SPring-8, the large synchrotron radiation facility

The research groups at Japan Synchrotron Radiation Research Institute (JASRI), RIKEN, and Panasonic Corporation - in collaboration with Hungarian Academy of Science and Yamagata University - have worked together to unravel microscopic mechanisms that enable long-term storage and fast rewriting of information in optical disks. The research successfully elucidated, for the first time in the world, the element-by-element roles played by each constituent element in DVD materials at the atomic level.

The Blu-ray Disc^TM (BD) standard is gaining increasingly wide ground as a recording media for long-term storage of high-resolution images obtained from such modern technologies as digital high-definition televisions and digital video camcorders. On the other hand, the precipitous proliferation in the volume of digital data is posing an increasingly serious concern to society, which has spurred the efforts toward developing recording media capable of providing higher writing speeds, larger data capacities, and longer storage lives. The current DVD and BD technologies in practical use are capable of instantaneous switching between the recording phase (recording state)^*2 and erasing phase (erasing state), i.e. recording and rewriting of information, through laser irradiation for an ultra-short period of time (several tens of nanoseconds). The recording phase can retain its state stably for several tens of years or longer at room temperature. The researchers at the SPring-8 ^*1 have successfully revealed the process of the fast atomic rearrangements that take place when information is optically written on a DVD or BD. However, the specific roles played by each element in this dynamic process - considered essential for realizing longer storage and quicker rewriting - have remained unknown. Clear knowledge of the roles played by each constituting element will be conducive for establishing development guidelines for element selections, leading to higher performance and economical alternatives.

Our research groups made high-intensity X-ray measurements, using the facilities at SPring-8, targeted on one of the matrix materials used in recording media such as DVDs and BDs - i.e. a germanium-antimony-tellurium compound (Ge₂Sb₂Te₅) - whereby the focus was placed on the behavior of antimony atoms and their surroundings, the knowledge of which helps elucidate the roles played by germanium (Ge) and tellurium (Te) atoms as well as antimony (Sb). The data obtained from these experiments were compared with the computer simulation conducted by the research group at Tampere University of Technology (Finland), leading to a successful 3D visualization of the matrix material's steric structure. The results indicated that the network structure consisting of germanium and tellurium is one factor that enables long-term retention of recorded information. In addition, the research showed, for the first time in the world, that the Sb-Te network structure lies in the backdrop to facilitating faster rewriting in the compounds with antimony as a component (typically Ge₂Sb₂Te₅) rather than those consisting only of Ge and Te.

The research results are expected to lead to element-selective designs of recording materials used in DVDs and BDs; the selection of constituent elements and the most suitable atomic arrangement design for each of them will contribute to the development of a higher performance recording material/medium for future use in post-BD media.

The study was conducted jointly by the research groups led by the following scientists: Shinji Kohara (principal researcher, JASRI), Koji Ohara (associate researcher, JASRI), Masaki Takata (principal researcher, RIKEN), Toshiyuki Matsunaga (chief engineer, Panasonic Corporation), and Dr. Noboru Yamada (Panasonic Corporation). The research results were published in the Advanced Online Publication (March 16, 2012) of the scientific journal “Advanced Functional Materials.”

Publication:
"The roles of the Ge-Te core network and the Sb-Te pseudo network during rapid nucleation-dominated crystallization of amorphous Ge₂Sb₂Te₅"
Koji Ohara, László Temleitner, Kunihisa Sugimoto, Shinji Kohara, Toshiyuki Matsunaga ,László Pusztai, Masayoshi Itou, Hiroyuki Ohsumi, Rie Kojima, Noboru Yamada, Takeshi Usuki, Akihiko Fujiwara, Masaki Takata