Multilevel Bipolar Electroforming-Free Resistive Switching Memory Based on Silicon Oxynitride

Audio Signal-Stimulated Multilayered HfOx/TiOy Spiking Neuron Network for Neuromorphic Computing

As the key hardware of a brain-like chip based on a spiking neuron network (SNN), memristor has attracted more attention due to its similarity with biological neurons and synapses to deal with the audio signal. However, designing stable artificial neurons and synapse devices with a controllable switching pathway to form a hardware network is a challenge. For the first time, we report that artificial neurons and synapses based on multilayered HfOx/TiOy memristor crossbar arrays can be used for the SNN training of audio signals, which display the tunable threshold switching and memory switching characteristics. It is found that tunable volatile and nonvolatile switching from the multilayered HfOx/TiOy memristor is induced by the size-controlled atomic oxygen vacancy pathway, which depends on the atomic sublayer in the multilayered structure. The successful emulation of the biological neuron's integrate-and-fire function can be achieved through the utilization of the tunable threshold switching characteristic. Based on the stable performance of the multilayered HfOx/TiOy neuron and synapse, we constructed a hardware SNN architecture for processing audio signals, which provides a base for the recognition of audio signals through the function of integration and firing. Our design of an atomic conductive pathway by using a multilayered TiOy/HfOx memristor supplies a new method for the construction of an artificial neuron and synapse in the same matrix, which can reduce the cost of integration in an AI chip. The implementation of synaptic functionalities by the hardware of SNNs paves the way for novel neuromorphic computing paradigms in the AI era.

Effects of Top and Bottom Electrodes Materials and Operating Ambiance on the Characteristics of MgFx Based Bipolar RRAMs

The effects of electrode materials (top and bottom) and the operating ambiances (open-air and vacuum) on the MgF_x-based resistive random-access memory (RRAM) devices are studied. Experiment results show that the device's performance and stability depend on the difference between the top and bottom electrodes' work functions. Devices are robust in both environments if the work function difference between the bottom and top electrodes is greater than or equal to 0.70 eV. The operating environment-independent device performance depends on the surface roughness of the bottom electrode materials. Reducing the bottom electrodes' surface roughness will reduce moisture absorption, minimizing the impact of the operating environment. Ti/MgF_x/p⁺-Si memory devices with the minimum surface roughness of the p⁺-Si bottom electrode show operating environment-independent electroforming-free stable resistive switching properties. The stable memory devices show promising data retentions of >10⁴ s in both environments with DC endurance properties of more than 100 cycles.

Reset First Resistive Switching in Ni1−xO Thin Films as Charge Transfer Insulator Deposited by Reactive RF Magnetron Sputtering

Reset-first resistive random access memory (RRAM) devices were demonstrated for off-stoichiometric Ni_1−xO thin films deposited using reactive sputtering with a high oxygen partial pressure. The Ni_1−xO based RRAM devices exhibited both unipolar and bipolar resistive switching characteristics without an electroforming step. Auger electron spectroscopy showed nickel deficiency in the Ni_1−xO films, and X-ray photoemission spectroscopy showed that the Ni³⁺ valence state in the Ni_1−xO films increased with increasing oxygen partial pressure. Conductive atomic force microscopy showed that the conductivity of the Ni_1−xO films increased with increasing oxygen partial pressure during deposition, possibly contributing to the reset-first switching of the Ni_1−xO films.

Vacuum and Low-Temperature Characteristics of Silicon Oxynitride-Based Bipolar RRAM

This study investigates the switching characteristics of the silicon oxynitride (SiOxNy)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances at temperatures ranging from 300 K to 77 K. The operating ambiances (open air or vacuum) and temperature affect the device’s performance. The electroforming-free multilevel bipolar Au/Ni/SiOxNy/p+-Si RRAM device (in open-air) becomes bilevel in a vacuum with an on/off ratio >104 and promising data retention properties. The device becomes more resistive with cryogenic temperatures. The experimental results indicate that the presence and absence of moisture (hydrogen and hydroxyl groups) in open air and vacuum, respectively, alter the elemental composition of the amorphous SiOxNy active layer and Ni/SiOxNy interface region. Consequently, this affects the overall device performance. Filament-type resistive switching and trap-controlled space charge limited conduction (SCLC) mechanisms in the bulk SiOxNy layer are confirmed.

Low-temperature characteristics of magnesium fluoride based bipolar RRAM devices

This study investigates the temperature-independent switching characteristics of magnesium fluoride (MgF_x) based bipolar resistive memory devices at temperatures ranging from 300 K down to 77 K. Filament type resistive switching at the interface of Ti/MgF_x and the trap-controlled space charge limited conduction (SCLC) mechanism in the bulk MgF_x layer are confirmed. The experimental results indicate that the operating environment and temperature critically control the resistive switching performance by varying the non-stoichiometry of the amorphous MgF_x active layer and Ti/MgF_x interface region. The gaseous atmosphere (open air or vacuum) affects device performances such as the electroforming process, on-state current, off-state current, on/off ratio, SET/RESET voltage and endurance of resistive-switching memory devices. After electroforming, the device performance is independent of temperature variation. The Ti/MgF_x/Pt memory devices show promising data retention for >10⁴ s in a vacuum at room temperature and 77 K with the DC endurance property for more than 150 cycles at 77 K. The devices have great potential for future temperature-independent electronic applications.

Effects of the Operating Ambiance and Active Layer Treatments on the Performance of Magnesium Fluoride Based Bipolar RRAM

This study investigates switching characteristics of the magnesium fluoride (MgFx)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances (open-air and vacuum). Operating ambiances alter the elemental composition of the amorphous MgFx active layer and Ti/MgFx interface region, which affects the overall device performance. The experimental results indicate that filament type resistive switching takes place at the interface of Ti/MgFx and trap-controlled space charge limited conduction (SCLC) mechanisms is dominant in both the low and high resistance states in the bulk MgFx layer. RRAM device performances at different operating ambiances are also altered by MgFx active layer treatments (air exposure and annealing). Devices show the better uniformity, stability, and a higher on/off current ratio in vacuum compared to an open-air environment. The Ti/MgFx/Pt memory devices have great potential for future vacuum electronic applications.

Artificial Neurons and Synapses Based on Al/a-SiNxOy:H/P+-Si Device with Tunable Resistive Switching from Threshold to Memory

As the building block of brain-inspired computing, resistive switching memory devices have recently attracted great interest due to their biological function to mimic synapses and neurons, which displays the memory switching or threshold switching characteristic. To make it possible for the Si-based artificial neurons and synapse to be integrated with the neuromorphic chip, the tunable threshold and memory switching characteristic is highly in demand for their perfect compatibility with the mature CMOS technology. We first report artificial neurons and synapses based on the Al/a-SiNxOy:H/P+-Si device with the tunable switching from threshold to memory can be realized by controlling the compliance current. It is found that volatile TS from Al/a-SiNxOy:H/P+-Si device under the lower compliance current is induced by the weak Si dangling bond conductive pathway, which originates from the broken Si-H bonds. While stable nonvolatile MS under the higher compliance current is attributed to the strong Si dangling bond conductive pathway, which is formed by the broken Si-H and Si-O bonds. Theoretical calculation reveals that the conduction mechanism of TS and MS agree with P-F model, space charge limited current model and Ohm's law, respectively. The tunable TS and MS characteristic of Al/a-SiNxOy:H/P+-Si device can be successfully employed to mimic the biological behavior of neurons and synapse including the integrate-and-fire function, paired-pulse facilitation, long-term potentiation and long-term depression as well as spike-timing-dependent plasticity. Our discovery supplies an effective way to construct the neuromorphic devices for brain-inspired computing in the AI period.

Electroforming-Free Bipolar Resistive Switching Memory Based on Magnesium Fluoride

Electroforming-free resistive switching random access memory (RRAM) devices employing magnesium fluoride (MgF_x) as the resistive switching layer are reported. The electroforming-free MgF_x based RRAM devices exhibit bipolar SET/RESET operational characteristics with an on/off ratio higher than 10² and good data retention of >10⁴ s. The resistive switching mechanism in the Ti/MgF_x/Pt devices combines two processes as well as trap-controlled space charge limited conduction (SCLC), which is governed by pre-existing defects of fluoride vacancies in the bulk MgF_x layer. In addition, filamentary switching mode at the interface between the MgF_x and Ti layers is assisted by O–H group-related defects on the surface of the active layer.

Functional bipolar resistive switching in AlN/Ni-Mn-In based magnetoelectric heterostructure

This report explores the influence of temperature on resistive switching characteristics in the AlN/Ni-Mn-In magnetoelectric (ME) heterostructure-based resistive random access memory (ReRAM) device. The fabricated Cu/AlN/Ni-Mn-In/Si device exhibits a sharp transition from a high resistance state (HRS) to low resistance state (LRS) at a SET voltage. The rupture of the filament from its weakest point at a RESET voltage turn the device back to its HRS. The stable bipolar resistive switching behavior is described by the current-voltage (I-V) characteristic. The HRS and LRS are explained by the trap-controlled space charge limited conduction mechanism and a well-known Ohmic conduction mechanism, respectively. The temperature-dependent resistance has been observed to further confirm the conduction mechanism in HRS and LRS. The current conduction in LRS is explained by an analytical model based on copper metallic filament formation via Cu⁺migration from the top to the bottom electrode. A significant change in the SET voltage has been observed with the decrease in temperature. This variation in the SET voltage is explained via strain-mediated coupling in interfacially connected AlN/Ni-Mn-In ME heterostructure. The fabricated device displays an appreciable OFF/ON ratio of the order ∼3 × 10³with good endurance and retention of ∼1000 cycles and ∼900 s, respectively. A slight variation (<40%) in SET and RESET voltages has been observed for total endurance cycles. This study demonstrates the importance of ME heterostructure for futuristic tuneable ReRAM applications.

High-Quality Single-Crystalline β-Ga2O3 Nanowires: Synthesis to Nonvolatile Memory Applications

One of the promising nonvolatile memories of the next generation is resistive random-access memory (ReRAM). It has vast benefits in comparison to other emerging nonvolatile memories. Among different materials, dielectric films have been extensively studied by the scientific research community as a nonvolatile switching material over several decades and have reported many advantages and downsides. However, less attention has been given to low-dimensional materials for resistive memory compared to dielectric films. Particularly, β-Ga₂O₃ is one of the promising materials for high-power electronics and exhibits the resistive switching phenomenon. However, low-dimensional β-Ga₂O₃ nanowires have not been explored in resistive memory applications, which hinders further developments. In this article, we studied the resistance switching phenomenon using controlled electron flow in the 1D nanowires and proposed possible resistive switching and electron conduction mechanisms. High-density β-Ga₂O₃ 1D-nanowires on Si (100) substrates were produced via the VLS growth technique using Au nanoparticles as a catalyst. Structural characteristics were analyzed via SEM, TEM, and XRD. Besides, EDS, CL, and XPS binding feature analyses confirmed the composition of individual elements, the possible intermediate absorption sites in the bandgap, and the bonding characteristics, along with the presence of various oxygen species, which is crucial for the ReRAM performances. The forming-free bipolar resistance switching of a single β-Ga₂O₃ nanowire ReRAM device and performance are discussed in detail. The switching mechanism based on the formation and annihilation of conductive filaments through the oxygen vacancies is proposed, and the possible electron conduction mechanisms in HRS and LRS states are discussed.