Nonpolar Resistive Switching Research Articles

Multistate nonpolar resistive switching in nickel embedded polyoxovanadate for high density data storage

The evolution of the electronic industry constantly relies on downscaling of electronic devices and integrating novel materials in active regions to accomplish ever-higher speeds and new features in device structures. Employing materials that display multistate switching for resistive-random-access-memory or simply resistive memory could be a simple and effective way to realize high density data storage. In this context, we report multistate “nonpolar” resistive switching in a nickel embedded polyoxovanadate cluster, (K2H5[NiV14O40]) – a molecule that belongs to the larger polyoxometalate family. We observed unique and distinctive nonpolar resistive switching behaviour for the first time in a multi-redox polyoxometalate cluster. The switching characteristics were repeatable for more than 200 cycles. Our two terminal Al/K2H5[NiV14O40])/ITO memory cells exhibited considerably high resistance window (105) and also long retention time (2000 s). This work holds promise for a novel strategy in order to achieve multilevel storage by exploiting different varieties of polyoxometalate molecules as active switching element that can possibly connect memory with neuromorphic computing.

Polarity independent resistive switching in MoS2 nanosheets and PEO-based nanocomposite films

Here, we report the exfoliation of bulk MoS2 (molybdenum disulfide) into few-layer nanosheets and then prepared nanocomposite films (MoS2-PEO) with poly(ethylene oxide) as the host. We observed nonpolar or polarity independent bistable resistive switching memory in two-terminal devices with indium tin oxide and aluminum (Al) as bottom and top electrodes, respectively. In both bipolar and unipolar operations, it is observed that the biasing direction controls the current conduction mechanism. When the positive bias is applied at the top Al electrode, the low resistance state (LRS) conduction is ohmic type. But in the opposite biasing condition, LRS conduction is space charge controlled. The current–voltage characteristics of bipolar and unipolar switching are distinctly different in terms of their RESET process. In bipolar, the RESET process is very sharp, whereas in unipolar operation it is staggered and step-wise.

Universal non-polar switching in carbon-doped transition metal oxides (TMOs) and post TMOs

Transition metal oxides (TMOs) and post-TMOs (PTMOs), when doped with carbon, show non-volatile current–voltage characteristics, which are both universal and repeatable. We have shown spectroscopic evidence of the introduction of carbon-based impurity states inside the existing larger bandgap effectively creating a smaller bandgap, which we suggest could be a Mott–Hubbard-like correlation effects. Our findings indicate new insights for yet to be understood unipolar and nonpolar resistive switching in the TMOs and PTMOs. We have shown that device switching is not thermal-energy dependent and have developed an electronic-dominated switching model that allows for the extreme temperature operation (from 1.5 to 423 K) and state retention up to 673 K for a 1 h bake. Importantly, we have optimized the technology in an industrial process and demonstrated integrated 1-transistor/1-resistor arrays up to 1 kbit with 47 nm devices on 300 mm wafers for advanced node CMOS-compatible correlated electron random access memory. These devices are shown to operate with 2 ns write pulses and retain the memory states up to 200 °C for 24 h. The collection of attributes shown, including scalability to state-of-the-art dimensions, non-volatile operation to extreme low and high temperatures, fast write, and reduced stochasticity as compared to filamentary memories, such as resistive random-access memories, shows the potential for a highly capable two-terminal back-end-of-line non-volatile memory.

Nonpolar Resistive Switching of Multilayer‐hBN‐Based Memories

AbstractResistive switching (RS) induced by electrical bias is observed in numerous materials, including 2D hexagonal boron nitride (hBN), which has been used in resistive random access memories (RRAMs) in recent years. For practical high‐density, cross‐point memory arrays, compared with bipolar memories, nonpolar (or unipolar) devices are preferable in terms of peripheral circuit design and storage density. The non‐volatile nonpolar RS phenomenon of hBN‐based RRAMs with Ti/hBN/Au structure as a prototype is reported. Stable manual DC switching for ≈103 cycles with an average window over five orders of magnitude is demonstrated. After identifying a possible mechanism related to the Joule heat that contributes to the rupture of conductive filaments in nonpolar RS operations, this mechanism is validated by analyzing the occurrence of the “Re‐set” process. Though the intriguing physical origin still requires more comprehensive studies, the achievement of nonpolar RS should make it more feasible to use hBN in practical RRAM technology.

Non-Polar and Complementary Resistive Switching Characteristics in Graphene Oxide devices with Gold Nanoparticles: Diverse Approach for Device Fabrication

Downscaling limitations and limited write/erase cycles in conventional charge-storage based non-volatile memories stimulate the development of emerging memory devices having enhanced performance. Resistive random-access memory (RRAM) devices are recognized as the next-generation memory devices for employment in artificial intelligence and neuromorphic computing, due to their smallest cell size, high write/erase speed and endurance. Unipolar and bipolar resistive switching characteristics in graphene oxide (GO) have been extensively studied in recent years, whereas the study of non-polar and complementary switching is scarce. Here we fabricated GO-based RRAM devices with gold nanoparticles (Au Nps). Diverse types of switching behavior are observed by changing the processing methods and device geometry. Tri-layer GO-based devices illustrated non-polar resistive switching, which is a combination of unipolar and bipolar switching. Five-layer GO-based devices depicted complementary resistive switching having the lowest current values ~12 µA; and this structure is capable of resolving the sneak path issue. Both devices show good retention and endurance performance. Au Nps in tri-layer devices assisted the conducting path, whereas in five-layer devices, Au Nps layer worked as common electrodes between co-joined cells. These GO-based devices with Au Nps comprising different configuration are vital for practical applications of emerging non-volatile resistive memories.

Reversible transitions among four modes of nonpolar resistive switching characteristics in nano-crystalline zinc ferrite magnetic thin films

Nano-crystalline zinc ferrite (ZnFe2O4) magnetic thin film was prepared on Pt/Ti/SiO2/Si substrate by chemical solution deposition method though spin coating technique. Reproducible nonpolar resistive switching (RS) characteristics in Pt/ZnFe2O4/Pt devices are reported. By changing the magnitude and polarity of the applied electric bias, reversible transitions among all the four nonpolar RS modes can be realized. For each switching mode, I-V characteristics were measured in the temperature range of 200 K–340 K. Temperature dependent I-V characteristics of high resistance state (HRS) have been found to support the conduction mechanism across the metal/insulator/metal (MIM) systems to be Schottky emission. Associated parameters such as activation energy and effective height of Schottky barrier at zero biasing (ϕo) have been evaluated at different temperature and electric field. In addition, the increase in resistance of low resistance state (LRS) with temperature revealed that the conduction through ZnFe2O4 in LRS is metallic. Based on the temperature-dependent characteristics, the reversible switching among the four modes of nonpolar RS via formation and rupture of the conducting filaments has been attributed to the joint effect of field-induced migration of oxygen vacancies and metallic Zn atoms.

Nonpolar Resistive Switching in Ag@TiO2 Core-Shell Nanowires.

Nonpolar resistive switching (RS), a combination of bipolar and unipolar RS, is demonstrated for the first time in a single nanowire (NW) system. Exploiting Ag@TiO2 core-shell (CS) NWs synthesized by postgrowth shell formation, the switching mode is controlled by adjusting the current compliance effectively, tailoring the electrical polarity response. We demonstrate ON/OFF ratios of 105 and 107 for bipolar and unipolar modes, respectively. In the bipolar regime, retention times could be controlled up to 103 s, and in the unipolar mode, >106 s was recorded. We show how the unique dual-mode switching behavior is enabled by the defect-rich polycrystalline material structure of the TiO2 shell and the interaction between the Ag core and the Ag electrodes. These results provide a foundation for engineering nonpolar RS behaviors for memory storage and neuromorphic applications in CSNW structures.

Stable nonpolar resistive switching characteristics in Cu/Cu-dispersed ZrO2/Pt memory devices

In this study, stable nonpolar resistive switching characteristics in Cu/Cu-dispersed ZrO2/Pt electrochemical metallization (ECM) memory devices were reported by dispersing metallic Cu into the ZrO2 matrix. Reversible transition among different switching modes can be realized simply by modulating the polarities and magnitudes of the voltages. Moreover, improved uniformity with lower switching voltages and forming-free behaviors were also demonstrated in this device. The experiment results confirmed that metallic Cu clusters were penetrated into the ZrO2 matrix during the annealing process, which would function as an effective cation source responsible for the nonpolar RS switches. These results suggest that controlling the distribution state of active metal atoms in ECM stacks is a feasible approach for reliable memory applications.

Detection of resistive switching behavior based on the Al2O3/ZnO/Al2O3 structure with alumina buffers

The single ZnO layer and a sandwich Al2O3/ZnO/Al2O3 structure were prepared via atomic layer deposition, and the single ZnO layer showed a feature of ohmic conduction only. When we added that together with top and bottom alumina buffers, a non-polar resistive switching (RS) behavior was accomplished, allowing investigation of its electric endurance and retention characteristics. Analysis of the conduction mechanism at both high and low resistance states using auxiliary mathematic tools resulted in the proposed model of conducting filaments formed in the insulating Al2O3 buffers. The effect of stopping voltage Vend on the resistance window at high resistance state is studied and discussed.

Proton exchange reactions in SiOx-based resistive switching memory: Review and insights from impedance spectroscopy

Abstract In this work, the AC admittance and conductance of non-polar SiOx-based resistive switching memory devices is measured as a function of temperature to investigate charge transport and potential switching mechanisms. After electroforming using a forward/backward voltage scan, devices were measured over the frequency range of 1 k–1 MHz and the temperature range of 200–400 K. For temperature (T) > 300 K, AC conductance follows σ(ω) = Aωs, where s is linearly dependent on temperature and close to, but less than, unity. For T

Nonpolar resistive memory switching with all four possible resistive switching modes in amorphous LaHoO3 thin films

We studied the resistive memory switching in pulsed laser deposited amorphous LaHoO3 (a-LHO) thin films for non-volatile resistive random access memory applications. Nonpolar resistive switching (RS) was achieved in Pt/a-LHO/Pt memory cells with all four possible RS modes (i.e., positive unipolar, positive bipolar, negative unipolar, and negative bipolar) having high RON/ROFF ratios (in the range of ∼104–105) and non-overlapping switching voltages (set voltage, VON ∼ ±3.6–4.2 V and reset voltage, VOFF ∼ ±1.3–1.6 V) with a small variation of about ±5–8%. Temperature dependent current-voltage (I–V) characteristics indicated the metallic conduction in low resistance states (LRS). We believe that the formation (set) and rupture (reset) of mixed conducting filaments formed out of oxygen vacancies and metallic Ho atoms could be responsible for the change in the resistance states of the memory cell. Detailed analysis of I–V characteristics further corroborated the formation of conductive nanofilaments based on metal-like (Ohmic) conduction in LRS. Simmons-Schottky emission was found to be the dominant charge transport mechanism in the high resistance state.

Characterization of hafnium oxide resistive memory layers deposited on copper by atomic layer deposition

Hafnium oxide-based resistive memory devices have been fabricated on copper bottom electrodes. The HfOx active layers in these devices were deposited by atomic layer deposition (ALD) at 250°C with tetrakis(dimethylamido)hafnium(IV) as the metal precursor and an O2 plasma as the reactant. Depth profiles of the HfOx by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a copper concentration on the order of five atomic percent throughout the HfOx film. In addition to the Cu doped HfOx, a thin layer (20nm) of CuxO is present at the surface. This surface layer is believed to have formed during the ALD process, and greatly complicates the analysis of the switching mechanism. The resistive memory structures fabricated from the ALD HfOx exhibited non-polar resistive switching, independent of the top metal electrode (Ni, Pt, Al, Au). Resistive switching current voltage (I–V) curves were analyzed using Schottky emission and ionic hopping models to gain insight into the physical mechanisms underpinning the device behavior. During the forming process it was determined that, at voltages in excess of 2.5V, an ionic hopping model is in good agreement with the I–V data. The extracted ion hopping distance ~4Å was within the range of interatomic spacing of HfO2 during the forming process consistent with ionic motion of Cu2+ ions. Lastly the on state I–V data was dominated at larger voltages by Schottky emission with an estimated barrier height of ~0.5eV and a refractive index of 2.59. The consequence of the Schottky emission analysis indicates the on state resistance to be a product of a Pt/Cu2O/Cu filament(s)/Cu2O/Cu structure.

Nonpolar resistive switching in Cu/SiC/Au non-volatile resistive memory devices

Amorphous silicon carbide (a-SiC) based resistive memory (RM) Cu/a-SiC/Au devices were fabricated and their resistive switching characteristics investigated. All four possible modes of nonpolar resistive switching were achieved with ON/OFF ratio in the range 106–108. Detailed current-voltage I-V characteristics analysis suggests that the conduction mechanism in low resistance state is due to the formation of metallic filaments. Schottky emission is proven to be the dominant conduction mechanism in high resistance state which results from the Schottky contacts between the metal electrodes and SiC. ON/OFF ratios exceeding 107 over 10 years were also predicted from state retention characterizations. These results suggest promising application potentials for Cu/a-SiC/Au RMs.

Using binary resistors to achieve multilevel resistive switching in multilayer NiO/Pt nanowire arrays

Reliable multilevel resistive switching in nanoscale cells is desirable for the wide adoption of resistive random access memory as the next-generation nonvolatile memory. We designed NiO-based cells in arrays of multilayered NiO/Pt nanowires to explore multilevel memory effects. Nonpolar resistive switching reproducibly occurs with significantly reduced switching voltages, narrow switching voltage distributions and a robust multilevel memory effect. A high resistance ratio (∼105) between the high- and low-resistance states in nanoscale cells enables stable multilevels that can be induced easily by a series of pulsed voltage. The existence of intermediate resistance states in NiO/Pt nanowire arrays can be well explained by the binary-resistor model combined with energy perturbations induced by the pulse voltage. We also verified that the conduction mechanism in multilayered NiO/Pt nanowires is dominated by the hopping of holes. Our bottom-up approach and proposed mechanism explain the controllable multilevel memory effect and facilitate sound device design to encourage their universal adoption. Metal-insulator-metal structures that can switch between low and high resistance states under electrical stimulus hold promise as components for future non-volatile memory devices. A Taiwan-based research team, led by Chih-Huang Lai at the National Tsing Hua University, has now devised a nanowire array that exhibits multilevel resistive switching behavior — that is, between low, high and also intermediate states — under voltage pulses, giving rise to robust memory effects. Each nanowire consists of multilayered, alternating nickel oxide and platinum parts, and the multilevel resistive switching phenomenon is attributed to averaging over all possible binary configurations for each nickel oxide segment in the multilayered nanowire array. The researchers explain these observations using a binary resistor model of conduction generated by the hopping of holes through defects, which may be applicable to other multilevel resistive switching systems. Nonpolar resistive switching reproducibly occurs in arrays of nanoscale cells composed of multilayered NiO/Pt nanowires with significantly reduced switching voltages, narrow switching voltage distributions and a robust multilevel memory effect. A high resistance ratio (∼105) between the high- and low-resistance states in nanoscale cells enables stable multilevels to be induced easily by a series of pulsed voltages. The existence of intermediate resistance states in NiO/Pt nanowire arrays can be well explained by the binary-resistor model combined with energy perturbations induced by the pulse voltage. Our bottom-up approach and proposed mechanism explain the controllable multilevel memory effect.

Hybrid aluminum and indium conducting filaments for nonpolar resistive switching of Al/AlOx/indium tin oxide flexible device

The nonpolar resistive switching characteristics of an Al/AlOx/indium tin oxide (ITO) device on a plastic flexible substrate are investigated. By analyzing the electron diffraction spectroscopy results and thermal coefficient of resistivity, it is discovered that the formation of aluminum and indium conducting filaments in AlOx film strongly depends on the polarity of the applied voltage. The metal ions arising from the Al and ITO electrodes respectively govern the resistive switching in corresponding operation polarity. After 104 times of mechanical bending, the device can perform satisfactorily in terms of resistance distribution, read sequence of high and low resistive states, and thermal retention properties.

Understanding the resistive switching characteristics and mechanism in active SiOx-based resistive switching memory

The resistive switching characteristics and mechanism in active SiOx-based resistive switching memory have been investigated by using a simple TaN/SiO2/n++ Si-substrate test structure. Controlling the oxygen content in SiOx layer not only improved device yield but also stabilized electrical switching characteristics. The current transport behavior in high- and low-resistance states, thickness effect in SiOx layer, device area effect, and multilevel effect by controlling compliance current limitation and stopped voltage values have been studied. The results indicate that resistive switching occurs in a localized region along a filament, rather than uniformly throughout the bulk. A general current flow model for nonpolar SiOx-based resistive switching memory has been proposed, which provides a simple physical concept to describe the resistive switching behavior and provides additional insights into optimization of resistive switching memory devices.

Cu-Embedded AlN-Based Nonpolar Nonvolatile Resistive Switching Memory

This letter covers the fabrication of a nonpolar resistive switching (RS) random access memory device using Cu-embedded AlN and its reproducible RS characteristics. The AlN-based memory device shows endurance of >; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , reliable retention time (ten years extrapolation at both room temperature and 85°C), and fast programming speed under 100-ns pulses in unipolar operation mode. The switching mechanism is believed to be mediated by the formation and rupture of conductive Cu filaments.

Improvement of resistive switching in NiO-based nanowires by inserting Pt layers

Nonpolar resistive switching is demonstrated in polycrystalline NiO-based nanowires. The lower switching voltages and narrower switching distributions are exhibited in multilayered NiO/Pt nanowires, compared to the monolithic NiO nanowires. The temperature dependence of resistance at low resistance state reveals the conduction is attributed to the hopping through percolation paths composed of oxygen-related defects. The inserted Pt layers behave as intermediate electrodes to reduce migration length of oxygen ions and to store the oxygen ions near the electrodes. Therefore, the localized formation/migration of oxygen ions confines the occurrence of percolation paths, leading to improvement of the switching parameters.

Nonpolar resistive switching in Mn-doped BiFeO3 thin films by chemical solution deposition

Nanoscale Mn-doped BiFeO3 (BFMO) films are prepared by chemical solution deposition on Pt/Ti/SiO2/Si substrates. Nonpolar nonvolatile resistive switching has been observed in Pt/BFMO/Pt structure. Analysis of the current-voltage relationship demonstrates that the space-charge-limited current conduction controlled by the localized traps should be of great importance to the resistance switching. X-ray photoelectron spectroscopy results suggest that the coexistence of Fe2+ and Fe3+ can be ascribed to the relatively oxygen vacancies. According to defect chemistry, the valence variation between Fe2+ and Fe3+ determined by the neutralization/ionization of oxygen vacancies is supposed to play a crucial role in conductive filament-related nonpolar resistive switching.

The Impact of Y Addition into TiO2 on Electronic States and Resistive Switching Characteristics

We have studied electronic structures and resistive switching characteristics of Y-doped TiO2 films as a function of Y content to improve the performance on resistive switching of TiO2-based ReRAM. TiYxOy films with different Y contents were deposited on a Pt layer by metal organic chemical vapor deposition (MOCVD) using dipivaloymethanato (DPM) precursors and followed by O2 anneal to densify the films. A fairly good compositional uniformity in each TiYxOy film was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The energy bandgap (Eg) of the TiYxOy films, which was determined by analyzing the absorption coefficient, was gradually increased with the Y content. Since a decrease in valence band offset between TiYxOy and Pt was almost the same as that in Eg, the conduction band offset was almost constant at ∼1.3 eV. The current–voltage (I–V) characteristics of TiYxOy films, which were measured by sweeping positive bias to a Au top electrode after forming process, show non-polar type resistive switching and its beneficial change with Y incorporation into the TiO2 matrix. The Y addition is quite effective to improve endurance in resistive switching and to reduce the variations of operation voltages (VSET and VRESET).