Random Access Memory Devices Research Articles

Design and reliability assessment of an ultra-thin body electrostatically doped bipolar transistor for mixed signal applications

Shrinking of the thickness of silicon on insulator (SOI) has been proposed as a potential solution for scaling down the physical base length of symmetric lateral electrostatically doped bipolar transistors. An ultra-thin body device that utilizes the full SOI thickness has been presented and the performance of the same is investigated in detail. The device features two distinct doping techniques: work function-induced electrostatic doping (WED) and bias-induced electrostatic doping (BED). The proposed design approach leads to significant improvements in gain and cut-off frequency compared to previously reported designs. The resulting devices exhibit peak current gain β values >1100, ft>500 GHz, fmax>1300 GHz. Moreover, these improved device performance matrices get translated into better performance of universal gates with low rise and fall time of ∼1.3 ns, and improved noise margin performance in static random access memory (SRAM) device of 0.43 and 0.41 for WED and BED based devices respectively. Furthermore, the study investigates the reliability of the device concerning breakdown voltage and its response to different temperature conditions. The findings reveal a decline in the β value for WED-based devices when subjected to temperatures exceeding 340 K. In contrast, BED-based devices demonstrate a comparatively smaller variation in β at temperatures above 340 K. These results show the potential of the proposed device for mixed-signal and digital circuit applications.

Field-Free Spin-Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer.

Current-induced spin-orbit torque (SOT) in a perpendicularly magnetized single layer has a strong potential to switch the magnetization using an extremely low current density, which is generally 2-3 orders of magnitude smaller than that required for conventional metal bilayer systems. However, an in-plane external magnetic field has to be applied to break the symmetry and achieve deterministic switching. To further enhance the high-density integration and accelerate the practical application of highly efficient SOT magnetic random-access memory (SOT-MRAM) devices, field-free SOT magnetization switching in a ferromagnetic single layer is strongly needed. In a spin-orbit ferromagnet (a ferromagnet with strong spin-orbit interaction) with crystal inversion asymmetry and a multi-domain structure, the internal Dzyaloshinskii-Moriya effective fields are considered to induce field-free switching. Here, combined with strong spin-orbit coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution and a possible magnetocrystalline anisotropy resulting from a slight substrate tilting, we successfully achieve magnetization switching in a spin-orbit ferromagnet (Ga,Mn)As single layer by utilizing SOT without applying any external magnetic field. Our findings help to deeply elucidate the SOT switching mechanism and can advance the development of a highly efficient MRAM with better scalability.

Flexible resistive memory device with egg-albumen/HfO x hybrid bilayer: fabrication and modeling of its switching variations

Egg-albumen, a natural polymer, in bilayer combination with ultrathin HfO x is explored as an active switching layer component in flexible resistive random access memory devices. The fabricated devices have shown excellent switching characteristics with a current on/off ratio of greater than 104, stable retention of both low resistance and high resistance states, reliable multiple cycle switching, and very low switching power (with set power as 0.5 µW and reset power as 3.1 mW). To investigate the electro-mechanical stability, devices were bent with different bending radii and it was found that negligible degradation in device performance was observed until a 5 mm bending radius. Furthermore, a simple mathematical model is used to simulate the devices’ characteristics and the values of fitting parameters were extracted with a root mean square error of less than 4.5%. Moreover, a switching variation was introduced by utilizing variations of the physical parameters, and a near practical physics based mathematical device model was demonstrated which can enable the strengthening of simulation capabilities for exploration of unique flexible resistive memory devices and related circuits.

Atomic layer annealing for spatial tailoring in sub-4 nm AlN RRAM devices with low-voltage operation

This article investigates the spatial tailoring of nitrogen vacancies by atomic layer annealing (ALA) on resistive random access memory (RRAM) devices with an AlN resistive switching layer. The ALA technique involves the in-situ layer-by-layer He/Ar plasma treatment in the cycle of atomic layer deposition for AlN. The AlN RRAM devices prepared with the ALA treatment near the TiN electrode have significantly superior uniformity of the operation voltages and the switching resistances, along with a lower cycle-to-cycle variation in endurance. This enhancement could be attributed to the relatively stable location of rupture and formation of conductive filaments, as a result of the film densification and the suppression of nitrogen vacancies in the AlN region treated with the ALA process. The AlN RRAM devices also demonstrate excellent retention of more than 106 seconds at 125°C. In addition, the ultra-thin 3.3 nm AlN switching layer contributes to low operation voltages of AlN RRAM. The outcomes manifest that the ALA technique could effectively improve the properties of nanoscale materials through precise spatial tailoring, with an accuracy up to atomic scale.

Low sheet resistance buried metal bit line realized by high-temperature metal CVD process in vertical channel transistor array

With the continuous evolution of dynamic random access memory (DRAM) devices, there is a growing demand for increased storage density per unit area. In this work, we aim to create a high-density array of vertical channel transistors using advanced DRAM process technology. A thickness of SiO2 (X+3 nm) was determined for the protective layer, which shows the best-protecting effect. We employed CVD to grow thin Ti films on the array’s bottom. To reduce the resistance of the buried bit line (BBL), we formed a high-quality metal silicide using a thermal annealing process combined with self-align technology. Nanoprobe measurement results show an average resistance of approximately 60 Ω of the bit line of each cell transistor, where the low series resistance can improve device performance. Our work involved optimizing the protective layers and achieving high-performance BBLs, paving the way for the development of high-density DRAMs.

Dielectric Properties and Carrier Transport Mechanism in Annealed HfOx‐Based Resistive Random Access Memory Devices

The diversity of the carrier transport mechanism in the resistive switching process remains a significant obstacle for the design and application of resistive random access memory (RRAM) devices. In this study, the influence of annealing on the dielectric properties of the Ti/HfOx/Pt RRAM device and its associated carrier transport mechanism is investigated. The results reveal that the current conduction in both annealed and unannealed devices is primarily attributed to grain boundary (GB) relaxation, with only the GB acting as a depletion layer that induces dielectric relaxation. Furthermore, annealing decreases the interfacial polarization and changes the relaxation time at the GB. In addition, the charge carriers exhibit nearest‐neighbor hopping conduction at a higher temperature in Ti/HfOx/Pt RRAM devices, whereas variable‐range‐hopping conduction occurs at lower temperatures. Long‐range charge carrier transport also plays a significant role in the conduction process. To offer a deeper understanding of the conduction processes in the Ti/HfOx/Pt RRAM device, a carrier transport model is proposed, which provides valuable insights into the intricate mechanisms governing conduction in these devices.

Effect of Electrochemically Active Top Electrode Materials on Nanoionic Conductive Bridge Y2O3 Random-Access Memory.

Herein, sol-gel-processed Y2O3 resistive random-access memory (RRAM) devices were fabricated. The top electrodes (TEs), such as Ag or Cu, affect the electrical characteristics of the Y2O3 RRAM devices. The oxidation process, mobile ion migration speed, and reduction process all impact the conductive filament formation of the indium-tin-oxide (ITO)/Y2O3/Ag and ITO/Y2O3/Cu RRAM devices. Between Ag and Cu, Cu can easily be oxidized due to its standard redox potential values. However, the conductive filament is easily formed using Ag TEs. After triggering the oxidation process, the formed Ag mobile metal ions can migrate faster inside Y2O3 active channel materials when compared to the formed Cu mobile metal ions. The fast migration inside the Y2O3 active channel materials successfully reduces the SET voltage and improves the number of programming-erasing cycles, i.e., endurance, which is one of the nonvolatile memory parameters. These results elucidate the importance of the electrochemical properties of TEs, providing a deeper understanding of how these factors influence the resistive switching characteristics of metal oxide-based atomic switches and conductive-metal-bridge-filament-based cells.

Resistive Switching Devices for Neuromorphic Computing: From Foundations to Chip Level Innovations.

Neuromorphic computing has emerged as an alternative computing paradigm to address the increasing computing needs for data-intensive applications. In this context, resistive random access memory (RRAM) devices have garnered immense interest among the neuromorphic research community due to their capability to emulate intricate neuronal behaviors. RRAM devices excel in terms of their compact size, fast switching capabilities, high ON/OFF ratio, and low energy consumption, among other advantages. This review focuses on the multifaceted aspects of RRAM devices and their application to brain-inspired computing. The review begins with a brief overview of the essential biological concepts that inspire the development of bio-mimetic computing architectures. It then discusses the various types of resistive switching behaviors observed in RRAM devices and the detailed physical mechanisms underlying their operation. Next, a comprehensive discussion on the diverse material choices adapted in recent literature has been carried out, with special emphasis on the benchmark results from recent research literature. Further, the review provides a holistic analysis of the emerging trends in neuromorphic applications, highlighting the state-of-the-art results utilizing RRAM devices. Commercial chip-level applications are given special emphasis in identifying some of the salient research results. Finally, the current challenges and future outlook of RRAM-based devices for neuromorphic research have been summarized. Thus, this review provides valuable understanding along with critical insights and up-to-date information on the latest findings from the field of resistive switching devices towards brain-inspired computing.

Ce-doping at Mn site to enhance resistive switching performance of spinel MnCo2O4 resistive random access memory devices

The spinel Ce-doping MnCo2O4 (MCO) thin films were fabricated on Pt/Ti/SiO2/Si substrates for resistive switching (RS) layer using sol-gel spin-coating deposition method. The Pt/Mn0.925Ce0.075Co2O4/Pt (Pt/7.5MCC/Pt) device exhibits stabler and superior bipolar RS performance, the concentrated distribution of Reset/Set voltage as low as – 0.62/1.175 V, good cycle endurance and excellent time retention capability. The improvement in the RS performance of the Pt/7.5MCC/Pt devices can be attributable to the effective limitation of the random formation and rupture of conductive filaments by appropriate Ce doping. The carrier transport mechanism of the device at high resistance state can be dominated by the Schottky emission, whereas the carrier transport mechanism at low resistance state consistent with the Ohmic conduction. This work provides a potential performance improvement approach to design the resistive memory devices for data storage applications.

Effect of metal insertion on the magnetic properties and anomalous Hall effect in MgO/CoFeB/Ta/MgO films

CoFeB-based nano-magnetic multilayers can be applied in magnetic sensors, magnetic random access memory and other logic devices, which have attracted wide attention. The magnetic properties and interface structure of the magnetic multilayers still need further research for future applications. The effects of Pt insertion on the perpendicular magnetic anisotropy (PMA) and the anomalous Hall effect (AHE) of MgO/CoFeB/Pt/Ta/MgO multilayers were studied. It is found that the sample with 0.2 nm Pt insertion at the CoFeB/Ta interface and annealed at 250 °C can transform the films from in-plane magnetic anisotropy to PMA. The effective magnetic anisotropy K eff can reach 1.82 × 106 erg cm−3. The Pt insertion and annealing heat treatment can promote the migration of oxygen from MgO at the top layer to CoFeB/Ta interface and combine with Fe to generate iron oxide, and improve the orbital hybridization of Fe 3d and O 2p, thus successfully inducing the PMA of the film samples. In addition, Pt insertion and annealing treatment can improve the side-jump mechanism and the skew scattering mechanism which contribute to the AHE. This work provides guidance for the application of PMA materials in magnetic sensor and memory devices.

Field-free spin–orbit torque switching in ferromagnetic trilayers at sub-ns timescales

Current-induced spin torques enable the electrical control of the magnetization with low energy consumption. Conventional magnetic random access memory (MRAM) devices rely on spin-transfer torque (STT), this however limits MRAM applications because of the nanoseconds incubation delay and associated endurance issues. A potential alternative to STT is spin-orbit torque (SOT). However, for practical, high-speed SOT devices, it must satisfy three conditions simultaneously, i.e., field-free switching at short current pulses, short incubation delay, and low switching current. Here, we demonstrate field-free SOT switching at sub-ns timescales in a CoFeB/Ti/CoFeB ferromagnetic trilayer, which satisfies all three conditions. In this trilayer, the bottom magnetic layer or its interface generates spin currents with polarizations in both in-plane and out-of-plane components. The in-plane component reduces the incubation time, while the out-of-plane component realizes field-free switching at a low current. Our results offer a field-free SOT solution for energy-efficient scalable MRAM applications.

Structural phase transition and resistive switching properties of Cu x O films during post-thermal annealing

We studied the phase change and resistive switching characteristics of copper oxide (Cu x O) films through post-thermal annealing. This investigation aimed to assess the material’s potential for a variety of electrical devices, exploring its versatility in electronic applications. The Cu x O films deposited by RF magnetron sputtering were annealed at 300, 500, and 700 °C in ambient air for 4 min by rapid thermal annealing (RTA) method, and then it was confirmed that the structural phase change from Cu2O to CuO occurred with increasing annealing temperature. Resistive random-access memory (ReRAM) devices with Au/Cu x O/p+-Si structures were fabricated, and the ReRAM properties appeared in CuO-based devices, while Cu2O ReRAM devices did not exhibit resistive switching behavior. The CuO ReRAM device annealed at 500 °C showed the best properties, with a on/off ratio of 8 × 102, good switching endurance of ∼100 cycles, data retention for 104 s, and stable uniformity in the cumulative probability distribution. This characteristic change could be explained by the difference in the grain size and density of defects between the Cu2O and CuO films. These results demonstrate that superior and stable resistive switching properties of RF-sputtered Cu x O films can be obtained by low-temperature RTA.

Investigation of Resistive Switching of Insulating Hafnium Nitride for Nonvolatile Memory Applications

In this study, nitrogen-rich Hafnium Nitride (HfN) featuring insulating properties was investigated for achieving resistive switching, crucial for the functionality of resistive random-access memory (ReRAM) devices. Devices were fabricated with a 15-nm HfN resistive switching layer using a Radio Frequency (RF) sputtering system. The fabricated devices successfully exhibited a bipolar switching characteristic with a high On/Off ratio (up to 104). An interesting 2-step behavior was also observed during the formation of the conduction filament which was suspected to be tied to the migration of the nitrogen ions. This is the first attempt at using HfN as the resistive switching material for nonvolatile memory applications.

Giant anisotropic Gilbert damping and spin wave propagations in single-crystal magnetic insulator

Gilbert damping in magnetic systems describes the relaxation of magnetization. This term was phenomenologically introduced into the Landau–Lifschitz–Gilbert (LLG) equation to describe spin dynamics. In most studies, such as magnetic random access memory, spin-wave propagations, and microwave devices, it has been assumed that the Gilbert damping is an isotropic constant. In this study, we uncover a giant anisotropic Gilbert damping parameter of up to 431% in single-crystal thin films of epitaxial [100]-oriented yttrium iron garnet (YIG) using angle-dependent ferromagnetic resonance. In contrast, the Gilbert damping parameter of a [111]-oriented YIG film is almost isotropic. The observed anisotropic damping is shown to have a similar fourfold symmetry with magneto-crystalline anisotropy. The anisotropic spin-wave group velocity (vg), relaxation time (τ), and decay length (ld) were also experimentally evaluated through spin-wave spectra of [100]-oriented YIG thin film. We developed the LLG equation with the introduction of an anisotropic orbital Gilbert damping term. This anisotropic orbital damping originates from the crystal-field dominated anisotropic spin–orbit coupling and orbital-related magnon–phonon coupling. Our results extend the understanding of the mechanism of anisotropic Gilbert damping in single-crystal magnetic insulators with strong magneto-crystalline anisotropy.

Self-Assembly of Delta-Formamidinium Lead Iodide Nanoparticles to Nanorods: Study of Memristor Properties and Resistive Switching Mechanism.

In the quest for advanced memristor technologies, this study introduces the synthesis of delta-formamidinium lead iodide (δ-FAPbI3) nanoparticles (NPs) and their self-assembly into nanorods (NRs). The formation of these NRs is facilitated by iodide vacancies, promoting the fusion of individual NPs at higher concentrations. Notably, these NRs exhibit robust stability under ambient conditions, a distinctive advantage attributed to the presence of capping ligands and a crystal lattice structured around face-sharing octahedra. When employed as the active layer in resistive random-access memory devices, these NRs demonstrate exceptional bipolar switching properties. A remarkable on/off ratio (105) is achieved, surpassing the performances of previously reported low-dimensional perovskite derivatives and α-FAPbI3 NP-based devices. This enhanced performance is attributed to the low off-state current owing to the reduced number of halide vacancies, intrinsic low dimensionality, and the parallel alignment of NRs on the FTO substrate. This study not only provides significant insights into the development of superior materials for memristor applications but also opens new avenues for exploring low-dimensional perovskite derivatives in advanced electronic devices.

Optimizing the spin Hall effect in Pt-based binary alloys

We present multicode calculations for the spin Hall effect in binary Pt-based alloys, where we explore the viability of alloying the archetype spin Hall material Pt with a large set of metals [Al, Ag, Au, Cu, Hf (hcp), Hf (fcc), Ir, Pd] in order to optimize the charge to spin current conversion for practical applications. To capture intrinsic and extrinsic mechanisms in material-specific calculations, we employ different first-principles codes based on density functional theory in the framework of Green's-function-based multiple scattering approaches. Capturing the transport properties within the relativistic and fully quantum mechanical Kubo-Bastin formalism as well as the semiclassical Boltzmann approach allows for a better understanding of the microscopic physics as well as a larger set of reliable data for the key transport parameters. If available, we compare our results to experimental data, where we generally find good agreement. As we access the full concentration range, we are able to identify the optimal doping regime, which will depend on the binary alloy but generally falls within a region of 60–90 at.% of Pt. When including the unavoidable experimental residual resistivities, the maximum spin Hall angle that we identified is 13% in Al0.2Pt0.8 and Hf0.1Pt0.9, which is comparable to the best spin Hall angles experimentally found in good metal systems. The longitudinal resistivities in this regime go beyond 70µΩ cm, which is compatible with metallic-based magnetic random access memory devices. Published by the American Physical Society 2024

Stable switching behavior of low-temperature ZrO2 RRAM devices realized by combustion synthesis-assisted photopatterning

We have realized efficient photopatterning and high-quality ZrO2 films through combustion synthesis and manufactured resistive random access memory (RRAM) devices with excellent switching stability at low temperatures (250 °C) using these approaches. Combustion synthesis reduces the energy required for oxide conversion, thus accelerating the decomposition of organic ligands in the UV-exposed area, and promoting the formation of metal-oxygen networks, contributing to patterning. Thermal analysis confirmed a reduction in the conversion temperature of combustion precursors, and the prepared combustion ZrO2 films exhibited a high proportion of metal-oxygen bonding that constitutes the oxide lattice, along with an amorphous phase. Furthermore, the synergistic effect of combustion synthesis and UV/O3-assisted photochemical activation resulted in patterned ZrO2 films forming even more complete metal-oxygen networks. RRAM devices fabricated with patterned ZrO2 films using combustion synthesis exhibited excellent switching characteristics, including a narrow resistance distribution, endurance of 103 cycles, and retention for 105 s at 85 °C, despite low-temperature annealing. Combustion synthesis not only enables the formation of high-quality metal oxide films with low external energy but also facilitates improved photopatterning.

Memory device based on a nano-granulars and nano-worms structured MoS2 active layer: The origin of resistive switching characteristics

This study systematically demonstrates resistive switching (RS) functionality of MoS2 nanostructured [nano-granulars (NGs) and nano-worms (NWs)] based non-volatile resistive random access memory (ReRAM) device. Interestingly, the SET and RESET voltage sharply changes from NGs to NWs device. The RS behavior of devices is interpreted by proposing and discussing a model. In the Ag/MoS2(NGs)/ITO device, a high resistance state (HRS) and low resistance state (LRS) could be ascribed due to the formation/disruption of conducting filament via trapping-detrapping of electrons across the NGs active layer. In contrast, the RS behavior of Ag/MoS2(NWs)/ITO device is caused by forming of an Ag metallic filamentary route between the electrodes. The NWs structured device show substantially better endurance and homogeneity than the device with an NGs structure. This nanostructural-dependent device functionality allows extended RS properties and establishes a relationship between the material's nano-level structural and electrical features.

Revealing the quantum nature of the voltage-induced conductance changes in oxygen engineered yttrium oxide-based RRAM devices

In this work, the quasi-analog to discrete transition occurring in the current–voltage characteristic of oxygen engineered yttrium oxide-based resistive random-access memory (RRAM) devices is investigated in detail. In particular, the focus of our research is not on the absolute conductance values of this characteristic but on the magnitude of its conductance changes occurring during the reset process of the device. It is found that the detected changes correspond to conductance values predominantly of the order of the quantum unit of conductance G0 = 2e2/h, where e is the electron charge and h the Planck constant. This feature is observed even at conductance levels far above G0, i.e. where electron transport is seemingly diffusive. It is also observed that such behavior is reproducible across devices comprising yttrium oxide layers with different oxygen concentrations and measured under different voltage sweep rates. While the oxygen deficiency affects the total number of quantized conductance states, the magnitude of the changes in conductance, close to 1 G0, is invariant to the oxygen content of the functional layer.

Interfacial modulation of magnetic relaxation and electrical characteristic in RuO2/CrO2 antiferromagnet-half metal bilayer

Half-metallic chromium dioxide (CrO2) is a promising candidate in magnetic random-access memory (MRAM) devices due to its nearly full spin polarization and ultralow Gilbert damping at room temperature. In this study, we succeeded in fabricating (110) and (100)-oriented RuO2/CrO2 bilayers with different thicknesses of CrO2 film using two-step method. The CrO2 film epitaxially grown on a RuO2 layer exhibits a perfect in-plane magnetic uniaxial anisotropy. The magnetic relaxation of CrO2 films can be strongly modulated via the proximity to RuO2 layer, which may relate to the enhancement of the two-magnon scattering due to effects of antiferromagnetic order in RuO2 film. In addition, electronic transport measurements indicate that the electronic characteristic is strongly related to the film growth orientation, and a linear magnetoresistance is observed stemming from intergrain tunneling magnetoresistance and double-exchange mechanism. Angle-dependent ferromagnetic resonance measurements indicates that the uniaxial anisotropy in CrO2 film is strongly related to the strain effect, which is consistent with reciprocal space mapping results. This work lays a foundation for current-drive MRAM based on RuO2/CrO2 bilayer and provides a feasible damping modulation method for CrO2 film.