SPring-8, the large synchrotron radiation facility

Skip to content
Personal tools

Antimony accelerates rewriting of information in DVD media! (Press Release)

Release Date
19 Mar, 2012
  • BL02B1 (Single Crystal Structure Analysis)
  • BL04B2 (High Energy X-ray Diffraction)
– Development of a new guideline for element selection in optical disk material designs

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

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 DiscTM (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.”

"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


Fig. 1. Overview of the element-selective X-ray anomalous scattering*4
Fig. 1. Overview of the element-selective X-ray anomalous scattering*4

The data from two X-ray scattering experiments - one using the X-ray energy that causes anomalous scattering of Sb, and another using the X-ray energy that causes anomalous scattering of Te - enables direct examination of atomic arrangement by comparing two sets of information obtained using different detection sensitivity. A selection of X-ray energy (wavelength) enables an atom-selective analysis: the light (shown in red) enables analysis of Sb (red point) arrangement utilizing anomalous scattering of Sb, and the light shown in blue enables analysis of Te (blue point) arrangement (abnormal scattering of Te). This method enables direct observation of atomic correlations in terms of particular atomic species; Sb-selective anomalous scattering provides a picture that correlates Sb and other atoms, and Te-selective anomalous scattering provides that of Te and other atoms.

Fig. 2. Structure of Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub>: reconstructed based on X-ray anomalous scattering data and computer simulation
Fig. 2. Structure of Ge2Sb2Te5: reconstructed based on X-ray anomalous scattering data and computer simulation

Previous research indicated that atomic disarrays occur in Ge2Sb2Te5 crystals when the recorded information is erased. The results of this research clearly indicate the occurrence of such atomic disarrays for the first time in 3-D visualization (the green arrow in (A)).

Fig. 3. Stability of recorded phase, and roles of each element in a fast rewriting process
Fig. 3. Stability of recorded phase, and roles of each element in a fast rewriting process

In the amorphous phase,*3 or recorded phase (see (A) in Fig.3), Ge and Te atoms form covalently bonded strong networks (blue lines), which help stabilize the amorphous phase and serve as a nucleus for developing a crystal phase (i.e. the process of erased phase creation). Some of the Sb-Te pairs are covalently bonded together (solid red lines), but others are not due to their larger inter-atomic distances (dotted red lines). Assuming that these dotted lines constitute potential bondages, these pairs can be viewed as forming pseudo regular atomic arrangement (potential network) as in the case of Ge-Te pairs. In the process of erasing memory, (B), only a small displacement of atoms enables the formation of Sb-Te bondages, restoring instantly the crystalline atomic arrays shown in (C).

*1 SPring-8

A RIKEN facility located in Harima Science Garden City (Hyogo prefecture) is capable of producing the world's highest intensity synchronous radiation. The management and promotion of utilization of this facility are undertaken by JASRI. The name “SPring-8” comes from “Super Photon ring-8GeV.” An electron flying at nearly the speed of light, if deflected from its original trajectory through the effect exerted by a magnet, emits an electromagnetic wave in a direction tangential to its trajectory, which is called radiation light (or synchrotron radiation). At present, there are three “3rd Generation” large scale synchronous radiation facilities in the world: SPring-8 (Japan), APS (USA) and ESRF (France). The acceleration energy available at SPring-8 (8 billion electron volts) enables the provision of an extremely wide spectrum of radiation light: from far infrared to visible, vacuum ultraviolet, and soft X-ray up to hard X-ray. SPring-8 provides a theater for collaborative works involving researchers inside and outside Japan, and the research conducted at this facility cover such diverse areas as material science, geoscience, life science, environmental science, and various applications in industrial sectors.

*2 Phase
A state of a matter characterized by homogeneity within an area separated from other matter by a distinguishable boundary. A solid is also called a “solid phase” and a gas, likewise, is also called a “gas phase.”

*3 Amorphous
A solid that is not crystalline: i.e. one that does not have 3-dimentionally ordered atomic arrays. Glass is a typical amorphous material. This type of solid is also called “amorphia.”

*4 X-ray anomalous scattering
When an X-ray spectrum is irradiated to a specimen, an exceptionally large scattering (anomalous scattering) occurs if a wavelength is selected that lies in the vicinity of an absorption edge of one of the constituent elements of the specimen. The X-ray anomalous scattering experiment is used to gather positional information of a specific element utilizing the fact that a scan of incident X-ray energy triggers a significant change of the element's atomic scattering factor in the vicinity of its absorption edge.

For more information‚ please contact:
  Dr. Shinji Kohara (JASRI)

  Dr. Masaki Takata (RIKEN)

  Dr. Toshiyuki Matsunaga (Panasonic Corporation)

Previous Article
Formation of a star-shaped polyhedron starting from a semi-regular polyhedron (Press Release)
Current article
Antimony accelerates rewriting of information in DVD media! (Press Release)
Next Article
Commonplace ceramics can absorb a large amount of hydrogen! (Press Release)