Supercomputer reveals the atomic structure of DVD materials
Crystallization model for AIST alloy in a DVD
Upper left: A laser pulse (hν arrow) causes the central antimony
atom to move and change its bonds to two neighbours.
Upper right: The green vector, sum of the three red bonds,
Below: A sequence of such processes leads from the amorphous (left)
to the crystalline form (right).
A new international study reveals the amorphous and crystalline structure of AIST alloys and the atomic mechanisms governing ultrafast phase change.
Although the storage of films and music on a DVD has become an integral part of our digital lifestyle, the physical basis of the storage mechanism has not been fully understood until recently. In the January issue of the leading journal Nature Materials, a group of researchers from Finland, Germany and Japan provide new insight into the read and write processes in DVDs.
"A deeper theoretical understanding of the processes involved in writing and erasing a DVD can help in the development of phase change storage media with longer life, larger capacity and shorter access times," says one of the group members, Academy Research Fellow Jaakko Akola from the Department of Physics at Tampere University of Technology.
Information is stored in a DVD in the form of microscopic bits, each with a diameter of less than 100 nanometres, in a thin layer of polycrystalline alloy. The bits can have a disordered (i.e. amorphous) or an ordered (i.e. crystalline) atomic structure. The transition between the two phases lasts only a few nanoseconds and can be triggered by a laser pulse.
Common alloys for storage materials, such as DVDs and Blu-ray Discs, contain germanium, antimony and tellurium, also known by their initials as GST. The most popular alloys for rewritable DVDs (DVD±RW) are AIST alloys made up of small amounts of silver and indium as well as antimony and tellurium. Both alloy families contain antimony and tellurium and appear to have much in common.
"However, their ultrafast phase change mechanisms are quite different," says Akola.
Amorphous and crystalline atomic structure of AIST alloys uncovered
The phase change in AIST alloys proceeds from the outside of the bit, where it adjoins the crystalline surroundings, towards its interior. The researchers explain this using a "bond exchange model", where the local environment in the amorphous bit is changed by the small movements of an antimony atom.
"A sequence of such movements has the avalanche effect of reorienting the atoms without requiring empty regions or large motions. The antimony atoms, stimulated by the laser pulse, have simply exchanged the strengths of the bonds to two neighbours, hence the name ‘bond exchange' model."
The researchers used both theoretical and experimental methods to achieve the results. The atomic and electronic structure simulations requiring immense computing power were performed with the IBM Blue Gene/P supercomputer at the Forschungszentrum Jülich in Germany. The X-ray spectroscopy experiments were conducted at SPring-8, the world's largest synchrotron radiation facility in Japan.
The collaboration project, which was funded by the Academy of Finland and the Japan Science and Technology Agency, also brought together industry representatives from Panasonic. The materials and products under scrutiny are commercially available and the measured samples came from the Panasonic research laboratory.