Engineering amorphous-crystalline interfaces in TiO2−x/TiO2−y-based bilayer structures for enhanced resistive switching and synaptic properties

Abstract

The operating principle of resistive random access memories (RRAMs) relies on the distribution of ionic species and their influence on the electron transport. Taking into account that formation and annihilation of conducting filaments (CFs) is the driving mechanism for the switching effect, it is very important to control the regions where these filaments will evolve. Thus, homolayers of titanium oxide with different oxygen contents were fabricated in order to tune the local electrical and thermal properties of the CFs and narrow down the potential percolation paths. We show that the oxygen content in the top layer of the TiO2−x/TiO2−y bilayer memristors can directly influence the morphology of the layers which affect the diffusion barrier and consequently the diffusivity and drift velocity of oxygen vacancies, yielding in important enhancement of switching characteristics, in terms of spatial uniformity (σ/μ < 0.2), enlarged switching ratio (∼104), and synaptic learning. In order to address the experimental data, a physical model was applied, divulging the crucial role of temperature, electric potential and oxygen vacancy density on the switching effect and offering physical insights to the SET/RESET transitions and the analog switching. The forming free nature of all the devices in conjunction with the self-rectifying behavior, should also be regarded as important assets towards RRAM device optimization.

Tuning Resistive, Capacitive, and Synaptic Properties of Forming Free TiO2‐x‐Based RRAM Devices by Embedded Pt and Ta Nanocrystals

“…In addition, series resistance effects could justify this behavior, which was also observed for the reference sample . Alternatively, capacitive changes have been attributed to gap modulation effect which, however, needs to change substantially (30 times) in order to explain our experiments, while CFs in multiple configuration is considered is incompatible with the area dependence characteristics, which imply the formation of limited number of CFs …”

“…In order to assess the applicability of our devices as synaptic learning modules, we measured the conductance change under the application of a train of pulses, with amplitude ±5 V and width 10 µs, in order to induce the potentiation/depreciation characteristics. The results, depicted in Figure , divulge that all samples respond to the application of each positive/negative pulses and more importantly, that synaptic plasticity properties have been measured within six orders of magnitude, while modulation of the CF diameter is considered the driving force for this effect . The small size of the NCs allows their facile integration into smaller devices, in terms of oxide thickness and electrode area, anticipating even lower operating voltages and enhanced uniformity characteristics, since smaller electrode areas will enclose less NCs.…”

Memristive Anodic Oxides: Production, Properties and Applications in Neuromorphic Computing

“…To improve homogeneity, often multiple deposition steps are performed, which increases overall film thickness and consequently electroforming voltage [59], while oxygen or argon plasma etching can be employed to introduce oxygen vacancies in the as-deposited materials, hence reducing electroforming voltages or even eliminating the need for this step [60,[66][67][68]. The possibility of applying multiple coating steps also opens the way to sol-gel processed double-layer structures [69], which brings potential benefits that span from increased endurance to reduced power consumption [44,[70][71][72]. For instance, in TiO 2 -based memristors oxygen vacancies migration can lead to oxygen gas evolution at the anode, which irreversibly compromises the oxide stoichiometry: the presence of a blocking layer can act as sink of oxygen ions and limit currents involved, avoiding oxide breakdown [3,73,74].…”

Enhancing Short-Term Plasticity by Inserting a Thin TiO2 Layer in WOx-Based Resistive Switching Memory

“…Non-filamentary type switching is more suitable for fine weight updates compared to filamentary type switching when implementing synaptic behaviors [27]. Metal oxides such as WO x [28][29][30][31][32][33], TiO x [34][35][36], and TaO x [37,38] show both filamentary and non-filamentary switching depending on the process conditions, film thickness, combination of electrodes, and switching operation.…”