Structure and the mechanism of rapid phase change in amorphous Ge ₂Sb₂Te₂

Understanding the mechanism of rapid phase change process in DVD (digital versatile disc) materials is one of the important topics in materials science, and hence numerous studies investigating the phase change process as well as structural analysis of the crystal and amorphous phases have been reported. Nevertheless, the mechanism of rapid phase change is still unclear, owing to the lack of detailed structure analysis, in particular on the amorphous phase and its crystallisation process. We have studied the amorphous structure of Ge ₂Sb₂Te₅ and the crystallisation process by high energy synchrotron x-ray diffraction with the aid of structure modelling and time-resolved synchrotron x-ray diffraction, in order to obtain key information for revealing the rapid phase change mechanism. We found a large fraction of 4-fold and 6-fold rings in the atomic configuration of the amorphous phase. Intriguingly the bond angle distributions of rings exhibit a peak at approximately 90°, which corresponds to that in the crystal phase. Therefore, it is suggested that the 4-fold and 6-fold rings are nuclei for crystallisation. Furthermore, time-resolved x-ray diffraction measurements combined with photoreflectivity measurements during the crystallisation of the amorphous phase suggest that the crystallisation of Ge₂Sb ₂Te₅ can be explained by a nucleation driven process. We conclude that a large amount of nuclei is the reason for rapid crystallisation in Ge₂Sb₂Te₅ with a nucleation driven process.