Chemical Structure of Conductive Filaments in Tantalum Oxide Memristive Devices and Its Implications for the Formation Mechanism

Abstract

Resistive switching in metal oxides is believed to be caused by a temperature and electric field driven redistribution of oxygen vacancies within a nanometer sized conductive filament. Accordingly, gaining detailed information about the chemical composition of conductive filaments is of key importance for a comprehensive understanding of the switching process. In this work, spectromicroscopy is used to probe the electronic structure of conductive filaments in Ta2O5‐based memristive devices. It is found that resistive switching leads to the formation of a conductive filament with an oxygen vacancy concentration of ≈20%. Spectroscopic insights provide detailed information about the chemical state of the tantalum cations and show that the filament is not composed of a metallic Ta0 phase. As an extreme case, devices after an irreversible dielectric breakdown are investigated. These devices feature larger conductive channels with higher oxygen vacancy concentrations. Using the experimental data as input for finite element simulations, the role of thermodiffusion for the formation process of conductive filaments is revealed. It is demonstrated that thermodiffusion is not the dominating effect for the filament formation here but might play a role in accelerating the forming process, as well as in the stabilization of the filament.

Filamentary TaOx/HfO2 ReRAM Devices for Neural Networks Training with Analog In‐Memory Computing

“…Instead, the high T generated upon forming the filament favor the lateral diffusion of the O 2 − in the TaO x layer. [ 41,65 ]…”

“…By reactive sputtering, it is possible to deposit various substoichiometric TaO x films, [40,41] corresponding to different resistivities. Such layers can be exploited to experimentally determine how different CMO resistivities impact the device switching properties.…”

“…Instead, the high T generated upon forming the filament favor the lateral diffusion of the O 2 − in the TaO x layer. [41,65] Thicker thicknesses of the TaO x layer further enhance the heat confinement effect, due to the increasing distance between the hot TaO x /filament interface and the thermally conductive TiN-TE. With higher T, the thermal diffusion is promoted, extending the highly oxidized TaO y>x region depicted in Figure 5b more laterally than vertically.…”

Multiscale Modeling of Ion Dynamics in Memristive Elements

“…Switching between different states of a memristive element based on a thin oxide layer is associated with the formation/destruction of filament structures, which are typically clusters of oxygen vacancies. [ 7–9 ] Oxygen vacancies act as traps for electrons and provide conditions for tunneling. [ 10 ]…”

Oxygen vacancy dynamics in Pt/TiOx/TaOy/Pt memristors: exchange with the environment and internal electromigration

“…This is based on its CMOS compatibility -which would ease the integration with standard electronics-, analog response -in order to mimic the adaptable synaptic weights of biological synapses- [9][10][11], very high endurance (from 10 10 to 10 12 cycles) [12,13], ultrafast switching time (≈ 10 ps) [14], large ON-OFF ratio (≈ 10 6 ) [15] and ultra-low power operation (≈ 60 fJ/bit) [15]. The already reported memristive mechanisms in TaO y -based devices include the formation of conducting nanofilaments [9,[16][17][18], the modulation of energy barriers present at metal-oxide interfaces [10,19] or the OV exchange between TaO y /TaO h bilayers (with different degree of oxidation) [13,20,21]. In the latter case, it is usually assumed that the more reduced layer acts as OV source/sink that eases the reduction/oxidation of the more oxidized one that drives the resistance changes -usually Ta 2 O 5 - [13].…”