NOMATEN HYBRID-SEMINAR June 13: Density functional simulations of threshold switching in non-volatile memory materials


In-person: NOMATEN seminar room



In-person: NOMATEN seminar room

Tuesday, June 13th  2023 13:00 CET

Density functional simulations of threshold switching in non-volatile memory materials

Dr. Jaakko Akola
Norwegian University of Science and Technology, Trondheim, Norway.



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/Å 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.

We also presents result for our latest electronic structure analysis of recrystallized Ge2Sb2Te5 model 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.

[1] J. Akola, K. Konstantinou, and R.O. Jones, Phys. Rev. Mater. 6, 035001 (2022). 
[2] J. Kalikka, J. Akola, and R.O. Jones, Phys. Rev. B 94, 134105 (2016).
[3] K. Konstantinou, F.C. Mocanu, and J. Akola, Phys. Rev. B 106, 184103 (2022).



Jaakko Akola is full professor in computational physics and group leader in the Norwegian University of Science and Technology (NTNU). He received his PhD in the University of Jyväskylä, Finland, in 2000 (theoretical physics), and worked as post-doctoral fellow in a polymer project in Forschungszentrum Jülich, Germany, in 2000-2005. After this, he returned Jyväskylä and worked as senior scientist until 2010 when he received a fellowship of the Academy of Finland to establish a research group in the Tampere University of Technology, Finland. He was appointed as associate professor (tenure-track) in 2016 before he decided to move to NTNU in 2017. Prof. Akola still holds a part-time affiliation in the Tampere University. He is an expert in atomistic simulations (in particular electronic structure methods) and his project portfolio is diverse, including topics related to (electro)catalysis, amorphous/glassy materials, metal alloys and nanoparticles, microelectronics, biochemical reactions, etc. 

Dr. Jaakko Akola



Dr. Jaakko Akola