Large Scale Molecular Dynamics Simulations of the Crystallization Dynamics of Amorphous and Liquid GeTe

In the last few years atomistic simulations based on density functional theory have provided useful insights on the properties of phase change materials. 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, require large simulation cells and long simulation times beyond the reach of fully DFT simulations. A route to overcome the limitations in system size and time scale of DFT molecular dynamics is the development of classical interatomic potentials. Traditional approaches based on the fitting of simple functional forms are very challenging due to the complexity and variability of the chemical bonding in the crystal and amorphous phases revealed by DFT simulations. A possible solution has been demonstrated recently by Behler and Parrinello [1] who developed empirical interatomic potentials with close to ab-initio accuracy for elemental carbon, silicon and sodium by fitting large DFT databases within a neural network (NN) scheme. By means of this technique, we have recently developed an interatomic potential for GeTe [2] which is one of the compounds under scrutiny for applications in phase change memories. The NN potential has allowed simulating several thousands of atoms for several tens of ns which provided insights on the thermal transport properties of amorphous GeTe [3] and on the fragility of the supercooled liquid phase [4]. We will present the results of NN simulations of the crystallization dynamics of the supercooled liquid and overheated amorphous states of GeTe in the homogeneous phase and at the interface with the crystal that allow extracting the crystal growth velocity as a function of temperature and density. [1] J. Behler and M.Parrinello, Phys. Rev. Lett. 14, 146401 (2007). [2] G. C. Sosso, G. Miceli, S. Caravati, J. Behler, and M. Bernasconi, Phys. Rev. B 85, 174103 (2012). [3] G. C. Sosso, D. Donadio, S. Caravati, J. Behler, and M. Bernasconi, Phys. Rev. B 86, 104301 (2012). [4] G. C. Sosso, J. Behler, and M. Bernasconi, Phy. Status Solidi B 249, 1880 (2012).