Cross-point Structure Research Articles

Exploration of electrode-modulated memory and threshold switching behaviour in Se-Te-Sn thin film devices

Due to their potential use in high-density, three-dimensional stackable cross-point array structures, the electrical switching ability of chalcogenide glasses has captured a lot of attention. Herein, the switching behaviour of unique Se86-xTe14Snx (x = 0, 2, 4, 6) chalcogenide glassy alloys in the form of a thin film were investigated. The electrode modulated dual functionality in switching was achieved by employing Aluminium (Al) and Silver (Ag), as top electrodes. The films with Al/Se86-xTe14Snx/Al interface exhibited memory-type switching due to the phase-changing properties of the material. The threshold voltage (Vth) decreased linearly from 12.75 V to 4.2 V at room temperature as Sn concentration in the glass increased. On the other hand, when the top electrode was replaced with Ag, the Ag/Se86-xTe14Snx/Al interface acted as a programmable metallization cell (PMC) showing threshold-switching properties. Ag/Se82Te14Sn4/Al thin film of thickness 200 nm showed promising results as a material for a unidirectional selector, due to the formation of temporary Ag filament inside chalcogenide material. The composition showed high selectivity (∼104), high endurance (>104 cycles), and low threshold voltage (∼1.6 V). The ability of the composition to exhibit electrode-dependent memory and threshold-switching phenomena makes the material an interesting case.

Thermal atomic layer deposition of ternary Ge-S-Se alloy for advanced ovonic threshold switch selectors in three-dimensional cross-point memory array

Two-terminal ovonic threshold switch (OTS) selectors based on chalcogenide alloys have attracted significant attention as alternatives to conventional silicon-based transistors. These selectors offer outstanding features that enable the mass production of three-dimensional cross-point arrays for advanced memory technologies. In this study, we developed a thermal atomic layer deposition (ALD) process for ternary Ge-S-Se alloys. By utilizing ALD super-cycles of Ge-Se and Ge-S, a broad range of film compositions can be achieved. We investigated the electrical characteristics and film properties of the deposited films, with a focus on their smooth surfaces and excellent conformality, which are indispensable characteristics for future vertical cross-point structures. By incorporating S into the Ge-Se film via an ALD super-cycle, we achieved a significant reduction in off-current of Ge0.50S0.15Se0.35 to approximately 35 nA. Additionally, the thermal stability of Ge0.50S0.15Se0.35 film is enhanced up to 420 ℃ due to the higher bandgap and shorter bonding length of S, exhibiting a cyclic endurance of more than 5 × 107. This research not only provides insight into the control of the film composition to improve and modify device characteristics but also constructs a foundation of ALD-based OTS research on ternary Ge-S-Se for a future X-point memory application.

Locally formed conductive filaments in an amorphous Ga2Te3 ovonic threshold switching device

Ovonic threshold switching (OTS) selector devices based on chalcogenide materials are promising candidates for addressing the sneak current in high-density cross-point array structures owing to their high selectivity, high endurance, and fast switching speed. However, the OTS mechanism remains controversial and needs to be clarified. In this study, the formation of local conductive filaments (CFs) during threshold switching in an amorphous Ga2Te3 OTS selector device was investigated by electrical measurements and conductive-atomic force microscopy (C-AFM). The amorphous Ga2Te3 OTS selector device requires a forming process before the threshold switching processes. In addition, the off-current density (JOFF) was dependent on the area of the bottom electrode. The difference between the threshold voltage (VTH) and the hold voltage (VH) increased as the applied higher electric field increased. The drift of VTH (VTH drift) depended on the relaxation time and measurement temperature. The requirements of the forming process, area dependence of the JOFF, the difference between the VTH−VH with the applied electric field, and VTH drift are expected to depend on locally formed CFs. In addition, the analysis of the C-AFM results strongly supports the formation of local CFs during threshold switching in an amorphous Ga2Te3 OTS selector device. The understanding of OTS behavior uncovered in this study may provide guidance for improving the characteristics of and designing materials for future applications of OTS selector devices.

Artificial synaptic and self-rectifying properties of crystalline (Na1-K )NbO3 thin films grown on Sr2Nb3O10 nanosheet seed layers

• Crystalline Na 1-x K x NbO 3 (CNKN) film grown on Sr 2 Nb 3 O 10 (SNO) seed layer at 370 °C. • CNKN film grown on one SNO monolayer shows typical bipolar switching properties. • CNKN film grown on two SNO monolayers shows self-rectifying switching properties. • The CNKN memristor with two SNO monolayers shows artificial synaptic properties. • The CNKN film can be used to fabricate ANNs with a cross-point array structure. Crystalline (Na 1- x K x )NbO 3 (NKN) thin films were deposited on Sr 2 Nb 3 O 10 /TiN/Si (S-TS) substrates at 370 °C. Sr 2 Nb 3 O 10 (SNO) nanosheets served as a template for the formation of crystalline NKN films at low temperatures. When the NKN film was deposited on one SNO monolayer, the NKN memristor exhibited normal bipolar switching characteristics, which could be attributed to the formation and destruction of oxygen vacancy filaments. Moreover, the NKN memristor with one SNO monolayer exhibited artificial synaptic properties. However, the NKN memristor deposited on two SNO monolayers exhibited self-rectifying bipolar switching properties, with the two SNO monolayers acting as tunneling barriers in the memristor. The conduction mechanism of the NKN memristor with two SNO monolayers in the high-resistance state is attributed to Schottky emission, direct tunneling, and Fowler–Nordheim (FN) tunneling. The current conduction in this memristor in the low-resistance state was governed by direct tunneling and FN tunneling. Additionally, the NKN memristor with two SNO monolayers exhibited large ON/OFF and rectification ratios and artificial synaptic properties. Therefore, an NKN memristor with two SNO monolayers can be used for fabricating artificial synaptic devices with a cross-point array structure.

Memristive Switching and Density-Functional Theory Calculations in Double Nitride Insulating Layers.

In this paper, we demonstrate a device using a Ni/SiN/BN/p+-Si structure with improved performance in terms of a good ON/OFF ratio, excellent stability, and low power consumption when compared with single-layer Ni/SiN/p+-Si and Ni/BN/p+-Si devices. Its switching mechanism can be explained by trapping and de-trapping via nitride-related vacancies. We also reveal how higher nonlinearity and rectification ratio in a bilayer device is beneficial for enlarging the read margin in a cross-point array structure. In addition, we conduct a theoretical investigation for the interface charge accumulation/depletion in the SiN/BN layers that are responsible for defect creation at the interface and how this accounts for the improved switching characteristics.

Material and Structural Engineering of Ovonic Threshold Switch for Highly Reliable Performance (Adv. Electron. Mater. 9/2022)

OTS Selection Devices Min Kyu Yang, Gun Hwan Kim and co-workers, in article number 2200161, successfully demonstrate a GeSeTe ovonic threshold switch (OTS) selector applied to a 3D cross-point structure. As a volatile memory, this OTS memory exhibits fast operation speed, low leakage current, and high endurance. OTS is also used as neurons in 3D cross-point structures for neuromorphic computing applications.

Nanoionics-based neuromorphic function of a Pt/Ti0.96Co0.04O2-δ /Pt multilayer device operating at low pulse voltage

We investigated the nanoionics-based neuromorphic function of Pt/Ti0.96Co0.04O2−δ /Pt multilayers with a cross-point structure prepared by RF magnetron sputtering. This multilayer exhibits electro-ion mixed conduction and a nonlinear current–voltage characteristic based on the Schottky barrier between Pt and Ti0.96Co0.04O2−δ layers. When the low electrical pulse of 0.8 V was applied with a short interval time of 14 s, current modulation corresponding to the long-term memorization (LTM) was observed, though the current response at the long interval time of 80 s was the short-term memorization (STM). The O 1 s photoemission spectrum showed the OH− peak that contributes to the electron-ion mixed conduction. The current responses with both LTM and STM are considered to be due to the local proton migration at near the Schottky barrier.

Physical Unclonable Functions Based on Transient Form of Memristors for Emergency Defenses

In this letter, transient form of diffusive memristors with cross-point structure of W/Ag/MgO/Ag/W for physical unclonable functions (PUF) were proposed for the first time. The stochastic dispersion of Ag in MgO switching layer made the randomness maximized, which achieved an inter-chip Hamming distance of 49.1%. Importantly, triggered failure of the transient PUF devices was activated in deionized water at room temperature after 3 min immersion. Additionally, the transient PUF devices were transferred onto flexible polyimide substrate by water-assisted transfer printing method without any bits flipped, which achieved physical disappearance in deionized water finally. This transient form of memristor provides powerful insights into next-generation PUF design and hardware security for emergency defenses.

Controlling the Grain Size of Dion–Jacobson-Phase Two-Dimensional Layered Perovskite for Memory Application

Organic-inorganic halide perovskites (OIHPs) have emerged as an active layer for resistive switching memory (RSM). Among various OIHPs, two-dimensional OIHPs are advantageous in RSMs because of their stability. This stability can be further improved using two-dimensional Dion-Jacobson OIHPs. Moreover, OIHP-based RSMs operated by the formation of halide-ion filaments are affected by grain boundaries because they can act as a shortcut for ion migration. Therefore, it is essential to control the grains in OIHPs for reliable memory operation. Here, we present RSMs using Dion-Jacobson OIHP with controlled grain sizes. The grain sizes of the OIHP are effectively controlled by adjusting the ratio of the N,N-dimethylformamide and dimethyl sulfoxide. The controlled grain sizes can modulate the paths for halide ion migration, which enables the change of the on/off ratio in RSM. In addition, cross-point array structure is essential for high-density memory applications. However, in the cross-point array structure, unwanted current flow through unselected memory cells can happen due to sneak-current paths, so it is necessary to suppress leakage current from neighboring cells by adopting selector devices. We demonstrate the application of selector devices to OIHP-based RSMs to prevent sneak current paths. These results provide the potential of OIHP for use in high-density memory applications.

Relaxation Oscillation Effect of the Ovonic Threshold Switch on the SET Characteristics of Phase-Change Memory in Cross-Point Structure

In this letter, the relaxation oscillation (RO) effect of the Ovonic Threshold Switch (OTS) on the RESET (amorphous)-to-Set (crystalline) characteristics of Phase-change memory (PCM) is discussed. RO, which happens when the current flowing through the OTS is below hold current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{h}{)}$ </tex-math></inline-formula> , may increase the minimum SET current and the fail bit rate (FBR) of RESET-to-SET transition in the Cross-point structure PCM (XP-PCM). The RO from the single XP-PCM cell was successfully measured using a low capacitance (low-z) probe and a high frequency oscilloscope, and the corresponding current oscillation is calculated from the voltage, discharge time, and write current. The voltage RO was proportionally increased as the applied current was decreased below the hold current of the OTS ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{h}{)}$ </tex-math></inline-formula> . The increasing RO behaviors were well matched with the increasing FBR of RESET-to-SET transition in the 64k bit array. The results indicate that the current oscillation not only makes the current supply for the crystallization unstable to generate SET fail but may even induce the partial amorphization (RESET) due to the series of high current – short pulses during the oscillation.

Self-selective ferroelectric memory realized with semimetalic graphene channel

A new concept of read-out method for ferroelectric random-access memory (FeRAM) using a graphene layer as the channel material of bottom-gated field effect transistor structure is demonstrated experimentally. The transconductance of the graphene channel is found to change its sign depending on the direction of spontaneous polarization (SP) in the underlying ferroelectric layer. This indicates that the memory state of FeRAM, specified by the SP direction of the ferroelectric layer, can be sensed unambiguously with transconductance measurements. With the proposed read-out method, it is possible to construct an array of ferroelectric memory cells in the form of a cross-point structure where the transconductance of a crossing cell can be measured selectively without any additional selector. This type of FeRAM can be a plausible solution for fabricating high speed, ultra-low power, long lifetime, and high density 3D stackable non-volatile memory.

Simulation and Calibration of Op-Amp Nonidealities in the Voltage Feedback Method for a Cross-Point Resistive Sensor Array

Sensor integration is widely demanded in many applications. For sensor fabrications, a cross-point structure is beneficial because of its low cost, while the readout interface circuit suffers from sneak currents, which can lead to enormous readout error. In this study, the developing electric olfaction system which integrates over 1,000 gas sensors was introduced. Compared to previous works, the array size tends to become large, and resistance values can be widely distributed. We employed the conventional voltage feedback method for the interface circuit because it can measure a wide range of resistance. However, since it constitutes the voltage divider circuit with the selected resistor and the reference resistance by an op-amp unity gain feedback, the readout accuracy is affected by its large load sensor resistance. Therefore, a calibration technique for the voltage feedback method was proposed. The feedback error caused by the load resistance and op-amp nonidealities was analyzed with a simplified circuit, in which circuit parameters such as an array scale, a reference resistance, and an op-amp output impedance were variable. The sneak currents were estimated in the calibration by utilizing the op-amp input difference, and their effects were eliminated. The simulation results showed that the readout error of 32×32 resistive array with 10% distributed resistance could be decreased from around 3% to less than 0.005%. The dependency of the readout accuracy on an op-amp's gain input offset voltage was evaluated systematically to apply the calibration to a practical system.

Bipolar and Complementary Resistive Switching Characteristics and Neuromorphic System Simulation in a Pt/ZnO/TiN Synaptic Device.

In this work, a ZnO-based resistive switching memory device is characterized by using simplified electrical conduction models. The conventional bipolar resistive switching and complementary resistive switching modes are accomplished by tuning the bias voltage condition. The material and chemical information of the device stack including the interfacial layer of TiON is well confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis. The device exhibits uniform gradual bipolar resistive switching (BRS) with good endurance and self-compliance characteristics. Moreover, complementary resistive switching (CRS) is achieved by applying the compliance current at negative bias and increasing the voltage at positive bias. The synaptic behaviors such as long-term potentiation and long-term depression are emulated by applying consecutive pulse input to the device. The CRS mode has a higher array size in the cross-point array structure than the BRS mode due to more nonlinear I–V characteristics in the CRS mode. However, we reveal that the BRS mode shows a better pattern recognition rate than the CRS mode due to more uniform conductance update.

Nonlinear Characteristics of Complementary Resistive Switching in HfAlOx-Based Memristor for High-Density Cross-Point Array Structure

In this work, we present the nonlinear current–voltage (I–V) characteristics of a complementary resistive switching (CRS)-like curve from a HfAlOx-based memristor, used to implement a high-density cross-point array. A Pt/HfAlOx/TiN device has lower on-current and larger selectivity compared to Pt/HfO2/TiN or Pt/Al2O3/TiN devices. It has been shown that the on-current and first reset peak current after the forming process are crucial in obtaining a CRS-like curve. We demonstrate transient CRS-like characteristics with high nonlinearity under pulse response for practical applications. Finally, after finding the optimal conditions for high selectivity, the calculated read margin proves that a Pt/HfAlOx/TiN device with a CRS-like curve is most suitable for use in a high-density cross-point array. Our results suggest that the built-in selector properties in a Pt/HfAlOx/TiN single layer device offer considerable potential in terms of the simplicity of the processes involved in the cross-point structure.

Effect of Pt top electrode deposition on the valence state and resistance switching behavior of NbO2-x

Resistive random-access memory (ReRAM) devices for cross-point array structures require selector elements to suppress the sneak current though neighboring cells. Niobium oxide is a promising candidate for one selector-one ReRAM (1S1R) device because of its diverse electrical characteristics such as resistance threshold switching in NbO2 and resistance memory switching in Nb2O5. In this study, a new method to fabricate a self-formed 1S1R device is suggested, which uses the top electrode Pt sputtering process. During the Pt sputtering of the top electrode, it was observed that the Nb4+ in NbO2-x layer was oxidized into Nb5+, resulting in Nb2O5-rich films. Such self-formed Nb2O5/NbO2-x layer exhibits dissimilar resistance switching behaviors depending on the compliance current during memory operation. A switching model is proposed based on the resistance switching characteristics of the Nb2O5/NbO2-x layer on the compliance current during memory operation.

Improving Multilevel Writes on Vertical 3-D Cross-Point Resistive Memory

Resistive memory is promising to be constructed as a high-density storage-class memory. Multilevel cell, access-transistor-free cross-point array structure, and 3-D array integration are three approaches to scale up the density of resistive memory. However, composing the three approaches together strengthens the interactions between array-level and cell-level nonidealities (interconnect resistance-induced IR drop, sneak current, and device variability) of resistive memory arrays during write operations and significantly degrades write performance and reliability. In this article, we analyze the dynamic voltage-dividing effect along a selected write current path in 3-D cross-point memory arrays. We propose a nonideality-tolerant high-density resistive memory (HD-RRAM) architecture, that can weaken the interactions between nonidealities and mitigate their degradation effects on the performance and reliability of array multilevel write operations. HD-RRAM is equipped with a double-transistor array architecture with two-transistor- $n$ -resistor (2TnR) cell organization along pillars to reduce the current driving requirement and the large undesired voltage drop across each vertical pillar access transistor. Moreover, multiside asymmetric bias improves the resistive switching velocity by leveraging current-dividing effects. Variability-aware multilevel state partition reduces the worst-case write error rate by leveraging target state dependency of variability. Proportional-control multilevel state tuning reduces the average number of required write-and-verify iterations by leveraging pulse amplitude dependency of variability. Multilevel cell parallel writing improves the cell-level parallelism by leveraging the pass-through feature of intermediate resistance states. The evaluations show that HD-RRAM reduces both memory access latency and energy consumption over an aggressive baseline.

Trap-controlled space-charge-limited conduction in amorphous AsxTe1−x thin films with ovonic threshold switching

Ovonic threshold switching (OTS) in chalcogenide materials has attracted considerable interest for application in electronic devices to suppress leakage current in cross-point array structures. Although OTS appears to originate from an electronic process, the exact mechanism of OTS remains unclear with respect to sub-threshold conduction and threshold switching. In this study, we demonstrated that the sub-threshold conduction characteristics is affected by the structures of the exponentially distributed trap states in amorphous chalcogenide materials. Using binary AsxTe1−x thin films, we investigated the effects of trap distribution on OTS characteristics, analyzed the band structures, and developed a trap-controlled space-charge-limited conduction model.

Electro-Thermal Model for Thermal Disturbance in Cross-Point Phase-Change Memory

We developed an electro-thermal model for cross-point phase-change memory (X-PCM) and compared the calculation values with the experimental results. In order to simulate the electro-thermal phenomenon such as thermal disturbance (TDB) of victim cells and reset the current ( ${I}_{{\text {RESET}}}$ ) of aggressor cells in a fully confined cross-point structure, a three by three mini-array was constructed with the finite-elemental method. Unlike the conventional thermal model, which only shows the temperature gradient for TDB, our new model can predict the crystallization behavior of victim cells by combining the crystallization model for nucleation and growth. This makes it possible to compare the calculation results with the experimental ones through the crystalline fraction of the victim cell. The simulation results clearly reveal that our new model can closely estimate the victim cell’s crystalline fraction for TDB, with the variation of both pulse height (PH) and pulse width (PW) during the RESET operation. Applying the confirmed model for devices of smaller dimensions shows that TDB increases as the device scales down. The increase of TDB is particularly severe as cells become smaller than 16 nm. We suggest that TDB can be reduced not only by decreasing the thermal conductivity ( $\kappa $ ) of the interlayer dielectric (ILD), but also by minimizing the PW of the aggressor.

3-D Synapse Array Architecture Based on Charge-Trap Flash Memory for Neuromorphic Application

In order to address a fundamental bottleneck of conventional digital computers, there is recently a tremendous upsurge of investigations on hardware-based neuromorphic systems. To emulate the functionalities of artificial neural networks, various synaptic devices and their 2-D cross-point array structures have been proposed. In our previous work, we proposed the 3-D synapse array architecture based on a charge-trap flash (CTF) memory. It has the advantages of high-density integration of 3-D stacking technology and excellent reliability characteristics of mature CTF device technology. This paper examines some issues of the 3-D synapse array architecture. Also, we propose an improved structure and programming method compared to the previous work. The synaptic characteristics of the proposed method are closely examined and validated through a technology computer-aided design (TCAD) device simulation and a system-level simulation for the pattern recognition task. The proposed technology will be the promising solution for high-performance and high-reliability of neuromorphic hardware systems.

Graphene-Boron Nitride-Graphene Cross-Point Memristors with Three Stable Resistive States.

Two-dimensional (2D) material-based memristors have shown several properties that are not shown by traditional ones, such as high transparency, robust mechanical strength and flexibility, superb chemical stability, enhanced thermal heat dissipation, ultralow power consumption, coexistence of bipolar and threshold resistive switching, and ultrastable relaxation when used as electronic synapse (among others). However, several electrical performances often required in memristive applications, such as the generation of multiple stable resistive states for high-density information storage, still have never been demonstrated. Here, we present the first 2D material-based memristors that exhibit three stable and well-distinguishable resistive states. By using a multilayer hexagonal boron nitride (h-BN) stack sandwiched by multilayer graphene (G) electrodes, we fabricate 5 μm × 5 μm cross-point Au/Ti/G/h-BN/G/Au memristors that can switch between each two or three resistive states, depending on the current limitation (CL) and reset voltage used. The use of graphene electrodes plus a small cross-point structure are key elements to observe the tristate operation, which has not been observed in larger (100 μm × 100 μm) devices with an identical Au/Ti/G/h-BN/G/Au structure nor in similar small (5 μm × 5 μm) devices without graphene interfacial layers (i.e., Au/Ti/h-BN/Au). Basically, we generate an intermediate state between the high resistive state and the low resistive state (LRS), named soft-LRS (S-LRS), which may be related to the formation of a narrower conductive nanofilament across the h-BN because of the ability of graphene to limit metal penetration (at low CLs). All the 2D materials have been fabricated using the scalable chemical vapor deposition approach, which is an immediate advantage compared to other works using mechanical exfoliated 2D materials.