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See detailAging in Phase Change Materials: Getting Insight from Simulation
Raty, Jean-Yves ULiege

in Physica Status Solidi. Rapid Research Letters (2019), 0(0), 1800590

Aging is one of the effects limiting the advent of phase change materials as acting components in non-volatile memories. This paper presents a review of recent simulation works allowing to describe the ... [more ▼]

Aging is one of the effects limiting the advent of phase change materials as acting components in non-volatile memories. This paper presents a review of recent simulation works allowing to describe the underlying microscopic mechanisms that are responsible for the aging of the semiconductor glass and the accompanying resistance drift. In comparison with other systems, the fragile character of phase change materials imposes the use of different methods to sample the space of configurations and the chemical ordering. The emerging picture is that both the evolution of coordination defects and of the underlying network are responsible for the evolution of the electronic properties. The advantage of simulations is that they allow to determine the relation between chemical ordering, the local geometry of atoms, and the nature of electronic states. From these correlations, one can extrapolate to obtain the structure of the “ideal” amorphous state and the relation between bonding in this phase and that of the more conductive crystalline phase. This understanding of microscopic phenomena is crucial to interpret experimental results, but also paves the way to the design of optimized glasses, that are less prone to aging, while preserving the unique properties that place phase change materials among the best candidates for high performance and scalable non-volatile memories. [less ▲]

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See detailA Quantum-Mechanical Map for Bonding and Properties in Solids
Raty, Jean-Yves ULiege; Schumacher, Mathias; Golub, Pavlo et al

in Advanced Materials (2018), (0), 1806280

Abstract A 2D map is created for solid-state materials based on a quantum-mechanical description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic ... [more ▼]

Abstract A 2D map is created for solid-state materials based on a quantum-mechanical description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary compounds; furthermore, it highlights a distinct region for a mechanism recently termed “metavalent” bonding. Then, it is shown how this materials map can be extended in the third dimension by including physical properties of application interest. Finally, it is shown how the map coordinates yield new insight into the nature of the Peierls distortion in phase-change materials and thermoelectrics. These findings and conceptual approaches provide a novel avenue to tailor material properties. [less ▲]

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See detailDecoding entangled transitions: Polyamorphism and stressed rigidity
Yildirim, Can; Raty, Jean-Yves ULiege; Micoulaut, Matthieu

in Journal of Chemical Physics (2018), 148(24), 244505

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See detailIncipient Metals: Functional Materials with a Unique Bonding Mechanism
Wuttig, Matthias; Deringer, Volker L.; Gonze, Xavier et al

in Advanced Materials (2018), 30(51), 1803777

Abstract While solid-state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. Phase-change materials (PCMs) for data storage are ... [more ▼]

Abstract While solid-state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. Phase-change materials (PCMs) for data storage are a prominent example: they have been claimed to show “resonant bonding,” but a clear definition of this mechanism has been lacking. Here, it is shown that these solids are fundamentally different from resonant bonding in the π-orbital systems of benzene and graphene, based on first-principles data for vibrational, optical, and polarizability properties. It is shown that PCMs and related materials exhibit a unique mechanism between covalent and metallic bonding. It is suggested that these materials be called “incipient metals,” and their bonding nature “metavalent”. Data for a diverse set of 58 materials show that metavalent bonding is not just a superposition of covalent and metallic cases, but instead gives rise to a unique and anomalous set of physical properties. This allows the derivation of a characteristic fingerprint of metavalent bonding, composed of five individual components and firmly rooted in physical properties. These findings are expected to accelerate the discovery and design of functional materials with attractive properties and applications, including nonvolatile memories, thermoelectrics, photonics, and quantum materials. [less ▲]

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See detailAb initio density functional theory study of the electronic, dynamic, and thermoelectric properties of the crystalline pseudobinary chalcogenide (GeTe)x/(Sb2Te3) (x=1, 2, 3)
Ibarra-Hernández, W.; Raty, Jean-Yves ULiege

in Physical Review. B (2018), 97(24),

We use ab initio density functional theory calculations to understand the electronic, dynamical, and thermoelectric behavior of layered crystalline phase-change materials. We perform calculations on the ... [more ▼]

We use ab initio density functional theory calculations to understand the electronic, dynamical, and thermoelectric behavior of layered crystalline phase-change materials. We perform calculations on the pseudobinary compounds (GeTe)x/(Sb2Te3) (GST) with x=1, 2, and 3. Since the stable configuration of these compounds remains somehow unsettled, we study one stacking configuration for GST124 (x=1), three for GST225 (x=2), and two for GST326 (x=3). A supercell approach is used to check the dynamical stability of the systems while thermoelectric properties are obtained by solving the Boltzmann transport equation. We report that the most accepted stacking configuration of GST124, GST225, and GST326 have metallic character and for the case of x=2 and 3, those are the ones with the lowest energy. However, we find the metallic of GST326 configuration to be dynamically unstable. In general, our values of the Seebeck coefficient and thermal conductivity for compounds with x=1 and 2 agree very well with the available experimental data. The small differences that we observe with respect to experimental data are attributed to the disorder that is present experimentally and that we have not taken into account. We do not find a Dirac cone in the electronic band structure of GST225, contrarily to previous reports. We attribute this due to the theoretical strain induced by the choice of the pseudopotential. © 2018 American Physical Society. [less ▲]

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See detailPhase Change Materials for Non-Volatile Memory devices: From Technological Challenges to Materials Science Issues
Noé, Pierre; Vallée, Christophe; Hippert, Francoise et al

in Semiconductor Science and Technology (2018)

Abstract Chalcogenide Phase-Change Materials (PCMs), such as Ge-Sb-Te alloys, are showing outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and ... [more ▼]

Abstract Chalcogenide Phase-Change Materials (PCMs), such as Ge-Sb-Te alloys, are showing outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, recently, in non-volatile resistive memories. The latter, known as Phase-Change Material memories or Phase-Change Random Access Memories (PCRAMs), are the most promising candidate among emerging Non-Volatile Memory (NVM) technologies to replace the current FLASH memories at CMOS technology nodes under 28 nm. Chalcogenide PCMs exhibit fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties leading to a unique set of features for PCRAMs, such as fast programming, good cyclability, high scalability, multi-level storage capability and good data retention. Nevertheless, PCM memory technology has to overcome several challenges to definitively invade the NVM market. In this review paper we examine the main technological challenges that PCM memory technology must face and we illustrate how new memory architecture, innovative deposition methods and PCM composition optimization can contribute to further improvements of this technology. In particular, we examine how to lower the programming currents and increase data retention. Scaling down PCM memories for large scale integration means incorporation of the phase-change material into more and more confined structures and raises material science issues to understand interface and size effects on crystallization. Other material science issues are related to the stability and ageing of the amorphous state of phase-change materials. The stability of the amorphous phase, which determines data retention in memory devices, can be increased by doping the phase-change material. Ageing of the amorphous phase leads to a large increase of the resistivity with time (resistance drift), which has hindered up-to-now the development of ultra-high multilevel storage devices. A review of the current understanding of all these issues is provided from a material science point of view. [less ▲]

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See detailDefining Resonant Bonding: Why Are Phase Change Materials Unique ?
Raty, Jean-Yves ULiege

Conference (2017, September 28)

Many of the most extraordinary properties of phase change materials (PCMs) can be attributed to the specific type of bonding found in the crystal structure and called ‘resonant bonding’. However, if ... [more ▼]

Many of the most extraordinary properties of phase change materials (PCMs) can be attributed to the specific type of bonding found in the crystal structure and called ‘resonant bonding’. However, if experimental studies have shown (using FTIR) the unique character of bonding in several PCMs, the ‘resonant’ nature of bonds remains a rather general concept. In this presentation, we use ab initio (DFT) methods to address the bonding properties of various types of crystalline compounds, among which popular phase change materials, and quantify the electronic properties using several indicators. We show the direct correlation between the high polarizability of the bonds and the localization properties of electrons close to the Fermi level which allow to better define resonant bonding. This allows to place resonantly bonded materials, thus PCMs, at a specific location on a scale that extends from ionic compounds to regular semiconductors and finally metals. Our indicator also illustrates clearly the difference between amorphous and crystalline PCMs. [less ▲]

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See detailPhase-change materials and rigidity
Piarristeguy, A.; Pradel, A.; Raty, Jean-Yves ULiege

in MRS Bulletin (2017), 42(1), 45-49

Rigidity theory is an extraordinary tool to understand glasses. This article demonstrates how this model can help in understanding the link between structure, dynamics, and subtler properties such as ... [more ▼]

Rigidity theory is an extraordinary tool to understand glasses. This article demonstrates how this model can help in understanding the link between structure, dynamics, and subtler properties such as drift and aging, in particular, in phase-change materials (PCMs). First, a map of flexible/rigid regions in the Ge-(Sb)-Te system is drawn on the basis of atomistic structures modeled either by ab initio or reverse Monte Carlo techniques. A clear link between the flexible/rigid nature of the glass and its aging behavior is shown through resistivity drift as a function of composition measurements in amorphous GexTe100-x. In the particular case of amorphous GeTe, application of rigidity theory indicates that the average number of mechanical constraints decreases during aging, making the glass less stressed-rigid. Finally, the stability of PCMs also depends on the topology of the materials. The increasing number of constraints in GeTe when doped with C or N results in increased stability of the PCM. © 2017 Materials Research Society. [less ▲]

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See detailAnomalous diffusion and non-monotonic relaxation processes in Ge-Se liquids
Yildirim, Can ULiege; Raty, Jean-Yves ULiege; Micoulaut, Matthieu

in Journal of Chemical Physics (2016), 144

We investigate the dynamical properties of liquid GexSe100−x as a function of Ge content by first-principles molecular dynamic simulations for a certain number of temperatures in the liquid state. The ... [more ▼]

We investigate the dynamical properties of liquid GexSe100−x as a function of Ge content by first-principles molecular dynamic simulations for a certain number of temperatures in the liquid state. The focus is set on ten compositions (where x ≤ 33%) encompassing the reported flexible to rigid and rigid to stressed-rigid transitions. We examine diffusion coefficients, diffusion activation energies, glassy relaxation behavior, and viscosity of these liquids from Van Hove correlation and intermediate scattering functions. At fixed temperature, all properties/functions exhibit an anomalous behavior with Ge content in the region 18%-22%, and provide a direct and quantitative link to the network rigidity. [less ▲]

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See detailUniversal amorphous-amorphous transition in GexSe100−x glasses under pressure
Yildirim, Can ULiege; Micoulaut, Matthieu; Boolchand, Punit et al

in Scientific Reports (2016), 6

Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments ... [more ▼]

Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa. The Ge-Se interatomic distances extracted from XAS data show a two-step response to the applied pressure; a gradual decrease followed by an increase at around 15–20 GPa, depending on the composition. This increase is attributed to the metallization event that can be traced with the red shift in Ge K edge energy which is also identified by the principal peak position of the structure factor. The densification mechanisms are studied in details by means of AIMD simulations and compared to the experimental results. The evolution of bond angle distributions, interatomic distances and coordination numbers are examined and lead to similar pressure-induced structural changes for any composition. [less ▲]

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See detailRevealing the role of network rigidity on the fragility evolution of glass-forming liquids
Yildirim, Can ULiege; Raty, Jean-Yves ULiege; Micoulaut, Mathieu

in Nature Communications (2016), 7(2016), 11086

If quenched fast enough, a liquid is able to avoid crystallization and will remain in a metastable supercooled state down to the glass transition, with an important increase in viscosity and relaxation ... [more ▼]

If quenched fast enough, a liquid is able to avoid crystallization and will remain in a metastable supercooled state down to the glass transition, with an important increase in viscosity and relaxation time towards equilibrium upon further cooling [1,2]. There are important differences in the way liquids relax as they approach the glass transition, rapid or slow variation in dynamic quantities under moderate temperature changes, and a simple means to quantify such variations is provided by the concept of "fragility". Here, we report molecular dynamics simulations of a typical network-forming glass, and find that the relaxation behaviour of the supercooled liquid is strongly correlated to the variation of rigidity with temperature and the spatial distribution of the corresponding topological constraints which, ultimately connect to fragility minima . This permits extending the fragility concept to aspects of topology/rigidity, and to the degree of homogeneity of the atomic-sale interactions for a variety of structural glasses. [less ▲]

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See detailTheoretical Aspects and Ab Initio Simulations
Raty, Jean-Yves ULiege

Conference (2016, March 28)

Part II focuses on ab initio simulation techniques and describe how these methods can yield realistic models of phase-change materials. Various quantities that can be obtained from such simulations will ... [more ▼]

Part II focuses on ab initio simulation techniques and describe how these methods can yield realistic models of phase-change materials. Various quantities that can be obtained from such simulations will be reviewed; these include bonding strength and structural, electronic, optical and dynamical quantities. There will be a discussion of how these quantities can shed light on technologically important features of phase-change materials, such as the optical and electronic contrast between the crystalline and amorphous phase, the stability and the ageing mechanisms of the glass, as well as its crystallization properties at high temperature. [less ▲]

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See detailTutorial : Theoretical Aspects and Ab Initio Simulations
Raty, Jean-Yves ULiege

Conference (2016, March 28)

Part II: Jean-Yves Raty Part II focuses on ab initio simulation techniques and describe how these methods can yield realistic models of phase-change materials. Various quantities that can be obtained from ... [more ▼]

Part II: Jean-Yves Raty Part II focuses on ab initio simulation techniques and describe how these methods can yield realistic models of phase-change materials. Various quantities that can be obtained from such simulations will be reviewed; these include bonding strength and structural, electronic, optical and dynamical quantities. There will be a discussion of how these quantities can shed light on technologically important features of phase-change materials, such as the optical and electronic contrast between the crystalline and amorphous phase, the stability and the ageing mechanisms of the glass, as well as its crystallization properties at high temperature. [less ▲]

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See detailAb Initio Perspective on Stability and Aging in Phase Change Materials
Raty, Jean-Yves ULiege

Scientific conference (2015, November 09)

Data recording with Phase Change Materials is a much studied topic as the writing/erasing characteristics, cyclability and downscaling properties of these materials allow for efficient data storage in ... [more ▼]

Data recording with Phase Change Materials is a much studied topic as the writing/erasing characteristics, cyclability and downscaling properties of these materials allow for efficient data storage in future generations of devices. Nevertheless, some aspects of phase change materials are limiting their performances and delaying their wider technological application. First, aging phenomena are common to all amorphous structures, but of special importance PCMs since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. One difficulty encountered in the simulation of these amorphous systems is that the direct generation of an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atoms, and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal. On the other hand its electronic properties drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal [1]. A second problem faced by PCMs is the fact that data recording is limited at high temperature due to the increased propensity to recrystallize. One approach to counter this is to stabilize the PCM using impurity atoms such as C or N. Using DFT and the analysis of the mechanical properties (constraints theory), we demonstrate how these impurity atoms modify the rigidity of the network, which is experimentally correlated with the activation energy for crystallization [2]. Finally, the crystal phase itself has been shown to have variable conductivities depending on the thermal history and annealing conditions. If this could be used profitably for multi-level recording, it also indicates that the crystal is undergoes some temporal evolution. Using DFT, we clarify the stability behavior of GST crystal and show that the metal-insulator transition is driven by the migration of intrinsic vacancies and an Anderson localization transition [3]. [1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nat. Comm. (2015) [2] G. Ghezzi, J.Y. Raty, S. Maitrejean, A. Roule, E. Elkaim and F. Hippert, Applied Physics Letters, 99 (2011) 151906 [3] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J-Y. Raty, M.Wuttig, S. Blügel et R. Mazzarello, Nature Materials 11 (2012) 952 [less ▲]

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See detailStability and Aging of Phase Change Materials : An Ab Initio Perspective
Raty, Jean-Yves ULiege

Conference (2015, September 17)

Data recording with Phase Change Materials is a much studied topic as the writing/erasing characteristics, cyclability and downscaling properties of these materials allow for efficient data storage in ... [more ▼]

Data recording with Phase Change Materials is a much studied topic as the writing/erasing characteristics, cyclability and downscaling properties of these materials allow for efficient data storage in future generations of devices. Nevertheless, some aspects of phase change materials are limiting their performances and delaying their wider technological application. First, aging phenomena are common to all amorphous structures, but of special importance PCMs since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. One difficulty encountered in the simulation of these amorphous systems is that the direct generation of an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atoms, and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal. On the other hand its electronic properties drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal [1]. A second problem faced by PCMs is the fact that data recording is limited at high temperature due to the increased propensity to recrystallize. One approach to counter this is to stabilize the PCM using impurity atoms such as C or N. Using DFT and the analysis of the mechanical properties (constraints theory), we demonstrate how these impurity atoms modify the rigidity of the network, which is experimentally correlated with the activation energy for crystallization [2]. Finally, the crystal phase itself has been shown to have variable conductivities depending on the thermal history and annealing conditions. If this could be used profitably for multi-level recording, it also indicates that the crystal is undergoes some temporal evolution. Using DFT, we clarify the stability behavior of GST crystal and show that the metal-insulator transition is driven by the migration of intrinsic vacancies and an Anderson localization transition [3]. [1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nat. Comm. (2015) [2] G. Ghezzi, J.Y. Raty, S. Maitrejean, A. Roule, E. Elkaim and F. Hippert, Applied Physics Letters, 99 (2011) 151906 [3] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J-Y. Raty, M.Wuttig, S. Blügel et R. Mazzarello, Nature Materials 11 (2012) 952 [less ▲]

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See detailAGING MECHANISMS IN AMORPHOUS PHASE CHANGE MATERIALS
Raty, Jean-Yves ULiege

Conference (2015, August 18)

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have ... [more ▼]

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. The direct melt-and-quench DFT based Molecular Dynamics approach leads to models with a few hundred atoms, and, hence a small number of atomic environments. Here we sample a large number of local atomic environments, and bonding schemes, by chemically substituting different alloys to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. GST alloys are known to display complex bonding that does not follow the chemist’s “octet-rule”. This lead to many controversies, especially concerning the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess their stability to find the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous [1]. [1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nature Comm. To appear. [less ▲]

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See detailAging mechanisms in amorphous phase-change materials
Raty, Jean-Yves ULiege; Zhang, wei; Luckas, Jennifer et al

in Nature Communications (2015), 6(7467), 1-8

Aging is a ubiquitous phenomenon in glasses. In the case of phase-change materials, it leads to a drift in the electrical resistance, which hinders the development of ultrahigh density storage devices ... [more ▼]

Aging is a ubiquitous phenomenon in glasses. In the case of phase-change materials, it leads to a drift in the electrical resistance, which hinders the development of ultrahigh density storage devices. Here we elucidate the aging process in amorphous GeTe, a prototypical phase-change material, by advanced numerical simulations, photothermal deflection spectroscopy and impedance spectroscopy experiments. We show that aging is accompanied by a progressive change of the local chemical order towards the crystalline one. Yet, the glass evolves towards a covalent amorphous network with increasing Peierls distortion, whose structural and electronic properties drift away from those of the resonantly bonded crystal. This behaviour sets phase-change materials apart from conventional glass-forming systems, which display the same local structure and bonding in both phases [less ▲]

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See detailAging mechanisms in amorphous phase-change materials
Raty, Jean-Yves ULiege

Conference (2015, June 23)

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have ... [more ▼]

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. The direct melt-and-quench DFT based Molecular Dynamics approach leads to models with a few hundred atoms, and, hence a small number of atomic environments. Here we sample a large number of local atomic environments, and bonding schemes, by chemically substituting different alloys to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. GST alloys are known to display complex bonding that does not follow the chemist’s “octet-rule”. This lead to many controversies, especially concerning the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess their stability to find the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous [1]. [1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nature Comm. To appear. [less ▲]

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See detailStructural properties of densified GexSe1-x glasses
Yildirim, Can ULiege; Micoulaut, Matthieu; Raty, Jean-Yves ULiege

Conference (2015, May 20)

The evolution in structural properties of liquid and densified glassy GexSe1-x is investigated by use of First Principles Molecular Dynamics (FPMD) combined with X-ray absorption (XAS) experiments for the ... [more ▼]

The evolution in structural properties of liquid and densified glassy GexSe1-x is investigated by use of First Principles Molecular Dynamics (FPMD) combined with X-ray absorption (XAS) experiments for the glassy state. Four different compositions (x=10%, 16%, 18% and 33%) representing the flexible and stressed rigid sides of the reversibility window are the focus of the study. The target systems were studied at pressures up to ~ 11 GPa under both annealed and cold compression conditions. We examine the structure factors, pair distribution functions, bond angle distributions, coordination numbers and neighbor distributions. The results show that the real and reciprocal space properties are in very good agreement with the experimental findings. The structural evolution during densification reveals the edge sharing tetrahedra is maintained upon compression whereas Ge-Ge homopolar bonds tend to increase in number. Ge-Se-Ge bond angular distributions show a transformation from tetrahedral octahedral geometry. We discuss the effect of thermal history on structural properties during densification. [less ▲]

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See detailAging mechanisms in amorphous GeTe
Raty, Jean-Yves ULiege; Zhang, Wei; Luckas, Jennifer et al

Conference (2015, May 18)

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have ... [more ▼]

Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or more elaborate Van der Waals approximation. New insight is provided on the stability of homopolar GeGe bonds and tetrahedral Ge bonding, in relation with the resistance drift phenomenon, that is investigated experimentally using photothermal deflection spectroscopy experiments [less ▲]

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