Large Scale molecular Dynamics Simulations of Phase Change Materials

In the last few years atomistic simulations based on density functional theory have provided useful insights on the properties of phase change materials (see ref. [1] for a review). However, several key issues such as the crystallization dynamics, the properties of the crystalline/amorphous interface and the thermal conductivity at the nanoscale, just to name a few, are presently beyond the reach of fully ab-initio simulations. A route to overcome the limitations in system size and time scale of ab-initio molecular dynamics is the development of classical interatomic potentials. Traditional approaches based on the fitting of simple functional forms turned out to be unfeasible due to the complexity and variability of the chemical bonding in the crystal and amorphous phases revealed by the ab-initio simulations. A possible solution has been demonstrated recently by Behler and Parrinello [2] who developed empirical interatomic potentials with close to ab-initio accuracy for elemental carbon, silicon and sodium by fitting large ab-initio databases within a neural network (NN) scheme. In general, a NN method is a non-linear technique that allows fitting any function to arbitrary accuracy and does not require any knowledge about the functional form of the underlying problem. By means of this technique, we have developed a classical interatomic potential for GeTe which is one of the compounds under scrutiny for applications in phase change memories. Simulations with the NN potential are from four to five order of magnitude faster than ab-initio ones for 4000-atom models. We will present results of NN simulations on the properties of liquid and amorphous GeTe including thermal conductivity and the homogeneous and heterogeneous crystallization of the amorphous. [1] D. Lencer, M. Salinga, and M. Wuttig, Advan. Mat. 23, 2030 (2011). [2] J. Behler and M.Parrinello, Phys. Rev. Lett. 14, 146401 (2007); J. Behler, R. Martonak, D. Donadio, M. Parrinello, Phys. Rev. Lett. 100, 185501 (2008); H. Eshet, R. Z. Khaliullin, T. D. Kuehne, J. Behler, and M. Parrinello, Phys. Rev. B 81, 184107 (2010); ibidem 81, 100103 (2010); N. Artrith, T. Morawietz, and J. Behler, Phys. Rev. B 83, 153101 (2011).