Nucleus-driven crystallization of amorphous Ge<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>Sb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>Te<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>5</mml:mn></mml:msub></mml:math>: A density functional study

Kalikka, Janne; Akola, Jaakko; Larrucea, Julen; Jones, R.

doi:10.1103/physrevb.86.144113

Cited by 83 publications

(71 citation statements)

References 37 publications

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“…The first (run0) had an ordered history, as it had the same starting structure as Ref. [12], with 58-atom crystalline seed embedded in the original a-GST structure. However, run0 was carried out without any structural constraints, and the seed disappeared rapidly.…”

Section: Resultsmentioning

confidence: 99%

“…The first simulations of this process supported this picture [11]. Our studies of crystallization of phase-change materials have used density functional (DF) calculations combined with molecular dynamics (MD) and include a 460-atom sample of a-GST at 500 K, 600 K, and 700 K, and a 648-atom sample at 600 K [12]. We used a fixed crystalline seed (58 atoms, six vacancies) and observed crystallization at 600 K and, somewhat faster, at 700 K. Ultrafast heating calorimetry indicated that crystallization is fastest at 670 K [13].…”

Section: Introductionmentioning

confidence: 87%

“…Cavities (vacancies, voids) are defined as in our previous work [8,9,12,26]. Cavity domains (regions where all points are at least 2.8Å from all atoms) are determined on a mesh of 0.057Å, and cavities are found by Voronoi construction from all points on a domain surface to nearby atoms.…”

Section: Cavitiesmentioning

confidence: 99%

“…The methods we use for calculation and data analysis are described in detail in Ref. [12] and are summarized in Sec. II.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

2014

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Crystallization of amorphous Ge 2 Sb 2 Te 5 (GST) has been studied using four extensive (460 atoms, up to 4 ns) density functional/molecular dynamics simulations at 600 K. This phase change material is a rare system where crystallization can be simulated without adjustable parameters over the physical time scale, and the results could provide insight into order-disorder processes in general. Crystallization is accompanied by an increase in the number of ABAB squares (A: Ge, Sb; B: Te), percolation, and the occurrence of low-frequency localized vibration modes. A sample with a history of order crystallizes completely in 1.2 ns, but ordering in others was less complete, even after 4 ns. The amorphous starting structures without memory display phases (>1 ns) with subcritical nuclei (10-50 atoms) ranging from nearly cubical blocks to stringlike configurations of ABAB squares and AB bonds extending across the cell. Percolation initiates the rapid phase of crystallization and is coupled to the directional p-type bonding in metastable GST. Cavities play a crucial role, and the final ordered structure is distorted rock salt with a face-centered cubic sublattice containing predominantly Te atoms. We comment on earlier models based on smaller and much shorter simulations.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 87%

Section: Cavitiesmentioning

confidence: 99%

“…The methods we use for calculation and data analysis are described in detail in Ref. [12] and are summarized in Sec. II.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

2014

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“…The large number of vacancies in Ge/Sb layers provides ample room to accommodate the adjacent Te. These antisite Te atoms are sometimes observed in the rapid crystallization from a supercooled GST liquid using AIMD simulations, [ 17,18 ] but are rarely seen in an equilibrated c -GST at ambient pressure because each antisite Te creates fi ve Te-Te homopolar bonds, which have much higher energy than the heteropolar Ge-Te and Sb-Te bonds. Under high pressure, however, Te atoms shift into the neighboring vacant sites to release the strain energy of the compressed heteropolar bonds.…”

Section: Anomalous Cooperative Antisite Hopping Under Pressurementioning

confidence: 99%

Disorder Control in Crystalline GeSb₂Te₄ Using High Pressure

Zhang

Mazzarello

et al. 2015

Advanced Science

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disordered as possible. However, usually such chemical disorder is discouraged by the concomitant enthalpy rise, caused by the formation of homopolar bonds. Disorder can induce localization of electronic wavefunctions (e.g., Anderson localization [ 1 ] and thus can be crucial for the electronic properties of materials. [2][3][4][5][6][7] The current applications of phasechange materials (PCMs) take advantage of the fast transformations and large property contrast between the amorphous and the crystalline states. [8][9][10][11] Such binary memory devices, however, may fail to meet the increasingly demanding requirements of data storage. A possible solution to this issue is to record data on each memory cell with multiple states of electrical resistivity, in addition to "on" and "off" switches. This may be achieved by tuning disorder in PCMs. Therefore, it is important to understand and control disorder in these materials. The development of a multistate memory device would signifi cantly increase the data density and could change the way electronic devices work. [ 12,13 ] To manipulate the disorder in data storage media, Siegrist et al. have modifi ed the atomic arrangement in crystalline PCMs such as Ge-Sb-Te (GST), a prototype of PCMs, by annealing. [ 2 ] At low annealing temperatures, these GST samples form metastable cubic rocksalt phases ( c -GST) with different levels of electrical resistivity. In this phase, one sublattice contains Te atoms, whereas Ge, Sb, and vacancies occupy the sites of the second sublattice in a random fashion. High annealing temperatures induce the ordering of the vacancies in c -GST, which gradually evolves into a metallic hexagonal phase ( h -GST) with all vacancies diffusing into layers. Ab initio simulations [ 3 ] show that such multiple resistive states are indeed due to the different degrees of vacancy ordering. In particular, strong disorder results in the localization of electron wave functions at the Fermi energy.In this article, we report that the compositional disorder in PCMs can also be tuned by pressure in lieu of the thermal treatment. Large-scale ab initio molecular dynamics (AIMD) simulations reveal that pressure can expedite the antisite hopping in the vacancy-ridden c -GST by lowering the migration barrier. Accumulation of these antisites leads to severe atomic distortions. The resulting strong misalignment of bonds may trigger the loss of the long-range order in the crystal [ 14 ] and contributes to the amorphization of c -GST under high pressure, as observed in experiments at 15 GPa. [ 15,16 ] Our simulations identify a new disorder-triggered mechanism of the amorphization Electronic phase-change memory devices take advantage of the different resistivity of two states, amorphous and crystalline, and the swift transitions between them in active phase-change materials (PCMs). In addition to these two distinct phases, multiple resistive states can be obtained by tuning the atomic disorder in the crystalline phase with heat treatment, because the disorder can lead to...

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Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

Skelton

Pallipurath

Lee

et al. 2014

Adv Funct Materials

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Phase‐change alloys are the functional materials at the heart of an emerging digital‐storage technology. The GeTe‐Sb2Te3 pseudo‐binary systems, in particular the composition Ge2Sb2Te5 (GST), are one of a handful of materials which meet the unique requirements of a stable amorphous phase, rapid amorphous‐to‐crystalline phase transition, and significant contrasts in optical and electrical properties between material states. The properties of GST can be optimized by doping with p‐block elements, of which Bi has interesting effects on the crystallization kinetics and electrical properties. A comprehensive simulational study of Bi‐doped GST is carried out, looking at trends in behavior and properties as a function of dopant concentration. The results reveal how Bi integrates into the host matrix, and provide insight into its enhancement of the crystallization speed. A straightforward explanation is proposed for the reversal of the charge‐carrier sign beyond a critical doping threshold. The effect of Bi on the optical properties of GST is also investigated. The microscopic insight from this study may assist in the future selection of dopants to optimize the phase‐change properties of GST, and also of other PCMs, and the general methods employed in this work should be applicable to the study of related materials, for example, doped chalcogenide glasses.

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Nucleus-driven crystallization of amorphous Ge2Sb2Te5: A density functional study

Cited by 83 publications

References 37 publications

Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

Disorder Control in Crystalline GeSb₂Te₄ Using High Pressure

Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

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Nucleus-driven crystallization of amorphous Ge2Sb2Te5: A density functional study

Cited by 83 publications

References 37 publications

Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

Disorder Control in Crystalline GeSb2Te4 Using High Pressure

Atomistic Origin of the Enhanced Crystallization Speed and n‐Type Conductivity in Bi‐doped Ge‐Sb‐Te Phase‐Change Materials

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Disorder Control in Crystalline GeSb₂Te₄ Using High Pressure