Structural basis for the fast phase change of Ge₂Sb₂Te₅, optical memory disk material

Abstract

A digital versatile disc-random access memory (DVD-RAM) involves the melt quenching (amorphizing for record) and annealing (crystallization for erase) of chalcogenide materials. It is well known that Ge₂Sb₂Te₅ is one of the well-established commercial media. In this study, we revealed the three-dimensional atomic configuration of amorphous Ge₂Sb₂Te₅ and GeTe by reverse Monte Carlo (RMC) simulation with synchrotron-radiation high-energy X-ray diffraction data. RMC models suggested that amorphous Ge₂Sb₂Te₅ can be regarded as an “even-numbered-ring structure”, because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. The unconventional ring statistics of amorphous Ge₂Sb₂Te₅ is the key structural information for the better understanding of fast crystallization mechanism of the material.