Ultra-low power non-volatile resistive crossbar memory based on pull up resistors

To realize a flexible and ultra-low power non-volatile resistive random access memory (NVRRAM), we propose a 3 × 3 resistive memory array comprised of crossbar memristors and a pull-up resistor connected to each column bar (1R-CM). This architecture forms a voltage divider network, which directly reads a parallel data of a row in the form of voltage instead of current. In the proposed structure, the optimum value of the pull-up resistor was found to be 10 MΩ. Poly(4-vinylphenol) (PVP) material is utilized in the memristor to achieve high OFF/ON resistance ratio of ∼1000 as a high resistance state (HRS) is 10 GΩ and a low resistance state (LRS) is 10 MΩ. The operating voltage for writing and reading are ±2 V and 0.5 V in current bound of 10–100 nA, respectively, and consumes ultra-low power (<8.33 nW) during operation. The proposed memory demonstrates bendability down to 10 mm with bending endurance of 1000 cycles and retention time for more than 180 days. It is fabricated on a plastic substrate by using direct-printing technique electrohydrodynamic (EHD) at ambient conditions that can be used in wearable electronics.     

Carrier transport and memory mechanisms of multilevel resistive memory devices with an intermediate state based on double-stacked organic/inorganic nanocomposites

Multilevel resistive memory devices with an intermediate state were fabricated utilizing a poly(methylmethacrylate) (PMMA) layer sandwiched between double-stacked PMMA layers containing CdSe/ZnS core–shell quantum dots (QDs). The current–voltage (I–V) curves on a Al/[PMMA:CdSe/ZnS QD]/PMMA/[PMMA:CdSe/ZnS QD]/indium-tin-oxide/glass device at low applied voltages showed current bistabilities with three states, indicative of multilevel characteristics. A reliable intermediate state was realized under positive and negative applied voltages. The carrier transport and the memory mechanisms of the devices were described on the basis of the I–V curves and energy band diagrams, respectively. The write-read-erase-read-erase-read sequence of the devices showed rewritable, nonvolatile, multilevel, and memory behaviors. The currents as functions of the retention time showed that three current states were maintained for retention times larger than 1 × 10⁴ s, indicative of the good stability of the devices.     

Twistable nonvolatile organic resistive memory devices

We fabricated an 8 × 8 cross-bar array-type organic nonvolatile memory devices on twistable poly(ethylene terephthalate) (PET) substrate. A composite of polyimide (PI) and 6-phenyl-C61 butyric acid methyl ester (PCBM) was used as the active material for the memory devices. The organic memory devices showed a high ON/OFF current ratio, reproducibility with good endurance cycle, and stability with long retention time over 5 × 10⁴ s on the flat substrate. The device performance remained well under the twisted condition with a twist angle up to ～30°. The twistable organic memory device has a potential to be utilized in more complex flexible organic device configurations.     

An organic nonvolatile resistive switching memory device fabricated with natural pectin from fruit peel

In this work, pectin, a by-product has not been recycled sustainably, is introduced as an insulator to fabricate a novel organic resistive switching memory devices with Ag/Pectin/FTO structure for the first time. The device exhibits superior switching endurance accompanied by an OFF/ON resistance ratio (storage density window) of ∼450. This work reveals for the first time that pectin from fruit peel is a promising material for nonvolatile memory applications.     

Data retention in organic ferroelectric resistive switches

Solution-processed organic ferroelectric resistive switches could become the long-missing non-volatile memory elements in organic electronic devices. To this end, data retention in these devices should be characterized, understood and controlled. First, it is shown that the measurement protocol can strongly affect the ‘apparent’ retention time and a suitable protocol is identified. Second, it is shown by experimental and theoretical methods that partial depolarization of the ferroelectric is the major mechanism responsible for imperfect data retention. This depolarization occurs in close vicinity to the semiconductor-ferroelectric interface, is driven by energy minimization and is inherently present in this type of phase-separated polymer blends. Third, a direct relation between data retention and the charge injection barrier height of the resistive switch is demonstrated experimentally and numerically. Tuning the injection barrier height allows to improve retention by many orders of magnitude in time, albeit at the cost of a reduced on/off ratio.     

All-printed and highly stable organic resistive switching device based on graphene quantum dots and polyvinylpyrrolidone composite

We propose all printed and highly stable organic resistive switching device (ORSD) based on graphene quantum dots (G-QDs) and polyvinylpyrrolidone (PVP) composite for non-volatile memory applications. It is fabricated by sandwiching G-QDs/PVP composite between top and bottom silver (Ag) electrodes on a flexible substrate polyethylene terephthalate (PET) at ambient conditions through a cost effective and eco-friendly electro-hydrodynamic (EHD) technique. Thickness of the active layer is measured around 97 nm. The proposed ORSD is fabricated in a 3 × 3 crossbar array. It operates switching between high resistance state (HRS) and low resistance state (LRS) with OFF/ON ratio ∼14 for more than 500 endurance cycles, and retention time for more than 30 days. The switching voltage for set/reset of the devices is ±1.8 V and the bendability down to 8 mm diameter for 1000 cycles are tested. The elemental composition and surface morphology are characterized by XPS, FE-SEM, and microscope.     

An organic multilevel non-volatile memory device based on multiple independent switching modes

The demand for higher data density memory structures is greater today than ever before. Multilevel resistive organic memory devices (OMD) provide an ideal solution, in being easily fabricated, cost-effective and at the same time promising high storage capacity. However, conventional methods for multilevel OMDs impose demanding requirements on material properties and attain only limited performance. We hereby provide an alternative design concept that combines multiple switching modes in one device to realize multilevel function. The device possesses a simple structure by using a ferroelectric phase-separated blend as the active layer. Two switching modes, the ferroelectric switching and the metallic filament switching, are realized simultaneously in this device, and enable a ternary storage function. The cross-section scanning electron microscope (SEM) images provide a strong evidence of the formation and annihilation of the metallic filament.     

Carrier transport mechanisms of multilevel nonvolatile memory devices with a floating gate consisting of hybrid organic/inorganic nanocomposites

Nonvolatile memory devices based on a poly(4-vinylphenol) (PVP) layer containing Cu₂ZnSnS₄ (CZTS) nanoparticles were fabricated by using a simple spin-coating method. An energy dispersive spectrum revealed that the CZTS nanoparticles were Cu poor and Zn rich. Transmission electron microscopy images showed that the CZTS nanoparticles were randomly distributed in the PVP layer. Capacitance–voltage (C–V) curves for Al/CZTS nanoparticles embedded in PVP layer/p-Si devices at 1 MHz showed a hysteresis with flat-band voltage (V_fb) shifts, which resulted from the existence of CZTS nanoparticles acting as trap sites in the memory devices. The magnitudes of the V_fb corresponding to the memory window shifts between 1.0 and 2.5 V, as determined from the C–V data at 1 MHz, were dependent on the voltages applied to the memory device, indicative of multilevel characteristics for the memory effect. The operating mechanisms of the writing and the erasing processes for Al/CZTS nanoparticles embedded in PVP layer/p-Si devices are described on the basis of the C–V results and the energy-band diagrams.     

Organic low voltage rewritable memory device based on PEDOT:PSS/f-MWCNTs thin film

We report on devices constructed using a small quantity (?0.01 wt.%) of functionalized multiwalled carbon nanotubes (f-MWCNTs) embedded in a conducting polymer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) matrix and aluminum top electrodes, prepared on indium-tin-oxide (ITO) substrates. Our ITO/(PEDOT:PSS + f-MWCNTs)/Al devices show current bistability. The low resistance ON-state, as well as the high resistance OFF state, retain the information for hours and are stable after hundreds of write–read–erase–read (WRER) cycles, being potentially interesting for erasable and rewritable volatile memory device applications. Moreover, the operation voltages used for performing these WRER cycles are very low. The threshold voltage for OFF to ON switching can be adjusted changing the f-MWCNTs concentration. Our results suggest that the nanotubes are necessary for the production of an inhomogeneous electric field playing a role in the electroforming (dielectric breakdown) of the aluminum oxide layer at the Al₂O₃/(PEDOT:PSS) interface.     

Thickness effect on the bipolar switching mechanism for nonvolatile resistive memory devices based on CeO2 thin films

Impact of switching layer thickness on the bipolar resistive memory performance, stability and uniformity has been investigated in Ti/CeO₂/Pt devices. XRD and FTIR analyses demonstrate polycrystalline nature of CeO₂ films and the formation of a TiO interface layer. The bipolar switching characteristics like HRS and LRS dispersion are found to be dependent on the thickness of CeO₂ layer. As it is noted that forming as well as SET voltages gradually increase with increasing CeO₂ layer thickness however RESET voltages are slightly affected. Oxygen gettering ability of Ti causes the formation of TiO layer, which not only extracts oxygen ions from the ceria film but also acts as ion reservoir, hence plays a key role in stable functioning of the memory devices. Current transport behavior is based upon Ohmic and interface modified space charge limited conduction. Based on unique distribution characteristics of oxygen vacancies in CeO₂ films, a possible mechanism of resistive switching in CeO₂ RRAM devices has been discussed.     

Evidence for compliance controlled oxygen vacancy and metal filament based resistive switching mechanisms in RRAM

We present electrical evidence on asymmetric metal-insulator-semiconductor (MIS) based test structures in support of the presence of two different independent switching mechanisms in a resistive random access memory (RRAM) device. The valid mechanism for switching depends on the compliance capping (I_gl) for forming/SET transition. Our results convincingly show that low compliance based switching only involves reversible oxygen ion drift to and from oxygen gettering gate electrodes, while high compliance switching involves formation and rupture of conductive metallic nanofilaments, as verified further by our physical analysis investigations. We have observed this unique dual mode switching mechanism only in NiSi-based gate electrodes, which have a moderate oxygen solubility as well as relatively low melting point.     

2T–1R STT-MRAM memory cells for enhanced on/off current ratio

Novel spin torque transfer magnetic tunnel junction (STT-MTJ) based memory cell topologies are introduced to improve both the sense margin and the current ratio observed by the sense circuitry. These circuits utilize an additional transistor per cell in either a diode connected or gate connected manner and maintain leakage current immunity within the data array. An order of magnitude increase in the current ratio over a traditional 1T–1R structure is observed. This improvement comes at a cost of 61% and 117% increase in area, respectively, for the diode and gate connected cells.     

Integration of organic based Schottky junctions for crossbar non-volatile memory applications

Small size Schottky junctions using two different synthesized organic semiconductors (oligophenylenevinylenes) were integrated by standard UV lithography into crossbar arrays. The proposed integration scheme can be applied to a wide class of organics without affecting material properties. Current-voltage characteristics were studied in order to investigate which of the tested compounds could possibly reach the requirements for non-volatile memory applications. All the investigated devices displayed good rectifying properties, ranging from 10² to 10⁴. On the other hand, one of the compounds reveals higher conductivity and possible reasons for this behavior are discussed.     

Resistive switching characteristics of solution-processed TiOx for next-generation non-volatile memory application; transparency, flexibility, and nano-scale memory feasibility

Solution-processed TiO_x layer was investigated as a candidate for next-generation resistive random access memory (ReRAM) application. TiO_x active layer was prepared by simple spin coating process of a titanium(IV) isopropoxide precursor using sol-gel chemistry. Through the introduction of indium-tin-oxide (ITO) coated glass and polyethersulfone (PES) substrates, tranparent and flexible ReRAM devices were demonstrated, respectively. In addition, using scalable via-hole structure with nano-scale active area, the feasibility for high-density memory application was investigated. All ReRAM devices formed using various substrates exhibited good memory performance, such as stable dc I-V, ac endurance, and retention characteristics during maintaining their own unique functions accomplished by substrate properties.     

Effect of the top electrode material on the resistive switching of TiO2 thin film

Effect of the top electrode (TE) metal on the resistive switching of (TE)/TiO₂/Pt structure was investigated. It was confirmed that the potential barrier height between the metal and TiO₂ is an important factor on the resistive switching characteristics. When high Schottky barrier was formed with the TiO₂ film, using Pt or Au as a top electrode, both stable URS (unipolar) and BRS (bipolar resistive switching) characteristics were observed depending on the current compliance level. In the case of Ag, which forms a relatively low Schottky barrier, only BRS characteristics were observed, regardless of the current compliance level. In the case of Ni and Al, which have similar work function as Ag, unstable URS and BRS at very low current compliance levels were observed due to a chemical reaction at the interface. For the Ti electrode, resistive switching was not observed, because the work function of Ti is lower than that of TiO₂ and TiO phase was formed at the interface (Ti/TiO_x contact is ohmic).     

Effect of post annealing on the resistive switching of TiO2 thin film

The effect of annealing on the resistive switching of 35-nm-thick TiO₂ thin film deposited with magnetron sputtering system was studied. Pt and Ag were used as a top electrode (TE), and Pt was used as a bottom electrode (BE). For Pt/as-deposited TiO₂/Pt structure, both unipolar (URS) and bipolar resistive switching (BRS) were observed depending on the current compliance level. For Pt/400 °C annealed TiO₂/Pt structure, only BRS was observed regardless of the current compliance level. The increase in the work function of the TiO₂ film after annealing lowers the potential barrier height and changes the electron transfer process which was also confirmed from Ag/as-deposited TiO₂/Pt structure. Above 600 °C, the film becomes leaky with the increase in grain size and roughness and the resistive switching behavior was not observed.     

Effect of the oxygen vacancy gradient in titanium dioxide on the switching direction of bipolar resistive memory

Resistive memory switching behavior depending on voltage sweep direction is studied by intentionally creating oxygen vacancies within titanium dioxide (TiO₂). By inserting a reactive Ti layer on the TiO₂, oxygen deficient TiO_2−x layer is created, which then causes TiO_2−x/TiO₂ which has an oxygen vacancy gradient. This gradient of oxygen vacancy makes it possible to create an insulating TiO₂ layer on the bottom electrode during the first reset with a negative bias at the top electrode. This insulating layer makes counterclockwise directional bipolar switching more stable. On the other hand, under the clockwise directional voltage sweeping, the first set switching is prevented by the insulating TiO₂ layers created during the first and second reset, which leads to a short circuit due to local heating eventually.     

Materials effects on the electrode-sensitive bipolar resistive switches of polymer:gold nanoparticle memory devices

Electronic devices with an polystyrene (PS) layer blended with Au nanoparticles capped with conjugated 2-naphthalenethiol (Au–2NT NPs) sandwiched between Au and Al electrodes exhibit bipolar resistive switches sensitive to the electrodes. This paper reports the effects of materials, including electrode materials, capping ligands of Au nanoparticles and matrix polymers, on the electrical behavior of the polymer:nanoparticle memory devices. Although the devices using Cu to replace Au as the top electrode exhibit resistive switches similar to those with Au, the threshold voltage for the resistive switch is higher, and the current density for the devices in the low conductivity state is lower. However, the threshold voltage and the current density are almost the same as those with Au as the top electrode, when a semiconductor, MoO₃, is used to replace Au as the top electrode of the devices. The effects of these electrodes are attributed to the charge transfer at the contacts between Au–2NT NPs and the electrodes. The resistive switches are also sensitive to the capping organic ligand of the Au nanoparticles. The threshold voltage decreases and the current density increases, when conjugated benzenethiol is used to replace 2-naphthalenethiol. However, the current density dramatically decreases and the threshold voltage increases, when 2-benzeneethanethiol, a partially conjugated molecule, is adopted as the capping ligand of the Au nanoparticles. The effect of the capping ligands is ascribed to their effect on the charge tunneling across the Au–2NT NPs in the active layer and the contacts between Au–2NT NPs and electrodes. The devices with poly(methyl methacrylate) (PMMA) replacing PS as the polymer matrix exhibit resistive switches almost the same as those with PS, which indicates that the Au–2NT NPs rather than the polymer is the active material responsible for the resistive switches.     

A shapeshifting evolvable hardware mechanism based on reconfigurable memFETs crossbar architecture

In this work, a new technologic strategy that allows implementing large crossbars formed with memFETs, a new device concept, is introduced. This memFET is an electrically reconfigurable field effect and resistive switching device that can be used to implement logic functions and memory blocks into a crossbar structure, allowing the dynamic logic configuration of the crossbar and simplifying both the design and the implementation of computing hardware. Moreover, taking the advantage of reconfiguration capability of such a technology and architecture we introduce a novel technique to design evolvable hardware where not only the logic functions are changeable (as is the case of the Field Programmable Gate Array, FPGA) but also the physical position of the components on the surface of the integrated circuit. This technology and principle leads towards a new computing paradigm based on what we name Shape Shifting Digital Hardware (SSDH).     

Structural effect on controllable resistive memory switching in donor–acceptor polymer systems

Controllable bistable electrical conductivity switching behavior and resistive memory effects have been demonstrated in Al/polymer/indium-tin oxide (ITO) sandwich structure devices, constructed from non-conjugated vinyl copolymers of PTPA_nOXD_m with pendant donor–acceptor chromophores. The observed electrical bistability can be attributed to the field-induced intra- and intermolecular charge transfer interaction between triphenylamine electron donor (D) and oxadiazole electron acceptor (A) entities, and is highly dependent on the chemical structure of the copolymers. The vinyl copolymers showed different memory behaviors, which depended on the loading of D/A ratios. The polymers containing only donor or acceptor moieties showed as insulators, the polymers containing both donor and acceptor moieties showed as WORM, flash and DRAM as D/A ratio increased. The structural effect on the physicochemical and electronic properties of the PTPA_nOXD_m copolymers, viz surface morphology, thermal stability, optical absorbance and photoluminescence, and molecular orbital energy levels, were investigated systematically to study the factors that influence the memory characteristics of the devices.