Write-once-read-many-times Memory Devices Research Articles

CsPbI3 Perovskite Quantum Dot-Based WORM Memory Device with Intrinsic Ternary States.

The migration of mobile ionic halide vacancies is usually considered detrimental to the performance and stability of perovskite optoelectronic devices. Taking advantage of this intrinsic feature, we fabricated a CsPbI3 perovskite quantum dot (PQD)-based write-once-read-many-times (WORM) memory device with a simple sandwich structure that demonstrates intrinsic ternary states with a high ON/OFF ratio of 103:102:1 and a long retention time of 104 s. Through electrochemical impedance spectroscopy, we proved that the resistive switching is achieved by the migration of mobile iodine vacancies (VIs) under an electric field to form conductive filaments (CFs). Using in situ conductive atomic force microscopy, we further revealed that the multilevel property arises from the different activation energies for VIs to migrate at grain boundaries and grain interiors, resulting in two distinct pathways for CFs to grow. Our work highlights the potential of CsPbI3 PQD-based WORM devices, showcasing intrinsic multilevel properties achieved in a simple device structure by rationally controlling the drift of ionic defects.

Enhancing the Resistive Switching Behavior of WORM Memory Devices Using D-π-A Based Ester-Flanked Quinolines.

Donor-Acceptor systems are highly appreciated in the field of organic memory devices due to their efficient charge transport within the systems. In this work, we have designed and synthesized a D-π-A system constituting ester-flanked quinolines and functionalized triarylamines (TAA) through a single-step cross-coupling reaction to fabricate memory devices via Write-Once Read-Many times (WORM) non-volatile memory. Structure-property relationships are reconnoitered for these conjugated D-π-A systems through a series of UV, fluorescence, XRD, DFT, and memory characterizations. The UV and CV data show efficient charge transfer with intramolecular charge transfer occurring at 407-417 nm and a short band gap of 2.56-2.65 eV. An enhancement in the resistive switching behavior of the memory devices is observed for the compounds with simple TAA-quinoline and tert-butylphenyl substituted TAA and fluorophenyl substituted quinoline due to balanced charge distribution in the compounds. This enhanced switching induces an on/off ratio of 103 by generating a highly ordered arrangement in the thin films. The HOMO, LUMO levels, and the ESP images together estimate a charge transfer and charge trapping as the plausible mechanism for the solution-processable WORM memory devices. The longer retention time (103 s) and lower threshold voltages (-1.21--2.12 V) of the devices makes them intriguing compounds for memory applications.

Memory effect of vertically stacked hBN/QDs/hBN structures based on quantum-dot monolayers sandwiched between hexagonal boron nitride layer

The characteristics of a flexible write-once-read-many-times (WORM) memory fabricated with monolayered 0-dimensional (0D) CdSe–ZnS quantum dots (QDs) layers sandwiched between two insulating 2-dimensional (2D) hexagonal boron nitride (hBN) multilayers were investigated by electrical measurement method. The hBN/QDs monolayer/hBN structure was fabricated in a vertical stacked structure using a technique which control the formation of the QDs monolayer. QDs monolayer was formed by electrostatic interaction between the negative charge group on the CdSe–ZnS QDs surface and the positive charge group on the hBN surface. The device has a WORM characteristic due to the presence of QDs in the current-voltage (I–V) measurement. When a bias is applied, carriers were initially trapped by tunneling due to the QDs and then a conductive filament was formed in the hBN, which were not detrapped and exhibit characteristics of write-once-read-many-times memory. The maximum ON/OFF ratio of the current for the devices was as large as 4 × 10, and the endurance was 5 × 10 4 cycles, and a retention time was larger than 1 × 10 5 s. In order to explain the carrier transport mechanism and conductive filament of the WORM memory device caused by QDs, it through various methods such as I–V fitting data, simulation, and conductive AFM. Unlike the conventional conductive filament mechanism, through random diffusion such as Ag filament, the Au/hBN/QD/hBN/ITO/PET structures implemented a consistent conductive filament using Au metal and QDs active layer.

TiO x -based self-rectifying memory device for crossbar WORM memory array applications* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61774079 and 61664001), the Science and Technology Plan of Gansu Province, China (Grant No. 20JR5RA307), and the Key Research and Development Program of Gansu Province, China (Grant No. 18YF1GA088).

Resistive switching with a self-rectifying feature is one of the most effective solutions to overcome the crosstalk issue in a crossbar array. In this paper, a memory device based on Pt/TiO x /W structure with self-rectifying property is demonstrated for write-once-read-many-times (WORM) memory application. After programming, the devices exhibit excellent uniformity and keep in the low resistance state (LRS) permanently with a rectification ratio as high as 104 at ± 1 V. The self-rectifying resistive switching behavior can be attributed to the Ohmic contact at TiO x /W interface and the Schottky contact at Pt/TiO x interface. The results in this paper demonstrate the potential application of TiO x -based WORM memory device in crossbar arrays.

Flexible and fully transparent WORM memory devices based on Ag nanoparticles blended with poly(ethylene-co-vinyl acetate)

Silver nanoparticles (Ag NPs) blended with poly(ethylene-co-vinyl acetate) (EVA) were used as an active layer to fabricate flexible and fully transparent memory devices by using the spin-coating method followed by the thermal roll lamination technique with the structure of ITO/EVA:Ag NPs/ITO. The devices exhibited a non-volatile write-once-read-many-times (WORM) memory type and possessed current bi-stability with ON/OFF current ratio within the range of 104-105 at a reading voltage of +1 V. The data continuous read operations reached more than 10 h, which revealed the stability of the memory devices over a long period of time. The conduction mechanisms of the memory device could be explained by theoretical models and proposed by electron trapping at the trapping center of Ag NPs inside the EVA matrix. In addition, the memory device showed a fast response of 231 ns and was fully transparent with maximum transmittance of the device at 83.2%. Further, the devices could be operated under the bending condition of more than 0.83% strain.

Transparent write-once-read-many-times memory devices based on an ITO/EVA:rGO/ITO structure

Fully transparent electronic devices play an important role in electronic applications. Well-known examples are the wearable devices and transparent displays; their applications are transparent memory devices of interest. The transparent write-once-read-many-times (WORM) memory devices based on synthesized graphene oxide (GO) blended in poly (ethylene-co-vinyl acetate) (EVA) had been fabricated by a thermal roll lamination technique in which the synthesized GO inside the EVA matrix was reduced into reduced graphene oxide (rGO) during the lamination process. The memory devices exhibited an optical transmittance of more than 60% in the visible light region. The conduction mechanisms of the memory devices were identified using theoretical models based on the current–voltage (I–V) curves. The fitted I–V results were described by the conductive filaments in which electrons were transported through the rGO direct contacts. In addition, the maximum ON/OFF current ratio of the transparent WORM memory device was approximately 5 × 105, and the retention time tests showed two current states with good data retention properties under different temperatures.

Thickness effect of nickel oxide thin films on associated solution-processed write-once-read-many-times memory devices

With self-prepared nickel acetate based solution, NiO thin films with different thicknesses have been fabricated by spin coating followed by thermal annealing. By forming a two-terminal Ag/NiO/ITO structure on glass, write-once-read-many-times (WORM) memory devices are realized. The WORM memory behavior is based on a permanent switching from an initial high-resistance state (HRS) to an irreversible low-resistance state (LRS) under the application of a writing voltage, due to the formation of a solid bridge across Ag and ITO electrodes by conductive filaments (CFs). The memory performance is investigated as a function of the NiO film thickness, which is determined by the number of spin-coated NiO layers. For devices with 4 and 6 NiO layers, data retention up to 104 s and endurance of 103 reading operations in the measurement range have been obtained with memory window maintained above four orders for both HRS and LRS. Before and after writing, the devices show the hopping and ohmic conduction behaviors, respectively, confirming that the CF formation could be the mechanism responsible for writing in the WORM memory devices.

Operating mechanisms of highly-reproducible write-once-read-many-times memory devices based on graphene quantum dot:poly(methyl silsesquioxane) nanocomposites

Resistive switching memory devices were fabricated utilizing graphene quantum dot (GQD):poly(methyl silsesquioxane) (PMSSQ) hybrid nanocomposites. Current-voltage curves for the Al/GQD:PMSSQ/indium-tin-oxide devices at room temperature showed write-once-read-many-times memory (WORM) characteristics with an ON/OFF ratio of as large as 106 due to the homogeneous dispersion of the GQDs in the PMSSQ matrix. The WORM devices maintained retention times larger than 2 × 104 s under ambient conditions. The devices showed high device-to-device reproducibility with threshold-voltage distributions between 3 and 5 V. The ON state currents remained between 10−6 and 10−3 A, and the OFF state currents maintained between 10−12 and 10−9 A. The operating mechanisms concerning the interaction between the GQDs and the PMSSQ matrix for the resistive-switch phenomenon were analyzed on the basis of the I–V results and with the aid of the energy band diagram.

Resistive switching memory devices based on electrical conductance tuning in poly(4-vinyl phenol)-oxadiazole composites.

Nonvolatile memory devices, based on electrical conductance tuning in thin films of poly(4-vinyl phenol) (PVP) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) composites, are fabricated. The current-voltage characteristics of the fabricated devices show different electrical conductance behaviors, such as the write-once read-many-times (WORM) memory effect, the rewritable flash memory effect and insulator behavior, which depend on the content of PBD in the PVP + PBD composites. The OFF and ON states of the WORM and rewritable flash memory devices are stable under a constant voltage stress or a continuous pulse voltage stress at a read voltage. The memory mechanism is deduced from the modeling of the nature of currents in both states in the devices.

Write-once-read-many-times characteristics of Pt/Al2O3/ITO memory devices

Nonvolatile write-once-read-many-times (WORM) Pt/Al2O3/ITO memory devices prepared at room temperature were demonstrated. The WORM memory devices show irreversible transition from the initial low resistance (ON) state to the high resistance (OFF) state, high ON/OFF ratio, long data retention, and good reading endurance in air at room temperature. The high performances are promising for employing the Pt/Al2O3/ITO WORM memory devices in permanent storage of information. The nonvolatile memory behaviors could be attributed to the formation and permanent rupture of conductive filament consisting of positively charged oxygen vacancies.

Electrical switching and conduction mechanisms of nonvolatile write-once-read-many-times memory devices with ZnO nanoparticles embedded in polyvinylpyrrolidone

We reported on the influence of zinc oxide nanoparticles (ZnO NPs) on the electrical bistable behavior of nonvolatile write-once-read-many-times (WORM) memory devices based on an indium-tin oxide/polyvinylpyrrolidone (PVP):ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure. The maximum ON/OFF current ratio of the nonvolatile WORM memory devices was approximately 3×103 and the devices remained in the ON state even after the applied voltage was turned off. In addition, reliability studies for response time and once write/continuous read operations of the optimal ZnO NPs concentration are presented. The response times of both rise-time and fall-time were about 3 and 6μs respectively. The conduction mechanisms of all voltage regions of the device were analyzed by theoretical models and electron trapping in the ZnO NPs of the electron tunneling among a PVP matrix was discussed.

Synthesis of Alternating Copolysiloxane with Terthiophene and Perylenediimide Derivative Pendants for Involatile WORM Memory Device

Alternating copolysiloxane with both electron donor terthiophene and electron acceptor perylenediimide derivative pendants (PTSi‐alt‐PDISi) is synthesized successfully. The polymer exhibits high decomposition and glass transition temperatures, good film‐forming ability, and high morphological stability. The estimated HOMO and LUMO energy levels of PTSi‐alt‐PDISi are –5.77 and –3.90 eV, respectively. The fabricated memory device with the configuration of ITO/PTSi‐alt‐PDISi/Au(Al) shows nonvolatile write‐once‐read‐many‐times (WORM) memory characteristics. Its turn‐on threshold voltage is 1.7 V, while its ON/OFF current density ratio is around 104 in ambient atmosphere. The well‐defined memory property of PTSi‐alt‐PDISi is attributed to the transition of the pendant terthiophene and perylenediimide groups from the disoriented state to the ordered face‐to‐face conformation at the threshold voltage and the charge transfer interaction between pendent terthiophene donor and perylenediimide acceptor moieties, which are confirmed by XRD patterns and fluorescence emission measurement. This suggests that the new donor–acceptor polysiloxanes have potential applications in the field of memory devices.

Formation of SiGe nanocrystals embedded in Al2O3 for the application of write-once-read-many-times memory

The structure of SiGe nanocrystals embedded in Al2O3 formed by sequential deposition of Al2O3/Si/Ge/Al2O3 and a subsequent annealing was confirmed by transmission electron microscopy and energy dispersive spectroscopy (EDS), and its application for write-once-read-many-times (WORM) memory devices was explored in this study. By applying a −10 V pulse for 1 s, a large amount of holes injected from Si substrate are stored in the nanocrystals and consequently, the current at +1.5 V increases by a factor of 104 as compared to that of the initial state. Even with a smaller −5 V pulse for 1 μs, a sufficiently large current ratio of 36 can still be obtained, verifying the low power operation. Since holes are stored in nanocrystals which are isolated from Si substrate by Al2O3 with good integrity and correspond to a large valence band offset with respect to Al2O3, desirable read endurance up to 105 cycles and excellent retention over 100 yr are achieved. Combining these promising characteristics, WORM memory devices are appropriate for high-performance archival storage applications.

Two-Terminal Charge-Trapping WORM Memory Device Using Anodic Aluminum Oxide

A two terminal write-once-read-many-times (WORM) memory device based on charge trapping near the interface of anodic aluminum oxide and p+Si is demonstrated. An investigation of the memory properties as a function of the interface morphology is presented. A memory window, defined as the ratio of current before and after the application of a given "write" pulse, of the order of 100 was demonstrated for the best devices studied. The devices also exhibited good stability with time both in terms of retention and in terms of measuring cycles in the "written" state. A variation of the results by less than 10% over 50 measured devices is reported. The current through the devices is analyzed and the mechanisms behind the observed behavior are discussed in detail.

Enhancing nonvolatile write-once-read-many-times memory effects with SiO2 nanoparticles sandwiched by poly(N-vinylcarbazole) layers

A nonvolatile write-once-read-many-times (WORM) memory device based on the ITO/poly(N-vinylcarbazole) (PVK)/SiO2 nanoparticles (SiO2 NPs)/PVK/Al structure is demonstrated. By employing SiO2 NPs, the ON/OFF current ratio of the device is enlarged from 20 to 7 × 102 at a low voltage, and the device shows obvious negative differential resistance (NDR) phenomenon at a high electric field. Compared with the ITO/PVK/Al and ITO/PVK/PVK/Al devices, it is found that the size of the SiO2 NPs sandwiched between two PVK layers plays an important role in enhancing the reproducibility and the ON state retention time of the WORM memory device. The WORM memory mechanisms of the device are investigated on the basis of the current–voltage (I–V) characteristics and scanning electron microscopy image. Here, the strength of the NDR effect in our devices is only related to the charge defects and traps of PVK films, and it can be enlarged by embedding charge traps into the PVK films.

Nonvolatile memory devices based on electrical conductance tuning in poly(N-vinylcarbazole)–graphene composites

Nonvolatile memory devices, based on electrical conductance tuning in thin films of poly(N-vinylcarbazole) (PVK)–graphene composites, are fabricated. The current density–voltage characteristics of the fabricated device show different electrical conductance behaviors, such as insulator behavior, write-once read-many-times (WORM) memory effect, rewritable memory effect and conductor behavior, which depend on the content of graphene in the PVK–graphene composites. The OFF and ON states of the WORM and rewritable memory devices are stable under a constant voltage stress or a continuous pulse voltage stress at a read voltage of −1.0V. The memory mechanism is deduced from the modeling of the nature of currents in both states in the devices.

Electrically permanent memory characteristics of an ionic conjugated polymer

In this work, an ionic conjugated polymer with propargyl side groups (poly(2-ethynylpyridinum bromide); PEP-P) was synthesized. High-quality thin films with smooth surfaces were prepared from this polymer on silicon substrates and metal electrodes, via a conventional, simple solution-coating and drying process. Synchrotron X-ray scattering analysis showed that the polymer in the nanoscale thin films was amorphous, but was somewhat preferentially oriented in the film plane, rather than randomly oriented. Using synchrotron X-ray reflectivity analysis, the electron densities and interfaces between the polymer film layers and the silicon substrate and metal electrodes were examined in detail. To our knowledge, PEP-P is the first ionic conjugated polymer to be shown to exhibit electrically nonvolatile memory behavior. The polymer in the nanoscale thin films showed excellent write-once-read-many-times (WORM) memory characteristics, without any polarity. The switching-ON voltage was lower than 1.5 V. WORM memory devices based on PEP-P were highly stable, even under ambient air conditions. PEP-P therefore has great potential as a candidate material for the low-cost mass production of high-performance, programmable unipolar WORM memory devices with very low power consumption.

Conjugated Fluorene Based Rod–Coil Block Copolymers and Their PCBM Composites for Resistive Memory Switching Devices

We report the fabrication and characterization of polymer resistive switching memory devices fabricated from conjugated rod-coil poly[2,7-(9,9-dihexylfluorene)]-block-poly(2-vinylpyridine) diblock copolymers (PF-b-P2VP) and their hybrids with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). PF(10)-b-P2VP(37) and PF(10)-b-P2VP(68)-based devices exhibited the volatile static random access memory (SRAM) characteristic with an ON/OFF current ratio up to 1 × 10(7), which was explained by the trapping/back transferring of charge carrier. PF(10)-b-P2VP(68) had a longer holding time in the ON state than PF(10)-b-P2VP(37) because of the delayed back transfer of trapping carriers originally from the longer P2VP blocks. The PCBM aggregated size in the composite thin films were effectively reduced by PF-b-P2VP compared to the homopolymer of PF or P2VP, because of the supramolecular charge transfer interaction, as evidenced by absorption and photoluminescence spectra. Their PCBM/PF-b-P2VP composite devices changed from the nonvolatile write-once-read-many-times (WORM) memory to the conductor behavior as the PCBM composition was increased. The electric-field induced charge transfer effect enabled the electrical bistable states for the applications in digital WORM memory device. The tunable memory characteristics through the block length ratio of block copolymers or PCBM composition provided the solution-processable charge storage nanomaterials for programmable high density memory device with a reducing bit cell size.

Memory effects and carrier transport mechanisms of write-once- read-many-times memory devices fabricated using poly(3-hexylthiophene) molecules embedded in a polymethylmethacrylate layer on a flexible substrate

The memory effects and the carrier transport mechanisms of write-once-read-many-times (WORM) memory devices fabricated using poly(3-hexylthiophene) (P3HT) molecules embedded in a polymethylmethacrylate (PMMA) polymer layer on a flexible substrate were investigated. Current-voltage (I-V) curves at 300 K for Al/P3HT:PMMA/indium-tin-oxide WORM device showed a permanent memory behavior with an ON/OFF ratio of 104. The estimated retention time of the ON state of the WORM device was more than 10 years. The carrier transport mechanisms of the WORM memory devices are described using several models to fit the experimental I-V data.

Nonvolatile Write-Once-Read-Many Times Memory Devices Based on the Composites of Poly(4-vinylphenol)/Vulcan XC-72

We fabricated write-once-read-many times (WORM) type organic memory devices in 8 x 8 cross-bar structure. The active material for organic based WORM memory devices is mixture of both poly(4-vinyphenol) (PVP) and Vulcan XC-72s. From the electrical characteristics of the WORM memory devices, we observed two different resistance states, low resistance state and high resistance state, with six orders of ON/OFF ratio (I(ON)/I(OFF) - 10(6)). In addition, the WORM memory devices were maintained for longer than 50000 seconds without any serious degradation.