Ratio Of High Resistance State Research Articles

Fabrication of ZnO-encapsulated porous alumina structure utilizing a two-step anodizing technique for memristor applications

Highly ordered porous alumina thin films with various aspect ratios were fabricated by a two-step anodizing technique. The dependence of the high resistance state to low resistance state ratio on the aspect ratio of the pores was observed in the memristors utilizing porous alumina. The reduction of threshold voltage and improvement in stability were achieved by properly encapsulating i-ZnO inside the porous alumina as the switching medium. A significant improvement in the ratio of high resistance state to low resistance state was achieved by i-ZnO encapsulation in porous alumina compared to the conventional planer switching layer of ZnO alone.

Enhanced resistive switching properties of HfAlOx/ZrO2- based RRAM devices

Resistive Random-Access Memory (RRAM) devices are recognized as potential candidates for next-generation memory devices, due to their smallest cell size, high write/erase speed, and endurance. Particularly, the resistive switching (RS) characteristics in oxide materials have offered new opportunities for developing CMOS-compatible high-density RRAM devices. In this study, the RS behavior of HfAlOx/ZrO2 thin films sandwiched structure between TiN bottom electrode and Au top electrodes were investigated. It was found that Au/HfAlOx/ZrO2/TiN stacks were superior in terms of RS performance when compare to Au/HfAlOx/TiN memory stacks. The devices demonstrated a good resistance ratio of high resistance state and low resistance state ∼103 for Au/HfAlOx/TiN and ∼105 for Au/HfAlOx/ZrO2/TiN stacks, respectively. Both stacks showed good retention characteristics (>104 ​s) and endurance (>103 cycles). The experimental current-voltage characteristics fitted with different conducting mechanisms, the linear lower bias region is dominated by ohmic conductivity, whereas the non-linear higher bias region was dominated by space-charge limited current conduction mechanism.

Nitrogen-doped titanium dioxide nanorod array memristors with synaptic features and tunable memory lifetime for neuromorphic computing

Titanium dioxide memristors are promising next generation devices for neuromorphic computing, which can mimic synapses of human brain. With respect to meet satisfactory performance, gradual conductance changes and stability of cycling are essential. In this work, nitrogen-doped titanium dioxide nanorod arrays (N-TiO2 NARs) memristors with high performance have been fabricated using a hydrothermal process. The X-ray photo electron spectra confirm the existence of Ti‒N bonds, with uniform distribution of nitrogen confirmed by elemental mapping. The bidirectional gradually changing conductance and endurance of over 8000 cycles have been achieved. The ratio of high resistance state (HRS) and low resistance state (LRS) exceeds 16. The synaptic features, such as potentiation, depression, paired pulse facilitation (PPF), experience-learning and STDP have been achieved using continuous sweeping voltage and pulse trains respectively. It is possible to emulate the learning processes by stimulating the memristor using pulses of different amplitude, duration and interval. By manipulating the stimulation pulse duration, voltage and current compliance, memory lifetimes (MLTs) can be tuned from seconds to tens of thousands of seconds. These transitional processes are associated with anion vacancy creation/accumulation and diffusion. Those synaptic features and tunable MLTs provide a promising approach for neuromorphic computing.

Ceria thin film memristive device by magnetron sputtering method

CeO2-x nanocrystal (NC) thin film memristive devices are fabricated by magnetron sputtering method. The influence of defect formation and carrier transport properties on memristive devices performances are characterized. The length of nanocrystal CeO2-x grain is about 8–12 nm and the diameter is about 6–8 nm with abundant grain boundaries (GBs). The memristive device exhibits an electronic conductivity. Under the forward bias, electrons accumulate in the vicinity of GBs due to space electrons zone effect and hop among oxygen vacancies to form the conductive filament (CF). The set voltage and reset voltage ranges from ~1.8 to 6 V and from ~-0.3 to -1 V, respectively. The cross-point memory shows an appropriate resistance ratio of high resistance state (HRS) to low resistance state (LRS) is about 104. The retention times of the devices can be more than 104 S, exhibiting a stable performance in terms of information storage capability.

Electric field-induced resistive switching, magnetism, and photoresponse modulation in a Pt/Co0.03Zn0.97O/Nb:SrTiO3 multi-function heterostructure

A Co0.03Zn0.97O (CZO) thin film was epitaxially grown on a Nb doped (001) SrTiO3 (NSTO) single-crystal substrate by pulsed laser deposition to form a Pt/CZO/NSTO heterostructure. This device exhibits stable bipolar resistive switching, well retention and endurance, multilevel memories, and a resistance ratio of high resistance state (HRS)/low resistance state (LRS) up to 7 × 105. Under the illumination of a 405 nm laser, the HRS of the device showed distinct photoelectricity with an open-circuit voltage of 0.5 V. A stronger ferromagnetism was observed at the HRS than at the LRS. The above phenomenon is attributable to the accumulation and migration of oxygen vacancies at the interface of CZO/NSTO. Our results demonstrated a pathway towards making multifunctional devices that simultaneously exhibit resistive switching, photoelectricity, and ferromagnetism.

Valence Change Bipolar Resistive Switching Accompanied With Magnetization Switching in CoFe2O4 Thin Film

Resistive Switching in oxides has offered new opportunities for developing resistive random access memory (ReRAM) devices. Here we demonstrated bipolar Resistive Switching along with magnetization switching of cobalt ferrite (CFO) thin film using Al/CFO/FTO sandwich structure, which makes it a potential candidate for developing future multifunctional memory devices. The device shows good retention characteristic time (>104 seconds) and endurance performance, a good resistance ratio of high resistance state (HRS) and low resistance state (LRS) ~103. Nearly constant resistance values in LRS and HRS confirm the stability and non-volatile nature of the device. The device shows different conduction mechanisms in the HRS and LRS i.e. Schottky, Poole Frenkel and Ohmic. Magnetization of the device is also modulated by applied electric field which has been attributed to the oxygen vacancies formed/annihilated during the voltage sweep and indicates the presence of valence change mechanism (VCM) in our device. It is suggested that push/pull of oxygen ions from oxygen diffusion layer during voltage sweep is responsible for forming/rupture of oxygen vacancies conducting channels, leading to switching between LRS and HRS and for switching in magnetization in CFO thin film. Presence of VCM in our device was confirmed by X-ray Photoelectron Spectroscopy at Al/CFO interface.

Fabrication and properties of Ag/Mg0.2Zn0.80/La0.67Ca0.33MnO/p+-Si resistive switching heterostructure devices

Mg0.2Zn0.8O (MZO)/La0.67Ca0.33MnO (LCMO) heterostructure was deposited on p+-Si substrates by sol-gel spin coating technique. The Ag/MZO/LCMO/p+-Si devices exhibit a bipolar, reversible, and remarkable current-voltage characteristic at room temperature. An obvious multilevel resistive switching effect is observed in the devices. The dominant conduction mechanism of the devices is trap-controlled space charge limited current. The resistance ratio of high resistance state and low resistance state of the devices is about six orders of magnitude, and the degradation is invisible in the devices after 250 successive switching cycles. The present results suggest that the Ag/MZO/LCMO/p+-Si devices may be a potential and multilevel candidate for nonvolatile memory application.

Bipolar resistive switching and its temperature dependence in the composite structure of BiFeO3 bilayer

In order to demonstrate the control of BiFeO3 thin film on the resistive switching effect and achieve the high-performance resistive switching device, the single layers and bilayer have been fabricated by chemical solution deposition method, respectively. In comparison with the single films, the composite film exhibits great performance of the resistive switching in endurance and repeatability, high stability and resistance ratio of high resistance state to low resistance state. Resistive switching effect in the BiFeO3 composite structure demonstrates an effective way to improve the endurance and repeatability of the resistive switching characteristics by designing the relative devices.

Conduction Mechanisms in Resistance Switching Memory Devices Using Transparent Boron Doped Zinc Oxide Films.

In this work, metal/oxide/metal capacitors were fabricated and investigated using transparent boron doped zinc oxide (ZnO:B) films for resistance switching memory applications. The optical band gap of ZnO:B films was determined to be about 3.26 eV and the average value of transmittance of ZnO:B films was about 91% in the visible light region. Experimental results indicated that the resistance switching in the W/ZnO:B/W structure is nonpolar. The resistance ratio of high resistance state (HRS) to low resistance state (LRS) is about of the order of 105 at room temperature. According to the temperature dependence of current-voltage characteristics, the conduction mechanism in ZnO:B films is dominated by hopping conduction and Ohmic conduction in HRS and LRS, respectively. Therefore, trap spacing (1.2 nm) and trap energy levels in ZnO:B films could be obtained.

A Metal Oxide Heterostructure for Resistive Random Access Memory Devices

We propose and investigate a metal oxide heterostructure (MOH) based resistive switching (RS) device. The driving mechanism of resistive switching (RS) in an MOH is more directly related to oxygen ion/vacancy migration around their interface. The performance of an MOH-based RS device depends on the oxygen mobility, oxygen vacancy concentration as well as its relation to the resistivity. An enhanced ratio of high resistance state to low resistance state can be achieved if the two involved metal oxides are mutually complemental metal oxides in which one of them has larger resistivity with increasing concentration of vacancy while the other one is the reverse.

Effect of annealing temperature on resistance switching behavior of Mg0.2Zn0.8O thin films deposited on ITO glass

Mg0.2Zn0.8O thin films for resistance random access memory application were prepared on ITO glass substrates by sol–gel method, and the effect of annealing temperature on resistance switching behavior has been studied. With the increase of annealing temperature, the crystallinity of Mg0.2Zn0.8O thin films can be improved, the turn on voltage, the turn off voltage, the resistance of low resistance state and the resistance ratio of high resistance state to low resistance state showed an increasing, but the endure of switching cycles decreased and the resistance of high resistance state exhibited a degradation.

Conduction mechanism of resistive switching films in MgO memory devices

In this work, nonpolar resistance switching behavior was demonstrated in Pt/MgO/Pt structure. The resistance ratio of high resistance state (HRS) and low resistance state (LRS) is about on the order of 105 for the compliance current (Icomp) of 1 mA at 300 K. Using enough Icomp (≥0.5 mA) during SET processes, the LRS resistances reach a minimum of about 102–103 Ω and the RESET currents reach a maximum of about 10−4–10−3 A. Experimental results indicate that the conduction mechanism in MgO films is dominated by the hopping conduction and the Ohmic conduction in HRS and LRS, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in MgO films were obtained.

Resistive switching effect for ZnO hybrid memory with metal-oxide nanocrystals

ZnO memory device with SnO2 nanocrystals embedded in a biphenyl-tertracarboxylic dianhydride–phenylen diamine polyimide layer was fabricated, and its electrical properties were evaluated. The complementary resistive switching effects with a current bistability appeared during voltage sweeping in the range of ±4V and ±5V, respectively. This switching effect of current–voltage may be originated from a resistance fluctuation due to the charge trapping into SnO2 nanocrystals. In the bipolar resistance switching behavior, the ratio of high-resistance state (HRS) and low-resistance state (LRS) currents was about 4.4×104 at 1V. The data retention of LRS/HRS currents was maintained about 2.2×103 after 103s.

Effects of ZnO buffer layer on GZO RRAM devices

Ag/GZO/ZnO/Pt structure resistive switching devices were fabricated by radio frequency (RF) magnetron sputtering, in which ZnO was used as a buffer layer. These devices have large ratio of high resistance state (HRS) to low resistance state (LRS), which is 2×103. The storage time measurement indicates that these devices have an excellent data retention characteristic. Moreover, the operation voltages are very low, which is 0.4V (ON state) and −0.35/−0.55V (OFF state). The electroforming process in initial state was not needed, and multistep reset process was found.

Low power consumption bipolar resistive switching characteristics of ZnO-based memory devices

Ag/ZnO/Pt structure resistive switching devices are fabricated by radio frequency (RF) magnetron sputtering at room temperature. The memory devices exhibit stable and reversible resistive switching behavior. The ratio of high resistance state to low resistance state can reach as high as 102. The retention measurement indicates that the memory property of these devices can be maintained for a long time (over 104 s under 0.1-V durable stress). Moreover, the operation voltages are very low, -0.4 V (OFF state) and 0.8 V (ON state). A high-voltage forming process is not required in the initial state, and multi-step reset process is demonstrated. Resistive switching device with the Ag/ZnO/ITO structure is constructed for comparison with the Ag/ZnO/Pt device.

Nonvolatile bipolar resistance switching effects in multiferroic BiFeO3 thin films on LaNiO3-electrodized Si substrates

We report on reversible bipolar resistance switching effects in multiferroic BiFeO3 thin films without electroforming. The BiFeO3 thin films with (110) preferential orientation were prepared on LaNiO3-electrodized Si substrates with a Pt/BiFeO3/LaNiO3 device configuration. The resistance ratio of high resistance state (HRS) to low resistance state (LRS) of the devices was as high as three orders of magnitude. The dominant conduction mechanisms of LRS and HRS were dominated by ohmic behavior and trap-controlled space charge limited current, respectively. The resistance switching mechanism of the devices was discussed using a modified Schottky-like barrier model taking into account the movement of oxygen vacancies.

Colossal resistance switching effect in Pt/spinel-MgZnO/Pt devices for nonvolatile memory applications

We reported the discovery of colossal resistance switching effect in polycrystalline spinel-like structure MgZnO thin films with high Mg contents sandwiched by Pt electrodes. The ultrahigh resistance ratio of high resistance state to low resistance state of about seven to nine orders of magnitude with a low reset voltage of less than 1 V was obtained in this thin film system. The resistance ratio shows an increase of several orders of magnitude compared with those of previously reported resistance switching material systems including metal oxides, semiconductors, and organic molecules. This colossal resistance switching effect will greatly improve the signal-to-noise ratio and simplify the process of reading memory state for nonvolatile memory applications. Our study also provides a material base for studying the origin of resistance switching phenomenon.

HfOx Thin Films for Resistive Memory Device by Use of Atomic Layer Deposition

AbstractThe materials properties and resistance switching characteristics of hafnium oxide-based binary oxide were investigated for next generation memory device application. A nonstoichometric hafnium oxide (HfOx) film with a mixture structure of monoclinic and tetragonal phase and some metallic Hf-Hf bonds on TiN/Si were prepared by atomic layer chemical vapor deposition (ALCVD). Resistance random access memory devices consisting of Pt/HfOx/TiN/Si with low power operation (< 0.4 mW) and reset current (< 100 mA) were fabricated. The resistance ratio of high resistance state to low resistance state maintains 100∼1000 and after 1000 cycles of repetitively switching. A 1-nm-thick Al2O3 film in the interface between top electrode and HfOx films, the Pt/Al2O3/HfOx/TiN/Si memory devices were found that soft-error of set and reset process often occurred. Interface states in the anode side play an important role in maintaining a stable resistive switching for HfOx-based memory devices.

Colossal electroresistance mechanism in aAu∕Pr0.7Ca0.3MnO3∕Ptsandwich structure: Evidence for a Mott transition

The resistive switching characteristics of $\mathrm{Au}∕{\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}(\mathrm{PCMO})∕\mathrm{Pt}$ sandwich structure were investigated by changing growth temperature of the PCMO film, adding an oxygen annealing process, and modifying the Au/PCMO/Pt sandwich structure by inserting a $\mathrm{Pr}\mathrm{Mn}{\mathrm{O}}_{3}$ (PMO) or $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_{3}$ (CMO) layer at the Au/PCMO interface. From these experiments, we obtained the following results. First, only crystalline PCMO films exhibited reversible resistive switching behavior in Au/PCMO/Pt sandwich structure. Secondly, the ${\mathrm{Mn}}^{4+}∕{\mathrm{Mn}}^{3+}$ ratio at the PCMO surface was changed by oxygen annealing of the PCMO film, resulting in an increase of the resistance ratio of high resistance state and low resistance state. Lastly, we could not observe the resistive switching behavior in Au/PMO/PCMO/Pt and Au/CMO/PCMO/Pt sandwich structures. The resistive switching behavior could be observed only in Au/PCMO/PMO(or CMO)/PCMO/Pt sandwich structure. These results indicate that the resistive switching of Au/PCMO/Pt sandwich structure depends on the mixed valence state ${\mathrm{Mn}}^{4+}∕{\mathrm{Mn}}^{3+}$ of Mn ions in the metal/PCMO interface domains. This result can be regarded as evidence for a Mott transition.

Effect of oxygen annealing on Pr0.7Ca0.3MnO3 thin film for colossal electroresistance at room temperature

We studied an appropriate annealing condition of Pr0.7Ca0.3MnO3 (PCMO) thin film that can show good resistive switching characteristics for resistance random access memory applications and also tried to elucidate the mechanism of the resistive switching of PCMO thin film at room temperature. We could observe that crystal structure of PCMO film was changed by oxygen annealing. From x-ray photoelectron spectroscopy measurements, we can conclude that the excess of oxygen by oxygen annealing of PCMO film leads to an increase of Mn4+ content at the PCMO surface with a subsequent change in the Mn4+∕Mn3+ ratio at the PCMO surface. The resistance ratio of high resistance state and low resistance state was increased by oxygen annealing of PCMO thin film. This can be explained as follows. The change of the Mn4+∕Mn3+ ratio at the PCMO surface by oxygen annealing leads to the change of characteristics of Au-PCMO interface domain and therefore results in the change of the resistance ratio.