Seeing DVDs in a new light: X-ray reading

Ezine

Published: Jan 15, 2011
Author: David Bradley
Channels: X-ray Spectrometry

thumbnail image: Seeing DVDs in a new light: X-ray reading

International DVD research

Although few of us will not have played a DVD and perhaps even fewer mused at the colourful reflections from the surface of an optical disk. However, little is known about the detailed structural changes that take place when data are stored and retrieved - the read-write process - on such optical media. Now, researchers in Finland, Germany and Japan have turned to synchrotrons, X-ray spectroscopy, and simulations to shed light on this phenomenon.

DVDs, Digital Versatile Disc (or sometimes Digital Video Disc) are the video successor to the optical data and music discs known as compact discs (CDs). They were developed in the mid-1990s and can store both video and data at much higher data capacity than CDs.

The polycrystalline "data" layer in DVD-RAM and Blu-Ray discs comprises several different materials including commonly germanium (Ge), antimony (Sb) und tellurium (Te) the so-called "GST" elements. DVD-RW use "AIST" alloys, which contain small amounts of silver (Ag) and indium (In) as well as antimony (Sb) and tellurium (Te). The DVD-RW alloys (AIST) were first developed by Ricoh in Japan. The material as a whole can be disordered, amorphous or an ordered, crystalline structure. Laser action lasting just nanoseconds triggers a transition between these two phases.

Alloys in common

"Both alloy families contain antimony and tellurium and appear to have much in common, but the phase change mechanisms are quite different", explains Robert Jones of the Juelich Research Centre, Germany, who has collaborated with an international team on the problem. The team included researchers from Forschungszentrum Juelich, Panasonic Corporation, the Japan Synchrotron Radiation Research Institute/SPring-8, Hyogo, Japan, the Nanoscience Center at the University of Jyväskylä and Tampere University of Technology, Finland. The team used X-ray diffraction studies, extended X-ray absorption fine structure and hard X-ray photoelectron spectroscopy experiments with density functional simulations to determine the crystalline and amorphous structures of an AIST and how it differs from GST.

"The structure of amorphous (a-) AIST shows a range of atomic ring sizes, whereas a-GST shows mainly small rings and cavities," the team explains. "The local environment of antimony in both forms of AIST is a distorted 3+3 octahedron. These structures suggest a bond-interchange model, where a sequence of small displacements of antimony atoms accompanied by interchanges of short and long bonds is the origin of the rapid crystallization of a-AIST. It differs profoundly from crystallization in a-GST." The experimental data were supported by extensive simulations on the Juelich supercomputer JUGENE, which allowed the structures of both phases to be determined for the first time in detail. This has now allowed the researchers to develop a model to explain how the phase change can occur so quickly.

The phase change in AIST alloys proceeds from the outside of the bit, where it abuts the crystalline surroundings, towards its interior. The team explains this using a "bond exchange model" in which the local environment in the amorphous bit changes because of small movements of an antimony atom. Many such small changes result in reorientation (crystallization), without the need for large movements or the formation of empty regions. The antimony atoms, stimulated by the laser pulse, simply swap bond allegiance with their two nearest neighbours.

GST and AIST

In earlier research, the team had identified the nature of the phase transition in GST materials (DOI: 10.1103/PhysRevB.80.020201) where the amorphous bit crystallizes through a nucleation process. The speed of the transition in those alloys could be explained on the basis of the amorphous and crystalline phases both containing the same structural "ABAB" rings. These four-membered rings contain two germanium or antimony atoms (A) and two tellurium atoms (B) and can rearrange in the available empty space without breaking many atomic bonds.

The deeper theoretical understanding of the processes involved in writing and erasing a DVD should aid the development of phase change storage media with longer life, larger capacity, and shorter access times.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.