Design of Electrodeposited Bilayer Structures for Reliable Resistive Switching with Self-Compliance

Self-Assembled Lanthanum Oxide Nanoflakes by Electrodeposition Technique for Resistive Switching Memory and Artificial Synaptic Devices

In recent years, many metal oxides have been rigorously studied to be employed as solid electrolytes for resistive switching (RS) devices. Among these solid electrolytes, lanthanum oxide (La2 O3 ) is comparatively less explored for RS applications. Given this, the present work focuses on the electrodeposition of La2 O3 switching layers and the investigation of their RS properties for memory and neuromorphic computing applications. Initially, the electrodeposited La2 O3 switching layers are thoroughly characterized by various analytical techniques. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky techniques are probed to understand the in situ electrodeposition, RS mechanism, and n-type semiconducting nature of the fabricated La2 O3 switching layers. All the fabricated devices exhibit bipolar RS characteristics with excellent endurance and stable retention. Moreover, the device mimics the various bio-synaptic properties such as potentiation-depression, excitatory post-synaptic currents, and paired-pulse facilitation. It is demonstrated that the fabricated devices are non-ideal memristors based on double-valued charge-flux characteristics. The switching variation of the device is studied using the Weibull distribution technique and modeled and predicted by the time series analysis technique. Based on electrical and EIS results, a possible filamentary-based RS mechanism is suggested. The present results assert that La2 O3 is a promising solid electrolyte for memory and brain-inspired applications.

Review of Electrochemically Synthesized Resistive Switching Devices: Memory Storage, Neuromorphic Computing, and Sensing Applications

Resistive-switching-based memory devices meet most of the requirements for use in next-generation information and communication technology applications, including standalone memory devices, neuromorphic hardware, and embedded sensing devices with on-chip storage, due to their low cost, excellent memory retention, compatibility with 3D integration, in-memory computing capabilities, and ease of fabrication. Electrochemical synthesis is the most widespread technique for the fabrication of state-of-the-art memory devices. The present review article summarizes the electrochemical approaches that have been proposed for the fabrication of switching, memristor, and memristive devices for memory storage, neuromorphic computing, and sensing applications, highlighting their various advantages and performance metrics. We also present the challenges and future research directions for this field in the concluding section.

Memristive Devices from CuO Nanoparticles

Memristive systems can provide a novel strategy to conquer the von Neumann bottleneck by evaluating information where data are located in situ. To meet the rising of artificial neural network (ANN) demand, the implementation of memristor arrays capable of performing matrix multiplication requires highly reproducible devices with low variability and high reliability. Hence, we present an Ag/CuO/SiO₂/p-Si heterostructure device that exhibits both resistive switching (RS) and negative differential resistance (NDR). The memristor device was fabricated on p-Si and Indium Tin Oxide (ITO) substrates via cost-effective ultra-spray pyrolysis (USP) method. The quality of CuO nanoparticles was recognized by studying Raman spectroscopy. The topology information was obtained by scanning electron microscopy. The resistive switching and negative differential resistance were measured from current–voltage characteristics. The results were then compared with the Ag/CuO/ITO device to understand the role of native oxide. The interface barrier and traps associated with the defects in the native silicon oxide limited the current in the negative cycle. The barrier confined the filament rupture and reduced the reset variability. Reset was primarily influenced by the filament rupture and detrapping in the native oxide that facilitated smooth reset and NDR in the device. The resistive switching originated from traps in the localized states of amorphous CuO. The set process was mainly dominated by the trap-controlled space-charge-limited; this led to a transition into a Poole–Frenkel conduction. This research opens up new possibilities to improve the switching parameters and promote the application of RS along with NDR.

Implementation of Simple but Powerful Trilayer Oxide-Based Artificial Synapses with a Tailored Bio-Synapse-Like Structure

The ultimate aim of artificial synaptic devices is to mimic the features of biological synapses as closely as possible, in particular, its ability of self-adjusting the synaptic weight responding to the external stimulus. In this work, memristors, based on trilayer oxides with a stack structure of TiN/TiON/HfO_y/HfO_x/TiN, are designed to function as the artificial synapses where intrinsically designed oxygen-deficient HfO_x layer, less oxygen-deficient HfO_y layer, and TiON layer, imitating the corresponding biological functionality of the pre-synapse, synaptic cleft, and post-synapse, respectively, resemble the features of bio-synapses most closely. Thus, diverse bio-synaptic functions and plasticity, including long-term potentiation and depression, spike-rate-dependent plasticity, spike-timing-dependent plasticity, and metaplasticity, are fulfilled in these devices. Moreover, they exhibit analogue plasticity in both potentiating and depressing, fully emulating the learning protocols of excitation and inhibition in the bio-synapses. The structure and Hf/O distribution of these devices, mimicking the structure and Ca²⁺ deployment of bio-synapses, are consolidated by the high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy, respectively. Powerful bio-realistic behavior, implemented in these simple artificial synaptic devices, make them tailored for neuromorphic hardware applications.

Realization of Self-Compliance Resistive Switching Memory via Tailoring Interfacial Oxygen

Self-compliance set switching (SCSS) offers the promise of a selector-less resistive random access memory (RRAM) implementation on flexible substrates, with great application in integrated flexible electronics. SCSS has been realized in RRAM devices with a series oxide layer incorporated into the memory stack. The series oxide acts as an in-built resistor, limiting the increase of the current during set transition. In this study, we show that SCSS can also be achieved in a bipolar RRAM cell without a series oxide layer, i.e., consisting of only a single switching oxide layer. This study reveals that oxygen pileup at the anode interface during the set evolution plays a crucial role in SCSS. The accumulation of oxygen gives rise to the increase of the switching oxide resistance, partially compensating the decrease of the filament resistance, and modulates the conduction barrier at the anode/oxide interface, which self-arrests the increase of the set switching current. Our results show interface engineering as a possible route for enabling SCSS in an RRAM device without the need for a complicated stack structure and careful thickness optimization.

Ultralow Power Consumption Flexible Biomemristors

Low power consumption is the important requirement in memory devices for saving energy. In particular, improved energy efficiency is essential in implantable electronic devices for operation under a limited power supply. Here, we demonstrate the use of κ-carrageenan (κ-car) as the resistive switching layer to achieve memory that has low power consumption. A carboxymethyl (CM) group is introduced to the κ-car to increase its ionic conductivity. Ag was doped in CM:κ-car to improve the resistive switching properties of the devices. Memory devices based on Ag-doped CM:κ-car showed electroforming-free resistive switching. This device exhibited low reset voltage (∼0.05 V), fast switching speed (50 ns), and high on/off ratio (>10³) under low compliance current (10^-5 A). Its power consumption (∼0.35 μW) is much lower than those of the previously reported biomemristors. The resistive switching may be a result of an electrochemical redox process and Ag filament formation in the CM:κ-car under an electric field. This biopolymer memory can also be fabricated on flexible substrate. This study verifies the feasibility of using biopolymers for applications to future implantable and biocompatible nanoelectronics.

High performance bi-layer atomic switching devices

Atomic switches, also known as conductive bridging random access memory devices, are resistive-switching devices that utilize the electrochemical reactions within a solid electrolyte between metal electrodes, and are considered essential components of future information storage and logic building blocks. In spite of their advantages as next generation switching components such as high density, large scalability, and low power consumption, the large deviations in their electrical properties and the instability of their switching behaviors hinder their application in information processing systems. Here, we report the fabrication of a uniform, low-power atomic switch with a bi-layer structure consisting of Ta₂O_5-x as the main switching layer (SL) and a relatively oxygen-deficient TaO_x as an oxygen vacancy control layer (VCL). The depth profiles of the filaments in the bi-layer device were obtained by performing conductive atomic force microscopy to assess the improvements in uniformity, reliability, and electrical performance that result from the insertion of the VCL. The coefficient of variation of the high resistance state of the bi-layer device was found to be drastically reduced from 60.92% to 2.77% in the cycle-to-cycle measurements and from 82.73% to 4.85% in the device-to-device measurements when compared with the values obtained for a single-layer device. The bi-layer device also exhibits a forming-free low operation voltage of ∼0.4 V, a high on/off ratio of ∼10⁶, and high reliability with 10 years data retention at 85 °C.

Reproducible and reliable resistive switching behaviors of AlOX/HfOX bilayer structures with Al electrode by atomic layer deposition

The resistive switching behaviors of AlO_X/HfO_X bilayer structures were investigated.

Tuning analog resistive switching and plasticity in bilayer transition metal oxide based memristive synapses

The existence of rich suboxide phases is favorable for increasing the number of weight states in transition metal oxide synapses.