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Atomic-Level Clarification of Differences in Recording Mechanism between Typical Optical Disc Materials (Press Release)

Release Date
10 Jan, 2011
  • BL04B2 (High Energy X-ray Diffraction)
  • BL14B2 (Engineering Science Research II)
  • BL47XU (HAXPES / uCT)
- Providing basic findings that will accelerate the development of next-generation materials

Japan Science and Technology Agency (JST)
Panasonic Corporation
Japan Synchrotron Radiation Research Institute (JASRI)

As part of JST Use-Inspired Fundamental Research, a research group led by Masaki Takata (Chief Scientist of RIKEN SPring-8 Center), Toshiyuki Matsunaga (Chief Engineer of Panasonic Corporation), and Noboru Yamada (General Manager of Panasonic Corporation) has succeeded in clarifying the differences in the recording mechanism between two typical phase*1-change materials that are commercially used for rewritable digital versatile discs (DVDs) and Blu-ray discsTM (BDs), i.e., germanium antimony telluride (GeSbTe) and silver indium antimony telluride (AgInSbTe). Specifically, they clarified, for the first time in the world, that the characteristic differences in crystallization process between these materials are caused by differences in the atomic arrangement between the crystalline and amorphous*2 phases and in their recombination mechanism by both atomic-level analytical experiments and theoretical simulations.

Currently, BDs are rapidly becoming a widespread large-capacity recording medium for high-quality images in digital high-vision broadcasting and digital video cameras. To record higher quality images over a longer time in the future, it is necessary to attempt to overcome the limits to the recording speed and density of current recording materials. However, the rewriting mechanism of GeSbTe and AgInSbTe materials still remains at the stage of hypothesis, and its atomic-level demonstration and clarification, which will provide important information for the development of materials, have not been achieved.

In 2008, this research group carried out pinpoint structural measurements*3 on two commercially used materials, i.e., GeSbTe and AgInSbTe, at SPring-8, and found that the phase-change processes between a recorded phase (amorphous phase) and an unrecorded phase (crystalline phase) associated with rewriting are clearly different between the above two materials on a nanosecond time scale (one-billionth of a second). However, the scientists did not clarify the structures of the materials at the atomic level. In this research, they combined high-energy X-ray diffraction,*4 X-ray absorption fine structure (XAFS),*5 and hard X-ray photoelectron spectroscopy*6 experiments using the SPring-8 beamline with theoretical simulations carried out at Tampere University of Technology (Finland) using a supercomputer at Forschungszentrum Jülich (Germany) to comprehensively analyze the atomic arrangement and electron structures of GeSbTe and AgInSbTe. From the results, the amorphous phase of GeSbTe contains many fine structures similar to the atomic bonds in the crystalline phase, which simultaneously form crystal nuclei. In contrast, the amorphous and crystalline phases of AgInSbTe have an atomic arrangement with a common basic structure and rearrangement is induced by a change in a certain atomic bond, resulting in an avalanche-like effect. These analytical results for GeSbTe and AgInSbTe revealed the relationships between their composition and crystallization process at the atomic level. This achievement of clarifying the rewriting mechanism of these commercially used materials at the atomic level is hoped to accelerate the development of new materials.

This research was carried out in cooperation with JASRI, the National Institute for Materials Science, Tampere University of Technology, and Forschungszentrum Jülich.

The results of this research were published online in the British scientific journal Nature Materials on 9 January 2011.

"From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials"
Toshiyuki Matsunaga, Jaakko Akola, Shinji Kohara, Tetsuo Honma, Keisuke Kobayashi, Eiji Ikenaga, Robert O. Jones, Noboru Yamada, Masaki Takata and Rie Kojima
Nature Materials 10, 129–134 (2011), published online 9 January 2011

*1 Phase

A phase is the state of a material; e.g., liquid phase, gas phase.

*2 Amorphous
An amorphous solid is a solid without the three-dimensionally ordered atomic arrangement that can be seen in crystals. Amorphous materials are also called noncrystalline materials.

*3 Pinpoint structural measurement
Pinpoint structural measurement refers to technologies that enable the measurement of material structures for a short time (temporal resolution, 40 ps) in an ultrasmall space (submicron spatial resolution, 100 nm) under an extreme environment (e.g., while being excited by high-intensity light, in electric fields, at a high pressure, during the operation of devices). By making full use of the high-brilliance synchrotron radiation source at SPring-8, pinpoint structural measurement technologies have been developed as a method of evaluating the mechanisms underlying the dynamic responses of nanomaterials under various environments for their research and development.

*4 High-energy X-ray diffraction
X-rays are electromagnetic waves with a short wavelength and are diffracted by objects with a repeat unit equivalent to their wavelength. Through the diffraction, it is possible to determine the structures of materials including crystals. SPring-8 beamlines can generate X-rays with a very short wavelength, i.e., a high energy, and can provide highly accurate experimental data that cannot be obtained with conventional laboratory X-ray diffraction equipment.

*5 X-ray absorption fine structure (XAFS)
All atoms have a certain energy at which the absorption of X-rays sharply increases. This is called an absorption edge. When absorption fine structures appearing at energies higher than the absorption edge are subjected to spectroscopy, information on the electron state and local structures of atoms can be obtained. XAFS technologies can be applied to materials in various phases, such as gas, solid, and liquid. In XAFS measurement, it is especially effective to use light sources that can generate high-intensity X-rays over a wide range of energies, similar to the SPring-8 synchrotron radiation source.

*6 Hard X-ray photoelectron spectroscopy
Photoelectron spectroscopy is a method of examining the electron state of solids by irradiating X-rays with a certain energy onto a material and measuring the energy of electrons expelled due to the photoelectric effect. When hard X-rays are used, the electron state and chemical bonding state of semiconductors and metals can be measured without being affected by the state of their surface.


Fig. 1 Ge2Sb2Te5 and Ag3.5In3.8Sb75.0Te17.7, commercially used DVD materials
Fig. 1 Ge2Sb2Te5 and Ag3.5In3.8Sb75.0Te17.7, commercially used DVD materials
Fig. 2 Crystallization (erasing of recording) of Ge2Sb2Te5 and Ag3.5In3.8Sb75.0Te17.7

Fig. 2 Crystallization (erasing of recording) of Ge2Sb2Te5 and Ag3.5In3.8Sb75.0Te17.7

For more information, please contact:
Dr. Masaki Takata (RIKEN)
E-mail: mail1
Dr. Noboru Yamada (Panasonic Corporation)
E-mail: mail2

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