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

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 ULg

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 ULg; 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 ULg

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 ULg; Micoulaut, Matthieu; Raty, Jean-Yves ULg

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 ULg; 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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See detailAging Mechanisms in Amorphous GeTe
Raty, Jean-Yves ULg; Zhang, Wei; Luckas, Jennifer et al

Conference (2015, April 08)

We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate Van der Waals ... [more ▼]

We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the 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. 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. Since the structural relaxations concerned with the drift take place on long time scales, the task of understanding them to limit their consequences is not a simple one. We successfully achieved this goal by developing new approaches to overcome a series of hurdles. A first problem is that directly generating an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atom (typically a few hundreds), 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. A second aspect is that GST alloys are known to display complex bonding mechanisms, for which the simple chemist’s “octet-rule” does not apply, leading a long series of controversies, concerning in particular 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 the stability of these local environments to obtain information of the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments that are presented here. 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, as compared to the crystal. [less ▲]

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See detailAb Initio DFT study of electronic and thermoelectric properties of crystalline Ge2Sb2Te5
Ibarra Hernandez, Wilfredo ULg; Raty, Jean-Yves ULg

Conference (2015, March 04)

Pseudo-binary phase change materials such as (GeTe)n/(Sb2Te3)m have been recently considered for thermoelectric applications. Among these, Ge2Sb2Te5 (GST225, n=2 and m=1) is very popular as it is the ... [more ▼]

Pseudo-binary phase change materials such as (GeTe)n/(Sb2Te3)m have been recently considered for thermoelectric applications. Among these, Ge2Sb2Te5 (GST225, n=2 and m=1) is very popular as it is the leading candidate for non-volatile memory devices such as phase change random access memory. It is well know that the stable crystal structure of GST225 is hexagonal, with atomic layers stacked in the c direction. The stacking sequence is however still under some debate, and structures varying from conventional semiconductor to Dirac semimetal have been claimed to differ only by the nature of the stacking sequence. Here we present electronic, dynamic and thermoelectric calculations on three different stacking sequences of crystalline GST225. We use ab-initio DFT calculations together with Boltzmann transport equations to access thermoelectric properties within the constant relaxation time approximation. Our results show that all three proposed stacking sequences are (meta-)stable. From the density of states we determine that two structures are metallic while the most stable structure has a 0.35 eV band gap. Above 100K, the computed Seebeck coefficient seems to indicate that the experimentally observed structure is the Dirac semimetal one, the doping level being of the order of 1 × 1020 cm−3. [less ▲]

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See detailAmorphous phase change materials: Structure, stability and relation with their crystalline phase
Raty, Jean-Yves ULg; Otjacques, C.; Peköz, R. et al

in Massobrio, Carlo; Jincheng, Du; Bernasconi, Marco (Eds.) et al Molecular Dynamics Simulations of Disordered Materials (2015)

PhaseChange Materials should be stable enough in their amorphous phase to achieve a durable data retention, however they should also be bad glass formers to be able to recrystallise at high speed. To ... [more ▼]

PhaseChange Materials should be stable enough in their amorphous phase to achieve a durable data retention, however they should also be bad glass formers to be able to recrystallise at high speed. To understand these contradicting properties, we construct models of amorphous Ge–Sb–Te systems using Ab Initio Molecular Dynamics and analyse the structures in relationwith the relevant crystalline state.We show that structural patterns that are precursors of the crystalline phase exist in the amorphous state and we identify the signature of the various types of local atomic orders in the X-ray absorption spectra that we compute using Density Functional Theory. We first analyse the mechanical properties of the amorphous phase in the framework of the Maxwell rigidity theory, showing that all efficient Phase Change Materials deviate from the perfect glass and are mechanically stressed-rigid. Additionally, we show that the stability of Phase Change Materials is related to the density of lowfrequency vibrational modes (Boson peak).We describe howan adequate doping can result in an increased stability of the amorphous phase while keeping intact the phase change ability of the material. © Springer International Publishing Switzerland 2015. [less ▲]

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

Conference (2014, September 08)

We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate van der Waals ... [more ▼]

We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate van der Waals approximation that proves more accurate in this system. 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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See detailThermoelectric properties of two stacking sequences of crystalline GST-225
Ibarra Hernandez, Wilfredo ULg; Raty, Jean-Yves ULg

Poster (2014, July 09)

Pseudobinary GeTe-Sb2Te3 compounds are widely used as phase-change optical materials for DVD-RAM.[3] Ge2Sb2Te5 (GST-225) is used for this propose but the stacking sequence of the stable crystal structure ... [more ▼]

Pseudobinary GeTe-Sb2Te3 compounds are widely used as phase-change optical materials for DVD-RAM.[3] Ge2Sb2Te5 (GST-225) is used for this propose but the stacking sequence of the stable crystal structure is motive of debate. Pseudobinary compounds there are claimed to be good thermoelectric materials due the large number of intrinsic structural vacancies.[4] Thermoelectric properties for two proposed stacking sequences of GST-225 are computed using DFT[5, 6] and Boltzmann transport equation in the constant relaxation time approximation. After phonon calculations, no dynamic instabilities were found in the Irreducible Brillouin Zone for either of the proposed stacking sequences. One of the stacking sequences shows semiconductor-like density of states (DOS) with a computed gap of 190 meV unlike the other stacking sequence which has a metallic-like DOS. Thermoelectric properties calculation reveals that semiconductor-like structure has the highest value of Seebeck coeffcient (SC). [less ▲]

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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 detailImportance of Dispersion Forces for the Simulation of Amorphous Phase Change Materials
Raty, Jean-Yves ULg; Wuttig, Matthias; Bichara, Christophe

Conference (2014, April 24)

GeTe and GST amorphous materials are re-investigated using ab initio dft molecular dynamics to compare the effect of various functionals that include the treatment of dispersion (Van der Waals) forces on ... [more ▼]

GeTe and GST amorphous materials are re-investigated using ab initio dft molecular dynamics to compare the effect of various functionals that include the treatment of dispersion (Van der Waals) forces on the structural and dynamical properties of the final amorphous structures. We show that the proportion of tetrahedral Ge to other types of environments as well as the ratio of 3-fold and 2-fold bonded Te atoms is much dependent on the choice of functional. The different functionals yield variable agreement with the available structural experimental data. Properties such as the diffusion coefficient and vibrational densities of states are computed, indicating that models that are extremely close in energy may have very different experimental signatures [less ▲]

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See detailEffect of hydrostatic pressure on the thermoelectric properties of Bi2 Te3
Ibarra-Hernández, Wilfredo; Verstraete, Matthieu ULg; Raty, Jean-Yves ULg

in Physical Review. B: Condensed Matter and Materials Physics (2014), 90(24), 245204

We use first-principles calculations to understand the behavior of the Seebeck coefficient (S) in Bi2Te3 as a function of isotropic pressure. We perform calculations up to 5 GPa using density functional ... [more ▼]

We use first-principles calculations to understand the behavior of the Seebeck coefficient (S) in Bi2Te3 as a function of isotropic pressure. We perform calculations up to 5 GPa using density functional theory and with thermoelectric properties extracted using Boltzmann transport equations. We find that with the increase in pressure the system becomes more metallic, in agreement with previous calculations on Sb2Te3. For p-type doping the overall behavior is a decrease in S with an increase in pressure. At small values of hole doping (p=1.8×1018cm-3), we obtain an anomalous variation of S under 2 GPa, which is an indication of the electronic topological transition. For n-type doping, S slightly increases with pressure. © 2014 American Physical Society. [less ▲]

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See detailLocal order, dynamics and stability of C and N doped phase change materials
Hippert, Françoise; Raty, Jean-Yves ULg; Noé, Pierre et al

Conference (2013, July 26)

Doping Chalcogenide Phase Change Materials, such as Ge2Sb2Te5 and GeTe used in non-volatile phase change memories, was shown to improve the stability of the amorphous phase and to strongly increase the ... [more ▼]

Doping Chalcogenide Phase Change Materials, such as Ge2Sb2Te5 and GeTe used in non-volatile phase change memories, was shown to improve the stability of the amorphous phase and to strongly increase the crystallization temperature. We combined total X-ray scattering experiments [1], Fourier Transform InfraRed (FTIR) spectroscopy and ab initio Molecular Dynamics simulations to address the stabilization of the amorphous phase of GeTe doped with Carbon or Nitrogen. The comparison between the simulation and exp erimental results allows in depth understanding of the role of dopants. They deeply modify the structure of the amorphous phase by introducing tetrahedral units centered on C or N, triangular environments and short C chains in C-doped GeTe, N-Ge3 pyramids and N2 molecules in N-doped GeTe. One major difference between N and C doping is the fact that C can form short bonds with Te, although in smaller proportion than C-Ge, while short N-Te bonds are absent in N-doped GeTe. The inclusion of C or N leads to an increase in high frequency vibrational modes, to a reduction of the density of floppy vibrational modes that drive the crystallization and to an increase of the rigidity. This stabilization mechanism could apply more generally to various GST materials as well as to other iono-covalent glasses. [1] G.E. Ghezzi, J.Y.Raty, S. Maitrejean, A. Roule, E. Elkaim and F. Hippert, Appl. Phys. Lett. 99 (2011) 151906 [less ▲]

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See detailStructure, dynamical properties and topology of GST phase change materials
Raty, Jean-Yves ULg; Bichara, Christophe

Conference (2013, June 04)

Ge-Sb-Te alloys (GST) have been successfully used as Phase Change Materials for data recording applications. They exhibit a unique collection of properties that make them adequate for high performance non ... [more ▼]

Ge-Sb-Te alloys (GST) have been successfully used as Phase Change Materials for data recording applications. They exhibit a unique collection of properties that make them adequate for high performance non-volatile RAM memories. Indeed, their crystalline phase(s) are extremely contrasted, both optically and electrically, against their easily generated amorphous phase(s). Understanding the fast switching mechanisms and the amorphous phase properties remains however quite challenging. In particular, the stability of the amorphous phase against phase separation, recrystallization, and drift of the electronic properties with time and cycling remain problematic. In this work, we generate models of a series of amorphous GST alloys using DFT molecular dynamics and simulated annealing. The obtained structures are compared and generic patterns are described. A new algorithm to determine the topological constraints, based on the analysis of the fluctuations of interatomic bond lengths and bond angles, has been proposed and applied to seven alloy compositions in the GeSbTe phase diagram [1]. A constraint map could then be drawn for the whole ternary system. According to this, the most commonly used GST amorphous phases are shown to be in the stressed rigid region [1]. In order to estimate the effect of rigidity on the recrystallization temperature (Tc), we performed a similar study for Carbon and Nitrogen doped GeTe [2]. The increase of the number of constraints upon doping is shown to be correlated with an increase of the recrystallization temperature and a reduction of the low frequency, or floppy, vibrational modes [3]. The generality of this stabilization mechanism is finally tested on other systems. [1] M. Micoulaut, J.Y. Raty, C. Otjacques, and C. Bichara, Physical Review B 81 (2010) 174206 [2] G. Ghezzi, J.Y. Raty, S. Maitrejean, A. Roule, E. Elkaim and F. Hippert, Applied Physics Letters, 99 (2011) 151906. [3] J.Y. Raty, P. Noe, G.Ghezzi, S. Maitrejean, C. Bichara and F. Hippert. To be published. [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 detailAb Initio Molecular Dynamics Simulation of Liquid and Amorphous Te
Raty, Jean-Yves ULg; Malcioglu, Osman Baris; Bichara, Christophe

Conference (2013, April)

Contrary to almost all other elements, liquid and amorphous phases of pure tellurium have proven difficult to simulate using ab initio molecular dynamics. Standard density functional theory calculations ... [more ▼]

Contrary to almost all other elements, liquid and amorphous phases of pure tellurium have proven difficult to simulate using ab initio molecular dynamics. Standard density functional theory calculations yield structures in relatively poor agreement with available diffraction experiments at low temperature, especially regarding first neighbor distance and coordination number, which are strongly overestimated in the simulations. Tellurium being a key component of many phase change materials, this poor structural description of its disordered phases raises important issues about the ability of ab initio molecular dynamics to generate accurate structural models of amorphous phases. In this work, we use ab initio molecular dynamics performed under constant volume (experimental values) conditions to simulate liquid Tellurium structure and dynamics along its density anomaly. We test different exchange correlation functionals and approximations, and show their influence on liquid and amorphous structures. In particular, we show that the treatment of dispersion forces is yielding a clear improvement over recent hybrid functional calculations [1], with significant local order modifications in both phases. Especially, the structure evolution along the density anomaly is shown to be related to the creation of many interconnections between Te chains, these chains having increasing lengths upon temperature reduction. In the amorphous phase, Te chains are almost perfectly isolated with specific dihedral angle distributions. These structural changes are reflected on dynamical properties, such as atomic diffusion coefficient and vibrational density of states. We then apply the same method to revisit the structure of some Te based alloys. [1] J. Akola, R. O. Jones, S. Kohara, T. Usuki, and E. Bychkov, Phys. Rev. B 81, 094202 (2010). [less ▲]

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See detailCdTe Nanocrystals: Dynamic Effect of Solvation on the Optical Properties of a CdTe Nanocrystal (Advanced Optical Materials 3/2013)
Malcioǧlu, O. B.; Raty, Jean-Yves ULg

in Advanced Optical Materials (2013), 1(3), 238-238

Molecular dynamics simulations are used to establish a correlation between the time evolution of spectral features with various structural components of a ligand-stabilized CdTe quantum dot. The ... [more ▼]

Molecular dynamics simulations are used to establish a correlation between the time evolution of spectral features with various structural components of a ligand-stabilized CdTe quantum dot. The localization of charge oscillations due to light absorption is investigated by O. B. Malci{dotless}oǧlu et al. on page 239 before and after embedding the quantum dot in an explicit solvent environment. The image shows a snapshot from the solvated molecular dynamics trajectory. The response charge density of a quantum dot to two different frequencies of irradiation is depicted as colored glass bubbles. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲]

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See detailDynamic Effect of Solvation on the Optical Properties of a CdTe Nanocrystal
Malcioǧlu, O. B.; Raty, Jean-Yves ULg

in Advanced Optical Materials (2013), 1(3), 239-243

Changes in the optical response of a CdTe quantum dot (QD) due to dynamic and surface effects are investigated using ab initio methods. The model successfully captures experimentally reported non-linear ... [more ▼]

Changes in the optical response of a CdTe quantum dot (QD) due to dynamic and surface effects are investigated using ab initio methods. The model successfully captures experimentally reported non-linear trends in the optical spectra. The combination of ab initio molecular dynamics, time-dependent DFT, and optical spectroscopy provides a very effective method to investigate various dynamic effects on very small QDs, and to explore generalizable trends for similar structures. Using TDDFT and ab-initio molecular dynamics, a correlation is established between the time evolution of spectral features with various structural components of a ligand-stabilized CdTe quantum dot. The localization of charge oscillations due to light absorption is investigated before and after embedding the quantum dot in an explicit solvent environment. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲]

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