Bidirectional threshold switching characteristics in Ag/ZrO2/Pt electrochemical metallization cells

A high-performance selector with bidirectional threshold switching (TS) characteristics of Ag/ZrO2/Pt structure was prepared by incorporating metallic Ag into the ZrO2 matrix. The bidirectional TS device exhibited excellent switching uniformity, forming-free behavior, ultra-low off current of <1 nA and adjustable selectivity (from 102 to 107). The experiment results confirmed that metallic Ag clusters were penetrated into the ZrO2 matrix during the annealing process, which would function as an effective active source responsible for the bidirectional TS. The volatile behavior could be explained by the self-dissolution of unstable filaments caused by minimization of the interfacial energy and thermal effect. Furthermore, a bipolar-type one selector-one resistor (1S-1R) memory device was successfully fabricated and exhibited significant suppression of the undesired sneak current, indicating the great potential as selector in a cross-point array.

Resistive switching memories based on the formation and dissolution of a metal filament in a simple metal/oxide/metal structure are attractive because of their potential high scalability, low‐power consumption, and ease of operation. From the standpoint of the operation mechanism, these types of memory devices are referred to as gapless‐type atomic switches or electrochemical metallization cells. It is well known that oxide materials can absorb moisture from the ambient air, which causes shifts in the characteristics of metal‐oxide‐semiconductor devices. However, the role of ambient moisture on the operation of oxide‐based atomic switches has not yet been clarified. In this work, current–voltage measurements were performed as a function of ambient water vapor pressure and temperature to reveal the effect of moisture on the switching behavior of Cu/oxide/Pt atomic switches using different oxide materials. The main findings are: i) the ionization of Cu at the anode interface is likely to be attributed to chemical oxidation via residual water in the oxide layer, ii) Cu ions migrate along grain boundaries in the oxide layer, where a hydrogen‐bond network might be formed by moisture absorption, and iii) the stability of residual water has an impact on the ionization and migration processes and plays a major role in determining the operation voltages. These findings will be important in the microscopic understanding of the switching behavior of oxide‐based atomic switches and electrochemical metallization cells.

This paper presents a comprehensive investigation of exploiting passive RRAM crossbar array with nonlinear selective elements for neuromorphic computing. Design considerations, including weight-mapping method, voltage-applying strategy, and optimization scheme, are discussed. By using these methods, the dysfunction of the non-linear selection characteristic during inference is removed. Moreover, we evaluate the IR-drop effect on the passive crossbar array with non-linear selection characteristic. Finally, this work provides insightful analysis for designing a neuromorphic computing system based on a passive crossbar with non-linear selection characteristics.

High‐performance selector devices are essential for emerging nonvolatile memories to implement high‐density memory storage and large‐scale neuromorphic computing. Device uniformity is one of the key challenges which limit the practical applications of threshold switching selectors. Here, high‐uniformity threshold switching HfO2‐based selectors are fabricated by using e‐beam lithography to pattern controllable Ag nanodots (NDs) with high order and uniform size in the cross‐point region. The selectors exhibit excellent bidirectional threshold switching performance, including low leakage current (<1 pA), high on/off ratio (>108), high endurance (>108 cycles), and fast switching speed (≈75 ns). The patterned Ag NDs in the selector help control the number of Ag atoms diffusing into HfO2 and confine the positions to form reproducible filaments. According to the statistical analysis, the Ag NDs selectors show much smaller cycle‐to‐cycle and device‐to‐device variations (C V < 10%) compared to control samples with nonpatterned Ag thin film. Furthermore, when integrating the Ag NDs selector with resistive switching memory in one‐selector‐one‐resistor (1S1R) structure, the reduced selector variation helps significantly reduce the bit error rate in 1S1R crossbar array. The high‐uniformity Ag NDs selectors offer great potential in the fabrication of large‐scale 1S1R crossbar arrays for future memory and neuromorphic computing applications.

The phenomenon of electric field mediated resistance switching in metal oxides is critical to nanoscale electronic devices ranging from conventional energy efficient MOSFETs to emerging memory devices as well as artificial neuron and synaptic devices. Such resistance transition can be facilitated by two main mechanisms: i) conduction of oxygen ions in non-stoichiometric compounds e .g. HfOx, AlOx. In this case the redox reaction of the oxide itself is manifested by oxygen anion transport, ii) the oxides serve as electrolyte dielectrics to conduct electrochemically active metal cations, such as Ag or Cu, forming the so-called electrochemical metallization memory (ECM) devices. So far inert and electrochemically active transition metals such as Pt and Cu/Ag respectively have been implemented to demonstrate resistive switching in Metal-Insulator-Metal (MIM) devices for application in steep slope transistors and as non-volatile memory and selector device. However, post transition metal e. g. Sn with lower cohesive energy as well as lower negative heat of formation of metal oxide offers viable alternative to Cu and Ag in ECM type devices. In this study, we fabricated asymmetric two terminal MIM structures in a cross-bar array with atomic layer deposited HfO2 thin film sandwiched between Pt and post-transition metal (Sn). We demonstrate reversible resistance switching in HfO2. Filamentary conduction by cations beyond a threshold voltage was verified by varying device areas. A recent report demonstrated coexistence of both oxygen anion and metal cation transport being responsible for resistance switching in a single oxide depending on the anode metal. We present a comparative investigation based on current-time (I-t) measurements to distinguish contributions from oxygen anions and metal cations in the resistance switching process. Using Sn anode we demonstrate bidirectional threshold switching as well as bipolar memory switching. We further evaluate viability of Sn anode in terms of switching speed in comparison with Cu and Ag. Furthermore, we discuss considerations for bidirectional threshold switching and bipolar memory switching with Sn anode substantiated with our ongoing molecular dynamic simulations.

The mechanism of memory and threshold switching in (GeTe4)100 − x(As2Se3)x glasses

Boron nitride and molybdenum disulfide as 2D composite element selectors with flexible threshold switching

A p-type ternary logic MOSFET (P-TMOS), which serves as the counterpart to a previously reported n-type ternary logic MOSFET (N-TMOS) but has been missing for the construction of a complementary standard ternary inverter, is demonstrated. The P-TMOS consists of a PMOS and an n-type threshold switch (TS) connected in series. While the PMOS determines the type of the ternary logic device, the TS, despite being an n-type device, provides both n-type and p-type characteristics, making it bidirectional with a tunable middle state in ternary logic. This tunable middle state is attractive for balancing noise margin and controlling power-delay product. By leveraging mature CMOS technology capable of large-scale fabrication, the Si-based P-TMOS facilitates the practical implementation of a ternary logic system.

Memristors are poised to revolutionize neuromorphic computing and next-generation memory, yet persistent challenges, including stochastic conductance control, limited endurance, and poor scalability, impede their practical adoption. Here, we report a self-rectifying memristor leveraging a GaOx/InOx heterostructure to overcome these barriers. The device demonstrates an ultrahigh rectification ratio and nonlinearity (&amp;gt;107), effectively suppressing sneak currents in passive crossbar arrays. Robust multi-level conductance modulation and cycling stability exceeding 106 cycles are achieved, alongside minimal inter-device variability (σ/μ &amp;lt; 10%) in a 6 × 6 passive array. Critically, it demonstrates dynamic tunability between short-term and long-term plasticity, modulated via pulse number/amplitude, a cornerstone for biologically inspired learning. By implementing the one-bitline pull-up scheme, the design achieves an integration density of 125.1 Tb at a 10% read margin, outperforming state-of-the-art resistive memories. This work establishes oxide heterostructures as a scalable platform for high-density neuromorphic systems, bridging the gap between functional material innovation and hardware-driven machine intelligence.

Effect of top electrode material on resistive switching properties of WN based thin films for non volatile memory application

Redox-based resistive switching memories (ReRAMs) are one of most promising candidates to be next generation nonvolatile memories. They meet all requirements for green IT e.g. low power consumption, high write, erase and read speed and can also be used for beyond von Neumann computing, alternative logic and neuromorphic operations. ReRAM devices have a simple structure, high scalability, data retention, endurance and can satisfy high-density integration criterion [1]. These systems are usually divided in two categories, VCM (Valence Change Memories) and ECM (Electrochemical Metallization cells), depending on the mechanism underlying the device operation [2]. The cell structure consists of a solid electrolyte, e.g. a metal oxide or a layered oxides structure, sandwiched between two electrodes with metallic behavior (metals, TiN or conducting transparent oxides). Despite the oxide films have macroscopic dielectric (high-k) properties, deposited as nanoscale thin films they behave as solid electrolytes [3]. Electrolyte for such devices are usually prepared by physical (e.g. sputtering, pulsed laser deposition) or chemical (atomic layer deposition, chemical vapor deposition) techniques. An alternative way to produce the bottom electrode/oxide junction is the anodizing, a low-cost and low temperature technique that allows to electrochemically prepare oxides on the surface of valve metals or valve metals alloys such as Ta, Ti, Nb, Al, Hf and so on [4]. It is possible to tailor oxides features, such as thickness, morphology, structure and composition by adjusting process parameters (e.g. growth rate, anodizing electrolyte, formation voltage). Barrier-type anodic films seem to be suitable solid electrolytes for ReRAM devices since they are smooth, uniform in thickness and composition and show perfect adhesion to the metallic substrate. In this work we want to present several valve metal/anodic oxide systems prepared by anodizing of metal thin films (namely Hf, Nb, Ta, Al and their alloys). Barrier-type anodic films were grown up to different formation voltages, in order to get oxides with different thickness and properties. An investigation based on electrochemical impedance and photoelectrochemical measurements was carried out to get information on the solid-state properties of the anodic oxides (i.e. band gap, flat band potential and dielectric constant) as a function of the formation conditions. Furthermore, we fabricated ReRAM-type devices (metal/oxide/metal junctions) by depositing Pt top electrodes on the anodic films surfaces. Electrical characterization was performed to check whether these devices exhibit resistive switching and, in addition, they are suitable for the use in redox-based memories. In particular, cyclic voltammetries, dc I-V sweeps and pulse measurements were carried out to study the performances of the devices and to establish the kind of mechanism underlying the device operation [5]. The experimental findings (I-V sweeps stability, data retention and endurance) prove that anodic oxides can be considered promising electrolytes for ReRAM devices. [1] Lee, M.-J.; Lee, C. B.; Lee, D.; Lee, S. R.; Chang, M.; Hur, J. H.; Kim, Y.-B.; Kim, C.-J.; Seo, D. H.; Seo, S.; Chung, U.-I.; Yoo, I.-K.; Kim, K. Nat. Mater. 10 (2011) 625-630. [2] Valov, I. ChemElectroChem 1 (2014) 26-36. [3] Valov, I.; Lu, W. D. Nanoscale 8 (2016) 13828-13837. [4] Zaffora, A.; Di Franco, F.; Santamaria, M.; Habazaki, H.; Di Quarto, F. Electrochim. Acta 180 (2015) 666–678. [5] Lübben, M.; Karakolis, P.; Ioannou-Sougleridis, V.; Normand, P.; Dimitrakis, P.; Valov, I. Adv. Mater. 27 (2015) 6202–6207.

We present 'unusual' resistive switching behaviours in electrochemical metallization (ECM) cells utilizing a dual-layer (SiOx/GeSex: SiOx on GeSex) solid electrolyte (SE). The observed switching behaviour markedly varies with the thickness of the upper SiOx layer and compliance current: (i) monostable switching, (ii) counter-eightwise bipolar switching, and (iii) combination of monostable and eightwise bipolar switching behaviours. Focusing on cases (i) and (iii), electrical and chemical analyses on these chameleonic cells were performed in an attempt to gain clues to the understanding of the observed complexity. The chemical analysis indicated the upper SiOx layer as a chemical potential well for Cu ions-Cu ions were largely confined in the well. This non-uniform distribution of Cu across the SE perhaps hints at the mechanism for the complex behaviour; it may be a 'zero-sum game' between SiOx and GeSex layers, in which the two layers fight over the limited number of Cu atoms/ions.

The electrochemical metallization cell, also referred to as conductive bridge random access memory, is considered to be a promising candidate or complementary component to the traditional charge based memory. As such, it is receiving additional focus to accelerate the commercialization process. To create a successful mass product, reliability issues must first be rigorously solved. In-depth understanding of the failure behavior of the ECM is essential for performance optimization. Here, we reveal the degradation of high resistance state behaves as the majority cases of the endurance failure of the HfO2 electrolyte based ECM cell. High resolution transmission electron microscopy was used to characterize the change in filament nature after repetitive switching cycles. The result showed that Cu accumulation inside the filament played a dominant role in switching failure, which was further supported by measuring the retention of cycle dependent high resistance state and low resistance state. The clarified physical picture of filament evolution provides a basic understanding of the mechanisms of endurance and retention failure, and the relationship between them. Based on these results, applicable approaches for performance optimization can be implicatively developed, ranging from material tailoring to structure engineering and algorithm design.

Resistive switching devices (ReRAM) represent a broad class of two-terminal continuously tunable resistors including memristors, phase change memory (PCM), valence change memory (VCM), and electrochemical metallization cells (ECM). Though these devices, especially PCM, are increasingly being commercialized by industry for use in next generation memories, they are also all actively studied for use as synaptic weights in next generation hardware-accelerated neuromorphic networks. We have previously investigated Electron Beam Induced Current Microscopy as a means of reliably characterizing resistive switches. In that investigation, we observed surprising electronic effects, such as internal secondary electron emission, in addition to more traditional electron-hole pair separation, and we broke those up into constituent currents based on their origin through Monte-Carlo modeling of the electron beam-matter interaction. Now, armed with a new understanding of the physics of EBIC imaging, we study the impact of manufacturing variations on resistive switches by continuously tuning the thickness of an Al2O3 interfacial barrier. Shifts in the apparent ratios of internal secondary electron emission from the top electrode to the bottom electrode and vice versa appear to indicate a continuous tuning of the apparent filament diameter as both a function of the injected current and the interfacial barrier thickness. This yields an apparent reduction in the current density, the primary effect of which is a reduction in the device damage from forming and a suppression of parasitic leakage currents imaged in devices without interfacial barriers. Figure 1

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application