Photoinduced ultrafast transition of the local correlated structure in chalcogenide phase-change materials

TL;DR

This study uncovers the ultrafast atomic structural transition in Ge2Sb2Te5, a key phase-change material, using femtosecond electron diffraction, revealing how local correlated structures respond to photoexcitation within hundreds of femtoseconds.

Contribution

It demonstrates the ultrafast suppression of local Peierls distortions and the transition from rhombohedral to cubic structure in Ge2Sb2Te5, providing microscopic insights into phase-change dynamics.

Findings

Local Peierls distortions are suppressed within ~0.3 ps.

The structural transition from rhombohedral to cubic occurs ultrafast.

Femtosecond electron diffraction effectively reveals local correlated structures.

Abstract

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5 has been widely used in optical data storage and non-volatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation and TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The randomly oriented Peierls distortion among unit cells in the averaged rock-salt phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ~ 0.3 ps. In addition, the impact of the carrier relaxation and the large amount of vacancies to the ultrafast structural response is quantified and discussed. Our work provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.