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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 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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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 detailStructure et dynamique de Te et GeTe liquides et amorphes : simulation par dynamique moléculaire ab initio
Raty, Jean-Yves ULg; Malcioglu, Osman Baris; Bichara, Christophe

Scientific conference (2012, December)

Contrairement à la plupart des éléments, les phases liquide et amorphe du tellure se sont avérées être assez mal reproduites par la méthode, pourtant éprouvée, de la dynamique moléculaire ab initio . Les ... [more ▼]

Contrairement à la plupart des éléments, les phases liquide et amorphe du tellure se sont avérées être assez mal reproduites par la méthode, pourtant éprouvée, de la dynamique moléculaire ab initio . Les calculs ‘standart’ utilisant la théorie de la fonctionnelle de la densité (DFT) produisent en effet des structures en relatif désaccord avec les expériences de diffraction, en particulier pour ce qui concerne le nombre de premiers voisins et les distances interatomiques, quantités qui apparaissent fortement surestimées dans les simulations. Le tellure étant composant essentiel de la plupart des matériaux à changement de phase (PCMs), cette mauvaise description de ses phases désordonnées soulève d’importantes questions au sujet de la capacité de la dynamique moléculaire ab initio à générer des structures d’amorphe réalistes pour les PCMs. Dans ce travail, nous utilisons la dynamique moléculaire ab initio réalisée à volume constant (valeurs expérimentales) pour simuler la structure et la dynamique du tellure liquide le long de l’anomalie de densité. Nous testons différentes approximations afin de démontrer leur importance sur la structure du liquide et de l’amorphe. En particulier, si l’interaction spin-orbite apparaît au final avoir un effet neutre, le traitement des forces de dispersion fournit une nette amélioration vis-à-vis de calculs récents utilisant des fonctionnelles hybrides, l’ordre atomique local étant fortement modifié dans les deux phases [1]. L’évolution de la structure avec l’anomalie de densité est clairement reliée à la création de nombreuses interconnections entre chaines de tellure, ces chaines étant de plus en plus longues avec l’abaissement de la température. Dans la phase amorphe, les chaines sont pratiquement intactes et présentent des distributions d’angle dièdre originales. Ces modifications sont reflétées dans les propriétés dynamiques, telles que le coefficient de diffusion et la densité d’états vibrationnels. [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 detailRole of vacancies in metal-insulator transitions of crystalline phase-change materials
Zhang, Wei; Thiess, Alex; Zalden, Peter et al

in Nature Materials (2012), 11(11), 952-956

The study of metal-insulator transitions (MITs) in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of ... [more ▼]

The study of metal-insulator transitions (MITs) in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of materials. Recently, a MIT governed by disorder was observed in crystalline phase-change materials. Here we report on calculations employing density functional theory, which identify the microscopic mechanism that localizes the wavefunctions and is driving this transition. We show that, in the insulating phase, the electronic states responsible for charge transport are localized inside regions having large vacancy concentrations. The transition to the metallic state is driven by the dissolution of these vacancy clusters and the formation of ordered vacancy layers. These results provide important insights on controlling the wavefunction localization, which should help to develop conceptually new devices based on multiple resistance states. © 2012 Macmillan Publishers Limited. All rights reserved. [less ▲]

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See detailPhase Change Memories challenges: A material and process perspective
Maitrejean, S.; Ghezzi, G.; Gourvest, E. et al

in 2012 IEEE International Interconnect Technology Conference, IITC 2012 (2012)

Among all the new memories concepts, Phase Change Memories (PCM) is one of the most promising. However, various challenges remain. This paper reviews the materials and processes required to face these ... [more ▼]

Among all the new memories concepts, Phase Change Memories (PCM) is one of the most promising. However, various challenges remain. This paper reviews the materials and processes required to face these challenges. As an example, attention will be made on the effect of Phase change material composition on stability of the amorphous phase i.e. on the retention of the information. Additionally, it is showed how specific processes such as CVD or ALD can be developed in order to minimize the current required to amorphize the phase change material i.e. to reset the device. Finally, with the perspectives of the advanced integration nodes, experimental results on the effect of scaling on phase transformation are presented and discussed. © 2012 IEEE. [less ▲]

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See detailTi impact in C-doped phase-change memories compliant to Pb-free soldering reflow
Perniola, L.; Noe, Pascal ULg; Hubert, Q. et al

in Technical Digest - International Electron Devices Meeting, IEDM (2012)

In this paper, we present a thorough physical-chemical analysis of an engineered PCM stack, where the integration of C-doping and the use of a Ti top layer allow obtaining an Amorphous As-Deposited (A-AD ... [more ▼]

In this paper, we present a thorough physical-chemical analysis of an engineered PCM stack, where the integration of C-doping and the use of a Ti top layer allow obtaining an Amorphous As-Deposited (A-AD) phase stable against Back End-Of-Line (BEOL) thermal budget. This PCM stack is then integrated in devices, which are extensively tested in order to validate a novel pre-coding technique compliant to the Pb-free soldering reflow issue. Finally, an original design to optimize the distribution dispersion is presented. © 2012 IEEE. [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 detailBand structure modulation of ZnSe/ZnTe nanowires under strain
Pekoz, Rengin; Raty, Jean-Yves ULg

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(16), 1654448-16544481654448

First-principles calculations have been used to investigate the structural and electronic properties of unpassivated ZnSe, ZnTe and ZnX/ZnY [X = Se(Te),Y = Te(Se)] core/shell nanowires oriented along the ... [more ▼]

First-principles calculations have been used to investigate the structural and electronic properties of unpassivated ZnSe, ZnTe and ZnX/ZnY [X = Se(Te),Y = Te(Se)] core/shell nanowires oriented along the [111] direction with hexagonal cross sections. The effects of quantum confinement and strain on the electronic properties of the nanowires have been explored for different diameters and core/shell thicknesses. We observe that strong band structure modulation is achievable through uniaxial strain. While for ZnTe nanowires, compression induces a direct-to-indirect band transition for diameters larger than 1.4 nm, there is no sign for a similar transition either for single component ZnSe or core/shell nanowires. The wave function analysis reveals a strong preference for localizing the electron states inside ZnSe rich regions. [less ▲]

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See detailEffect of carbon doping on the structure of amorphous GeTe phase change material
Ghezzi, G. E.; Raty, Jean-Yves ULg; Maitrejean, S. et al

in Applied Physics Letters (2011), 99(15), 1519063-15190631519063

Carbon-doped GeTe is a promising material for use in phase change memories since the addition of C increases the stability of the amorphous phase. By combining x-ray total scattering experiments and ab ... [more ▼]

Carbon-doped GeTe is a promising material for use in phase change memories since the addition of C increases the stability of the amorphous phase. By combining x-ray total scattering experiments and ab initio molecular dynamics, we show that carbon deeply modifies the structure of the amorphous phase through long carbon chains and tetrahedral and triangular units centered on carbon. A clear signature of these units is the appearance of an additional interatomic distance, around 3.3 Aring in the measured pair distribution function. Besides, the first Ge-Ge and Ge-Te distances are almost not affected by doping. [less ▲]

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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 detailFirst-principles design of efficient solar cells using two-dimensional arrays of core-shell and layered SiGe nanowires
Pekoz, Rengin; Malcioglu, Osman Baris ULg; Raty, Jean-Yves ULg

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(3), 0353176-03531760353176

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using ... [more ▼]

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using first-principles methods, Ge/Si and Si/Ge core/shell and Si-Ge layered nanowires are shown to possess the required qualities for an efficient use in photovoltaic applications. We investigate the details of their band structure, effective mass, absorption property, and charge-carrier localization. The strong charge separation and improved absorption in the visible spectrum indicate a remarkable quantum efficiency that, combined with new designs, compares positively with bulk Si. [less ▲]

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See detailDFT studies of solvation effects on the nanosize bare, thiolated and redox active ligated Au55 cluster
Periyasamy, Ganga ULg; Durgun, Engin; Raty, Jean-Yves ULg et al

in Journal of Physical Chemistry C: Nanomaterials, Interfaces, and Hard Matter (2010), Protected Metallic Clusters, Quantum Wells and Metal-Nanocrystal Molecules Symposium

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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 detailStructural and vibrational study of the negative thermal expansion in liquid As2Te3
Otjacques, C.; Raty, Jean-Yves ULg; Hippert, F. et al

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82(5), 0542029-05420290542029

We present an experimental and theoretical study of liquid As 2Te 3. This alloy exhibits a negative thermal expansion (NTE) in a 250 K range above the melting temperature T m = 654 K. We evidence the ... [more ▼]

We present an experimental and theoretical study of liquid As 2Te 3. This alloy exhibits a negative thermal expansion (NTE) in a 250 K range above the melting temperature T m = 654 K. We evidence the changes in As 2Te 3 structure by measuring neutron-diffraction spectra at five temperatures in the NTE range and perform first principles molecular dynamics simulations at the same temperatures and densities to study the local order evolution in the liquid. Our calculated structures show an increase in the coordination numbers and a symmetrization of the first neighbors shell around atoms when the temperature rises. To confirm these results, we performed inelastic neutron scattering to obtain the vibrational density of state (VDOS) along the NTE. We see a clear change in the VDOS, consisting in a redshift of the highest frequencies with temperature. Finally, electrical conductivity evolution was obtained from the simulated structures, to compare with the semiconductor to metal transition measured experimentally. [less ▲]

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See detailThe mayonnaise droplet
Terwagne, Denis ULg; Mack, Nicolas; Dorbolo, Stéphane ULg et al

Poster (2009, March)

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