X-raying a DVD

Little is known about the detailed structural changes that take place when data are stored and retrieved - the read-write process - on an optical disc like a DVD. Now, researchers in Finland, Germany and Japan have used a synchrotron to study the materials used in DVDs.

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

The two alloy families contain antimony and tellurium and superficially have much in common, but the phase change mechanisms are quite different, according to Robert Jones of the Juelich Research Centre, Germany. 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 they differ from GST.

'The structure of amorphous AIST, a-AIST, shows a range of atomic ring sizes, whereas a-GST shows mainly small rings and cavities,' the researchers explain. The local environment around the antimony atoms in both forms of AIST is a distorted 3+3 octahedron, which suggests a certain type of bonding between atoms. A sequence of small displacements of antimony atoms caused by the laser light allows short and long bonds to swap places easily and this could explain the rapid crystallization of a-AIST. This differs profoundly from the crystallization of a-GST.

In earlier research, the team had shown that a phase transition in GST materials proceeds via a nucleation process. The speed of the transition in those alloys is explained as the shifting of "ABAB" rings of two antimony atoms and two tellurium atoms which are found in both amorphous and crystalline GST. These rearrangements of the four-membered rings occur without the need for atomic bonds to made or broken.

The team suggests that the deeper theoretical understanding of the processes involved in writing and erasing a DVD could help in the development of novel materials for optical discs that rely on a phase change that could be more persistent than current technology and so improve the usable lifespan of the discs. The same understanding might also improve the data capacity of the discs and allow them to be read more quickly.