Nanosized Conducting Filaments Formed by Atomic-Scale Defects in Redox-Based Resistive Switching Memories

Abstract

Redox-based resistive switching phenomena are found in many metal oxides and hold great promise for applications in next-generation memories and neuromorphic computing systems. Resistive switching involves the formation and disruption of electrically conducting filaments through ion migration accompanied by local electrochemical redox reactions. These structural changes are often explained by point defects, but so far clear experimental evidence of such defects is missing. Here, nanosized conducting filaments in Fe-doped SrTiO3 thin-film memories are visualized, for the first time, by scanning transmission electron microscopy and core-loss spectroscopy. Conducting filaments are identified by a high local concentration of trivalent titanium ions correlating to oxygen vacancies. Strontium vacancies and lattice distortions also exist in the filaments. Despite a high concentration of defects in the filaments, their general SrTiO3 perovskite structure is essentially preserved. First insights into the switching mechanism are deduced from a snapshot simultaneously showing multiple nanosized filaments in different evolutionary stages. The coexistence of a high Ti3+ concentration along with Sr- and O-vacancies in the conducting filaments provides atomic scale explanations for the resistive switching mechanisms. The results shed unique light on the complexity of the conducting filament formation that cation and anion defects need to be considered jointly.

The ultimate switching speed limit of redox-based resistive switching devices

“…This value is in agreement with different spectro-microscopic observations of laments in resistive switching devices. 24,35,45,46 To calculate the temperature distribution in this structure, the transient heat equation,…”

Study on the Connection Between the Set Transient in RRAMs and the Progressive Breakdown of Thin Oxides

“…Briefly, the energy transfer from the CF to its surroundings promotes the diffusion of the fastest atomic species, which gradually enlarges the CF connecting the electrodes of the stack causing a progressive increase in the leakage current. Remarkably, SET and BD events show the same statistical behavior [9], and similar micro structural changes in the dielectric [15]- [19]. Several authors [15]- [17] showed that the BD spot is characterized by the formation of a Sirich (for poly-Si/SiO x N y /Si stacks) or a metal-rich region (for metal gate/high-K/Si stacks) in the gate dielectric.…”

“…Previously reported current compliance studies have shown a clear dependence of the CF characteristics on the maximal current flowing through the device, both for the BD of gate oxides [7] and the SET event in RRAM devices [22]. In addition, TEM imaging of Si-based MOS capacitors prior to and post dielectric BD [15]- [17] and HfO 2 -based RRAM cells after forming and cycling [18], [19] show comparable microstructural changes in the oxide, suggesting the diffusion of the anodic atomic species into the oxide layer during both events.…”

Conductivity of iron‐doped strontium titanate in the quenched and degraded states

“…The most reduced sample ( = 4.5 × 10 −18 Pa) shows a bulk activation energy for conduction of 0.23 eV in agreement with literature values from theoretical simulations of polaron hopping and does not exhibit a metal‐to‐insulator transition as suggested by other literature. This low oxygen activity regime is relevant to more aggressive resistance degradation such as resistive switching in thin films, where the concentration can increase substantially in conductive filament paths …”

“…SrTiO 3 is commercially used as a dielectric in both high‐voltage varistors and capacitors and may be a candidate for solid‐state memory based on resistive switching mechanisms . Control of resistive switching from the drift of ionic oxygen vacancies () relies on understanding the equilibrium initial conditions, transport kinetics of ionic defects and local boundary conditions at the electrodes . The spatial redistribution of is also important to the electrical degradation of SrTiO 3 .…”