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Phase Change Materials from First Principles

Titel Englisch Phase Change Materials from First Principles
Gesuchsteller/in Parrinello Michele
Nummer 119882
Förderungsinstrument Projektförderung (Abt. I-III)
Forschungseinrichtung Computational Science Dept. of Chemistry and Applied Biosciences ETH Zürich
Hochschule ETH Zürich - ETHZ
Hauptdisziplin Physikalische Chemie
Beginn/Ende 01.04.2008 - 31.03.2010
Bewilligter Betrag 192'813.40
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Keywords (9)

Ab-initio molecular dynamics; Linear scaling; Car-Parrinello molecular dynamics; electronic structure calculations; chalcogenide alloys; materials for microelectronics; phase change materials; amorphous materials; non-volatile memories

Lay Summary (Englisch)

Lay summary
Phase change materials based on chalcogenide alloys are presently used in optical storage devices (DVD) and are promising materials for non-volatile electronic memories of new concept, the Phase Change Memory (PCM) device. Both applications rely on the reversible and fast transition between the amorphous and crystalline phases whose difference in optical and electronic properties are exploited to store information. Among the chalcogenide glasses, Ge2Sb2Te5 (GST) is the material of choice for PCM applications due to its superior performances in terms of speed of transformation and difference in electronic conductivity between the (insulating) amorphous and crystalline (metallic) phases. In spite of the great technological importance of GST and related materials, their microscopic structures and the detailed mechanism of the phase transformation are largely unknown. In this project we will investigate GST and related binary materials (GeTe, GeSb, Sb2Te3) by atomistic quantum-mechanical simulations. Phase Change Memories were included in the International Technology Roadmap for Semiconductor (ITRS) in 2005, due to their superior scalability in size with respect to non-volatile memories presently in use and based on silicon (Flash) technology. Although commercialization of the first PCM devices by major semiconductor industries are envisaged in few years, a better understanding of the microscopic properties of this class of materials than currently exists is strongly needed to improve the performances of PCM devices and enlarge their potential market. To make the PCM suitable for applications in automotive technologies, for instance, the stability of the amorphous phase against recrystallization (and thus the data recorded) must be extended above 150 C, but still preserving a fast phase switching. Based on density functional molecular dynamics simulations, we aim at elucidating the structure, the electronic properties and transformation mechanism of the amorphous phase of GST and related binary materials. In the first six months of the project we have gained insight on the structure of amorphous GeTe and GST and on the mechanism of the reversible crystal-to-amorphous transition from the simulation of pressure induced amorphization of GST, recently demonstrated experimentally. In particular, the simulations of amorphous GeTe and GST have revealed the coexistence of the two types of local environment for Ge, a tetrahedral site and a defective octahedral site, the latter recalling the structure of the crystalline (rocksalt) phase. We might envisage that tetrahedral sites, absent in the crystalline phase, would partially hinder the crystallization process. Would this be the case, the larger concentration of tetrahedral sites we have found in GeTe with respect to GST might explain the difference in stability of the amorphous phase of two compounds measured experimentally. The simulations have then provided a hint in the search of alloys with a higher stability of the amorphous phase well needed for better performing PCM devices.
Direktlink auf Lay Summary Letzte Aktualisierung: 21.02.2013

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