Improved Switching Uniformity Research Articles

High-performance resistive random access memory using two-dimensional electron gas electrode and its switching mechanism analysis

In this study, a two-dimensional electron gas (2DEG), which is a conductive layer formed at the interface of Al2O3 and TiO2, was used as an electrode for resistive random access memory (RRAM) and implemented in a cell size down to 30 nm. For an RRAM device comprising W/2DEG/TiO2/W, we confirmed that the dominant switching mechanism changed from interfacial to filamentary as the cell size decreased from 500 nm to 30 nm. Through analyses of changes in forming characteristics and conduction mechanisms in the low resistive state depending on the cell size, it was identified that the 2DEG acted as an oxygen-scavenging layer of TiO2 during the resistive switching process. By comparing the switching characteristics of RRAM devices with and without 2DEG for a 30 nm cell size, we confirmed that a high-performance 2DEG RRAM was realized, with highly uniform current–voltage characteristics, a low operating voltage (∼1 V), and a high on/off ratio (>102). Finally, the applicability of the proposed device to a crossbar array was validated by evaluating 1S1R operation with an NbO2-based selector. Considering the improved switching uniformity, the 2DEG RRAM shows promise for high-density memory applications.

Enhanced resistive switching characteristics of conductive bridging memory device by a Co–Cu alloy electrode

One of the main challenges in the development of conductive bridging random access memory (CBRAM) is the large stochastic nature of ion movement that ultimately leads to large parameter variability. In this study, the resistive switching variability of CBRAM devices is significantly improved by employing Co–Cu alloy as the active electrode. By comparing with Pt/Ta2O5/Co devices, the Co70Cu30 alloy exhibited lower forming voltage (<2 V), lower SET voltage (<0.70 V), and faster response time (∼70 ns). The filament stability indicated by the distribution of SET/RESET voltage and high resistance state/low resistance state variation was significantly improved. Our experimental results suggest the formation of Co filaments, and the proposed mechanism is governed by the galvanic effect. In addition, a comparison between Co70Cu30 and Co30Cu70 alloys highlights that the relative proportion between Co and Cu plays an essential role in the device performance. A physical model based on different electrochemical activities of the alloys has been proposed to explain the filament formation and the improved switching uniformity in the Co70Cu30 alloy. This study not only develops a CBRAM with enhanced performance but also advances the implementation of suitable alloy systems for the application of such devices.

Deterministic Conductive Filament Formation and Evolution for Improved Switching Uniformity in Embedded Metal-Oxide-Based Memristors─A Phase-Field Study

The extreme device-to-device variation of switching performance is one of the major obstacles preventing the applications of metal-oxide-based memristors in large-scale memory storage and resistive neural networks. Recent experimental works have reported that embedding metal nano-islands (NIs) in metal oxides can effectively improve the uniformity of the memristors, but the underlying role of the NIs is not fully understood. Here, to address this specific problem, we develop a physical model to understand the origin of the variability and how the embedded NIs can improve the performance and uniformity of memristors. We find that due to the dimension confinement effect, embedding metal NIs can modulate the electric field distribution and lead to a more deterministic formation of the conductive filament (CF) from their vicinity, in contrast to the random growth of CFs without embedded NIs. This deterministic CF formation, via vacancy nucleation, further reduces the forming, reset, and set voltages and enhances the uniformity of the operation voltages and current ON/OFF ratios. We further demonstrate that modifying the shapes of the metal NIs can modulate the field strengths/distributions around the NIs and that choosing the NI metal composition and shape that chemically facilitate vacancy formations can further optimize the CF morphology, reduce the operation voltages, and improve the switching performance. Our work thus provides a fundamental understanding of how embedded metal NIs improve the resistive switching performance in oxide-based memristors and could potentially guide the selection of embedded NIs to realize a more uniform memristor that also operates at a higher efficiency than present materials.

Vertically oriented 2D layered perovskite-based resistive random access memory (ReRAM) crossbar arrays

Resistive random-access memory (ReRAM) based on 2D layered perovskites have recently emerged as a new class of data storage devices, and the switching materials used in these devices have attracted extensive attention in recent years. There is a certain amount of variability in ReRAM owing to the stochastic nature of the formation and rupture of the conductive path, causing failure of the switching operation in the memory array. In this work, we fabricated both randomly and vertically oriented PEA2MA4Pb5I16-based ReRAM devices and conducted comparative studies of their switching variability. We demonstrated the formation of vertical alignment in PEA2MA4Pb5I16 films using methylammonium chloride (MACl) additives. Thereafter, the fabrication of 8×8 ReRAM crossbar arrays by patterning perovskites (∼600μm) and electrodes (line width ∼ 5 μm) using photolithography and e-beam lithography is demonstrated, and the resistive switching characteristics of both samples are discussed. Both cycle-to-cycle and device-to-device switching uniformity were improved in the vertically oriented film-based ReRAM devices, which can be explained by the narrow distribution of the energy barriers for VI migration owing to the vertically aligned charge transport path. These results suggest that vertically oriented 2D layered perovskites have great potential in ReRAM devices with improved switching uniformity.

Modulation of oxygen transport by incorporating Sb2Te3 layer in HfO2-based memristor

The oxygen transport plays an important role on the uniformity of the transition metal oxides (TMOS) memristors. Here, the effect of incorporating Sb2Te3 layer into TiN/HfO2/Pt memristor on oxygen transport has been systematically explored. The experimental results reveal that the memristor with Sb2Te3 incorporation at TiN/HfO2 interface has improved switching uniformity and memory window. Further theoretical calculations demonstrate that Sb2Te3 is a proper oxygen reservoir as oxygen possesses very low formation energy and migration barrier in Sb2Te3 with many vacancies. During the operation process, the Sb2Te3 will gain more oxygen from the HfO2 layer than TiN once the applied voltage reaches up to forming voltage, producing more oxygen vacancies (VOs) in the HfO2 layer, compared with the device without the Sb2Te3 layer. Thus, the VOs conductive filaments (CF) in the HfO2 layer will be thick, resulting in a decrease in the randomness of CF's formation/rupture and, in turn, improving the device uniformity. Our findings provide an in-depth understanding of the oxygen reservoir in TMOS memristors, which is of great significance for the design and development of memristors.

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Cross‐Point Arrays with Low‐Power ITO‐HfO2 Resistive Memory Cells Integrated on Vertical III‐V Nanowires

AbstractVertical nanowires with cointegrated metal‐oxide‐semiconductor field‐effect‐transistor (MOSFET) selectors and nonvolatile resistive random access memory (RRAM) cells represent a promising candidate for fast, energy‐efficient, cross‐point memory cells. This paper explores indium‐tin‐oxide‐hafnium‐dioxide RRAM cells integrated onto arrays of indium‐arsenide (InAs) vertical nanowires with a resulting area of 0.06 µm2 per cell. For low current operation, an improved switching uniformity over the intrinsic self‐compliant behavior is demonstrated when using an external InAs nanowire MOSFET selector in series. The memory cells show consistent switching voltages below ±1 V and a switching cycle endurance of 106 is demonstrated. The developed fabrication scheme is fully compatible with low‐ON‐resistance vertical III‐V nanowire MOSFET selectors, where operational compatibility with the initial high‐field filament forming is established. Due to the small footprint of a vertical implementation, high density integration is achievable, and with a measured programming energy for 50 ns pulses at 0.49 pJ, the technology promises fast and ultralow power cross‐point memory arrays.

Resistive switching performance improvement of amorphous carbon-based electrochemical metallization memory via current stressing

Current stressing treatment on amorphous carbon (a-C)-based electrochemical metallization (ECM) memory cells before the electroforming process is proposed. The resistive switching (RS) performance of a Cu/a-C/Pt ECM cell after low current (10 μA) stressing treatment is improved, including reduced forming voltages, improved switching uniformity, enhanced cycling endurance, and enhanced switching speed. MicroRaman mapping and conductive-atomic force microscopy measurements reveal that current stressing can cause clustering of sp2 sites in a-C and the conductive filament (CF) randomness is reduced. The reduction of CF nucleation site randomness and enhancement of the local electric field through sp2-clustering are assumed to be responsible for the observed RS performance improvement. The results demonstrate that current stressing is a feasible approach for optimizing the RS performance of a-C-based ECM memory, and it can also be extended to the development of carbon-based electronic devices.

Resistive switching modification by ultraviolet illumination in amorphous SrO-based resistive random access memory

The novel amorphous strontium oxide (α-SrO) resistive random access memory (RRAM) device based on the ITO/α-SrO/Pt sandwich-like structure was presented. Photosensitivity to ultraviolet (UV) light for α-SrO based RRAM was investigated. The resistive switching characteristics were affected through the modification of the number of oxygen vacancy by the UV irradiation. Ultraviolet light-assisted oxygen vacancy manipulation in the SrO film is an effect method to adjust device resistive switching behavior. The current conduction mechanism of the SrO-based RRAM device was changed from the Schottky emission mechanism to Poole-Frankel mechanism after the UV light illumination. In order to unveil the stability of the SrO samples after the UV light illumination, the physical models for different switching processes were proposed. Additionally, the improved switching uniformity via UV light irradiation gives physical insight into designing resistive memory devices.

Effect of ultraviolet illumination on metal oxide resistive memory

We investigate the photoelectrical and resistive switching properties of Pt/ZnO/Pt capacitor operated in unipolar mode under ultraviolet (UV) illumination. The oxygen photodesorption under UV illumination explains the photoconduction observed in initial and high resistance states. Meanwhile, oxygen readsorption at surface-related defects justifies the different photoresponses dynamics in both states. Finally, UV illumination significantly reduces the variations of resistance in high resistance state, set voltage and reset voltage by 58%, 33%, and 25%, respectively, stabilizing Pt/ZnO/Pt capacitor. Our findings in improved switching uniformity via UV light give physical insight into designing resistive memory devices.

Localized resistive switching in a ZnS–Ag/ZnS double-layer memory

Bipolar resistive-switching memories were demonstrated based on a Ag/ZnS/ITO stack structure, where the electrolyte layer is made into a double-layer structure composed of a Ag-nanoclusters-embedded ZnS layer and a pure ZnS layer. The introduction of Ag-nanoclusters results in thicker Ag conducting filaments (CFs) formed in ZnS–Ag than in ZnS. This difference in CFs’ size between two sub-layers makes the resistive switching localized in the ZnS sub-layer. By reducing its thickness, the switching region is reduced and the degree of localization is enhanced. Therefore, compared with the single-layer ZnS memory, the double-layer device shows improved switching uniformity, reduced forming/switching voltages and better endurance.

Resistive switching in zinc–tin-oxide

Bipolar resistive switching is demonstrated in the amorphous oxide semiconductor zinc–tin-oxide (ZTO). A gradual forming process produces improved switching uniformity. Al/ZTO/Pt crossbar devices show switching ratios greater than 103, long retention times, and good endurance. The resistive switching in these devices is consistent with a combined filamentary/interfacial mechanism. Overall, ZTO shows great potential as a low cost material for embedding memristive memory with thin film transistor logic for large area electronics.

Improved switching uniformity in resistive random access memory containing metal-doped electrolyte due to thermally agglomerated metallic filaments

We demonstrate improved switching uniformity in resistive random-access memory (RRAM) containing metal-doped electrolyte due to thermally agglomerated metallic filaments. Rapid thermal annealing (RTA) produced copper-doped carbon (CuC) devices that exhibited better switching parameters, such as on/off resistance and set/reset voltage, than a control device. High-resolution transmission electron microscopy, electron dispersive spectroscopy, and conductive atomic force microscopy revealed that Cu atoms were agglomerated during the RTA process and formed a Cu filament in the CuC film. Consequently, the forming process can be eliminated, which is desirable for practical RRAM applications.

Effect of Scaling $\hbox{WO}_{x}$-Based RRAMs on Their Resistive Switching Characteristics

We investigated the effect of scaling down the device area of WOx resistive random-access memory (RRAM) devices on their switching characteristics. Device dimensions were successfully scaled down to 50 nm using a via-hole structure with additional Al2O3 sidewall process. As compared to the microscale devices, the nanoscale devices exhibited a distinct switching mechanism and better memory performance, such as improved switching uniformity, larger memory window, and stable endurance characteristics for up to 107 cycles. This improvement can be explained by a uniform interfacial switching mechanism in nanoscale device; this is in contrast with the defect-induced filamentary switching mechanism observed in microscale devices. In this way, the intrinsic switching properties of RRAMs were obtained by scaling down of the device area, indicating that RRAMs hold considerable promise for future applications.

New Set/Reset Scheme for Excellent Uniformity in Bipolar Resistive Memory

We proposed a new set/reset operation scheme to improve the switching uniformity of filament-type resistive memory. By controlling the magnitude of the sweep voltage in the pulse-sweep mode at each set and reset operation, similar resistance of a conducting filament in a low-resistance state and a homogeneously dissolved conducting filament in high resistance state were obtained continuously. As compared with a normal operation scheme involving a fixed voltage, our new operation scheme exhibits dramatically improved switching uniformity. By combining the new operation scheme with a gradual reset operation, we successfully achieved a stable multibit operation.

Improved Switching Uniformity and Speed in Filament-Type RRAM Using Lightning Rod Effect

Improved switching uniformity and speed were demonstrated using a filament-type resistive memory. By using a gradual reset operation, a leaky high resistance, which has a smaller gap distance confirmed by C-AFM, was successfully obtained. The leaky high-resistance state shows significantly improved switching uniformity compared to the high-resistance state, which has a higher resistance than the leaky high-resistance state, because of the confinement of the randomly formed conducting filaments. A faster operation speed was achieved using the smaller gap distance. To confirm the improved switching speed, we monitored the real-time oscilloscope response.

Improved switching uniformity of a carbon-based conductive-bridge type ReRAM by controlling the size of conducting filament

We investigated the resistive switching properties of a amorphous carbon-based ReRAM device. In order to minimize the fluctuations of switching parameters, we introduced an external load resistor ( R Load) in series, which indirectly acts as a current limiter. Reduced reset current ( I reset) and improved switching uniformity were obtained when the proper external R Load was connected. The voltage drop at the ReRAM device during switching was directly monitored using an oscilloscope. We have confirmed that fluctuation of the effective voltage applied across the conducting filament was dramatically reduced by adding R Load. In contrast, we observed degradation of retention characteristic of sample with R Load. To meet both switching uniformity and retention characteristics, we need to optimize the resistance of low resistance state.