Unipolar Switching Behavior Research Articles

Resistive switching characteristics of polycrystalline SrTiO3 films

Strontium titanate (STO) thin films 90 nm in thickness were grown on a Pt substrate through atomic layer deposition (ALD). The as-deposited ALD STO grown with an ALD cycle ratio of 1:1 (Sr:Ti) was in an amorphous phase, and annealing at 800 °C in air crystallized the films into the perovskite phase. This phase change was confirmed by x-ray diffraction and transmission electron microscopy. The as-deposited ALD STO exhibited no discernible switching mechanism, whereas unipolar switching behavior was reproducibly observed with a high resistance ratio (108–109) and strict separation of the set/reset voltages and currents in the annealed ALD STO. Mechanisms for charge transport in both the low- and high-resistance states and for resistive switching in the annealed ALD STO are also proposed.

Single- and bi-layer memristive devices with tunable properties using TiOx switching layers deposited by reactive sputtering

The authors systematically studied reactive sputtering deposition of TiOx thin films using a mixture of Ar and O2 gases under different ratios of O2 flow. As directly revealed by X-ray photoelectron spectroscopy, the deposition changed from a metallic Ti target mode to an oxide target mode when the O2 flow ratio was beyond 40%, resulting in TiOx thin films with different chemical compositions. Consequently, metal/oxide/metal devices with a single TiOx layer exhibited a broad spectrum of electrical characteristics such as Ohmic, rectifying, and memristive behavior. The reactive sputtering deposited TiOx thin films were also used in a bilayer memristive device structure, and a transition from bipolar to unipolar switching behavior was observed for devices based on thin films prepared with different oxygen flow.

RESET-first unipolar resistance switching behavior in annealed Nb2O5 films

In this work, the effect of thermal annealing on the resistance switching behavior of Nb2O5 films was investigated in conjunction with an analysis of the chemical bonding states and crystal structure. The Nb2O5 films were deposited via reactive sputtering, and annealed via rapid thermal annealing at various temperatures up to 650°C. The crystal structure of the as-deposited Nb2O5 films transformed from amorphous to a hexagonal Nb2O5 crystalline phase with tetragonal NbO2 following thermal annealing at 500°C. The conductivity of the Nb2O5 films increased drastically as the annealing temperature increased. An increase in the non-lattice oxygen in the Nb2O5 films was also observed with thermal annealing. Pt/Nb2O5/Pt stacks with the as-deposited Nb2O5 showed typical unipolar resistance switching behaviors after electro-forming; however, the Nb2O5 film devices annealed at 500°C showed RESET-first resistance switching behavior without prior electro-forming. The RESET-first resistance switching in annealed Nb2O5 is believed to be due to the nano-scale conductive path formed in the annealed Nb2O5 films.

Unipolar Switching Behavior in Highly Crystalline Hexagonal Boron Nitride

ABSTRACTWe observed resistive switching in highly crystalline layered insulator hexagonal boron nitride (h-BN) under electric field in a nano-device configuration. Two distinct resistive states were observed in the 2D material heterostack. Electrical characterization using capacitance-voltage approach further revealed the role of h-BN as the active switching element. The switching behavior could be attributed to substitutional doping of h-BN under electric field present in the active region, possibly resulting in the formation of multi-element complex in which electrical conductivity depends on the amount of substituted dopant in the boron nitride crystal lattice. Since switching is observed independent of the direction of electric field, it is unipolar in nature. The observed memristance phenomenon in layered insulator may be potentially used in the form of NVM, providing possible direction to implement information storage or reconfigurable logic applications.

Study of the bipolar resistive‐switching behaviors in Pt/GdOx/TaNx structure for RRAM application

The bipolar resistive switching (RS) behaviors in Pt (top)/GdOx/TaNx (bottom) memory cells were investigated systematically for RRAM applications. Compared to that of the memory cells in Pt/GdOx/Pt structure showing unipolar switching behaviors, the memory cells in Pt/GdOx/TaNx structure with bipolar switching behaviors show better switching properties, such as better switching reliability, enhanced cycling endurance, improved low resistive state value and more uniform RS parameters. The composition and chemical bonding states of the prepared GdOx and TaNx films were analyzed by X‐ray photoelectron spectroscopy to elucidate the underlying resistive switching mechanisms for Pt/GdOx/TaNx structures. The results show that both the oxygen vacancies in the GdOx film and the interfacial TaON layer formed between the dielectric GdOx film and the TaNx bottom electrode play important roles in the RS performance improvement.

Effect of thermal annealing on resistance switching characteristics of Pt/ZrO2/TiN stacks

In this study, the effect of thermal annealing on both the physical properties and the resistive switching properties of ZrO2 films deposited by atomic layer deposition (ALD) method were investigated for its potential application to non-volatile memory devices. The ZrO2 films in the Pt/ZrO2/TiN structure exhibited unipolar and bipolar resistance switching behaviors depending on the nature of the bias applied to Pt top electrodes for the electro-forming process. For unipolar switching, the resistance of the high resistance state (HRS) was reduced with increasing annealing temperature, accompanied with the increase of metallic Zr in the annealed ZrO2 films. In contrast, the HRS resistance in the bipolar switching was increased while the low resistance state (LRS) resistance was decreased with increasing annealing temperature, producing a greater change in resistance. SIMS and EDX showed that the thickness of interfacial TiOxNy layer between the ZrO2 and the TiN bottom electrode was enlarged with annealing. The enlarged TiOxNy layer was expected to produce the reduction of LRS resistance with the increase of HRS resistance in the bipolar resistance switching.

Nanogap Resistance Random Access Memory Based on Natural Aluminum Oxide

We report unipolar and bipolar resistive switchings in naturally oxidized AlxO1-x thin films. We find a relationship between the switching behavior and the electrode gap distance. The macro-gap device with the electrode separation of 20 µm shows a unipolar switching behavior while the nano-gap device with the separation of 40 nm shows a bipolar behavior. The result is explained by a model in which the unipolar or the bipolar switching is governed by the way of the carrier injection into oxygen vacancies.

Observation of conductive filaments in a resistive switching nonvolatile memory device based on amorphous InGaZnO thin films

Resistive switching mechanism was investigated on a model nonvolatile memory device structure composed of an amorphous InGaZnO (a-IGZO) insulator layer and TiN and indium tin oxide (ITO) electrodes by directly observing the microstructural and compositional changes in the device during the switching operation. In the pristine state, microstructure of the device was characterized by well-defined smooth interface between the a-IGZO switching layer and electrodes. Once the device is in a set state by applying positive bias to the TiN electrode, the TiN/a-IGZO interface became markedly rough with irregularly localized structures protruded into the a-IGZO layer from the TiN electrode. Transmission electron microscopy revealed that these protruded structures are polycrystalline TiN which have been formed by the migration of Ti species during the voltage sweep due to the electric field between positively biased TiN and grounded ITO electrodes. The device structures showed bistable unipolar switching behavior while they were transparent to visible light with reasonable transmittance of about 77%.

Variation of switching mechanism in TiO2 thin film resistive random access memory with Ag and graphene electrodes

We report the fabrication of resistive random access memory (ReRAM) on both Si and PET flexible substrates using TiO2 as the dielectric spacer between Ag electrodes. Ag/TiO2/Ag ReRAM shows unipolar switching behavior with a ramping rate of 50 mV. We further examined the switching mechanism for Ag based ReRAM in the low resistive state (LRS) and high resistive states (HRS). To elucidate the impact of electrode material on the switching mechanism, we fabricated a graphene/TiO2/graphene ReRAM device and observed that the switching behavior changed from unipolar to bipolar due to the unique physical properties of graphene. This study demonstrates ReRAM based on Ag and graphene electrodes on both Si and PET substrates, and directly demonstrates the strong dependence of electrode materials on the switching mechanism.

Role of InGaOx resistive switching characteristics on the performances of resistance random access memory of Pt/IGO/TiN device

In this study, the Pt/IGO/TiN resistance random access memory (ReRAM) is investigated by the fabrication of co-sputtering the Ga2O3 and In2O3 targets to form the InGaOx (IGO) film. The Pt/GaOx/TiN structure was fabricated as comparison device. In addition to the bipolar resistance switching characteristic shown in GaOx film, the unipoar switching characteristic is also obtained in IGO device. The switching mechanisms between bipolar and unipolar in the IGO device are investigated by measuring the resistance values in various temperatures. From the results, the proportional relationship between resistance value and temperature among the bipolar switching characteristics illustrates that the switch is caused by the generation/recombination of oxygen vacancies. However, the unipolar switching behaviors can be attributed to metallic filament due to the trend of increasing resistance with the increasing temperature.

Unipolar resistance switching and abnormal reset behaviors in Pt/CuO/Pt and Cu/CuO/Pt structures

The effects of Pt and Cu top electrodes on resistance switching properties were investigated for CuO thin films with Pt/CuO/Pt and Cu/CuO/Pt sandwich structures. Typical unipolar resistance switching (URS) behaviors and two different kinds of resistance changes in the reset process were observed in both structures. When voltages were applied to the film, the low-resistance state (LRS) with relatively low resistance value (<30Ω) was switched to the high-resistance state (HRS), exhibiting normal reset behavior. For LRS with relatively high resistance value (>50Ω), the resistance first decreased then increased to HRS, showing abnormal reset behavior. The former variation of LRS could be ascribed to the decrease in filament size induced by Joule heating, while the latter one could be ascribed to the growth of disconnected filaments induced by high electric fields. This study indicates that the switching modes and the abnormal reset behaviors in CuO thin films are not due to Pt and Cu top electrodes, but the intrinsic properties of CuO film.

Self-Assembly-Induced Formation of High-Density Silicon Oxide Memristor Nanostructures on Graphene and Metal Electrodes

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.

Programmable Bipolar and Unipolar Nonvolatile Memory Devices Based on Poly(2‐(N‐carbazolyl)ethyl methacrylate) End‐Capped with Fullerene

A novel polymer, poly(2-(N -carbazolyl)ethyl methacrylate) end-capped with fullerene (PCzMA-C(60) ), has been synthesized via living anionic polymerization. Electrically programmable flash memory devices were easily fabricated with this polymer by using solution coating and metal deposition. This polymer was found in these devices to exhibit bipolar and unipolar switching behaviors with a high ON/OFF current ratio, a long retention time, high reliability, and low power consumption. The excellent properties and easy processability of this polymer open up the possibility of the mass production of high performance nonvolatile memory devices at low cost.

Synthesis of Resistive Memory Oxides by Ion Implantation

ABSTRACTIn this work, we report on the use ion of implantation to synthesize resistive memory oxides. The surface of copper thin films was converted to copper oxide using oxygen implantation. Devices fabricated from the copper oxide (CuxO) layers exhibited unipolar switching behavior without the need for a forming voltage. Technology scaling was demonstrated by oxygen implanting copper damascene vias. Unipolar switching was observed in via-based devices down to 48 nm. The current-voltage data of devices scaled from 100 μm to 48 nm suggests that the RESET transition is related to localized Joule heating. Tantalum oxide (TaxOy) was also created by oxygen implantation but exhibited bipolar resistive switching. Analysis of the conduction suggests that the difference between the two resistance states in these devices is largely due to a lowering of the Pt-TaxOy Schottky barrier.

The application of conventional photolithography to microscale organic resistive memory devices

We demonstrate the application of conventional photolithography to fabricate organic memory devices in an array structure with a cell area of 4 × 4 μm2 without damaging the underlying organic memory layer. Applying photolithography to organic electronic devices is not trivial because the solvents used during lithography may dissolve and damage the previously coated organic layers. The application of photolithography to our organic devices was possible because of the introduction of polymethyl methacrylate (PMMA)/polyvinyl alcohol (PVA) onto the memory active layer, where PMMA functions as a buffer layer to prevent dissolution of the PVA layer during developing process, and PVA acts as a striped layer during metal lift-off process. Embedded Al bottom electrodes were particularly constructed to minimize the switching failure. The completed organic memory devices exhibited typical unipolar switching behavior and excellent memory performance in terms of their statistical memory parameters (ON and OFF currents and threshold voltages), ON/OFF ratio (>102), endurance (>230 cycles), and retention (>104 s). This convenient photolithography patterning technique is applicable for the further scaling of many types of organic devices.

Structural and Electrical Characterization of a Block Copolymer‐Based Unipolar Nonvolatile Memory Device

Electronic devices based on a series of synthesized block copolymers are demonstrated. In particular, a block copolymer system with a lamellar structure exhibits unipolar switching behavior. This study provides a simple strategy based on the adjustment of the block ratio in block copolymers to control the polymer morphology and thus the electrical and switching properties of polymer-based memory devices.

Unipolar resistive switching and mechanism in Gd-doped-TiO2-based resistive switching memory devices

Gd-doped-TiO2-based resistive switching random-access memory (RRAM) devices with MOM structure of top metal electrode (W, Ti)/Gd-doped-TiO2/Pt/Ti/SiO2/Si were fabricated, and resistive switching characteristics were investigated. The W/Gd-doped-TiO2/Pt RRAM device showed typical unipolar resistive switching behavior irrespective of the bias polarity on the W top electrode. However, no unipolar switching was observed in the Ti/Gd-doped-TiO2/Pt devices when positive bias was applied on the Ti top electrode. A new resistive switching model was proposed to explain the unipolar switching behavior in the W/Gd-doped-TiO2/Pt RRAM device, based on the formation and destruction of conducting filaments which result from pinning and unpinning effects of surface trap states of oxide film on the Fermi level of metal electrode. The model clearly elucidates the mechanism of the low resistance state (LRS) and the high resistance state (HRS) as well as the switching behavior between LRS and HRS in the W/Gd-doped-TiO2/Pt RRAM device. The model also indicates the important role of the metal electrode material in unipolar resistive switching of oxide-based RRAM devices.

Rewritable and Non‐Volatile Memory Effects Based on Polyimides Containing Pendant Carbazole and Triphenylamine Groups

AbstractAs part of a strategy to explore highly functionalized macromolecular semiconductors, two novel polyimides consisting of electron‐donating carbazole and triphenylamine side groups are synthesized and characterized. Polyimide films sandwiched between an indium‐tin oxide (ITO) bottom electrode and Al top electrode exhibit two accessible conductivity states and can be switched from the low‐conductivity state to the high‐conductivity state. Memory devices with the configuration of Al/PI(CzBD‐DPBPDA)/ITO exhibit bipolar flash‐type memory characteristics while that of Al/PI(TPABD‐DPBPDA)/ITO demonstrate excellent unipolar ON and OFF switching behavior. Both devices show high ON/OFF current ratios up to 104 and long retention times of 104 s in ambient atmosphere.

Different Resistive Switching Characteristics of a Cu/SiO2/Pt Structure

A 20-nm SiO2 thin film was deposited using a radio-frequency magnetron sputter to form a Cu/SiO2/Pt structure. Resistance of the Cu/SiO2/Pt device was reversibly switched between the high resistance-state and the low resistance-state using dc voltages. The switching behaviors and the conduction mechanisms suggested that a conducting filament model would best explain the resistance switching. Both unipolar and bipolar switching behaviors were observed in the identical device by using different voltage operations. The detailed switching mechanism can not be differentiated by the switching behaviors directly. This study adopted current sweeping mode, reset current, and sweeping speed to distinguish the conducting model from the thermochemical and the electrochemical reactions.

A non-filamentary model for unipolar switching transition metal oxide resistance random access memories

A model for resistance random access memory (RRAM) is proposed. The RRAM under research utilizes certain transition metal oxide (TMO) such as NiO which shows unipolar switching behavior. The existence of metal/insulator states is not explained by filaments but attributed to different Hubbard U values, which stems from an electron correlation effect. Current-voltage formulae are given both on the metal and insulator sides by putting the appropriate solutions of Hubbard model into the mesoscopic Meir-Wingreen transport equation. The RESET phenomenon is explained by a sufficient separation of Fermi levels in the electrodes and hence a Mott transition can be triggered in the anodic region due to a lack of electrons. The SET behavior originates from a tunneling current which removes the insulating region near the anode. Several experimental evidences are also presented to support this model. The model also serves as the theoretical prototype of Correlated Electron Random Access Memory (CeRAM) which is defined to be a TMO RRAM whose working mechanism is based on the strong electron correlation effects.