Impact of Pt embedded nanocrystals on the resistive switching and synaptic properties of forming free TiO2 – x/TiO2 – y-based bilayer structures

Abstract

The resistive switching characteristics of forming free TiO2 – x/TiO2 – y memory devices containing Pt nanocrystals (NCs) beneath the top electrode were systematically investigated through experiments and numerical simulation insights. By embedding Pt nanocrystals, we have the possibility to narrow down the possible locations where the switching effect will evolve and thus significantly improve the inherent variability of the devices. Besides, the deployment of bilayer structures can tune the resistance levels, since the presence of the layer with low oxygen content (TiO2 – y) acts practically as series resistance, limiting the operating currents and at the same time forcing the switching effect to evolve in the layer with the higher oxygen content (TiO2 – z). A numerical model is implemented, in order to shed light into the origin of the SET/RESET transitions and illustrate the direct impact of NCs on the conducting filament (CF) shape and distribution of oxygen vacancies. It is demonstrated that a higher density of oxygen vacancies is created in the vicinity of NCs, which can directly impact the operating current values and the uniformity of the switching characteristics. The presence of NCs also facilitates the reduction of the operating voltages (∼3 V), and, as a result, it significantly improves power consumption, without sacrificing the switching ratio (∼103), temporal/spatial variability (σ/μ < 0.2), and pulse endurance (108 cycles) characteristics of our memory cells. Evidence about the impact of the NCs position within the material configuration are also presented. The direct impact of Pt NCs on the depression and potentiation characteristics of the synaptic weight denotes similarly the huge applicability of our approach to tune a wide range of resistive switching properties.

Toward Controlling Filament Size and Location for Resistive Switches via Nanoparticle Exsolution at Oxide Interfaces

“…This improvement in the coefficient of variability (CV) in the low resistance state of about 90% compares well with previous reports on switching in HfO x (x < 2) with embedded Pt nanocrystals [23] and exceeds the improvements in reports of Pt nanocrystals in TiO 2 . [32] The improvement in the resistive switching characteristics is possible by the local enhancement of the electrical field, which confines the conductive filaments at the Ni nanoparticles, and the possibility of controlled oxygen exchange between the particle and the switching oxide during device operation.…”

“…Nanoparticles at the electrode interface are often integrated into the device by depositing ultrathin metallic films-via electron beam evaporation, [34] pulsed laser deposition, [37] or sputtering [32] and utilizing island growth. Another option is to disperse previously synthesized nanoparticles by spin coating [35] or electrostatic immobilization by organic aids.…”

“…In order to address the challenge of stochastic filament location, several strategies have been put forward to spatially confine filaments either in the switching or electrode materials. These include 1) switching within a single dislocation in SrTiO 3 [9] and in SiGe, [10] 2) fabricating electrodes into tips, [11][12][13][14][15][16][17][18] 3) integrating nanoporous graphene into the switching material, [19][20][21][22] 4) embedding nanoparticles into the switching material, [23][24][25][26][27][28][29][30][31][32][33] 5) introducing nanoparticles at the metal-oxide interface, [34][35][36][37][38][39][40] and 6) engineering the edges of the devices, which has been shown to be industrially viable. [41] These strategies are analyzed in terms of their opportunities, processing challenges and materials universality in Table 1, and summarized through device [35] sketches in Figure 1a.…”

Synaptic Plasticity and Quantized Conductance States in TiN-Nanoparticles-Based Memristor for Neuromorphic System

“…Bousoulas et al described that multilevel memory state, better variability, and long retention were achieved by Pt-nanocrystals (NCs) inclusion inside TiO x due to local field enhancement [ 10 ]. Also, embedding Pt-NCs inside switching layers, the local electric field leads to narrow and controlled CFs formation, further enhancing the switching performances described by Sakellaropoulos et al [ 9 ]. In RRAM, gradual synaptic weight change manipulation by applying external stimuli is essential for high-density memory storage and artificial synaptic device for neuromorphic application.…”

Emulating Artificial Synaptic Plasticity Characteristics from SiO2-Based Conductive Bridge Memories with Pt Nanoparticles

“…More specifically, while a t SET ~50 ns is required in order to switch the memory device into the LRS under the application of +0.8 V/100 ns square pulse, the same transition takes place at a t SET ~20 ns when a higher pulse, in terms of amplitude is applied (+2.5 V/100 ns). In order to gain valuable insights into the origins of this effect, a series of measurements were carried out by enforcing various positive square pulses, with amplitude ranging from 0.5 to 2.5 V and step 100 mV [ 20 ]. The results depicted in Supplementary Materials Figure S3 reveal a highly nonlinear connection between the pulse amplitude and the measured delay time, which indicates that the SET kinetics is governed by ionic hopping procedures.…”