and temperatures, and compared to the simulated counterpart which exhibits a reasonable agreement at low frequency. [less ▲]

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See detailThe role of C and N dopants incorporation in phase change materials
Noé, Pierre; Raty, Jean-Yves ULg; Ghezzi, Giada et al

Conference (2013, April 05)

Phase change memory (PCM) technology is considered to be among the most promising alternatives to conventional technologies in embedded memories [1]. To allow operation at relatively high temperatures in ... [more ▼]

Phase change memory (PCM) technology is considered to be among the most promising alternatives to conventional technologies in embedded memories [1]. To allow operation at relatively high temperatures in embedded applications, it is crucial to improve the stability of the amorphous phase. Carbon and nitrogen doping have been shown to significantly increase the crystallization temperature [1-3]. Moreover, the high RESET current requirement [2], which is a limit to the scalability of GeTe and GST, can be reduced by the incorporation of a dopant element [4]. In this presentation we focus on correlating experimental results and ab initio simulations to understand the effect of C and N incorporation in GeTe and GST PCM devices. Understanding the effect of dopants on the change of electronic properties and the mechanisms of the phase transformation requires analysis of the local order and structure of the amorphous to crystalline phases. In this context, we demonstrate that carbon and nitrogen deeply affects the structure and the dynamical properties of the amorphous phase of GeTe. In particular, the inclusion of N and C dopant elements in GeTe has a drastic effect on the vibrational modes of GeTe therefore improving the stability of the glass. This effect goes with an increased mechanical rigidity explaining why these doped GeTe compounds have a higher crystallization temperature than the undoped ones. Finally we will explore, mainly by FTIR and XRD measurements, the effect of C and N dopants during the annealing of amorphous PCMaterials towards their crystalline phases. These results will be discussed in order to understand the origin of the differences of the doped PCMaterials amorphous phase stability (data retention) observed between full sheet materials and the materials integrated in PCM devices. [1] A. Fantini et al., 2010 IEEE International Electron Devices Meeting (IEDM), 2010, pp. 29.21.21-29.21.24. [2] G. Betti Beneventi et al., Solid-State Electronics, 65-66 (2011) 197-204. [3] V. Sousa et al., EPCOS 2011. [4] Q. Hubert et al., IMW 2012. [less ▲]

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See detailVibrational properties and stabilization mechanism of the amorphous phase of doped GeTe
Raty, Jean-Yves ULg; Noé, Pierre; Ghezzi, G. et al

in Physical Review B (2013), 88(1),

Doping chalcogenide phase change materials was shown to improve the stability of the amorphous phase at high temperature and to strongly increase the crystallization temperature. In this work, we use ab ... [more ▼]

Doping chalcogenide phase change materials was shown to improve the stability of the amorphous phase at high temperature and to strongly increase the crystallization temperature. In this work, we use ab initio molecular dynamics together with Fourier transform infrared spectroscopy to address the stabilization of GeTe doped with nitrogen and carbon in the amorphous phase. The comparison between the simulation and experimental results allows in-depth understanding of the mechanisms. The inclusion of C and N leads to an increase in high frequency vibrational modes and to a lowering of the boson peak intensity. The reduction of the density of floppy vibrational modes and the computed increase of the mechanical rigidity are responsible for the higher activation energy for crystallization. The mechanism described here could apply more generally to stabilize other Ge-Sb-Te phase change materials and ionocovalent glasses at high temperature. © 2013 American Physical Society. [less ▲]

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See detailComment on "new structural picture of the Ge 2Sb 2Te 5 phase-change alloy"
Raty, Jean-Yves ULg; Bichara, C.; Mazzarello, R. et al

in Physical Review Letters (2012), 108(23),

[No abstract available]

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See detailMaxwell rigidity and topological constraints in amorphous phase-change networks
Micoulaut, M.; Otjacques, C.; Raty, Jean-Yves ULg et al

in AIP Conference Proceedings (2011), 1393

By analyzing first-principles molecular-dynamics simulations of different telluride amorphous networks, we develop a method for the enumeration of radial and angular topological constraints, and show that ... [more ▼]

By analyzing first-principles molecular-dynamics simulations of different telluride amorphous networks, we develop a method for the enumeration of radial and angular topological constraints, and show that the phase diagram of the most popular system Ge-Sb-Te can be split into two compositional elastic phases: a tellurium rich flexible phase and a stressed rigid phase that contains most of the materials used in phase-change applications. This sound atomic scale insight should open new avenues for the understanding of phase-change materials and other complex amorphous materials from the viewpoint of rigidity. [less ▲]

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See detailUnderstanding amorphous phase-change materials from the viewpoint of Maxwell rigidity
Micoulaut, M.; Raty, Jean-Yves ULg; Otjacques, C. et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(17), 17420610-1742061017420610

Phase-change materials (PCMs) are the subject of considerable interest because they have been recognized as potential active layers for nonvolatile memory devices, known as phase-change random access ... [more ▼]

Phase-change materials (PCMs) are the subject of considerable interest because they have been recognized as potential active layers for nonvolatile memory devices, known as phase-change random access memories. By analyzing first-principles molecular-dynamics simulations we develop a method for the enumeration of mechanical constraints in the amorphous phase and show that the phase diagram of the most popular system (Ge-Sb-Te) can be split into two compositional regions having a well-defined mechanical character: a tellurium rich flexible phase and a stressed rigid phase that encompasses the known PCMs. This sound atomic scale insight should open new avenues for the understanding of PCMs and other complex amorphous materials from the viewpoint of rigidity. [less ▲]

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See detailAmorphous structure and electronic properties of the Ge1Sb2Te4 phase change material
Raty, Jean-Yves ULg; Otjacques, C.; Gaspard, Jean-Pierre ULg et al

in Solid State Sciences (2010), 12(2), 193-198198

Ge1Sb2Te4 is one of the most commonly used phase change materials, due to the large optical and electrical contrast between a metastable crystalline phase and the amorphous phase. We use ab initio ... [more ▼]

Ge1Sb2Te4 is one of the most commonly used phase change materials, due to the large optical and electrical contrast between a metastable crystalline phase and the amorphous phase. We use ab initio molecular dynamics to generate an amorphous Ge1Sb2Te4 structure. By analysing the distance distributions, we show that the structure can be analysed in terms of 21% of tetrahedrally coordinated Ge atoms and 79% of 3-fold Ge atoms. These are involved in distorted octahedral shells with bond length correlations that are similar to the a-GeTe structure as a consequence of a Peierls-distortion. The electronic properties are shown to be in reasonable agreement with the experiment with an electronic gap of 0.45 eV with. The optical conductivity curve is also in agreement with the experiment, with a maximal conductivity at an energy of ~3 eV. [All rights reserved Elsevier]. [less ▲]

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See detailDynamics of the negative thermal expansion in tellurium based liquid alloys
Otjacques, C.; Raty, Jean-Yves ULg; Coulet, M-V et al

in Physical Review Letters (2009), 103(24), 2459014-24590142459014

Negative thermal expansion (NTE) in tellurium based liquid alloys (GeTe 6 and GeTe 12) is analyzed through the atomic vibrational properties. Using neutron inelastic scattering, we show that the ... [more ▼]

Negative thermal expansion (NTE) in tellurium based liquid alloys (GeTe 6 and GeTe 12) is analyzed through the atomic vibrational properties. Using neutron inelastic scattering, we show that the structural evolution resulting in the NTE is due to a gain of vibrational entropy that cancels out the Peierls distortion. In the NTE temperature range, these competing effects give rise to noticeable changes in the vibrational density of states spectra. Additional first principles molecular dynamics simulations emphasize the role of the temperature dependance of the Ge atomic environment in this mechanism. For comparison, we extended our study to Ge 2Sb 2Te 5 and Ge 1Sb 2Te 4 phase-change materials. [less ▲]

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See detailTemperature-induced density anomaly in Te-rich liquid Germanium tellurides: p versus sp 3 bonding?
Bichara, C.; Johnson, M.; Raty, Jean-Yves ULg

in Physical Review Letters (2005), 95(26), 2678011-26780142678014

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental ... [more ▼]

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp 3 hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases [less ▲]

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See detailA thermodynamic investigation of selenium confined in silicalite zeolite
Bichara, C.; Raty, Jean-Yves ULg; Pellenq, R.

in Molecular Simulation (2004), 30(9), 601-606

In this paper, we study the practical feasibility of selenium adsorption in silicalite-1 zeolite by performing Grand Canonical Monte Carlo (GCMC) simulations on a simulation box including the porous ... [more ▼]

In this paper, we study the practical feasibility of selenium adsorption in silicalite-1 zeolite by performing Grand Canonical Monte Carlo (GCMC) simulations on a simulation box including the porous matrix and its outer surface. This work aims at gaining insight on the stability of semi-conductor wires in microporous materials. The simulations at two different temperatures show two distinct behaviors: adsorption occurs inside the pores at 200degreesC while solely on the external surface at 650degreesC. This indicates that adsorption inside the pore network can only proceed below the pseudo-wetting transition temperature that lies between 200 and 650degreesC. The existence of such transition temperature is thus crucial if one aims to produce nanowires from microporous materials by adsorption from a gas phase. [less ▲]

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See detailProperties of molten Ge chalcogenides: an ab initio molecular dynamics study
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Bichara, C.

in Journal of Physics-Condensed Matter (2003), 15(1 Sp. Iss. SI), 167-173

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available ... [more ▼]

In this study, we perform first-principles molecular dynamics simulations of the eutectic alloy Ge15Te85 at five different densities and temperatures. We obtain structures in agreement with the available diffraction data and obtain a new view of the molten Ge chalcogenides. We show that the anomalous volume contraction observed in the liquid 30 K above the eutectic temperature corresponds to a significant change of the Ge-Te partial structure factor. The detailed structural analysis shows that volume variations observed upon melting in Ge15Te85, as in liquid, GeSe and GeTe, can be explained in terms of the competition between two types of local environment of the germanium atoms. A symmetrical coordination octahedron is entropically favoured at high temperature, while an asymmetrical octahedron resulting from the local manifestation of the Peierls distortion is electronically favoured at lower temperatures. [less ▲]

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See detailLocal structure of liquid GeTe via neutron scattering and ab initio simulations
Raty, Jean-Yves ULg; Godlevsky, V. V.; Gaspard, Jean-Pierre ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2002), 65

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare ... [more ▼]

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare these results with ab initio molecular dynamics simulations. The simulations were based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. [less ▲]

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See detailAdsorption of selenium wires in silicalite-1 zeolite: a first order transition in a microporous system
Bichara, C.; Raty, Jean-Yves ULg; Pellenq, RJ-M

in Physical Review Letters (2002), 89(1), 016101-1-016101-016101-4016101-016101-4

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of ... [more ▼]

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of a first order transition, unexpected for adsorption in a microporous system with pore size of the order of 0.5 to 0.6 nm. We analyze this behavior as a result of the favored twofold coordinated chain structure of selenium that grows inside the complex three-dimensional microchannel network of silicalite. This analysis is confirmed by simpler calculations of a lattice gas-type model [less ▲]

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See detailComputer simulation of liquid semiconductors
Bichara, C.; Gaspard, Jean-Pierre ULg; Raty, Jean-Yves ULg

in Journal of Non-Crystalline Solids (2002), 312

We discuss two examples of computer simulation of liquid semiconductors by two different techniques. Both examples are concerned with the relationship between thermodynamic properties and the atomic ... [more ▼]

We discuss two examples of computer simulation of liquid semiconductors by two different techniques. Both examples are concerned with the relationship between thermodynamic properties and the atomic structure in the liquid state. By means of ab initio molecular dynamics we analyze the atomic structure of liquid Ge15Te85 eutectic alloys. We show that the changes observed in the experimental total structure factor S(q) are located in the GeTe partial structure factor, the TeTe partial structure factor remaining essentially unaltered. Using a semi-empirical tight binding approach, coupled with Gibbs ensemble and constant pressure Monte Carlo calculations, we can calculate the liquid vapor equilibrium of selenium. We obtain a liquid-vapor equilibrium curve and a critical point in the correct range of magnitude and an atomic structure of the liquid phase in good agreement with the experimental data. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

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See detailDistance correlations and dynamics of liquid GeSe: An ab initio molecular dynamics study
Raty, Jean-Yves ULg; Godlevsky, V. V.; Gaspard, Jean-Pierre ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2001), 64

We analyze the structure and dynamics of semiconducting liquid GeSe using ab initio molecular-dynamics simulations. We show the local order of the liquid to be close to that of the low-temperature ... [more ▼]

We analyze the structure and dynamics of semiconducting liquid GeSe using ab initio molecular-dynamics simulations. We show the local order of the liquid to be close to that of the low-temperature crystalline phase. alpha -GeSe. In particular, we show that the Peierls distortion, which defines the a phase and vanishes in the high-temperature beta crystalline phase, reenters GeSe in the melt. Examining the distance histograms allows one to analyze the Ge environment as consisting of a Gese(3) unit and having one Ge-Ge defective bond. Evidence is presented that Peierls distortion is directly responsible for the semiconducting behavior of the melt. The calculated viscosity and electrical conductivity are in agreement with the experiment. An additional neutron-diffraction experiment indicates that this liquid structure is unmodified 200 K above the melting point. [less ▲]

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See detailEvidence of a reentrant Peierls distortion in liquid GeTe
Raty, Jean-Yves ULg; Godlevsky, V.; Ghosez, Philippe ULg et al

in Physical Review Letters (2000), 85(9), 1950-1953

The local atomic order of semiconducting liquid GeTe is studied using first-principles molecular-dynamics simulations. Our work points out a high degree of alternating chemical order in the liquid and ... [more ▼]

The local atomic order of semiconducting liquid GeTe is studied using first-principles molecular-dynamics simulations. Our work points out a high degree of alternating chemical order in the liquid and demonstrates the presence of a Peierls distortion close above the melting temperature. This distortion, absent in the high temperature crystalline structure of NaCl type, is a remnant of the atomic arrangement in the A7 low temperature crystalline phase. It disappears slowly with temperature, as the liquid evolves from a semiconducting to a metallic state. [less ▲]

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See detailStructural and electronic properties of high-temperature fluid selenium
Raty, Jean-Yves ULg; Saul, A.; Gaspard, Jean-Pierre ULg et al

in Computational Materials Science (2000), 17(2-4), 239-242242

A semi-empirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces. The band structure parameters are ... [more ▼]

A semi-empirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces. The band structure parameters are obtained by fitting ab initio calculations. The simulated liquid structures are in very good agreement with the most recent X-ray scattering and EXAFS measurements. The Monte Carlo simulations performed show that the complex liquid structures observed result from the breaking and branching of the selenium chains. The total coordination number is shown to result from the balance between one-, two- and three-fold coordinated atoms. The role of these defects is discussed in relationship with the electrical conductivity of the liquid, i.e. the semiconductor-metal and metal-non-metal transitions observed at high pressure and temperature [less ▲]

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See detailRe-entrant Peierls distortion in IV-VI compounds
Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg; Bichara, C. et al

in Physica B: Condensed Matter (2000), 276-278

At room temperature, the local structure of crystalline group V elements (As, Sb,middotmiddotmiddot) and their IV-VI isoelectronic compounds (GeSe,middotmiddotmiddot) is governed by a Peierls distortion ... [more ▼]

At room temperature, the local structure of crystalline group V elements (As, Sb,middotmiddotmiddot) and their IV-VI isoelectronic compounds (GeSe,middotmiddotmiddot) is governed by a Peierls distortion of the simple cubic or NaCl structure which is a symmetry breaking electronic instability. The morphology of the distortion is determined by the filling ratio of the p-band; for a half-filled p-band, the sixfold coordination becomes 3 (short, covalent) +3 (long, van der Waals). In general, at high temperature, the structure recovers its higher coordination number. Neutron diffraction experiments have been made in the liquid state at lambda=0.7 Aring. It is observed that the Peierls distortion is still present in the liquid for most IV-VI compounds. This behavior is observed and discussed for a series of Sn and Ge chalcogenides: SnS, SnSe, GeS, GeSe and GeTe and their temperature evolution is discussed. GeSe and GeTe show an interesting re-entrant phase behavior. The heaviest IV-VI compound SnTe does not show a distorted state. We demonstrate that the hardness of the repulsive potential is a key parameter in this mechanism [less ▲]

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See detailStructure of high temperature fluid selenium
Bichara, C.; Raty, Jean-Yves ULg; Gaspard, Jean-Pierre ULg

in Journal of Non-Crystalline Solids (1999), 250-252

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures ... [more ▼]

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures obtained are in agreement with the X-ray scattering and extended X-ray absorption fine structure (EXAFS) data in a range of temperatures and densities. A correlation between the conductivity of high temperature fluid selenium and the degree of branching and breaking of the selenium chains is observed [less ▲]

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See detailStructure of high-temperature fluid selenium
Raty, Jean-Yves ULg; Saul, A.; Gaspard, Jean-Pierre ULg et al

in Physical Review. B : Condensed Matter (1999), 60(4), 2441-24482448

A semiempirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces that proves necessary at the liquid ... [more ▼]

A semiempirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces that proves necessary at the liquid densities under study. The band-structure parameters are obtained by fitting abinitio calculations. The simulated liquid structures are in very good agreement with the most recent X-ray scattering and extended X-ray absorption fine-structure experiments in a broad temperature and density range. The Monte Carlo simulations performed show that the complex liquid structures observed result from the breaking and branching of the selenium chains. The total coordination number is shown to result from the balance between one-, two-, and threefold coordinated atoms. The role of these defects is discussed in relationship with the electrical conductivity of the liquid, i.e., the semiconductor-metal and metal-nonmetal transitions observed at high pressures [less ▲]

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