Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials

• Published in: Nature materials Vol. 7; no. 5; pp. 399 - 405
• Main Authors: Elliott, S. R, Hegedüs, J
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.05.2008; Nature Publishing Group
• Subjects: Biomaterials; Chemistry and Materials Science; Condensed Matter Physics; Crystallization; Data storage; Information storage; Materials Science; Molecular structure; Nanotechnology; Nucleation; Optical and Electronic Materials; Phase transitions; Simulation
• ISSN: 1476-1122; 1476-4660
• DOI: 10.1038/nmat2157

Ge–Sb–Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge 2 Sb 2 Te 5 composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge 2 Sb 2 Te 5 . As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on. Phase-change materials are of commercial interest for their use in rewritable optical disks and as non-volatile memories, although little is known about the dynamics of the phase transition. The numerical simulation of the entire write-erase cycle therefore provides important clues towards the development of new phase-change materials.