Download Structural basis for the fast phase change of DVD-RAM

October 17, 2006
Japan Synchrotron Radiation Research Institute (JASRI)
Japan Science and Technology Agency (JST)
RIKEN
Matsushita Electric Industrial Co., Ltd.
Tsukuba University
Structural basis for the fast phase change of
DVD-RAM
The research group of Japan Synchrotron Radiation Research Institute (JASRI), Riken
Harima Institute, Matsushita Electric Industrial Co., Ltd. and Tsukuba University et al.
has discovered the key structure of DVD-RAM, which highly contributes to the
improvement of the phase-change speed in the materials. This finding will provide
important clues for the design of faster phase-change materials. This work was
supported by CREST "X-ray pinpoint structural measurement project -Development of
the spatial- and time-resolved structural study for nano-materials and devices-" from
Japan Science and Technology Agency.
The development of fast phase-change materials in the last century was accomplished
by the landmark studies on GeTe and Au-Ge-Sn-Te compounds in a single-phase
structure by Chen et al. and Yamada et al., respectively. They reported that these
materials show a high phase-stability of the amorphous phase and a very short
crystallization time. These approaches discovered the way to develop new
phase-change rewritable materials and led to the discovery in 1987 of the GeTe-Sb2Te3
single-phase material by Yamada and co-workers. The pseudo-binary compound
2GeTe-Sb2Te3 (Ge2Sb2Te5) is one of the well-established mother materials for
commercial DVD-RAM (digital versatile disc-random access memory) device which
utilizes the phase change of crystal-liquid-amorphous (record) and amorphous-crystal
(erase) in chalcogenide materials (Fig. 1). In order to develop faster phase-change
materials, it is necessary to understand the structural origins of the phase change,
especially for the 3-dimensional structure of the amorphous phase in the phase change
from amorphous to crystal. In this study, we derived the 3-dimensional atomic
configuration of amorphous Ge2Sb2Te5 (a-Ge2Sb2Te5, crystallization speed: 20 nsec)
and amorphous GeTe a-GeTe, crystallization speed: 100 nsec) by reverse Monte Carlo
(RMC) computer simulation with synchrotron-radiation x-ray diffraction data.
The high-energy x-ray diffraction experiments were carried out at the SPring-8
high-energy x-ray diffraction beamline BL04B2 and powder diffraction beamline BL02B2.
The measured data were analyzed by Rietveld analysis and RMC simulation.
Figure 2 shows the measured structure factors S(Q). The diffraction pattern of
Ge2Sb2Te5 and GeTe crystals consists of sharp Bragg reflections, indicating the
long-range periodicity in the atomic arrangement. On the other hand, the diffraction
pattern of Ge2Sb2Te5 (953 K) and GeTe (1073 K) liquids show a typical halo pattern
peculiar to non-crystalline materials, reflecting the lack of the long-range periodicity.
These diffraction patterns show a highly disordered state (liquid state) in which both
Ge2Sb2Te5 and GeTe have to undergo during a recording process (crystal-amorphous
phase change) by a laser-heated melt-quench process.
The ring statistics for the amorphous and crystal Ge2Sb2Te5 and GeTe were calculated
up to 12-fold ring as shown in Fig. 3(a). It is remarkable that a-Ge2Sb2Te5 can be
regarded as "even-numbered rings structure", because the ring statistics is dominated
by 4- and 6-fold rings analogous to the crystal phase. On the other hand, we find that
a-GeTe has various size (both odd- and even-numbered) rings. These differences in the
network of the ring structure are clearly visible in 3-dimensional atomic configurations
obtained from the RMC simulation as shown in Fig. 3(b).
From the comparison of the ring statistics, the mechanism of fast
crystal-liquid-amorphous (record) and amorphous-crystal (erase) phase changes in
Ge2Sb2Te5 are proposed by the schematic presentation as shown in Fig. 4. In the
crystal-liquid phase-change process (stage I), atomic configuration in the crystal phase
is disarranged by the laser heating and melted in liquid, where there is no significant
structural difference between Ge2Sb2Te5 and GeTe (see also Fig. 2). Meanwhile in the
liquid-amorphous phase-change process (stage II), only even-numbered rings are
constructed in a-Ge2Sb2Te5. In the amorphous-crystal phase-change process (stage III),
a-Ge2Sb2Te5 transforms to the crystal phase due to the transformation of only the
large-size even-numbered (8, 10, 12-fold) rings into crystal structure (4- and 6-fold
rings). On the other hand, a-GeTe exhibits various size (both odd- and even-numbered)
rings in stage II. Therefore the recombination of the various size is required in stage III.
Thus the construction of odd-numbered rings in a-GeTe disturbs the fast crystallization
of the amorphous phase, and it is worth mentioning that the addition of Sb2Te3 to GeTe
can effectively prevent the formation of Ge-Ge bonds in a-Ge2Sb2Te5
In conclusion, we found that the ring statistics of amorphous Ge2Sb2Te5 is dominated by
4- and 6-fold rings analogous to the crystal phase, by means of the RMC analysis. We
believe such a unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast
crystallization speed of the material.
This work was conducted by Dr. Masaki Takata (Riken Harima Institute), Dr. Shinji
Kohara (Japan Synchrotron Radiation Research Institute), Dr. Noboru Yamada
(Matsushita Electric Industrial Co., Ltd.) and Yutaka Moritomo (Tsukuba University) et al.
and will be presented in the following conference.
International Symposium on Optical Memory 2006
RMC Analyses Solve High-Speed Phase-Change Mechanism
T. Matsunaga, R. Kojima, N. Yamada, S. Kohara, M. Takata
This work will be published in the following article.
Applied Physics Letters, 89, (2006). November 06, 2006.
Structural basis for the fast phase change of Ge2Sb2Te5
- Ring statistics analogy between the crystal and amorphous states Shinji Kohara, Kenichi Kato, Shigeru Kimura, Hitoshi Tanaka, Takeshi Usuki, Kentaro
Suzuya, Hiroshi Tanaka, Yutaka Moritomo, Toshiyuki Matsunaga, Noboru Yamada,
Yoshihito Tanaka, Hiroyoshi Suematsu, Masaki Takata*
*: Corresponding author
For more information, please contact:
RIKEN Public Relations Office
Email: [email protected]
Fig. 1 Record / Erase process in DVD-RAM.
Fig. 2 Total structure factors S(Q) of crystal (300 K), liquid (953 K), and
amorphous Ge2Sb2Te5 (300 K) and crystal (300 K), liquid (1073 K), and
amorphous GeTe.
Fig. 3 Ring size distribution in a-Ge2Sb2Te5 and a-GeTe (a). A 16 Å x 16 Å x 16 Å atomic
configuration and enlarged framework atomic configuration of a-Ge2Sb2Te5 and
a-GeTe obtained from the RMC snapshot (b).
Fig. 4 A schematic presentation for the possible ring size transformation in
crystal-liquid-amorphous phase change (record) and amorphous-crystal phase
change (erase) in Ge2Sb2Te5 and GeTe. Stage I and II: recording process, stage
III: erasing process.