Coexistence of memory resistance and memory capacitance in TiO2 solid-state devices

This work exploits the coexistence of both resistance and capacitance memory effects in TiO₂-based two-terminal cells. Our Pt/TiO₂/TiO _x /Pt devices exhibit an interesting combination of hysteresis and non-zero crossing in their current-voltage (I-V) characteristic that indicates the presence of capacitive states. Our experimental results demonstrate that both resistance and capacitance states can be simultaneously set via either voltage cycling and/or voltage pulses. We argue that these state modulations occur due to bias-induced reduction of the TiO _x active layer via the displacement of ionic species.     

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I₃⁻)/iodide (I⁻) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I₃⁻ on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.     

Mechanism of nonvolatile resistive switching in graphene oxide thin films

The mechanism of the resistive switching (RS) effect in graphene oxide (GO) thin films prepared by the vacuum filtration method has been investigated by macroscopic current–voltage (I–V) measurements and conducting atomic force microscopy (CAFM). Detailed I–V measurements show that in metal/GO/Pt sandwiches, the RS originates from the formation and rupture of conducting filaments. An analysis of the temperature dependence of the ON-state resistance reveals that the filaments are composed of metal atoms due to the diffusion of the top electrodes under a bias voltage. Moreover, the RS is found to occur within confined regions of the metal filaments. The RS effect is also observed in GO/Pt structures by CAFM. It is attributed to the redox reactions between GO and adsorbed water induced by external voltage biases.     

Low threshold voltage,highly stable electroforming-free threshold switching characteristics in VOx films-based device

Threshold switching (TS) devices have evolved as one of the most promising elements in memory circuit due to their important significance in suppressing crosstalk current in the crisscross array structure. However, the issue of high threshold voltage (V_th) and low stability still restricts their potential applications. Herein, the vanadium oxide (VO_x) films deposited by the pulsed laser deposition (PLD) method are adopted as the switching layer to construct the TS devices. The TS devices with Pt/VO_x/Pt/PI structure exhibit non-polar, electroforming-free, and volatile TS characteristics with an ultralow V_th (+0.48 V/−0.48 V). Besides that, the TS devices also demonstrates high stability, without obviously performance degradations after 350 cycles of endurance measurements. Additionally, the transition mechanism is mainly attributed to the synergistic effect of metal-insulator transition of VO₂ and oxygen vacancies. Furthermore, the nonvolatile bipolar resistance switching behaviors can be obtained by changing oxygen pressure during the deposition process for switching films. This work demonstrates that vanadium oxide film is a good candidate as switching layer for applications in the TS devices and opens an avenue for future electronics.     

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.     

The origin of nonlinear current-voltage behavior in fiber-reinforced cement composites

Two distinct slopes (resistances) are obtained in current-voltage (I-V) plots of discontinuous, conductive fiber-reinforced cement composites. The low-field resistance correlates with the DC resistance (R_DC) of each composite. The high-field resistance correlates with the intermediate frequency cusp resistance (R_cusp) in Nyquist (−Z_imag vs. Z_real) plots obtained using impedance spectroscopy (IS). A model is developed that is based on passive oxide film formation on copper or steel fiber surfaces at low fields (I-V) or low frequencies (IS) due to the high pH pore solution of cement paste. With increase of field, leading to film breakdown (active or transpassive corrosion behavior), or increase of frequency, leading to short-circuiting of the passive layer, the fibers act as short-circuiting elements in the composite microstructure, resulting in a decrease in overall resistance.     

The internal resistance of a microbial fuel cell and its dependence on cell design and operating conditions

The internal resistance R_int of a mediator-less microbial fuel cell (MFC) has been determined as a function of cell voltage using electrochemical impedance spectroscopy (EIS) for a MFC with and without Shewanella oneidensis MR-1. The same tests were performed for a MFC containing small stainless steel (SS) balls in the anode compartment with a graphite feeder electrode as in a packed bed cell. It has been found that R_int decreased with decreasing cell voltage as the increasing current flow decreases the polarization resistance of the anode and the cathode. The ohmic components of R_int played a very minor role. In the presence of MR-1 R_int was lower by a factor of about 100 than R_int of the MFC with buffer and lactate as anolyte. R_int was also significantly lower for the anode containing SS balls with buffer and lactate as anolyte. For the MFC containing SS balls in the anode compartment no significant further decrease of R_int could be obtained when MR-1 was added to the anolyte since in this case the polarization resistance of the anode was lower than that of the cathode. Similar trends were observed in the cell voltage (V)-current (I) curves that were obtained using potentiodynamic sweeps and the power (P)-V curves that were calculated from the V-I curves.     

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.     

Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy

Effects of the electrolyte of DSCs on impedance spectra were evaluated by changing concentration of redox couple, viscosity, and additives to electrolyte. The relation with current-voltage characteristics (I-V characteristics) was investigated. In many cases, the impedance component attributed to charge transfer at TiO₂|electrolyte interface demonstrated strong relation with the I-V characteristics. The recombination of electrons in TiO₂ with I₃⁻ in electrolyte was a key factor in determining performance of DSCs. To evaluate the effect of I₃⁻, diffusion-limiting current in the electrolyte for various viscosities was evaluated by cyclic voltammetry. When the short circuit current (SCC) was almost equal to the diffusion-limiting current, strong influence of the diffusion coefficient on the impedance spectra was observed: impedance arcs were enlarged as the diffusion coefficient was decreased. On the other hand, when the diffusion-limiting current was larger than the SCC, photo-excitation and electron injection processes became dominating factors in the DSCs performance. The SCC was regulated by the charge recombination process at TiO₂|electrolyte interface, and thus the impedance component ω₃ was related to the performance in such condition.     

Stability scheme of ZnO-thin film resistive switching memory: influence of defects by controllable oxygen pressure ratio

We report a stability scheme of resistive switching devices based on ZnO films deposited by radio frequency (RF) sputtering process at different oxygen pressure ratios. I-V measurements and statistical results indicate that the operating stability of ZnO resistive random access memory (ReRAM) devices is highly dependent on oxygen conditions. Data indicates that the ZnO film ReRAM device fabricated at 10% O₂ pressure ratio exhibits the best performance. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) of ZnO at different O₂ pressure ratios were investigated to reflect influence of structure to the stable switching behaviors. In addition, PL and XPS results were measured to investigate the different charge states triggered in ZnO by oxygen vacancies, which affect the stability of the switching behavior.     

Memristive Switching Characteristics in Biomaterial Chitosan-Based Solid Polymer Electrolyte for Artificial Synapse

This study evaluated the memristive switching characteristics of a biomaterial solid polymer electrolyte (SPE) chitosan-based memristor and confirmed its artificial synaptic behavior with analog switching. Despite the potential advantages of organic memristors for high-end electronics, the unstable multilevel states and poor reliability of organic devices must be overcome. The fabricated Ti/SPE-chitosan/Pt-structured memristor has stable bipolar resistive switching (BRS) behavior due to a cation-based electrochemical reaction between a polymeric electrolyte and metal ions and exhibits excellent endurance in 5 × 10² DC cycles. In addition, we achieved multilevel per cell (MLC) BRS I-V characteristics by adjusting the set compliance current (I_cc) for analog switching. The multilevel states demonstrated uniform resistance distributions and nonvolatile retention characteristics over 10⁴ s. These stable MLC properties are explained by the laterally intensified conductive filaments in SPE-chitosan, based on the linear relationship between operating voltage margin (ΔV_switching) and I_cc. In addition, the multilevel resistance dependence on I_cc suggests the capability of continuous analog resistance switching. Chitosan-based SPE artificial synapses ensure the emulation of short- and long-term plasticity of biological synapses, including excitatory postsynaptic current, inhibitory postsynaptic current, paired-pulse facilitation, and paired-pulse depression. Furthermore, the gradual conductance modulations upon repeated stimulation by 10⁴ electric pulses were evaluated in high stability.     

Forming-free bipolar resistive switching in nonstoichiometric ceria films

The mechanism of forming-free bipolar resistive switching in a Zr/CeO _x /Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrO _y layer at the Zr/CeO _x interface. X-ray diffraction studies of CeO _x films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeO _x film and in the nonstoichiometric ZrO _y interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).     

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.     

Unipolar resistive switching behavior of amorphous SrMoO4 thin films deposited at room temperature

Amorphous SrMoO₄ (SMO) thin films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature by pulsed laser deposition and the resistive switching (RS) behavior of the Au/SMO/Pt devices was investigated. The Au/SMO/Pt devices exhibit typical unipolar RS behavior with excellent switching parameters as follows: high resistance ratio (~10⁵) between the low resistance state (LRS) and high resistance state (HRS), non-overlapping switching voltages, and good endurance and retention characteristics. Detailed analysis of their current-voltage characteristics reveals that the conduction mechanisms are Ohmic conduction in the LRS and lower voltage region of HRS, and Poole-Frenkel emission in the higher voltage region of the HRS. Temperature dependent resistance measurements, combined with x-ray photoelectron spectroscopy and model analysis indicate that the unipolar RS behavior of the Au/SMO/Pt devices could be understood by a conical conducting filaments (CFs) model in which the conical CFs are composed of oxygen vacancies. The conical CFs extend from the cathode to anode during the forming process and the observed RS behavior occurs in the localized region near the anode. These results suggest that the room-temperature- deposited amorphous SMO thin films could find potential application in nonvolatile RS memory.     

Low voltage resistive memory devices based on graphene oxide–iron oxide hybrid

Low cost resistive switching memory devices using graphene oxide–iron oxide (GF) hybrid thin films, sandwiched between platinum (Pt) and indium-tin-oxide (ITO) electrodes, were demonstrated. The fabricated devices with Pt/GF/ITO structure exhibited reliable and reproducible bipolar resistive switching performance, with an ON/OFF current ratio of 5 × 10³, excellent retention time longer than 10⁵ s, SET voltage of 0.9 V, and good endurance properties. In all aspects of the device characteristics, the GF based devices outperformed graphene oxide (GO) based devices. Ohmic conduction was found to be dominant current conduction mechanism in all switching regions except for the high voltage regime where space charge limited conduction and trap charge limited conduction were found to be the main current conduction mechanism. X-ray photoelectron spectroscopy and transmission electron microscopy/selected area diffraction analysis revealed γ-Fe₂O₃ and Fe₃O₄ iron oxide phases coexist in the hybrid films. While the desorption/adsorption of oxygen-related functional groups on the GO sheets is the dominant resistive switching mechanism in Pt/GO/ITO devices, the formation/rupture of multiple highly conducting Fe₃O₄ filaments at the iron oxide/GO interface additionally facilitate the switching in the present Pt/GF/ITO devices. Thereby, excellent electrical switching performance was achieved.     

Detection of oxygen ion drift in Pt/Al2O3/TiO2/Pt RRAM using interface-free single-layer graphene electrodes

Resistive switching random access memory (RRAM) with oxygen ion drift under electric (E)-field has been intensively studied. However, the findings are insufficient because redox reaction by oxygen ion drift occurs beneath the top electrode, and it is difficult to analyze with a nondestructive method. Therefore, an effective method to circumvent this difficulty is suggested in this study with a Pt/Al₂O₃/TiO₂/Pt device using a single layer graphene (SLG) top electrode. Based on results from spectroscopic analyses, the SLG serves as not only an interface free electrode, but also as a highly effective indicator for proving O⁻ ion drift motion in response to the E-field in RRAM. The origin of asymmetric resistive switching is due to a redox reaction at the interface by oxygen ion drift. The endurance and operation-current distribution are significantly improved with increased thickness of the Al₂O₃ insertion layer, which provides carrier tunneling barrier height. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) is greater than one order of magnitude in a log scale within 1800 cycles. This result demonstrates that control of a localized charge tunneling barrier is a key factor for reliable resistive switching of the scaled-down RRAM.     

Reproducible switching effect of an all-inorganic halide perovskite CsPbBr3 for memory applications

All-inorganic halide perovskite CsPbBr₃ have been developed and investigated. We have further demonstrated the using of this stable all-inorganic halide perovskite as storage media in memristors. Reproducible typical bipolar resistance switching behaviors in two different structures of resistance random access memory devices (Pt/CsPbBr₃/FTO and Pt/CsPbBr₃/Cu₂O/FTO) are observed. Particularly, the Pt/CsPbBr₃/Cu₂O/FTO device based on CsPbBr₃/Cu₂O heterojunction exhibits a remarkably high resistance switching effect with low set and reset voltages. Such appealing characteristics are comparable with those of frequently-used transition metal oxides perovskites like BaTiO₃ and SrZrO₃, etc. Possible conduction mechanisms are also proposed to understand the resistance switching behaviors of the studied devices.     

The effects of the time-dependent on the characteristic parameters of polypyrrole/p-type Si/Al diode

A detailed study of the effects of the time-dependent or aging on the characteristic parameters of polypyrrole/p-type Si/Al structure has been presented. The polypyrrole film has been formed on a p-type Si substrate by means of an anodization process. The polypyrrole/p-Si contact has demonstrated clearly rectifying behavior by the current-voltage curves studied at room temperature. The current-voltage (I-V) curves of the diode have been measured immediately, 7, 15, 30, 60 and 90 days after fabrication of the polypyrrole/p-Si contact. It has been seen that the characteristics parameters such as barrier height, ideality factor and series resistance of polypyrrole/p-type Si/Al structure have changed with increasing ageing time. Furthermore, the density distribution of interface states of the device was obtained from the forward bias I-V characteristics. The fact that the diode shows non-ideal I-V behavior with increasing ageing time may be ascribed to a slow replacement of the initial doping agent by oxygen and this process certainly plays a role in the aging of the diode.     

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.     

Resistive random access memory based on gallium oxide thin films for self-powered pressure sensor systems

The resistive switching (RS) behavior of a gallium oxide (Ga₂O₃) thin film for use in resistive random access memory (RRAM) was investigated. Ta/Ga₂O₃/Pt memory devices exhibited favorable RS behavior, such as a small distribution of switching parameters and switching cycles of more than 3 × 10⁶. X-ray photoelectron spectroscopy and the current transport mechanism indicated that that the RS behavior was attributed to the local variation on the Schottky barrier near the Pt electrode interface due to oxygen vacancies. A hybrid system for self-powered data storage and deletion was built by combining the RRAM device with a commercial Pb(Zr_1-xTi_x)O₃ piezoelectric ceramic as a pressure sensor/power generator. The excellent anti-interference and reuse performance of the system indicated promising potential for the application of this memory device.