First-principles simulations of crystallization of phase-change materials
Prof. Dr. Riccardo Mazzarello
Physics Department, RWTH Aachen University, Germany
Time: 10:30-11:30, June 5 (Friday)
Place: MSE building Room 205
Abstract:
Phase-change materials (PCMs) are capable of switching very rapidly and reversibly between the amorphous and crystalline phase at high temperatures. Yet, the two phases are stable at room temperature and exhibit pronounced optical and electrical contrast. These unique properties have resulted in applications in rewritable optical devices and electronic non-volatile random access memories (PRAM), where heating is induced by laser irradiation and the Joule effect, respectively. Recently, sub-nanoseconds crystallization of phase-change memory cells of size of less than 10 nanometers has been reported. These time and length scales are almost within reach of first-principles methods based on density functional theory.
In this talk, I will present our ab initio molecular dynamics simulations of crystallization of two technologically important PCMs, namely Ag,In-doped Sb2Te [1] and Ge2Sb2Te5 [2]. I will first discuss the links between the fragility of the supercooled liquid phase of these materials and the ability of the glass to crystallize rapidly at high temperature and yet to be extremely stable at room temperature. I will then show that high-temperatures simulations yield crystal growth velocities in good agreement with experimental data. They indicate that fast crystal growth stems from the large diffusion constants and sticking coefficients and the presence of a sharp crystalline-liquid interface.
On the other hand, at lower temperatures, discrepancies between simulations and experiments are observed. We have attributed these deviations to the high fragility of the system, in combination with the very fast quenching rates employed in the simulations. I will briefly discuss our recent efforts to circumvent this problem and generate relaxed models of the amorphous state by alternative approaches to simulated quenching [3].
[1] W. Zhang, I. Ronneberger, P. Zalden, M. Xu, M. Salinga, M. Wuttig, and R. Mazzarello, Sci. Rep. 4, 6529 (2014).
[2] I. Ronneberger, W. Zhang, and R. Mazzarello, Adv. Funct. Mater. (2015) DOI: 10.1002/adfm.201500849.
[3] J.-Y. Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara, and M.Wuttig, Nature Comm. (2015) DOI: 10.1038/ncomms8467
Speaker:
Riccardo Mazzarello is a Junior Professor of Physics at RWTH Aachen University in Germany since 2009. He leads the research group on Theoretical Nanoelectronics. His research interests include phase change materials, organic materials, graphene, magnetism, and so on. He has authored or co-authored more than 40 peer-reviewed journal articles with 14 of them published on high-impact academic journals (IF>7), such as Science, Nature Materials, Nature Communications, Phys. Rev. Lett., Angew. Chem. Int. Ed. and Advanced Materials family. The total SCI citation of these articles is 4400+ and the highest single-article citation is 3600+. He has been invited more than 30 times to give presentations at international conferences and renowned academic institutes.
