Atomistic simulations of thermal transport and vibrational properties in phase-change materials

Phase change materials are a class of compounds employed for data storage applications such as rewritable optical disks (DVD-RW, Blue-Ray disks) and more recently for non-volatile electronic memories of new generation, named phase change memories (PCM)[1]. These applications rely on a fast (50 ns) and reversible transition between a crystalline and the amorphous phases upon heating. The strong optical and electronic contrast between the crystal and the amorphous allows discriminating between the two phases that correspond to the two states of the memory, i.e. the 0 and 1 bits. In this work I have studied, by means of first principle and classical calculations, the structural, vibrational and thermal properties of some of the most promising and widely used phase-change materials such as GeTe, Ge2Sb2Te5 (GST), GeTe-Sb2Te3 superlattices and InSbTe (IST) alloys. The first part of the thesis is focused on the calculation of bulk thermal conductivity and thermal boundary resistances(TBR) between the active media and the surrounding dielectrics and metallic electrodes. Since in PCMs the phase changes corresponding to the memory writing/erasing processes are induced by Joule heating, heat dissipation and transport are key factors that greatly affect the power consumption and the switching speed of the memory cell. Moreover these quantities also influence the thermal cross-talks among the different bits in a memory array which can rise serious reliability issues, especially in ultrascaled devices. Bulk thermal has been computed on the basis density functional calculations [2] for crystalline GeTe, Sb2Te3 and GST. These calculations allowed to identify the origin of the large variability in experimental measurement for GeTe and the origin of the glass-like thermal conductivity in GST. An estimate of IST thermal conductivity was also obtained based on the minimal thermal conductivity model and ab-initio phonons. Thermal boundary resistance at different interface of crystalline GST, IST and GeTe with dielectrics and metals have been estimated from ab-initio phonons and the Diffuse Mismatch Model. The calculations revealed that an important contribution to the TBR comes from the electron-phonon coupling within GST and GeTe. For the GeTe amorphous/crystalline interface, which is also present in the device, we used an interatomic potential generated with a Neural Network (NN) method [3] and non-equilibrium molecular dynamics simulations. In the second part of the thesis we calculated the vibrational Raman spectra GeTe multilayers and of different GeTe-Sb2Te3 superlattices and intermixed compounds which are proposed to be the basis of the so called interfacial phase-change memories, a new type of device with very low power consumption[4]. The comparison between theoretical and experimental spectra allowed the identification of the growth mechanism of GeTe thin films on silicon and could allow the identification of the structures in the superlattices. In the last part we studied the nanowires of Sb2Te3 and GeTe. In particular we studied the energetic of Sb2Te3 surfaces by mean of ab initio calculations in order to explain the observation of a new Sb2Te3 crystal structure in nanowires that turned out to be stabilized by the low dimensionality. Finally we extended the bulk NN potential for GeTe previously developed in our group, enabling the possibility to study the properties of GeTe surfaces and nanowires. [1] M. Wuttig and N. Yamada, Nature Materials 6, 824 (2007). [2] L. Paulatto et al., Phys. Rev. B 87, 214303 (2013). [3] G.C. Sosso et al., Phys. Rev. B 86, 104301 (2012). [4] R.E.Simpson et al., Nature Nanotechnology 6, 501 (2011).