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Jülich supercomputer identifies key factors for improved optical storage materials

Structure of phase change material unravelled

[8. Juli 2009]

Jülich, 9 July 2009 - An international team of researchers from science and industry – with decisive participation from Jülich – has achieved a breakthrough in materials research. A German-Japanese team has found a convincing explanation for the long disputed structure of a material used in optical storage, for example, in DVDs. The new insights should simplify the search for improved storage media and were made possible by simulations on the "JUGENE" supercomputer in Jülich and experiments at the "SPring-8" synchrotron in Japan. The journal "Physical Review B - Rapid Communications" reports on this in its current online issue (DOI: 10.1103/PhysRevB.80.020201).

Physicists at Forschungszentrum Jülich – a member of the Helmholtz Association – have succeeded in clarifying previously unsolved problems concerning the structure of so-called phase change materials as exemplified by the alloy Ge2Sb2Te5 (GST). GST is the “memory” of rewritable optical storage media such as DVD-RAM. Similar materials are also used in Blu-ray Discs. GST can be switched between two states (“phases”): an ordered “crystalline” form and a disordered “amorphous” form. Information can be written and stored in this way, and the different reflectivities of the two states allow the stored information to be “read”. Both switching and reading occur using a laser.

“Optical storage media based on GST have been marketed since the 1990s, but it was unclear until now how switching occurred on an atomic scale,” explained Dr. Robert Jones, theoretical physicist at Forschungszentrum Jülich. “The rearrangement of the atoms occurs on a very short time scale, within a few nanoseconds. This raised the question as to what sort of structure makes this possible. Only when this is known, can we focus on finding better materials.” However, structures without a regular crystalline arrangement are very difficult to determine. Numerous, often contradictory theories have been proposed in recent years.

The Jülich physicists solved this problem using a stepwise procedure. They used the JUGENE supercomputer to simulate the cooling of molten GST to an amorphous state, as occurs when information is written. Based on the results of the first calculation, they adjusted their model bit by bit using the experimental data of their Japanese colleagues at the SPring-8 synchrotron until the final structure was obtained.

The difference between this study and others is the sheer scale: 460 atoms were simulated over the comparatively long time of 300 picoseconds, approximately the same time scale required for the atoms to reorder in the experiment. Over 4,000 processors of the Jülich computers were occupied with this task for nearly four months. “There are only a few places in the world where so much computer power is available,” said Jones. “This put us in a position to identify the key factors that make the fast phase change in GST possible.”

The key factors are square building blocks composed of atoms that occur in both amorphous and crystalline states. Each component consists of four atoms ordered in the shape of a ring. Every second atom is tellurium, while the others are either antimony or germanium. Cavities play an essential role in providing space to allow these building blocks to rearrange without breaking too many bonds. A laser creates the conditions for rapid phase change.

The results are an important step towards a better understanding of phase change materials and for the development of rules for designing new storage media. The ultimate goal is to predict important properties of materials on the basis of chemical composition and to design tailor-made materials.

Original Publications

Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change material Ge2Sb2Te5
Phys. Rev. B 80, 020201(R) (2009); DOI: 10.1103/PhysRevB.80.020201

Structure of liquid phase change material AgInSbTe from density functional/molecular dynamics simulations
Appl. Phys. Lett. 94, 251905 (2009); DOI:10.1063/1.3157166

Structure of amorphous Ge8Sb2Te11: GeTe-Sb2Te3alloys and optical storage
Phys. Rev. B 79, 134118 (2009); DOI: 10.1103/PhysRevB.79.134118

Structural phase transitions on the nanoscale: The crucial pattern in the phase change materials Ge2Sb2Te5 and GeTe
Phys. Rev. B 76, 235201 (2007); DOI: 10.1103/PhysRevB.76.235201

Further information: Institute of Solid State Research


Image of GST produced by simulation. The ring structures and the blue cavities are clearly visible. Their interplay makes rapid phase change possible in this class of materials. The elements are marked in colour: tellurium: yellow; germanium: red; antimony: blue.

Press contact

Angela Wenzik
Science Journalist
Forschungszentrum Jülich
Institute of Solid State Research
52425 Jülich, Germany
tel. +49 2461 61-6048