Unravelling the interplay of local structure and physical properties in phase-change materials

Wełnic, Wojciech; Pamungkas, Ariesto; Detemple, Ralf; Steimer, C.; Blügel, Stefan; Wuttig, Matthias

doi:10.1038/nmat1539

Cited by 311 publications

(220 citation statements)

References 19 publications

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“…38 Amorphization in semiconductors usually results in broader valence and conduction bands that can overlap, so a larger band gap is unusual. 10 The overall form of the DOS ͑Fig. 10͒ can be understood in terms of a simple model for systems with average valence of 5 ͑GeTe͒ or near this value ͑GST͒.…”

Section: E Electronic Structurementioning

confidence: 99%

“…A model of amorphous Ge 1 Sb 2 Te 4 where a spinel structure ͑tetrahedral Ge͒ was compared to the octahedral rocksalt phase has also been studied. 10 Most recently, x-ray fluorescence holography of an epitaxial layer of GST indicated a cubic structure with tetrahedral site symmetry about Ge atoms. 11 It is remarkable that phase-change materials could become the basis of commercially successful products with so much uncertainty about the structures of the phases involved.…”

Section: Introductionmentioning

confidence: 99%

“…19 Nevertheless, their demands on computational resources have restricted simulations on GST systems to relatively small unit cells ͑up to 56 atoms in Ref. 10, 108 atoms and 12 vacancies in Ref. 15, and 168 atoms in Ref.…”

Section: Introductionmentioning

confidence: 99%

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Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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Phase-change materials are of immense importance for optical recording and computer memory, but the structure of the amorphous phases and the nature of the phase transition in the nanoscale bits pose continuing challenges. Massively parallel density functional simulations have been used to characterize the amorphous structure of the prototype materials Ge 2 Sb 2 Te 5 and GeTe. In both, there is long-ranged order among Te atoms and the crucial structural motif is a four-membered ring with alternating atoms of types A ͑Ge and Sb͒ and B ͑Te͒, an "ABAB square." The rapid amorphous-to-crystalline phase change is a reorientation of disordered ABAB squares to form an ordered lattice. There are deviations from the "8 − N rule" for coordination numbers, with Te having near threefold coordination. Ge atoms are predominantly fourfold coordinated, but-contrary to recent speculation-tetrahedral coordination is found in only approximately one-third of the Ge atoms. The average coordination number of Sb atoms is 3.7, and the local environment of Ge and Sb is usually "distorted octahedral" with AB separations from 3.2 to 4 Å in the first coordination shell. The number of A -A bonds is significantly greater in GeTe than in Ge 2 Sb 2 Te 5 . Vacancies ͑voids͒ in the disordered phases of these materials provide the necessary space for the phase transitions to take place. The vacancy concentration in Ge 2 Sb 2 Te 5 ͑11.8%͒ is greater than in GeTe ͑6.4%͒, which is consistent with the better phase-change performance of the former.

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Section: E Electronic Structurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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“…Kolobov et al [8], however, proposed that Ge and Sb atoms are displaced from their ideal positions, enabling the order-disorder transition to occur as an ''umbrella flip'' of Ge atoms from octahedral to tetrahedral positions. A model of amorphous Ge 1 Sb 2 Te 4 where a spinel structure (tetrahedral Ge) was compared to the octahedral rock salt phase has also been studied [9], and X-ray fluorescence holography of an epitaxial layer of GST indicated a cubic structure with tetrahedral site symmetry about Ge atoms [10]. It is astonishing that PC materials could become the basis of commercially successful products with so much uncertainty about the structures of the phases involved.…”

mentioning

confidence: 99%

“…Nevertheless, their demands on computational resources restricted simulations on GST systems up to 2007 to relatively small unit cells (up to 56 atoms in Ref. [9], 108 atoms and 12 vacancies in Ref. [12], 168 atoms in Ref.…”

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confidence: 99%

Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

Akola

Jones

2012

Physica Status Solidi (b)

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Since their first development in the 1960s, phase change (PC) memory materials have become essential components of optical memories (DVD‐RW, Blu‐ray Disc, …) used in countless households worldwide. They are now poised to play a decisive role in future non‐volatile computer memories. PC memory materials are restricted to those with an extremely rapid and reversible transition between the amorphous and crystalline phases of an appropriate recording medium, and their development and optimization has been hindered by the inherent difficulties in determining amorphous structures. Alloys of germanium, antimony, and tellurium (“GST” alloys) are among the most widely used in the above contexts, and there has been much speculation concerning their amorphous structures. Theoreticians were slow to appreciate the rich array of problems awaiting their attention, but the past few years have seen a dramatic change in this situation. Density functional (DF) calculations, which are generally free of adjustable parameters, have now been reported on numerous systems. We review here the information that has resulted about the amorphous structures of GST‐alloys.

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Phase Change Memories: Principles and Applications

Pirovano

2016

Wiley Encyclopedia of Electrical and Electronics Engineering

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The phase‐change memory (PCM) is a nonvolatile semiconductor technology based on thermally induced phase transitions of a thin‐film material, typically a chalcogenide. PCM relies on a resistance change to store data permanently. Although the historical origin of the PCM concept dates back to the 1970s, nowadays it is considered one of the most promising candidates for the next‐generation nonvolatile memories (NVM). In comparison with flash, PCM has the potential to improve the random access time, read throughput (versus NOR flash), write throughput (versus NAND flash), direct write, bit granularity, and endurance, as well as to be scalable beyond flash technology. The interest of the semiconductor industries for the PCM technology has led to several research programs that culminated in the mass production in 2012 of a 1 GB PCM chip for mobile applications.

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Unravelling the interplay of local structure and physical properties in phase-change materials

Cited by 311 publications

References 19 publications

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

Phase Change Memories: Principles and Applications

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