Understanding the multistate SET process in Ge-Sb-Te-based phase-change memory

Multilevel operation is a topic of much current research in the field of phase-change memory materials, representing the most feasible method for increasing memory density beyond the ultimate scaling limits of the cell size. In this work, we present a combined experimental and ab initio molecular dynamics study of the formation of intermediate states during the crystallisation of Ge2Sb2Te5 (GST). A single intermediate resistance level is formed within a narrow voltage window, and simulations suggest this consists of microscopic crystallites embedded in a bulk amorphous phase. These findings are interpreted within the framework of classical nucleation theory, and a mechanism is proposed to explain the formation of the intermediate state. Our findings suggest that it may be difficult to obtain multiple intermediate states reliably during the crystallisation of Ge2Sb2Te5 and shed light on the fundamental limitations of using this method for multilevel programming.

[1]  R. O. Jones,et al.  Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change material Ge 2 Sb 2 Te 5 , 2009 .

[2]  T. Lee,et al.  Structural role of vacancies in the phase transition of Ge 2 Sb 2 Te 5 memory materials , 2011 .

[3]  Matthias Wuttig,et al.  Towards a universal memory? , 2005, Nature materials.

[4]  Y.C. Chen,et al.  Write Strategies for 2 and 4-bit Multi-Level Phase-Change Memory , 2007, 2007 IEEE International Electron Devices Meeting.

[5]  R. Shelby,et al.  Phase change materials and their application to random access memory technology , 2008 .

[6]  Myong R. Kim,et al.  Crystallization behavior of sputter-deposited amorphous Ge2Sb2Te5 thin films , 1999 .

[7]  K. Gopalakrishnan,et al.  Phase change memory technology , 2010, 1001.1164.

[8]  Burke,et al.  Generalized Gradient Approximation Made Simple. , 1996, Physical review letters.

[9]  Y. Yeo,et al.  Multi-level phase change memory devices with Ge2Sb2Te5 layers separated by a thermal insulating Ta2O5 barrier layer , 2011 .

[10]  S. Elliott,et al.  Ab Initio computer simulation of the early stages of crystallization: application to Ge(2)Sb(2)Te(5) phase-change materials. , 2011, Physical review letters.

[11]  J. H. Coombs,et al.  Laser‐induced crystallization phenomena in GeTe‐based alloys. II. Composition dependence of nucleation and growth , 1995 .

[12]  S. Elliott,et al.  Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials. , 2008, Nature materials.

[13]  Sumio Hosaka,et al.  Programming margin enlargement by material engineering for multilevel storage in phase-change memory , 2009 .

[14]  K Schulten,et al.  VMD: visual molecular dynamics. , 1996, Journal of molecular graphics.

[15]  Ming-Jinn Tsai,et al.  Ga2Te3Sb5—A Candidate for Fast and Ultralong Retention Phase‐Change Memory , 2009 .

[16]  Hafner,et al.  Ab initio molecular dynamics for liquid metals. , 1995, Physical review. B, Condensed matter.

[17]  M. Breitwisch,et al.  Estimation of amorphous fraction in multilevel phase change memory cells , 2009, 2009 Proceedings of the European Solid State Device Research Conference.

[18]  S. Braga,et al.  Experimental Analysis of Partial-SET State Stability in Phase-Change Memories , 2011, IEEE Transactions on Electron Devices.

[19]  Songlin Feng,et al.  Nitrogen-implanted Ge2Sb2Te5 film used as multilevel storage media for phase change random access memory , 2004 .

[20]  Blöchl,et al.  Projector augmented-wave method. , 1994, Physical review. B, Condensed matter.

[21]  H.-S. Philip Wong,et al.  Phase Change Memory , 2010, Proceedings of the IEEE.