The amorphization and crystallization of Ge 2 Sb 2 Te 5 : an ab initio molecular dynamics study
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The amorphization and crystallization of Ge2Sb2Te5: an ab initio molecular dynamics study Paulo S. Branicio,1 Kewu Bai,1 H. Ramanarayan,1 David.T. Wu,1 Wendong Song,2 Weijie Wang,2 Minghua Li,2 Rong Zhao,2 Luping Shi,2 and David J. Srolovitz1 1 Institute of High Performance Computing, 1 Fusionopolis Way, 16-16, 138632 Singapore. 2 Digital Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore. ABSTRACT The reversible switching between the amorphous and crystalline phases of Ge2Sb2Te5 (GST) is investigated with ab initio molecular dynamics. We apply different quench rates (-16 K/ps, -5 K/ps, -2 K/ps, and -0.45 K/ps) and different annealing temperatures (500 K, 600 K, 700 K, and 800 K) to amorphize and crystallize GST respectively. Results show that the generated amorphous is strongly dependent on the quench rate. For -16 K/ps and -5 K/ps, generated amorphous samples have different density of crystal seeds, higher in the later. The amorphous structure formed at -2 K/ps contains a single crystalline cluster, while that formed at the quench rate of -0.45 K/ps had sufficient time to completely crystallize the amorphous phase. Annealing the amorphous systems formed at different rates shows that crystallization depends both on the annealing temperature and on the structure of the initial system (i.e., whether or not it contains crystalline clusters or crystal seeds). At 500 K, formation of crystalline clusters occurs readily within a few ps while the rate at which they grow is slow, taking 0.9 ns to complete the crystallization. In contrast, crystalline cluster formation is inhibited at 800 K. In the intermediate temperature range, both crystalline cluster formation and growth occur within a few hundred ps indicating that these temperatures leads to the fastest crystallization. The crystallization of a 63atom at ~900 K resulted in a highly relaxed crystal structure showing a clear tendency for separation of Ge and Sb species in layers. This model also indicates a tendency of segregation of vacancies, suggesting that vacancy layering may play a key role in the crystallization process. INTRODUCTION Phase change materials such as Ge2Sb2Te5 (GST) show exceptional promise in new memory device (PCRAM) applications that have the potential to be a substitute for Flash technology within a decade [1, 2]. PCRAM works through fast, reversible switching between a highly resistive amorphous phase and a low resistance crystalline phase. PCRAM has many outstanding properties such as non-volatility, high-endurance, and long data retention. In addition to which it allows for random access, multilevel programming, is highly scalable, and switches on the time scale of a few nanoseconds. Several challenges remain for PCRAM technology to mature to the point that it achieves profitable commercial application as compared with Flash and other memory technologies. One key to further advances is the development of an atomistic understanding of the amorphization and crystallization processes and their exploitation in the
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