Crystal growth within a phase change memory cell

Sebastian, Abu; Gallo, Manuel Le; Krebs, Daniel

doi:10.1038/ncomms5314

Cited by 207 publications

(186 citation statements)

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“…[ 4 ] In another study, [ 6 ] the growth velocity of meltquenched, doped GST was measured in the temperature range between 353 K and 543 K, using phase-change memory cells. [ 6 ] From their measurements, a velocity of about 0.1 m s −1 was extrapolated for T = 600 K. This value is smaller than the one yielded by our simulations. However, the extrapolated value depends sensitively on the parameters and the type of model used (due to the fragile behavior of GST, the growth velocity does not follow the Arrhenius law).…”

Section: Discussionmentioning

confidence: 99%

Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations

Ronneberger

Zhang

Eshet

et al. 2015

Adv Funct Materials

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6407wileyonlinelibrary.com strong dependence of the crystallization speed on temperature has recently been linked to the high fragility of PCMs. [4][5][6][7][8][9][10] More specifi cally, this behavior has been attributed to a pronounced change in the activation energy and the prefactor for growth velocity as a function of temperature, which is a fi ngerprint of fragility.The GeSbTe compounds lying on the pseudobinary line GeTe-Sb 2 Te 3 are the most widely studied family of PCMs, due to their applications in DVD-RAM, rewritable Blu-Ray discs, and phase-change memories. [11][12][13][14][15][16][17][18][19][20][21] Recrystallization of GST is known to be triggered by nucleation events. However, in state-of-the-art memory cells, it may be governed by crystal growth at the interface with the crystalline matrix, due to the small size of the amorphous mark. For system sizes of the order of nanometers, crystallization occurs on the sub-nanosecond time scale at elevated temperatures, opening up the possibility of investigating this phenomenon by ab initio molecular dynamics (AIMD) based on density functional theory (DFT). [22][23][24][25][26] In previous studies [22][23][24] amorphous models of the prototypical compound Ge 2 Sb 2 Te 5 (GST) containing 60-180 atoms were crystallized using AIMD. These works provided valuable information about the microscopic mechanisms of crystallization, although they did not assess fi nite size effects, which are expected to play an important role in such small samples. Larger models of GST containing 460 and 640 atoms were later studied: [ 25,26 ] in one study, [ 25 ] crystallization was facilitated by inserting a crystalline seed inside the amorphous network, whereas, in another [ 26 ] extremely long simulations (4 ns) of 460-atoms models without structural constraints were carried out.The stable crystalline phase of GST is hexagonal and consists of alternating Ge, Sb, and Te layers. [ 27 ] However, there is experimental evidence that, upon fast crystallization of the glass or the supercooled liquid, a metastable rocksalt phase is formed. In this phase, Te atoms occupy one face-centered cubic sublattice, whereas Ge, Sb, and vacancies are randomly arranged in the second one. The amount of disorder in this sublattice is difficult to ascertain. Recently, it has been shown experimentally, [ 28 ] and subsequently corroborated by theory, [ 29 ] that the degree of randomness in crystalline GeSbTe and similar compounds (such as Ge 1 Sb 2 Te 4 ) can be reduced by thermal annealing, so as to induce disorder-driven insulator-metal transitions.In this work, we carry out AIMD simulations to study the crystallization kinetics of GST at high temperature (≈600 K) and estimate its growth velocity, as well as to determine the structural properties of the recrystallized phase, including the layer stacking Ge 2 Sb 2 Te 5 (GST) is an important phase-change material used in optical and electronic memory devices. In this work, crystal growth of GST at 600 K is investigated by ab initio molecular dynamics. S...

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Section: Discussionmentioning

confidence: 99%

Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations

Ronneberger

Zhang

Eshet

et al. 2015

Adv Funct Materials

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“…Laser-driven experiments revealed that shorter pulses can induce crystallization in the MQ but not in the AD state 12,13 . However, approaches to study crystal growth velocities based on optical lasers or electrical excitation were limited to pulse lengths longer than a nanosecond and therefore could not determine growth velocities over the entire range of temperatures 11,[13][14][15][16] . Ultrashort laser pulses have also been employed to excite PCMs, but only the amorphization process was studied systematically as a function of fluence 17,18 .…”

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confidence: 99%

How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

et al. 2015

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Glass forming materials are employed in information storage technologies making use of the transition between a disordered (amorphous) and an ordered (crystalline) state. With increasing temperature the crystal growth velocity of these phase-change materials becomes so fast that prior studies have not been able to resolve these crystallization dynamics. However, crystallization is the time limiting factor in the write speed of phase-change memory devices. Here, for the first time, we quantify crystal growth velocities up to the melting point by using the relaxation of photo-excited carriers as an ultrafast heating mechanism.During repetitive femtosecond optical excitation, each pulse enables dynamical evolution for tens of picoseconds before the intermediate atomic structure is frozen-in as the sample rapidly cools. We apply this technique to Ag 4 In 3 Sb 67 Te 26 (AIST) and compare the dynamics of as-deposited and application-relevant melt-quenched glass. Both glasses retain their different kinetics even in the supercooled liquid state, thereby revealing differences in their kinetic fragilities. This approach enables the characterization of application-relevant properties of phase-change materials up to the melting temperature, which has not been possible before.Mankind has utilized glasses during the last five thousand years. They can be prepared by cooling a liquid fast and far enough below the glass transition temperature -to a temperature where its viscosity is sufficiently high that the atomic arrangement is kinetically frozen-in 1 . Until recently, research and technology have focused on good glass formers, i.e. materials which can be vitrified by cooling their liquid state at moderate rates. But in recent decades poor glass formers such as metallic glasses and certain chalcogenide glasses have gained interest due to their remarkable property portfolio 2,3 . These materials need to be cooled at rates in excess of around 3*10 9 K/s to bypass crystallization and to quench the atoms in an amorphous arrangement 4 . This so-called glass transition at temperature is commonly observed at a

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“…10) A viscosity model, namely, the Mauro-Yue-Ellison-GuptaAllan (MYEGA) model based on the temperature-dependent configurational entropy, 11) has been developed to study the crystallization kinetics. There are three parameters in the MYEGA model, i.e., fragility (m), T g , and viscosity at infinite temperature (η ∞ ).…”

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confidence: 99%

Resolving glass transition in Te-based phase-change materials by modulated differential scanning calorimetry

Chen

Wang

et al. 2017

Appl. Phys. Express

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Glass transitions of Te-based phase-change materials (PCMs) were studied by modulated differential scanning calorimetry. It was found that both Ge 2 Sb 2 Te 5 and GeTe are marginal glass formers with ΔT (= T x % T g ) less than 2.1°C when the heating rate is below 3°C min %1. The fragilities of Ge 2 Sb 2 Te 5 and GeTe can be estimated as 46.0 and 39.7, respectively, around the glass transition temperature, implying that a fragile-to-strong transition would be presented in such Te-based PCMs. The above results provide direct experimental evidence to support the investigation of crystallization kinetics in supercooled liquid PCMs.

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Crystal growth within a phase change memory cell

Cited by 207 publications

References 46 publications

Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations

Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations

How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

Resolving glass transition in Te-based phase-change materials by modulated differential scanning calorimetry

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Crystal growth within a phase change memory cell

Cited by 207 publications

References 46 publications

Crystallization Properties of the Ge2Sb2Te5 Phase‐Change Compound from Advanced Simulations

Crystallization Properties of the Ge2Sb2Te5 Phase‐Change Compound from Advanced Simulations

How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

Resolving glass transition in Te-based phase-change materials by modulated differential scanning calorimetry

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Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations

Crystallization Properties of the Ge₂Sb₂Te₅ Phase‐Change Compound from Advanced Simulations