First-principles calculation of lattice thermal conductivity in crystalline phase change materials: GeTe, Sb2Te3 , and Ge2

Abstract

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds Ge 2 Sb 2 Te 5 and GeTe in their crystalline form. The related Sb 2 Te 3 compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal Ge 2 Sb 2 Te 5 of about 0.45 W m −1 K −1 measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Ab initio density functional theory study of the electronic, dynamic, and thermoelectric properties of the crystalline pseudobinary chalcogenide (GeTe)x/(Sb2Te3)Ibarra-Hernández1, Raty2 2018Phys. Rev. B

“…In this work we have not included long-range dispersion forces; instead, we have chosen to use PBESol exchange-correlation functional to improve accuracy on the lattice parameters. It has been shown that negative frequencies reported for GST225 become positive once the overestimation of the c parameter is reduced closer to experimental values [31]. Here we report that the use of the PBEsol xc functional improves the agreement of lattice parameters of GST225 with respect to experimental measures, which leads to similar results as those obtained by Campi et al through van der Waals interactions [31].…”

“…It has been shown that negative frequencies reported for GST225 become positive once the overestimation of the c parameter is reduced closer to experimental values [31]. Here we report that the use of the PBEsol xc functional improves the agreement of lattice parameters of GST225 with respect to experimental measures, which leads to similar results as those obtained by Campi et al through van der Waals interactions [31]. Table II summarizes our relaxed structure parameters together with the available experimental data.…”

“…8(c). It has been shown that the overestimation of the lattice thermal conductivity obtained from ab initio calculations can be overcome by the inclusion of an effective disorder on the cation masses [31]. The overestimation was expected since, experimentally, there are many other different sources of scattering that we do not take into account in our calculations.…”

“…In these PDOS, the states increase almost linearly up to 33 cm −1 and the maximum is reached around 100 cm −1 . As we mentioned before, Campi et al [31] show that the inclusion of vdW corrections in GST225 leads to only positive phonons at the point. In comparison with our results, the negative frequency shown in the mentioned work was around 30 cm −1 .…”

“…[50]. Phonon band structure has been reported for some of these structures in the works of Campi et al [31,51].…”