Unipolar Resistive Switching Behavior Research Articles

Isoindigo-Based Donor–Acceptor Conjugated Polymers for Air-Stable Nonvolatile Memory Devices

Nonvolatile resistive memory devices based on a new low bandgap donor-acceptor (D-A) conjugated polymer, poly((E)-6,6'-bis(2,3-dihydrothieno[3,4-b][1,4]dioxine-5-yl)-1,1'-bis(2-octyldodecyl)-[3,3'-biindolinyi-dene]-2,2'-dione) (PIDED), which are fabricated and operated in ambient air, are reported. The D-A conjugated polymer is synthesized from 2,3-dihydrothieno[3,4-b][1,4]dioxine and isoindigo as an electron donor and an electron acceptor, respectively, using CH-arylation polymerization. The devices show nonvolatile, unipolar resistive switching behaviors with a high on/off current ratio (∼104), excellent endurance cycles (>200 cycles), and a long retention time (>104 s) in ambient air. These properties remain stable in ambient air over one year, demonstrating that the device performance is significantly unaffected by exposure to air as the isoindigo has strong electron-withdrawing character and the PIDED exhibits a high degree of crystallinity. This study may pave the way for use of practical nonvolatile organic memory devices operating in ambient air.

Unipolar resistive switching behavior of amorphous gallium oxide thin films for nonvolatile memory applications

Amorphous gallium oxide thin film with heavy oxygen deficiency was deposited on Pt/Ti/SiO2/Si substrate by pulsed laser deposition in order to explore the resistive switching behavior of the Pt/Ga2O3-x/Pt sandwich structure. A well unipolar resistive switching behavior was obtained in this structure, which exhibits a high resistance ratio of OFF/ON up to 104, non-overlapping switching voltages, and excellent repeatability and retention. Both I-V relation plots of ON and OFF states and temperature dependent variation resistances indicate that the observed resistive switching behavior can be explained by the formation/rupture of conductive filaments formed out of oxygen vacancies.

Unipolar resistive switching behaviors and mechanisms in an annealed Ni/ZrO2/TaN memory device

The effects of Ni/ZrO2/TaN resistive switching memory devices without and with a 400 °C annealing process on switching properties are investigated. The devices exhibit unipolar resistive switching behaviors with low set and reset voltages because of a large amount of Ni diffusion with no reaction with ZrO2 after the annealing process, which is confirmed by ToF-SIMS and XPS analyses. A physical model based on a Ni filament is constructed to explain such phenomena. The device that undergoes the 400 °C annealing process exhibits an excellent endurance of more than 1.5 × 104 cycles. The improvement can be attributed to the enhancement of oxygen ion migration along grain boundaries, which result in less oxygen ion consumption during the reset process. The device also performs good retention up to 105 s at 150 °C. Therefore, it has great potential for high-density nonvolatile memory applications.

Unipolar resistive switching behavior of high-k ternary rare-earth oxide LaHoO3 thin films for non-volatile memory applications

ABSTRACTRare-earth oxides have attracted considerable research interest in resistive random access memories (ReRAMs) due to their compatibility with complementary metal-oxide semiconductor (CMOS) process. To this end we report unipolar resistive switching in a novel ternary rare-earth oxide LaHoO3 (LHO) to accelerate progress and to support advances in this emerging densely scalable research architecture. Amorphous thin films of LHO were fabricated on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition, followed by sputter deposition of platinum top electrode through shadow mask in order to elucidate the resistive switching behavior of the resulting Pt/LHO/Pt metal-insulator-metal (MIM) device structure. Stable unipolar resistive switching characteristics with interesting switching parameters like, high resistance ratio of about 105 between high resistance state (HRS) and low resistance state (LRS), non-overlapping switching voltages with narrow dispersion, and excellent retention and endurance features were observed in Pt/LHO/Pt device structure. The observed resistive switching in LHO was explained by the formation/rupture of conductive filaments formed out of oxygen vacancies and metallic Ho atom. From the current-voltage characteristics of Pt/LHO/Pt structure, the conduction mechanism in LRS and HRS was found to be dominated by Ohm’s law and Poole-Frenkel emission, respectively.

Resistance switching mode transformation in SrRuO3/Cr-doped SrZrO3/Pt frameworks via a thermally activated Ti out-diffusion process.

This work reports on a mechanism for irreversible resistive switching (RS) transformation from bipolar to unipolar RS behavior in SrRuO3 (SRO)/Cr-doped SrZrO3 (SZO:Cr)/Pt capacitor structures prepared on a Ti/SiO2/Si substrate. Counter-clockwise bipolar RS memory current-voltage (I–V) characteristics are observed within the RS voltage window of −2.5 to +1.9 V, with good endurance and retention properties. As the bias voltage increases further beyond 4 V under a forward bias, a forming process occurs resulting in irreversible RS mode transformation from bipolar to unipolar mode. This switching mode transformation is a direct consequence of thermally activated Ti out-diffusion from a Ti adhesion layer. Transition metal Ti effectively out-diffuses through the loose Pt electrode layer at high substrate temperatures, leading to the unintended formation of a thin titanium oxide (TiOx where x < 2) layer between the Pt electrode and the SZO:Cr layer as well as additional Ti atoms in the SZO:Cr layer. Cross-sectional scanning electron microscopy, transmission electron microscopy and Auger electron spectroscopy depth-profile measurements provided apparent evidence of the Ti out-diffusion phenomenon. We propose that the out-diffusion-induced additional Ti atoms in the SZO:Cr layer contributes to the creation of the metallic filamentary channels.

Resistive switching characteristics of HfO2-based memory devices on flexible plastics.

In this study, we examine the characteristics of HfO2-based resistive switching random access memory (ReRAM) devices on flexible plastics. The Pt/HfO2/Au ReRAM devices exhibit the unipolar resistive switching behaviors caused by the conducting filaments. From the Auger depth profiles of the HfO2 thin film, it is confirmed that the relatively lower oxygen content in the interface of the bottom electrode is responsible for the resistive switching by oxygen vacancies. And the unipolar resistive switching behaviors are analyzed from the C-V characteristics in which negative and positive capacitances are measured in the low-resistance state and the high-resistance state, respectively. The devices have a high on/off ratio of 10(4) and the excellent retention properties even after a continuous bending test of two thousand cycles. The correlation between the device size and the memory characteristics is investigated as well. A relatively smaller-sized device having a higher on/off ratio operates at a higher voltage than a relatively larger-sized device.

Effect of oxidizable electrode material on resistive switching characteristics of ZnO(x)S(1-x) films.

We investigate the memory characteristics of ZnO(x(S(1-x) based resistive switching random access memory (ReRAM) devices with Al and Pt bottom electrodes (BEs). Both the ReRAM devices with Al and Pt BEs exhibit unipolar resistive switching behaviors, regardless of the materials of the BEs. The ratios of the high resistance state (HRS) to the low resistance state (LRS) of the Au/annealed ZnO(x)S(1-x)/Al and the Au/annealed ZnO(x)S(1-x)/Pt devices are more than 10(6) and 10(4), respectively. The HRS depends more significantly on the material of the BE than the LRS. The resistance in the HRS of the device with the Al BE is more stable in the endurance characteristics and higher in magnitude than that of the device with the Pt BE. For an anealed ZnO(x)S(1-x)/Al film, the oxygen signal in the auger depth profile shows the formation of an AIO(x) layer at the interface between the annealed ZnO(x)S(1-x) layer and the Al BE. The difference between the memory characteristics of the annealed ZnO(x)S(1-x) devices with the Al and Pt BEs is explained with the presence or absence of the oxidized layers formed in the interfaces between the annealed ZnO(x)S(1-x) films and the BEs.

Nanocrystalline Si pathway induced unipolar resistive switching behavior from annealed Si-rich SiNx/SiNy multilayers

Adding a resistive switching functionality to a silicon microelectronic chip is a new challenge in materials research. Here, we demonstrate that unipolar and electrode-independent resistive switching effects can be realized in the annealed Si-rich SiNx/SiNy multilayers with high on/off ratio of 109. High resolution transmission electron microscopy reveals that for the high resistance state broken pathways composed of discrete nanocrystalline silicon (nc-Si) exist in the Si nitride multilayers. While for the low resistance state the discrete nc-Si regions is connected, forming continuous nc-Si pathways. Based on the analysis of the temperature dependent I-V characteristics and HRTEM photos, we found that the break-and-bridge evolution of nc-Si pathway is the origin of resistive switching memory behavior. Our findings provide insights into the mechanism of the resistive switching behavior in nc-Si films, opening a way for it to be utilized as a material in Si-based memories.

Lateral photovoltaic effect co-observed with unipolar resistive switching behavior in Cu-doped ZnO film

Unipolar resistive switching (RS) behavior was first observed in Cu-doped ZnO film based on ZnO/SiO2/Si structure, which was a novel phenomenon as memory films grown on Si substrate usually showed a bipolar one. The results demonstrate Cu-doped ZnO a new candidate for memory material. By introducing an external electric-field before the sweeping process, we have verified that the RS behavior was a localized effect. The non-linear I-V character, which suggested a junction of the proposed Cu-doped ZnO/SiO2/Si structure, leads to the lateral photovoltaic effect (LPE) investigation. In photovoltaic mode, which is the simplest configuration, the position sensitivity of lateral photovoltage observed on Cu-doped ZnO film achieves 24.82 mV/mm and the nonlinearity is within 9.95%, indicating that Cu-doped ZnO could serve as a LPE material directly. The dual effects accommodate functions of detector and memristor in the same structure and make Cu-doped ZnO a competitive material for advanced multi-functional device.

Unipolar resistive switching behavior of amorphous YCrO3 films for nonvolatile memory applications

Amorphous YCrO3 (YCO) films were prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition in order to investigate resistive switching (RS) phenomenon. The Pt/YCO/Pt device showed stable unipolar RS with resistance ratio of ∼105 between low and high resistance states, excellent endurance and retention characteristics, as well as, non-overlapping switching voltages with narrow dispersions. Based on the x-ray photoelectron spectroscopy and temperature dependent switching characteristics, observed RS was mainly ascribed to the oxygen vacancies. Moreover, current-voltage characteristics of the device in low and high resistance states were described by Ohmic and trap controlled space–charge limited conduction mechanisms, respectively.

Unipolar resistive switching of ZnO-single-wire memristors.

Well unipolar resistive switching (RS) behaviors were observed from Ag/ZnO single-microwire/Ag memristors. The reset voltages were larger than the set voltages, and all of them were less than 1 V. The resistance ratios of high-resistance state (HRS) to low-resistance state (LRS) reached 103. The bistable RS behaviors were entirely reversible and steady within 100 cycles. It was found that the dominant conduction mechanisms in LRS and HRS were ohmic behavior and space-charge-limited current (SCLC), respectively.

Resistive Switching Behaviors of Sol‐Gel‐Derived MgNb2O6 Thin Films on ITO/glass Substrate

In this study, transparent amorphous MgNb2O6 (MNO) films were fabricated via the sol‐gel method to form an Al/MNO/indium tin oxide/glass structure. The resistive switching (RS) behavior of the devices was investigated. From the DC voltage sweep test, the air‐annealed MNO samples exhibited stable and reproducible bipolar resistive switching (BRS) behavior; however, the samples annealed in an O2‐rich environment showed no RS property. These results suggest that the RS behavior of the MNO memory devices is highly related to the oxygen vacancy concentration and distribution within the MNO films. In addition, forming‐free unipolar resistive switching (URS) behavior was observed when the MNO films were annealed under an N2H2 atmosphere. In order to determine the origin of the BRS and URS behaviors, cross‐sectional high‐resolution transmission electron microscopy images of the MNO samples were acquired. The RS behavior of the MNO films can be ascribed to the release and recombination of electrons and oxygen vacancies.

Confinement of ferroelectric domain-wall motion at artificially formed conducting-nanofilaments in epitaxial BiFeO3 thin films.

We report confinement of ferroelectric domain-wall motion at conducting-nanofilament wall in epitaxial BiFeO3 thin film on Nb-doped SrTiO3 substrate. The BiFeO3 film exhibited well-defined ferroelectric response and unipolar resistive switching behavior. We artificially formed conducting-nanofilaments in the BiFeO3 via conducting atomic force microscope techniques. The conducting-nanofilament wall, which does not possess any ferroelectric polarization, is then able to block domain propagation. Consequently, we demonstrate that the domain-wall motion is effectively confined within the conducting-nanofilament wall during polarization switching. This significant new insight potentially gives an opportunity for the artificial manipulation of nanoscale ferroelectric domain.

Scaling-down characteristics of nanoscale diamond-like carbon based resistive switching memories

A nonvolatile resistive switching random access memory (RRAM) device based on the diamond-like carbon (DLC) films and the inert metal electrodes was demonstrated. A typical unipolar resistive switching (RS) behavior without high voltage “forming process” is observed. It exhibits good scaling-down properties when negligible dependence upon the cell area is observed for VSET and IRESET decreases with the reduction of the cell area, which is suitable for practical nonvolatile memory applications. Investigations on the electron transport characteristics at HRS and LRS indicate that Frenkel–Poole emission and Ohmic Laws dominate the LRS and HRS states, respectively. Based on the conduction mechanism studies, the RS behavior is found to arise from the formation and rupture of conductive sp2-like graphitic filaments originating from the connection of conductive sp2-like carbon bonds in the predominantly sp3-like insulating carbon matrix through the electric field induced dielectric breakdown process and thermal fuse effects.

Negative Differential Resistance Behavior and Memory Effect in Laterally Bridged ZnO Nanorods Grown by Hydrothermal Method

A novel memory device based on laterally bridged ZnO nanorods (NRs) in the opposite direction was fabricated by the hydrothermal growth method and characterized. The electrodes were defined by a simple photolithography method. This method has lower cost, simpler process, and higher reliability than the traditional focused ion beam lithography method. For the first time, the negative differential resistance and bistable unipolar resistive switching (RS) behavior in the current-voltage curve was observed at room temperature. The memory device is stable and rewritable; it has an ultra-low current level of about 1 × 10(-13) A in the high resistance state; and it is nonvolatile with an on-off current ratio of up to 1.56 × 10(6). Moreover, its peak-to-valley current ratio of negative differential resistance behavior is greater than 1.76 × 10(2). The negative differential resistance and RS behavior of this device may be related to the boundaries between the opposite bridged ZnO NRs. Specifically, the RS behavior found in ZnO NR devices with a remarkable isolated boundary at the NR/NR interface was discussed for the first time. The memory mechanism of laterally bridged ZnO NR-based devices has not been discussed in the literature yet. In this work, results show that laterally bridged ZnO NR-based devices may have next-generation resistive memories and nanoelectronic applications.

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.

Improvement of Unipolar Resistive Switching Characteristics in Ti Embedded ZrO&lt;sub&gt;2&lt;/sub&gt; Thin Film

Due to the electrical and physical analyses, the robust negative voltage unipolar resistive switching behaviors in Ti/ZrO2/Ti/ZrO2/Pt device have been investigated. The Ti diffusion in ZrO2 film can generate more amounts of oxygen vacancies in it, which leads the lower value and narrower variation in forming voltage. Moreover, the good performances with more than 1000 switching cycles and long retention test can be achieved in the present device.

Resistive switching characteristics of ferroelectric BiFeO3 nanodot prepared by dip-pen nanolithography

We report the ferroelectric response and resistive switching characteristics of a BiFeO3 nanodot on a Nb-doped SrTiO3 substrate, prepared by elaborately controllable dip-pen nanolithography (DPN) process. By performing piezoresponse force microscopy (PFM) measurements, it is confirmed that the BiFeO3 nanodot exhibited good ferroelectric response and switching properties. More interestingly, based on current–voltage characterizations using a conducting atomic force microscope (CAFM) technique, we observed unipolar resistive switching behavior in the BiFeO3 nanodots. In addition, the bistable resistive switching characteristics were found to be reproducible with increasing the switching cycle up to 200 cycles, suggesting the future applicability for next generation RRAM memory devices.

Unipolar resistive switching properties of amorphous Pr0.7Ca0.3MnO3 films grown on a Pt/Ti/SiO2/Si substrate

Amorphous Pr0.7Ca0.3MnO3 (APCMO) films were grown on a Pt/Ti/SiO2/Si (Pt–Si) substrate at temperatures below 500 °C and the Pt/APCMO/Pt–Si device showed unipolar resistive switching behavior. Conduction behavior of the low resistance state (LRS) of the Pt/APCMO/Pt–Si device followed Ohm's law, and the resistance in LRS was independent of the size of the device, indicating that the conduction behavior in LRS can be explained by the presence of the conductive filaments. On the other hand, the resistance in the high resistance state (HRS) decreased with increasing the device size, and the conduction mechanism in the HRS was explained by Schottky emission.

Retention behaviour of graphene oxide resistive switching memory

Graphene oxide (GO) has recently been proposed as one of the potential resistive switching materials for future nonvolatile memory applications. In this work, we present a flexible resistive memory device with the structure of aluminium (Al)/GO/ indium tin oxide (ITO) fabricated at room temperature. The Al/GO/ITO devices show unipolar resistive switching behaviour with resistance ratio to over 30, and SET and RESET voltages for 5 V and 3 V, respectively. In addition, the Al/GO/ITO flexible memory can sustain over 250 cycling without any resistance window closure. However, the retention failures after 5000 s were observed owing to the sharp resistance increase of LRS, which is first observed in GO based resistive memory. The retention behaviour is examined intensively through various READ operations, including changing read interval time, read voltage and environment temperature. Supported by the results of these electrical measurements, it has been proved that NEM (Nano-Electromechanical) effects are responsible for the resistive switching and retention failure. The stacked GO flakes in the film joined to each other due to the electrostatic attraction and lead the device to LRS while separated by voltage-induced break and drive the device to HRS. Therefore, short read interval time, large read voltage and high temperature can help hold LRS resistance.