Scientists spotlight secrets of data storage structures
Researchers have unravelled the details of precisely how DVDs and similar devices store data. The findings, published in the journal Nature Materials, are set to boost the development of easily accessible storage media with a larger capacity and a longer life. Rewritable, high-density storage devices such as the DVD-RAM (digital versatile disc - random access memory), the DVD-RW (digital versatile disc - re-recordable), the Blu-ray disc and personal computer's RAM (random access memory) have been widely used for a number of years. These systems all rely on phase change recording; in DVDs, data is stored as 'bits' less than 100 nanometres (nm) across in a thin layer of alloy made up of a number of elements. The bits come in two varieties, or phases - an ordered 'crystalline' phase and a disordered 'amorphous' phase. A bit can be switched from one phase to another in just a fraction of a second by using a laser pulse. Lasers are also used to read data stored on a disc, as the two states have different reflectivities. DVD-RAMs and Blu-ray discs have a GST data storage layer; the letters GST come from the symbols for the elements that make up the layer, namely germanium (Ge), antimony (Sb) and tellurium (Te). Meanwhile, DVD-RW devices use AIST alloys; these are made up of small amounts of silver (Ag) and indium (In) as well as antimony (Sb) and tellurium (Te). Despite the widespread nature of devices using GST and AIST storage systems, little is known about what is going on in these systems at the atomic level when the bits switch from one phase to another. In this study, scientists in Finland, Germany and Japan probed the workings of the AIST data storage system. The team drew on experimental data and X-ray spectra from the Japanese SPring-8 synchrotron facility, the world's most powerful X-ray source. This information was complemented by extensive simulations using the JUGENE supercomputer at Forschungszentrum Jülich in Germany. The analyses revealed that in AIST alloys, the phase change starts at the outside of the bit, where it is connected to the crystalline surroundings, towards the interior. According to the team, a 'bond exchange model' is in operation; when the bit is stimulated with a laser, the antimony atoms in the bit swap the strength of their bonds to two neighbours. The phase transition in AIST storage is therefore quite different to that found in GST, which the researchers elucidated in earlier research. In GST, the amorphous bit crystallises through nucleation; in other words, the change starts in the middle of the bit and grows until it covers the entire bit. In GST systems, both phases are characterised by groups of four atoms arranged 'ABAB' in a ring, where 'A' is either germanium or antimony and 'B' is tellurium. This structure includes enough empty space for the atoms to rearrange themselves without breaking many atomic bonds. 'Both alloy families contain antinomy and tellurium and appear to have much in common, but the phase change mechanisms are quite different,' commented Dr Robert Jones of Forschungszentrum Jülich. According to the team, the calculation of the structure of AIST's amorphous phase is the largest ever carried out in this area of research; the team used some 4,000 processors of the JUGENE supercomputer over a period of 4 months.For more information, please visit: Forschungszentrum Jülich:http://www.fz-juelich.de/ Nature Materials:http://www.nature.com/naturematerials
Countries
Germany, Finland, Japan