Unipolar Resistive Switching Research Articles

Ionic conduction and unipolar resistance switching in [formula omitted]-phase [formula omitted] thin films

Fast-ion conductors are of broad interest in diverse fields such as electrochemical energy and non-volatile memory technologies. In this study, δ-phase Bi2O3 thin film was stabilized to room temperature via reactive magnetron sputter deposition and characterized by X-ray diffraction and Raman spectroscopy. The δ-phase Bi2O3 thin films were highly textured along (111) direction on various substrates and stable during annealing up to 400 °C in air and 200 °C in 5% H2 atmosphere. Electrochemical impedance measurements on δ-phase Bi2O3 thin films show an ionic conductivity of 0.001 S/cm at 400 °C and an activation energy of 0.98 eV. δ-phase Bi2O3 thin films based resistance switching cells (Ag/δ-Bi2O3/Pt or Ag/δ-Bi2O3/Au) show unipolar resistance switching phenomena. The voltage difference between the RESET and SET voltage is over 0.8 V and the resistance contrast between high resistance state (HRS) and low resistance state (LRS) is greater than 103. The resistance switching parameters, like FORMING voltage, SET voltage and RESET voltage can be controlled by the thickness of δ-Bi2O3. In the low resistance state, both the absence of superconductivity down to 2 K and magnetoresistance value indicates metallicity originates from non-stoichiometric bismuth oxide and not pure bismuth metal. The electric field required for SET and RESET operations are estimated to be 7 MV/m and 3.25 MV/m respectively.

Nonvolatile Resistive Switching and Physical Mechanism in LaCrO3 Thin Films**Supported by the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences’ Large-Scale Scientific Facility under Grant No U1532149, and the National Basic Research Program of China under Grant No 2014CB931704.

Polycrystalline LaCrO3 (LCO) thin films are deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching (RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of ∼ 104 between the high resistance state (HRS) and low resistance state (LRS) and exhibits excellent endurance/retention characteristics. The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies, which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results. Further analysis shows that the reset current IR and reset power IR in the reset processes exhibit a scaling law with the resistance in LRS (R0), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.

Control the switching mode of Pt/HfO2/TiN RRAM devices by tuning the crystalline state of TiN electrode

In order to increase the resistive switching performance of memory devices based on TiN electrode, it is desirable to fabricate TiN with good oxygen reservoir ability. In the present paper, we report that the crystallized (200) textured TiN electrode were achieved by epitaxial grown on CrRu intermediate layer, which had better oxygen reservoir ability than the amorphous TiN. It is found that the Pt/HfO2/TiN devices with crystallized TiN electrode exhibited a typical bipolar resistive switching behavior, while the I-V curves of the Pt/HfO2/TiN devices with amorphous TiN electrode showed the unipolar resistive switching behavior regardless of voltage polarity, which is similar to the Pt/HfO2/Pt devices. It is believed that this originates from the different oxygen reservoir ability of TiN with different fabricating process. Moreover, the RRAM device with the new structure of TiN (amorphous)/HfO2/TiN (crystallized) exhibited the bipolar resistive switching behavior. This further proved the amorphous TiN electrode had poorer ability of oxygen reservoir than crystallized TiN, which was similar to a noble electrode like Pt electrode. Tuning the resistive switching behaviors of RRAM devices by controlling the crystalline state of TiN electrode may offer a method for fabrication of transition metal oxides based RRAM device with good performances.

In situ observation of conducting filament in NiO memristive devices by electroluminescence

By synchronously measuring the electroluminescence (EL) of Pt/NiO/Pt unipolar resistive switching (RS) devices during switching, we have nondestructively observed the dynamic evolution of conducting filaments (CFs) across the device. We demonstrate that both the random distribution and the subtle change in the chemical composition of CFs contribute to fluctuation of switching parameters. The EL emission during RS is attributed to the radiative transition through oxygen vacancy related defect levels of NiO rather than from the thermal radiation. Our findings offer a method to investigate the RS mechanism which further can be utilized to improve the stability and scalability of RS devices.

Nonvolatile Memory Device Based on Bipolar and Unipolar Resistive Switching in Bio‐Organic Aloe Polysaccharides Thin Film

AbstractGrowing environmental awareness has prompted a paradigm shift toward using green materials for various electronic applications. Bio‐organic materials are promising candidates attributable to their inherent biocompatibility, biodegradability, and environmental friendliness. Here, a nonvolatile memory device based on resistive switching in a bio‐organic polysaccharides film is presented. The polysaccharides are extracted from natural Aloe vera gel using facile alcohol precipitation. Optimal film deposition is achieved through spin‐coating followed by drying at 120 °C, which is higher than the usual processing temperature of commercial Aloe vera gel. Both bipolar and unipolar resistive switching can be induced in the polysaccharides film depending on the top electrode of the device. A comprehensive elucidation of the switching mechanism reveals the existence of various conduction processes that are electronic, electrochemical, and thermochemical in nature resulting in the unique resistive switching behaviors. The device is capable of delivering competitive performances, featuring wide read memory windows and large ON/OFF ratios that render it with potential as a next‐generation nonvolatile memory. The demonstration represents an important milestone toward the realization of green electronics.

Bias polarity-dependent unipolar switching behavior in NiO/SrTiO3 stacked layer**Project supported by the National Natural Science Foundation of China (Grant No. 11404093), the Foundation of Henan Provincial Science and Technology Department, China (Grant No. 132102210258), the Research Funding from Henan Province, China (Grant Nos. 15A140001, 2015GGJS-021, and 17HASTIT014), and the Henan University Emerging Cross and Characteristic Discipline Cultivation Program, China (Grant

The identification of the switching location has been a key technology to tune the physical properties of unipolar resistive switching (RS) cells. Here we report the RS properties of Au/NiO/SrTiO3(STO)/Pt memory cells. The switching repeatability is closely related to the applied bias polarity, which is different from the scenario of the Au/STO/Pt cells reported in our previous researches. The high resistance in positive bias is greater than that in negative bias. The plot of the on-state I–V curve shows a regular shape only with a slight bend and an abnormal shape with an abrupt increase and decrease under negative and positive bias, respectively. These comparative experimental results reveal that the conductance filament consisting of oxygen vacancies grows from the cathode to the anode. The spatial RS location is identified with the weaker part along the conductance filament length direction, which should be near the NiO/STO interface and STO/Pt interface under positive and negative bias, respectively.

First principles investigation of the unipolar resistive switching mechanism in an interfacial phase change memory based on a GeTe/Sb2Te3 superlattice

The interfacial phase change memory (iPCM) based on a GeTe/Sb2Te3 superlattice is one of the candidates for future storage class memories. However, the atomic structures of the high and low resistance states (HRS/LRS) remain unclear and the resistive switching mechanism is still under debate. Clarifying the switching mechanism is essential for developing further high-reliability and low-power-consumption iPCM. We propose, on the basis of the results of first-principles molecular dynamics simulations, a mechanism for resistive switching, and describe the atomic structures of the high and low resistance states of iPCM for unipolar switching. Our simulations indicated that switching from HRS to LRS occurs with Joule heating only, while that from LRS to HRS occurs with both hole injection and Joule heating.

Coexistence of unipolar and bipolar resistive switching behaviors in NiFe2O4 thin film devices by doping Ag nanoparticles

The coexistence of unipolar and bipolar resistive switching (RS) behaviors of Ag-nanoparticles (Ag-NPs) doped NiFe2O4 (NFO) based memory devices was investigated. The switching voltages of required operations in the unipolar mode were smaller than those in the bipolar mode, while ON/OFF resistance levels of both modes were identical. Ag-NPs doped NFO based devices could switch between the unipolar and bipolar modes just by preferring the polarity of RESET voltage. Besides, the necessity of identical compliance current during the SET process of unipolar and bipolar modes provided an additional advantage of simplicity in device operation. Performance characteristics and cycle-to-cycle uniformity (>103 cycles) in unipolar operation were considerably better than those in bipolar mode (>102 cycles) at 25 °C. Moreover, good endurance (>600 cycles) at 200 °C was observed in unipolar mode and excellent nondestructive retention characteristics were obtained on memory cells at 125 °C and 200 °C. On the basis of temperature dependence of resistance at low resistance state, it was believed that physical origin of the RS mechanism involved the formation/rupture of the conducting paths consisting of oxygen vacancies and Ag atoms, considering Joule heating and electrochemical redox reaction effects for the unipolar and bipolar resistive switching behaviors. Our results demonstrate that 0.5% Ag-NPs doped nickel ferrites are promising resistive switching materials for resistive access memory applications.

Unipolar resistive switching in sol-gel synthesized strontium titanate thin films

We report repeatable and robust unipolar resistive switching (URS) in strontium titanate (SrTiO3 ∼ STO) thin films fabricated on conducting ITO coated glass substrate. The Atomic Force Microscopy (AFM) image of the top surface of the films suggests very smooth surface over a large area. The conduction mechanisms responsible for the resistive switching phenomenon were also thoroughly analyzed by current-voltage and impedance spectroscopy analysis. Frequency-dependent various impedance parameters were analyzed for each resistive states (high resistance state (HRS) and low resistive state (LRS)). The oxygen vacancies may be one of the dominating factors for robust URS phenomenon in the device. The presented Au/STO/ITO memory devices have exhibited long retention characteristics of >104 s and OFF/ON resistance ratio of >103 with a distinguishable SET and RESET voltage window of ∼1 V.

Improving Unipolar Resistive Switching Uniformity with Cone-Shaped Conducting Filaments and Its Logic-In-Memory Application.

Resistive random access memory (RRAM) with inherent logic-in-memory capability exhibits great potential to construct beyond von-Neumann computers. Particularly, unipolar RRAM is more promising because its single polarity operation enables large-scale crossbar logic-in-memory circuits with the highest integration density and simpler peripheral control circuits. However, unipolar RRAM usually exhibits poor switching uniformity because of random activation of conducting filaments and consequently cannot meet the strict uniformity requirement for logic-in-memory application. In this contribution, a new methodology that constructs cone-shaped conducting filaments by using chemically a active metal cathode is proposed to improve unipolar switching uniformity. Such a peculiar metal cathode will react spontaneously with the oxide switching layer to form an interfacial layer, which together with the metal cathode itself can act as a load resistor to prevent the overgrowth of conducting filaments and thus make them more cone-like. In this way, the rupture of conducting filaments can be strictly limited to the tip region, making their residual parts favorable locations for subsequent filament growth and thus suppressing their random regeneration. As such, a novel "one switch + one unipolar RRAM cell" hybrid structure is capable to realize all 16 Boolean logic functions for large-scale logic-in-memory circuits.

Self-rectifying resistive switching behavior observed in Al2O3-based resistive switching memory devices with p-AlGaN semiconductor bottom electrode

In this work, self-rectifying unipolar resistive switching (RS) behavior is demonstrated in Al2O3-based resistive random access memory (RRAM) devices by employing wide bandgap compound semiconductors, such as p-GaN and p-AlGaN, as a bottom electrode (BE). Compared to an RS characteristic of the Al2O3 RRAM with p-GaN BE, the memory cell with p-AlGaN BE shows a lager current ratio while a current-limited region is also more clearly obtained in a low voltage region, which can result in one diode type RRAM with self-selecting properties. Consequently, these results infer that the proposed Al2O3-based RRAM cells with a wide bandgap BE warrant the realization of selector-free RRAM cell without any additional peripheral elements to suppress a disturbance in reading operation.

Unipolar resistive switching in porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films

In this study, the unipolar resistive switching (RS) effect is exhibited in porous perovskite-like Bi3.15Nd0.85Ti3O12 (BNT) thin films fabricated on Pt/Ti/SiO2/Si substrates through chemical solution deposition technique employed PEG template. The unipolar RS characteristics can be adjusted by the size and density of the pores in the porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films. The size and density of the pores are controlled by the processing parameters of the annealing temperature or the dwell time or the both during the preparation of the porous perovskite-like Bi3.15Nd0.85Ti3O12 thin films. The porous BNT thin films annealed at 720 °C for 10 min show large current ratio of low/high resistance state of four orders and low operating voltage of 5 V for SET and 1.5 V for RESET. Moreover, the LRS/HRS can be reversed repeatedly without large decay. After the retention test of 104 s for low/high resistance state, the devices still show as large current ratio of low/high resistance state as about four orders. The present study should have guide to develop a unipolar RS memory using the perovskite oxides.

Reversible alternation between bipolar and unipolar resistive switching in Ag/MoS2/Au structure for multilevel flexible memory

A new operating scheme for voltage-polarity-controlled multilevel memory based on Ag/MoS2/Au flexible structure with reversible alternation between bipolar and unipolar resistive switching modes was demonstrated.

Resistive Switching Characteristics of Al 2 O 3 /ZnO Bilayer Thin Films for Flexible Memory Applications

Unipolar resistive switching behaviors of the ZnO and Al2O3/ZnO films fabricated on flexible substrates by pulse laser deposition were studied in this paper. The films were deposited at room temperature without post-annealing treatment during the process. X-ray diffraction results indicated that ZnO film has a dominant peak at (002). Scanning electron microscopy observation showed a columnar grain structure of the ZnO film to the substrate. The bilayer device of Al2O3/ZnO films had stable resistive switching behaviors with a good endurance performance of more than 200 cycles, high resistive switching ratio of over 103 at a read voltage of 0.1 V, which is better than that of the single oxide layer device of ZnO film. A possible resistive switching filamentary mode was demonstrated in this paper. The conduction mechanisms of high and low resistance states can be explained by space charge limited conduction and Ohmic’s behaviors. The endurance of the bilayer (BL) device was not degraded upon bending cycles, which indicates the potential of the flexible resistive switching random access memory applications.

Improved unipolar resistive switching characteristics of Au-doped nickel ferrite magnetic thin films for nonvolatile memory applications

Au-doped nickel ferrite (NiFe2O4-Au, NFO-Au) magnetic thin films were prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method to explore the effect of Au doping on unipolar resistive switching (RS) behavior. Experimental results reveal that 1% Au doped NFO film demonstrated considerably optimum switching parameters such as lower electroforming voltage, narrow distribution of SET/RESET voltages, good cycle-to-cycle endurance (103 cycles) and long data retention (105 s at 125 °C) as compared to NFO and Au-(0.5% and 2%) doped NFO films. The improved performance is attributed to suppressing of randomness of oxygen vacancies based filament by Au doping. Current transport conduction mechanisms are found to be Ohmic in low resistance state and Schottky emission in high resistance state. Physical model concerns for the rupture and formation of Au atoms assisted conductive filament inside the Au-doped NFO films based on the thermal effect of oxygen vacancies. Magnetic properties correlated with RS were measured and analyzed. The variation of saturation magnetization with different resistance state may be attributed to Au-doping induced different oxygen vacancies concentrations and make the conversion of valence states of Fe3+ and Fe2+ ions in octahedral sites easier. Our results provide a pathway towards the applications in nonvolatile RS memory and magnetoelectronic coupling devices.

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.

A Study on Set Process and Its Influence on Performance of Resistive Switching

In this paper, it has been found that resistive switching (RS) types are dominated by magnitude and polarities of set voltage (Vset). The difference in Vset and its influences on RS behaviors between bipolar and unipolar RS are studied. There are two types of polarity dependence of Vset in devices, which comes from the influence of defects on breakdown. The resistances of low resistance state (LRS) of bipolar are much smaller than these of unipolar RS under the same current compliance. The LRS resistances of bipolar RS show complex dependence on Vset. It reveals that the oxygen vacancy (VO) filaments properties and RS types are controlled by the intrinsic properties of breakdown and remnant defects.

Studies on dielectric, optical, magnetic, magnetic domain structure, and resistance switching characteristics of highly c-axis oriented NZFO thin films

With the rapid development of new device miniaturization technology, there is invigorated interest in magnetic nanostructures for potential application in novel multifunctional devices. In continuation to our search for a suitable magnetic material having Curie temperature (Tc) well above room temperature for multifunctional applications, we have studied the dielectric, optical, magnetic, and resistance switching characteristics of Ni0.65Zn0.35Fe2O4 (NZFO) thin films. The observation of only (004) reflection in the X-ray diffraction patterns confirms the c-axis orientation and high quality growth of NZFO thin films. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photon spectroscopy, which supports the cationic ordering-mediated large dielectric response. Our investigations reveal NZFO to be an indirect band gap material (∼1.8 eV) with a direct gap at ∼2.55 eV. These nanostructures exhibit high saturation magnetization and a low coercive field with a ferrimagnetic–paramagnetic phase transition of ∼713 K. Magnetic force microscopy studies revealed the stripe-like domain structure of the investigated thin films. In addition, these thin films exhibit reliable and repeatable unipolar resistive switching characteristics. The observed high dielectric permittivity with low loss tangent, large magnetization with soft magnetic behavior, striped magnetic domain structure and reliable resistance switching in NZFO thin films above room temperature suggest potential application in memory, spintronics, and multifunctional devices.

Resistive switching in a metal-insulator-metal device with γ-APTES as the insulator layer

Gamma-aminopropyltriethoxysilane (γ-APTES) is an organosilane material commonly used for biomedical sensing. Sensors with a γ-APTES surface layer have been reported for use in pH, DNA, and cell detection. However, no application of γ-APTES on resistive switching random access memory (RRAM) devices has yet been reported. In this paper, we report, for the first time, the resistive switching characteristics of using γ-APTES as the insulator layer in an RRAM device. The resistive switching of the γ-APTES layer embedded with ZnO nanoparticles is also investigated in this work. A unipolar resistive switching characteristic is found when the γ-APTES is employed as an insulator layer in a device with a metal-insulator-metal (MIM) structure. The stability and reliability of the resistive switching characteristics of the device can be improved after adding zinc oxide (ZnO) nanoparticles at the expense of reducing the ratio of the resistance of a high-resistance state (RHRS) to the resistance of a low-resistance state (RLRS).

Impact of ultrathin Al2O3 interlayers on resistive switching in TiOx thin films deposited by atomic layer deposition

TiOx and TiOx/Al2O3/TiOx sandwiched thin films were fabricated by using atomic layer deposition at 250 °C. As compared to single-layered TiOx, TiOx/Al2O3/TiOx sandwiched thin films exhibit stable resistive switching with a substantially improved OFF/ON ratio and elongated endurance. Essentially, the embedded ultrathin Al2O3 interlayer will suppress the crystallization of amorphous TiOx and thus reduce the channels for current leakage. As a result, the resistive switching properties are substantially enhanced. The TiOx/Al2O3/TiOx sandwiched thin films with the Al2O3 thickness of more than 2 nm show stable unipolar resistive switching, and the rupture and reformation of conductive filaments mainly occur at the top interface between Al2O3 and TiOx layers. A physical model is proposed to understand the resistive switching behaviors.