Density Functional Simulations of Ag Migration in a Conductive Bridging Random Access Memory Cell and Electron Localization Effects in Recrystallized Ge₂Sb₂Te₅

We have performed density functional/molecular dynamics (DF/MD) simulations to investigate the drift of Ag atoms in an amorphous GeS₂ solid-state electrolyte between Ag and Pt electrodes in the presence of a finite electric field [1]. The atomistic model structure represents a conductive bridging random access memory (CBRAM) device, where the electric field induces the formation of conductive filaments across the chalcogenide. Simulations of a 1019-atom structure under an external electrostatic potential of 0.20 eV/Angst. at 480 and 680 K show significant atomic diffusion within 500 ps. Ag migration and the formation of percolating filaments occur in both cases. The electronic structure analysis of selected snapshots shows that dissolved Ag atoms become markedly cationic, which changes when Ag clusters form at the Pt electrode. The electrolyte does not conduct, despite percolating single-atom Ag wire segments. Sulfur becomes anionic during the migration as a result of Ag-S bonding, and the effect is most pronounced near the active (Ag) electrode. The formation of conductive filaments requires a percolating network of Ag clusters to grow from the Pt interface, and the weakest link of this network appers to be at the Ag electrode.<br/><br/>We also presents result for our latest electronic structure analysis of recrystallized Ge₂Sb₂Te₅model systems based on our prior DF/MD simulations [2]. Understanding the relation between the structural disorder in the atomic geometry of the recrystallized state of PCMs and the localized states in the electronic structure is essential for fundamental understanding. Hybrid density-functional theory simulations are employed to ascertain the impact of antisite defects on the spatial localization of the electronic states in the bottom of the conduction band in recrystallized models of Ge₂Sb₂Te₅. Te−Te homopolar bonds are the local defective atomic environments mainly responsible for the electron localization of the conduction-band-edge states in the simulated structures, while Sb−Te chains can also induce spatial localization. Unoccupied defect-related electronic states can emerge in the band gap during a crystallization event, while Sb−Sb homopolar bonds have been identified in the defect environment of a deep localized state [3]. These findings are in accordance with the latest results for recrystallized model systems of Ge₁Sb₂Te₄ in Ref. [4]<br/><br/>[1] J. Akola, K. Konstantinou, and R.O. Jones, Phys. Rev. Mater. <b>6</b>, 035001 (2022).<br/>[2] J. Kalikka, J. Akola, and R.O. Jones, Phys. Rev. B <b>94</b>, 134105 (2016).<br/>[3] K. Konstantinou, F.C. Mocanu, and J. Akola, Phys. Rev. B 2022 (in press).<br/>[4] Y. Xu, Y. Zhou, X.-D. Wang, W. Zhang, E. Ma, V. L. Deringer, and R. Mazzarello, Adv. Mater. <b>34</b>, 2109139 (2022).