Data Storage

Optical data storage and rewritable DVDs

Several Tellurides alloys containing Ge and Sb exhibit a pronounced contrast in the optical absorption of the crystalline and the amorphous state. This phenomenon is the basis for their application in optical data storage e.g. in rewritable DVDs. These two phases also present a profound change in electrical properties 'such as the resistivity change' which is one of the crucial features that would be used in phase change random access memories, a very promising candidate for future non-volatile memories.

Fig.1 Setup to measure electrical switching in PCRAM's

To understand the change in optical properties upon amorphization of the phase change materials (PCM), experimentalists in Germany [1] wanted to correlate a macroscopic measurement like the optical properties with microscopic details like the local atomic structure. Using ab initio calculations on structural models with different local order, theoreticians in Palaiseau node [2,3] proved that the optical contrast between the two phases is due to the change of the number of first neighbours of Ge atom and that it should be possible to adjust the optical contrast by changing the composition of the PCM. These results provide a fundamentally new insight in the physics of the optical absorption of amorphous materials and in addition they represent important contributions to a systematic material optimization of phase change alloys.

This example illustrates how first principles calculations can provide an ideal complementary tool in technological issues.

[1] A. Kolobov, P. Fons, A. Frenkel, A. Ankudinov, J. Tominaga, and T. Uruga, Nature Materials 3, 703 (2004).

[2] W. Welnic, A. Pamungkas, R. Detemple, C. Steimer, S. Blügel, and M. Wuttig, Nature Materials 5, 56 (2006).

[3] W. Welnic, S. Botti, L. Reining and M. Wuttig, Optical contrast in phase change materials, submitted.