Electronic and thermal properties of the phase-change memory material, Ge2Sb2Te5, and results from spatially resolved transport calculations

TL;DR

Ge2Sb2Te5 (GST) exhibits rapid amorphous–crystalline switching with large contrasts in conductivity; this paper combines realistic KMLE GST structural models with hybrid-functional DFT (HSE06) and machine-learned interatomic potentials to study electronic structure, lattice dynamics, and spatially resolved transport. The authors demonstrate strong electron–phonon coupling near the Fermi level, substantial thermally driven fluctuations of conduction-tail states, and highly heterogeneous electronic transport tied to Te-rich motifs and Sb-vacancy environments. They introduce and apply space-projected conductivity (SPC) and site-projected thermal conductivity (SPTC) to map charge and heat flow at the atomic scale, revealing filamentary Te/Sb networks that dominate heat transport and conductive channels that percolate through defective motifs. Collectively, these results provide a mechanistic picture of transport in amorphous GST and offer a generalizable framework for tailoring phase-change materials via atomic-scale structural motifs.

Abstract

We report new insights into the electronic, structural, and transport (heat and charge) properties of the phase-change memory material Ge2Sb2Te5. Using realistic structural models of Konstantinou et. al. [Nat. Commun. 10, 3065 (2019)], we analyze the topology, electronic states, and lattice dynamics with density functional methods, including hybrid-functional calculations and machine-learned interatomic potentials. The Kohn-Sham orbitals near the Fermi level display a strong electron-phonon coupling, and exhibit large energy fluctuations at room temperature. The conduction tail states exhibit larger phonon-induced fluctuations than the valence tail states. To resolve transport at the atomic scale, we employ space-projected electronic conductivity and site-projected thermal conductivity methods. Local analysis of heat transport highlights the role of filamentary networks dominated by Te, with Sb and Ge making progressively smaller contributions.

Figures (7)

• Figure 1: Coordination analysis for the local environment of (a) germanium, (b) antimony, and (c) tellurium, respectively, averaged over twelve GST models from KMLE after HSE06 relaxation (Red color). Coordination number was calculated by using a cut-off bond distance of 3.2 Å for each atom species. The blue histogram shows the average coordination calculated from before HSE06 relaxation.
• Figure 2: (a) The total and projected density of states near the band edges for M3. The projected density of states for atomic species reveals that Te dominates the band edges. (b) The IPR indicates the extent of electronic localization of states near the band edges. (c) Atom-projection of the states at the conduction band edge (LUMO), labeled in (b). The conduction tail state is predominantly due to Ge-Te and Ge-Ge, Te-Te forming a network illustrated by a large sphere. The volume of the sphere represents the atomic contribution to the conduction band tail state. (d) The total and projected density of states near the band edges for M7 show a shallow-gap defect near the conduction band edge. (e) The shallow gap state is localized (IPR $\approx$ 0.25). (f) Atom-projection of the shallow gap state is predominantly due to a Te atom (the largest brown sphere in (f)), and Sb-Sb bonds. Color code: teal - Ge, purple - Sb, and brown - Te.
• Figure 3: (a) The total and projected density of states near the band edges for the GST model M8 shows multiple defects in the HOMO-LUMO gap. (b) Mid-gap gap states ("i-ii") and a shallow-gap defect ("iii") are localized, indicated by high IPR values. (c)[i-iii] Atom-projection of the mid-gap states and a shallow-gap state. The mid-gap state "i" is predominantly due to a cluster of over-coordinated Ge atoms with Te atoms (cluster is highlighted). Second mid-gap state "ii" 0.11 eV above the mid-gap level is localized on a Sb-Sb-Sb trimer structure, while shallow-defect state "iii" 0.15 eV below the LUMO level is centered at a group of Sb and Te atoms, shown in (c) [ii-iii], respectively. The volume of the sphere in (c) indicates the weight of atomic contributions to the defect state. Color code: teal - Ge, purple - Sb, and brown - Te.
• Figure 4: All plots correspond to M5 thermally equilibrated at 300K using a Nosé-Hoover thermostat with a time step of 1.5 fs. (a) Fluctuation in Kohn-Sham orbitals near the Fermi level, depicting fluctuations in LUMO (pink), mid gap (green), and HOMO (dashed-yellow). See legends for fluctuations in different orbitals. (b) Energy-resolved orbital properties for selected states in GST model M5. Black markers and lines show the inverse participation ratio (IPR), indicating the degree of localization of each state. The red dashed line with diamond markers represents the thermal fluctuation of the corresponding orbital energies. (c-d) Snapshots of mid-gap and LUMO energy level as an isosurface plot at times "t$_1$" and "t$_2$", respectively, labeled in (a).
• Figure 5: Thermally driven electronic conductivity fluctuations for M5 at 300K. The SPC isosurface plot reveals fluctuations in conduction-active regions in the model at different times. Color code: teal -Ge, purple - Sb, and brown - Te. Sb is shown as large spheres to identify antimony-rich and poor sites.