Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaOx interface

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >10⁵ cycles, and good data retention of >10⁴ s with a good resistance ratio of >10² at 85°C are obtained using a Ti nanolayer to form a W/TiO_x/TaO_x/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaO_x/W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm² is observed by transmission electron microscopy. The presence of oxygen-deficient TaO_x nanofilament in a W/TiO_x/TaO_x/W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaO_x interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.².     

Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaOx interface

Excellent resistive switching memory characteristics were demonstrated for an Al/Cu/Ti/TaO_x/W structure with a Ti nanolayer at the Cu/TaO_x interface under low voltage operation of ± 1.5 V and a range of current compliances (CCs) from 0.1 to 500 μA. Oxygen accumulation at the Ti nanolayer and formation of a defective high-κ TaO_x film were confirmed by high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy. The resistive switching memory characteristics of the Al/Cu/Ti/TaO_x/W structure, such as HRS/LRS (approximately 10⁴), stable switching cycle stability (>10⁶) and multi-level operation, were improved compared with those of Al/Cu/TaO_x/W devices. These results were attributed to the control of Cu migration/dissolution by the insertion of a Ti nanolayer at the Cu/TaO_x interface. In contrast, CuO_x formation at the Cu/TaO_x interface was observed in an Al/Cu/TaO_x/W structure, which hindered dissolution of the Cu filament and resulted in a small resistance ratio of approximately 10 at a CC of 500 μA. A high charge-trapping density of 6.9 × 10¹⁶ /cm² was observed in the Al/Cu/Ti/TaO_x/W structure from capacitance-voltage hysteresis characteristics, indicating the migration of Cu ions through defect sites. The switching mechanism was successfully explained for structures with and without the Ti nanolayer. By using a new approach, the nanoscale diameter of Cu filament decreased from 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 μA, resulting in a large memory size of 7.6 T to 28 Pbit/sq in. Extrapolated 10-year data retention of the Ti nanolayer device was also obtained. The findings of this study will not only improve resistive switching memory performance but also aid future design of nanoscale nonvolatile memory.     

Self-compliance-improved resistive switching using Ir/TaOx/W cross-point memory

Resistive switching properties of a self-compliance resistive random access memory device in cross-point architecture with a simple stack structure of Ir/TaO _x /W have been investigated. A transmission electron microscope and atomic force microscope were used to observe the film properties and morphology of the stack. The device has shown excellent switching cycle uniformity with a small operation of ±2.5 V and a resistance ratio of >100. The device requires neither any frorming-process nor current compliance limit for repeatable operation in contrast to conventional resistive random access memory devices. The effect of bottom electrode morphology and surface roughness is also studied. The improvement is due to the enhanced electric field at the nanotips in the bottom electrode and the defective TaO _x switching layer which enable controlled filament formation/rupture. The device area dependence of the low resistance state indicates multifilament formation. The device has shown a robust alternating current endurance of >10⁵ cycles and a data retention of >10⁴ s.     

Enhanced resistive switching phenomena using low-positive-voltage format and self-compliance IrOx/GdOx/W cross-point memories

Enhanced resistive switching phenomena of IrO_x/GdO_x/W cross-point memory devices have been observed as compared to the via-hole devices. The as-deposited Gd₂O₃ films with a thickness of approximately 15 nm show polycrystalline that is observed using high-resolution transmission electron microscope. Via-hole memory device shows bipolar resistive switching phenomena with a large formation voltage of -6.4 V and high operation current of >1 mA, while the cross-point memory device shows also bipolar resistive switching with low-voltage format of +2 V and self-compliance operation current of <300 μA. Switching mechanism is based on the formation and rupture of conducting filament at the IrO_x/GdO_x interface, owing to oxygen ion migration. The oxygen-rich GdO_x layer formation at the IrO_x/GdO_x interface will also help control the resistive switching characteristics. This cross-point memory device has also Repeatable 100 DC switching cycles, narrow distribution of LRS/HRS, excellent pulse endurance of >10,000 in every cycle, and good data retention of >10⁴ s. This memory device has great potential for future nanoscale high-density non-volatile memory applications.     

Comparison of resistive switching characteristics using copper and aluminum electrodes on GeOx/W cross-point memories

Comparison of resistive switching memory characteristics using copper (Cu) and aluminum (Al) electrodes on GeO_x/W cross-points has been reported under low current compliances (CCs) of 1 nA to 50 μA. The cross-point memory devices are observed by high-resolution transmission electron microscopy (HRTEM). Improved memory characteristics are observed for the Cu/GeO_x/W structures as compared to the Al/GeO_x/W cross-points owing to AlO_x formation at the Al/GeO_x interface. The RESET current increases with the increase of the CCs varying from 1 nA to 50 μA for the Cu electrode devices, while the RESET current is high (>1 mA) and independent of CCs varying from 1 nA to 500 μA for the Al electrode devices. An extra formation voltage is needed for the Al/GeO_x/W devices, while a low operation voltage of ±2 V is needed for the Cu/GeO_x/W cross-point devices. Repeatable bipolar resistive switching characteristics of the Cu/GeO_x/W cross-point memory devices are observed with CC varying from 1 nA to 50 μA, and unipolar resistive switching is observed with CC >100 μA. High resistance ratios of 10² to 10⁴ for the bipolar mode (CCs of 1 nA to 50 μA) and approximately 10⁸ for the unipolar mode are obtained for the Cu/GeO_x/W cross-points. In addition, repeatable switching cycles and data retention of 10³ s are observed under a low current of 1 nA for future low-power, high-density, nonvolatile, nanoscale memory applications.     

Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films

ABSTRACT: In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified.     

Studies on resistive switching characteristics of aluminum/graphene oxide/semiconductor nonvolatile memory cells

We report semiconductor-based resistive switching nonvolatile memory devices with graphene oxide (GO) as an active layer which is sandwiched between aluminum (Al) metal and semiconductors such as Si and Ge. Semiconductors (p-Si or p-Ge) are used as bottom electrodes on which a layer of GO is deposited and Al electrodes are then formed on the top of it by thermal evaporation. From current–voltage characteristics, it is found that the devices show diode like rectifying switching behavior, which can suppress the cross talk between adjacent cells. In these structures, during initial voltage biasing, the current conduction is found to be due to thermionic emission and in later stages, it is driven by space charge. The maximum on/off ratio in Al/GO/p-Si and Al/GO/p-Ge structures is 110 (at −1.2 V) and 76 (at −1.7 V), respectively. However, breakdown occurs in the memory cells fabricated on p-Ge after switching to low resistance state due to lack of stable oxide at the interface between Ge and GO unlike in the cells on Si where stable native SiO₂ prevents such breakdown. The mechanism of resistive switching in semiconductor based memory cells has been explained using X-ray photoelectron spectroscopy and capacitance–voltage characteristics.     

Formation polarity dependent improved resistive switching memory characteristics using nanoscale (1.3 nm) core-shell IrOx nano-dots

ABSTRACT: Improved resistive switching memory characteristics by controlling the formation polarity in an IrOx/Al2O3/IrOx-ND/Al2O3/WOx/W structure have been investigated. High density of 1 × 1013/cm2 and small size of 1.3 nm in diameter of the IrOx nano-dots (NDs) have been observed by high-resolution transmission electron microscopy. The IrOx-NDs, Al2O3, and WOx layers are confirmed by X-ray photo-electron spectroscopy. Capacitance-voltage hysteresis characteristics show higher charge-trapping density in the IrOx-ND memory as compared to the pure Al2O3 devices. This suggests that the IrOx-ND device has more defect sites than that of the pure Al2O3 devices. Stable resistive switching characteristics under positive formation polarity on the IrOx electrode are observed, and the conducting filament is controlled by oxygen ion migration toward the Al2O3/IrOx top electrode interface. The switching mechanism is explained schematically based on our resistive switching parameters. The resistive switching random access memory (ReRAM) devices under positive formation polarity have an applicable resistance ratio of > 10 after extrapolation of 10 years data retention at 85°C and a long read endurance of 105 cycles. A large memory size of > 60 Tbit/sq in. can be realized in future for ReRAM device application. This study is not only important for improving the resistive switching memory performance but also help design other nanoscale high-density nonvolatile memory in future.     

Bipolar resistance switching characteristics with opposite polarity of Au/SrTiO3/Ti memory cells

Two types of bipolar resistance switching with eightwise and counter eightwise polarities are observed to coexist in Au/SrTiO₃/Ti memory cells. These two types of switching can be induced by different defect distributions which are activated by controlling the electric process. The analyses of I-V and C-V data reveal that the resistance switching with eightwise polarity originates from the change of Schottky barrier at the Au/SrTiO₃ interface caused by trapping/detrapping effects at interface defect states, while the switching with counter eightwise polarity is caused by oxygen-vacancy migration.     

Self-rectifying performance in the sandwiched structure of Ag/In-Ga-Zn-O/Pt bipolar resistive switching memory

We reported that the resistive switching of Ag/In-Ga-Zn-O/Pt cells exhibited self-rectifying performance at low-resistance state (LRS). The self-rectifying behavior with reliability was dynamic at elevated temperature from 303 to 393 K. The Schottky barrier originated from the interface between Ag electrode and In-Ga-Zn-O films, identified by replacing Ag electrode with Cu and Ti metals. The reverse current at 1.2 V of LRS is strongly suppressed and more than three orders of magnitude lower than the forward current. The Schottky barrier height was calculated as approximately 0.32 eV, and the electron injection process and resistive switching mechanism were discussed.     

Influence of the voltage window on resistive switching memory characteristics based on g-C3N4 device

The resistance change of an insulator or semiconductor under applied current or voltage is defined as resistive switching effect, which is a significative physical performance in the exploit of new concept nonvolatile resistance random access memory (RRAM). In our work, the g-C₃N₄ powder was firstly fabricated by calcination method, and continuously a device with Ag/g-C₃N₄/FTO structure was prepared using drop-coated g-C₃N₄ powder to form a film onto FTO. It can be observed that the as-prepared cell exhibits an excellent resistive switching memory characteristic (HRS/LRS resistance ratio can be reached to ~?52) and good reliability under applied voltage window of 4.0?V. Finally, it is believed that the space charge limited conduction is appropriate to understanding such the memory behavior.     

Self-rectifying and forming-free resistive switching behaviors in Pt/La2Ti2O7/Pt structure

In this work, we report the resistive switching behavior of an amorphous La₂Ti₂O₇ (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process.     

Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation

A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.     

Nonvolatile/volatile behaviors and quantized conductance observed in resistive switching memory based on amorphous carbon

Coexistence of nonvolatile and volatile resistive switching behaviors was demonstrated in Cu/amorphous carbon/Pt sandwich-structure memory devices by adjusting compliance currents (CCs) to control the size of Cu conductive filament (CF). It was observed that the retention time of the volatile switching strongly depends on the CF’s size, and can be tuned in a wide range from hundreds of milliseconds to tens of seconds. When the nanoscale CF contains only a small number of Cu atoms, the conductance quantization occurs in the relaxation process of resistance state. By quantitatively studying the dependence of relaxation time on CF’s size and temperature, the volatile behavior can be well understood within the framework of the Rayleigh instability, where the Cu-CF spontaneously dissolves to minimize the surface energy. The observed nonvolatile/volatile behaviors, as well as the spontaneous relaxation effect, bear many resemblances to the long-term/short-term plasticity of biological synapses, and thus can be fully utilized to develop artificial synaptic devices.     

Carrier transport mechanism and bipolar resistive switching behavior of a nano-scale thin film TiO2 memristor

A titanium dioxide (TiO₂) based memristor device having an active layer thickness of 10?nm was fabricated using radio frequency (RF) reactive sputtering and its resistive switching characteristics and carrier transport mechanisms were investigated. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to confirm the structural properties of the device. Measurement of the time-dependent current-voltage (I-V-t) was used to characterize resistive switching and memristive behavior. The characteristic pinched hysteresis I-V loops of the memristor were apparent. Bipolar and homogeneous resistive switching characteristics and a forming voltage of 2?V were detected in the device. The retention time exceeded 10³ s and the endurance test was reasonably acceptable. In addition, the carrier transport mechanism of the device was revealed. The linear region of the low electric field demonstrated ohmic behavior, whereas the non-linear high electric field region was dominated by a Schottky emission carrier transport mechanism. A Poole-Frenkel emission mechanism acted as a secondary conduction mechanism. It was proposed that the Poole-Frenkel and Schottky emission mechanisms were associated with oxygen vacancies in the TiO₂.     

All-inorganic perovskite Cs4PbBr6 thin films in optoelectronic resistive switching memory devices with a logic application

Perovskite-based Cs₄PbBr₆ has potential applications in optoelectronic devices, such as photodetectors, electroluminescence devices and color converters. All-inorganic perovskite Cs₄PbBr₆ thin films were successfully prepared by a simple low-temperature synthesis method and were first implemented as an insulating layer in Au/Cs₄PbBr₆/PEDOT:PSS/Pt devices. The memory devices possess reproducible bipolar resistive switching behavior, low operating voltages, good endurance, and long retention times. Furthermore, the novel sandwich architecture enables the application of the Cs₄PbBr₆ films as memristors and photoresponsive behaviors. Considering the distinct photoresponses of the resistance state as a nonvolatile memory, the devices can be used as a logical "OR" gate by applying bias voltages and light illumination as input signals. The formation and annihilation of Br^- ion vacancy filaments induced by the external bias and light illumination can result in pronounced resistive switching performance. It is believed that solution-processed Cs₄PbBr₆-based devices have great potential for technological deployment at the forefront as a photonic nonvolatile memory as well as integrated modulating and arithmetic functions.     

New mechanism of gas transport at the interface solid/liquid

It was recently shown that an abnormally fast transport of CO molecules takes place at the electrode/electrolyte interface of Pt and PtRu electrodes in H₂SO₄ and HClO₄ solutions. In the present paper, this phenomenon is tested for other gases, such as hydrogen and oxygen. The fast transport is also observed at the solid/electrolyte solution interface of other electrode materials and at the glass/electrolyte interface. Several experiments are shown, demonstrating that mass transfer takes place at a velocity, which is more than one order of magnitude higher than expected for usual diffusion conditions.Assuming radial mass transfer at the interface of a Pt disc, the activation energy, E_a = 23 kJ mol⁻¹, was calculated from Arrhenius plots. The same value was measured in H₂SO₄ and HClO₄ as supporting electrolytes. The mass transport parameter, Y, at 298 K was 4.8 × 10⁻³ cm² s⁻¹ and 2.9 × 10⁻³ cm² s⁻¹ in 0.5 M H₂SO₄ and 1 M HClO₄ respectively.     

Adsorption characteristics of carboxymethylated lignin at a hydrophobic solid/water interface

Carboxymethylated lignin (CML), a water-soluble lignin derivative with more carboxylic groups introduced by chemical modification, can be used as dispersant in industry. In this paper, using the methods of zeta potential measurement, X-ray photoelectron spectroscopy and UV spectroscopy, natural graphite (NG) and high-purity graphite (HPG) were selected to investigate the adsorption characteristics of CML at the hydrophobic solid/water interface. The adsorption isotherm of CML on HPG was more suitable for the Langmuir model than the Freundlich model, whereas the opposite result was obtained for the adsorption of CML on NG. Moreover, the amounts adsorbed of CML on HPG increased with higher ionic strength and lowering solution pH. However, the uptake of CML on NG increased dramatically with adding NaCl under acidic condition, while the ionic strength had no obvious influence on the adsorption under neutral condition. With increasing solution pH, the amounts of CML adsorbed on NG were dropped under acidic condition followed by increases under alkaline condition. It is suggested that the interactions between CML and the minor metallic impurities on NG surface probably contribute considerably to the differences between adsorption behaviors of CML on NG and HPG in varying solution conditions. Moreover, the results showed that the adsorption mechanism of CML at graphite/water interface could be described by Lennard-Jones potential.     

Enhanced pyroelectric and piezoelectric responses in W/Mn-codoped Bi4Ti3O12 Aurivillius ceramics

The pyroelectric and piezoelectric properties of 4 at% Mn-doped Bi₄Ti_2.9W_0.1O₁₂ (BiTW-Mn) Aurivillius ceramic were investigated and compared to Bi₄Ti_2.9W_0.1O₁₂ (BiTW) counterpart, which were fabricated using a conventional solid state reaction method. High resistivities of 4.9?×?10¹² and 2.5?×?10¹¹ Ω?cm at 100?°C were obtained in the W-doped and W/Mn-codoped BiT ceramics, respectively. They showed similar activation energies and ionic-p-type mixed conduction mechanisms. Higher pyroelectric coefficients of 57.1?μC/m²K and piezoelectric coefficients of 21 pC/N, as well as much lower dielectric loss of 0.003 were achieved in W/Mn-codoped ceramics. These property changes were mainly induced by $\text{M}{\text{n}}_{\text{Ti}}^{}?{\text{V}}_{\text{o}}^{}$ defect dipoles. The effect of acceptor doping was evidenced by an internal bias field, shown by a horizontal offset in the polarization-field behavior. The improved properties together with high thermal stability indicate that BiTW-Mn may be a promising candidate for pyroelectric and piezoelectric devices at elevated temperatures.     

Copper pillar and memory characteristics using Al2O3 switching material for 3D architecture

A novel idea by using copper (Cu) pillar is proposed in this study, which can replace the through-silicon-vias (TSV) technique in future three-dimensional (3D) architecture. The Cu pillar formation under external bias in an Al/Cu/Al₂O₃/TiN structure is simple and low cost. The Cu pillar is formed in the Al₂O₃ film under a small operation voltage of <5 V and a high-current-carrying conductor of >70 mA is obtained. More than 100 devices have shown tight distribution of the Cu pillars in Al₂O₃ film for high current compliance (CC) of 70 mA. Robust read pulse endurances of >10⁶ cycles are observed with read voltages of −1, 1, and 4 V. However, read endurance is failed with read voltages of −1.5, −2, and −4 V. By decreasing negative read voltage, the read endurance is getting worst, which is owing to ruptured Cu pillar. Surface roughness and TiO _x N _y on TiN bottom electrode are observed by atomic force microscope and transmission electron microscope, respectively. The Al/Cu/Al₂O₃/TiN memory device shows good bipolar resistive switching behavior at a CC of 500 μA under small operating voltage of ±1 V and good data retention characteristics of >10³ s with acceptable resistance ratio of >10 is also obtained. This suggests that high-current operation will help to form Cu pillar and lower-current operation will have bipolar resistive switching memory. Therefore, this new Cu/Al₂O₃/TiN structure will be benefited for 3D architecture in the future.