Fig. 1: Schematic illustration of the bond-interchange-based recrystallization process in AIST upon laser irradiation© 2011 NPG

Blu-ray discs have the capacity to store 25 gigabytes of data, almost six times that of a DVD and equivalent to roughly 38 CDs. All three technologies encode information by changing the atomic arrangement of a host medium using laser light. As the name implies, Blu-ray uses blue light, which can be focused to a smaller spot than the red and infrared pulses used in preceding technologies, providing higher storage densities. Further improvements in data storage lifetime and density, however, will require a comprehensive understanding of how the atomic make-up is altered by the laser light.

The two materials commonly used for rewritable data storage, known as AIST and GST, react to light in fundamentally different ways, but precisely how has been unclear. Noboru Yamada from Panasonic Corporation and a team of researchers from Japan, Germany and Finland have now provided an explanation.1

Silver indium antimony telluride (AIST) and germanium antimony telluride (GST) both start as crystalline solids with a regular atomic structure that can be destroyed by an intense pulse of laser light. This phase change is localized, very fast and long lasting, making it a convenient way to store binary information. The crystalline phase can be restored by irradiating the material with longer pulses of weaker laser light. It is known that AIST recrystallizes in a little as 10 ns, but exactly why the process in AIST is so fast compared to GST was unknown. “By clarifying this mystery we hope to accelerate the development of faster and higher capacity data storage,” explains Yamada.

The research team probed the atomic structure of AIST using high-energy X-rays and compared the results with supercomputer simulations, which allowed them to develop a model of AIST recrystallization. They were able to show that the tendency for AIST to begin recrystallizing from the edge of the laser spot — whereas GST recrystallizes from nucleation sites within the spot — is due to a process of bond interchange (see image), confirming the fundamental difference between the recrystallization processes in these two materials. “We now plan to extend this analysis to create an animation of the atomic movements during crystallization,” says Yamada.