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 Ge2Sb2Te5 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 Ge2Sb2Te5. 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.
This is a preview of subscription content, access via your institution
Subscribe to this journal
Receive 12 print issues and online access
$259.00 per year
only $21.58 per issue
Rent or buy this article
Prices vary by article type
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
Yamada, N., Ohno, E., Nishiuchi, K., Akahira, N. & Takao, M. Rapid phase transitions of GeTe–Sb2Te3 . J. Appl. Phys. 69, 2849–2856 (1991).
Lankhorst, H. R., Ketelaars, W. S. M. M. & Wolters, R. A. M. Low-cost and nanoscale non-volatile memory concept for future silicon chips. Nature Mater. 4, 347–352 (2005).
Ovshinsky, S. R. Reversible electrical switching phenomena in disordered structures. Phys. Rev. Lett. 21, 1450–1453 (1968).
Lee, S.-H., Jung, Y. & Agarwal, R. Highly scalable non-volatile and ultra-low power phase-change nanowire memory. Nature Mater. 2, 626–630 (2007).
Kohara, S. et al. Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous phases. Appl. Phys. Lett. 89, 201910 (2006).
Volkert, C. A. & Wuttig, M. Modelling of laser pulsed heating and quenching in optical data storage media. J. Appl. Phys. 86, 1808–1816 (1999).
Lang, C., Song, S. A., Manh, D. N. & Cockayne, D. J. H. Building blocks of amorphous Ge2Sb2Te5 . Phys. Rev. B 76, 054101 (2007).
Eom, J.-H. et al. Global and local structures of the Ge–Sb–Te ternary alloy system for a phase-change memory device. Phys. Rev. B 73, 214202 (2006).
Caravati, S., Bernasconi, M., Kuhna, T. D., Krack, M. & Parrinello, M. Coexistence of tetrahedral and octahedral-like sites in amorphous phase change materials. Appl. Phys. Lett. 91, 171906 (2007).
Wełnic, W., Botti, S., Reining, L. & Wuttig, M. Origin of the optical contrast in phase-change materials. Phys. Rev. Lett. 98, 236403 (2007).
Welnic, W. et al. Unraveling the interplay of local structure and physical properties in phase-change materials. Nature Mater. 5, 56–62 (2006).
Wuttig, M. et al. The role of vacancies and local distortions in the design of new phase-change materials. Nature Mater. 6, 122–128 (2007).
Sun, Z., Zhou, J. & Ahuja, R. Structure of phase change materials for data storage. Phys. Rev. Lett. 96, 055507 (2006).
Sun, Z., Zhou, J. & Ahuja, R. Unique melting behaviour in phase-change materials for rewritable data storage. Phys. Rev. Lett. 98, 055505 (2007).
Kresse, G. & Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558–561 (1993).
Coombs, J. H., Jongenelis, A. P. J. M., van Es-Spiekman, W. & Jacobs, B. A. J. Laser-induced crystallization phenomena in GeTe-based alloys. II. Composition dependence of nucleation and growth. J. Appl. Phys. 78, 4918–4927 (1995).
Kalb, J. A., Spaepen, F. & Wuttig, M. Kinetics of crystal nucleation in undercooled droplets of Sb- and Te-based alloys used for phase change recording. J. Appl. Phys. 98, 054910 (2005).
Chen, Y. C. et al. International Electron Devices Meeting, 2006. IEDM’06 Vols 1,2, 531–534 (IEEE, New York, 2006).
Matsunaga, T., Yamada, N. & Kubota, Y. Structures of stable and metastable Ge2Sb2Te5, An intermetallic compound in GeTe–Sb2Te3 pseudobinary systems. Acta Crystallogr. B 60, 685–691 (2004).
Akola, J. & Jones, R. O. Structural phase transitions on the nanoscale: The crucial pattern in the phase materials Ge2Sb2Te5 and GeTe. Phys. Rev. B 76, 235201 (2007).
Kooi, B. J., Groot, W. M. G. & De Hosson, J. Th. M. In situ transmission electron microscopy study of the crystallization of Ge2Sb2Te5 . J. Appl. Phys. 95, 924–932 (2003).
Weidenhof, V., Friedrich, I., Ziegler, S. & Wuttig, M. Laser induced crystallization of amorphous Ge2Sb2Te5 films. J. Appl. Phys. 89, 3168–3176 (2001).
van Pieterson, L., Lankhorst, M. H. R., van Schijndel, M., Kuiper, A. E. T. & Roosen, J. H. J. Phase-change recording materials with a growth-dominated crystallization. J. Appl. Phys. 97, 083520 (2005).
Privitera, S., Bongiorno, C., Rimini, E. & Zonca, R. Crystal nucleation and growth processes in Ge2Sb2Te5 . Appl. Phys. Lett. 84, 4448–4450 (2004).
Kolobov, A.V. et al. Understanding the phase-change mechanism of rewritable optical media. Nature Mater. 3, 703–708 (2004).
Jovari, P. et al. ‘Wrong bonds’ in sputtered amorphous Ge2Sb2Te5 . J. Phys. Condens. Matter 19, 335212 (2007).
Segall, M. D. et al. First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys. Condens. Matter 14, 2717–2744 (2002).
Troullier, N. & Martins, J. L. Efficient pseudopotentials for plane-wave calculations. Phys. Rev. B 43, 1993–2006 (1991).
Klein, A. et al. Changes in electronic structure and chemical bonding upon crystallization of the phase change material Ge1Sb2Te4 . Phys. Rev. Lett. 100, 016402 (2008).
Baker, D., Paesler, M., Lucovsky, G. & Taylor, P. C. EXAFS study of amorphous Ge2Sb2Te5 . J. Non-Cryst. Solids 32, 1621–1623 (2005).
Yeh, T.-T., Hsieh, T.-E. & Shieh, H.-P. D. Enhancement of data transfer rate of phase change optical disk by doping nitrogen in Ge–In–Sb–Te recording layer. Japan. J. Appl. Phys. 43, 5316–5320 (2004).
Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758–1775 (1999).
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett 77, 3865–3868 (1996).
Njoroge, W. K., Woltgens, H.-W. & Wuttig, M. Density changes upon crystallization of Ge2Sb2.04Te4.74 films. J. Vac. Sci. Technol. A 20, 230–233 (2000).
Bichara, C., Johnson, M. & Gaspard, J.-P. Octahedral structure of liquid GeSb2Te4 alloy: First-principles molecular dynamics study. Phys. Rev. B 75, 060201(R) (2007).
Stimulating discussions with A. L. Greer, J. Hafner, G. Kresse, D. Cockayne, J.-Y. Raty, T. Bucko, S. Kugler, G. Csanyi, K. Borisenko and P. Jovari are gratefully acknowledged. J.H. is grateful for the award of a Marie-Curie Fellowship. All simulations were carried out using the Cambridge High-Performance Computer Facility.
About this article
Cite this article
Hegedüs, J., Elliott, S. Microscopic origin of the fast crystallization ability of Ge–Sb–Te phase-change memory materials. Nature Mater 7, 399–405 (2008). https://doi.org/10.1038/nmat2157
This article is cited by
Nature Electronics (2023)
Some novel perspectives of iso-conversional analysis in the study of Meyer–Neldel energy for thermally governed crystallization by using Johnson–Mehl–Avrami (JMA) theory
Journal of Thermal Analysis and Calorimetry (2023)
Nature Communications (2022)
Nano Research (2022)
Nature Communications (2021)