SPring-8, the large synchrotron radiation facility

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.

(Publication)
"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