Write-once-read-many-times Device 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.

Barium titanate write-once read-many times resistive memory with an ultra-high on/off current ratio of 108

Write-once read-many times (WORM) resistive memories fabricated using perovskite barium titanate (BaTiO3, BTO) resistive switching (RS) layer are demonstrated in this study. The BTO RS layers with thicknesses of 3.2–8.5 nm are fabricated by radio frequency sputtering at room temperature. Annealing processes at 400–600 °C are used to modify the oxygen vacancy content in the BTO layer and thereby control the OFF-state current of an Al/BTO/n+-Si WORM device. The ION/IOFF ratio increases with increasing annealing temperature until 500 °C. A negative effect is observed when the temperature further rises to 600 °C. The optimal Al/BTO/n+-Si device shows an ultra-high ION/IOFF ratio of 108 and a low coefficient of variation of 6% for set voltages. The writing speed is 80 ns and the power consumption is 400 pJ. In addition, retention and stress tests are performed to confirm high data storage reliability of the device. Mechanisms for voltage-polarity independent writing process and the related energy band diagrams are explored.

Two-Terminal Nonvolatile Write-Once-Read-Many-Times Memory Based on All-Inorganic Halide Perovskite.

Write-once-read-many-times (WORM) memory belonging to an important non-volatile memory type achieves the read-only state after the write operation and is used in the fields of data security storage widely. WORM memory has been developed based on a variety of materials. In recent years, halide perovskites have become the research hotspot material for this memory due to its excellent properties. Here, the all-inorganic CsPbBr3 perovskite thin film was prepared on a FTO substrate by using a two-step method. The prepared CsPbBr3 thin films have the characteristics of densely packed crystal grains and smooth surface. The device, having the FTO/CsPbBr3/Al sandwich structure by evaporating the Al electrode onto the CsPbBr3 thin film, represents the typical WORM behavior, with long data retention time (104 s), a low operation voltage (2.1 V) and a low reading voltage (0.1 V). Additionally, the resistance transition mechanism of the resulting WORM devices was analyzed.

Write-once-read-many-times resistive switching behavior of amorphous barium titanate based device with very high on-off ratio and stability

Write once read many times (WORM) memory devices based on the resistive switching mechanism of a sputtered amorphous BaTiO3 (am-BTO) thin film in a metal–insulator–metal structure is fabricated on a FTO coated glass substrate with a silver top contact. Fabricated devices show the switching from a low-conductance state to a high-conductance state with the formation of conductive filament(s) in the am-BTO layer. The memory characteristics are investigated as a function of thickness of am-BTO layer, which is determined by varying the deposition time. Devices with all deposited thicknesses show data retention for more than 4000 s and 300 reading cycle. Devices with 180 nm thickness show a high on-off ratio on the order of 106. The fabricated WORM devices exhibit good reading-endurance and data-retention characteristics.

Robust and Transient Write‐Once‐Read‐Many‐Times Memory Device Based on Hybrid Perovskite Film with Novel Room Temperature Molten Salt Solvent

AbstractOrganic–inorganic hybrid perovskites (OIHPs) have emerged as a novel class of functional materials with applications in solar cells, photodetectors, light‐emitting diodes, and resistive switching memories. Generally, perovskite films are fabricated with toxic solvents and antisolvent technique under inert atmosphere, which limits the commercial applications of OIHP‐based devices. To address this issue, uniform CH3NH3PbBr3 (MAPbBr3) film is fabricated through a facile one‐step spin‐coating method by directly dissolving perovskite precursor in a room‐temperature molten salt methylammonium acetate under air ambient condition. The nonvolatile resistive switching devices based on MAPbBr3 exhibit robust and reproducible write‐once‐read‐many‐times (WORM) memory properties with a large ON/OFF ratio (106), reliable retention properties (104 s), and somewhat ternary resistive characteristic. The conductive filaments in the perovskite thin film are proposed to induce the electrical conductivity switching. Additionally, the MAPbBr3 films can be dissolved rapidly in deionized water within 5 s, showing the transient characteristics. This work demonstrates a new perspective on the perovskite film fabrication and shows the potential application of MAPbBr3 in transient WORM devices.

A reversible bipolar WORM device based on AlOxNy thin film with Al nano phase embedded

An Al-rich AlOxNy thin film based reversible Write-Once-Read-Many-Times (WORM) memory device with MIS structure could transit from high resistance state (HRS, ∼1011Ω) to low resistance state (LRS, ∼105Ω) by sweeping voltage up to ∼20V. The first switching could be recorded as writing process for WORM device which may relate to conductive path are formed through the thin film. The conductive path should be formed by both Al nano phase and oxygen vacancies. Among of them, Al nano phases are not easy to move, but oxygen vacancies could migrate under high E-field or at high temperature environment. Such conductive path is not sensitive to charging effect after it formed, but it could be broken by heating effect, which may relate to the migration of excess Al ions and oxygen vacancies at high temperature. After baking LRS (ON state) WORM device at 200°C for 2min, the conductivity will decrease to HRS which indicates conductive path is broken and device back to HRS (OFF state) again. This phenomenon could be recorded as recovery process. Both writing and recovery process related to migration of oxygen vacancies and could be repeated over 10 times in this study. It also indicates that there is no permanent breakdown occurred in MIS structured WORM device operation. We suggest that this conductive path only can be dissolved by a temperature sensitive electro-chemical action. This WORM device could maintain at LRS over 105s with on-off ratio over 4 orders.

Realization of write-once-read-many-times memory device with O2 plasma-treated indium gallium zinc oxide thin film

A write-once-read-many-times (WORM) memory devices based on O2 plasma-treated indium gallium zinc oxide (IGZO) thin films has been demonstrated. The device has a simple Al/IGZO/Al structure. The device has a normally OFF state with a very high resistance (e.g., the resistance at 2 V is ∼109 Ω for a device with the radius of 50 μm) as a result of the O2 plasma treatment on the IGZO thin films. The device could be switched to an ON state with a low resistance (e.g., the resistance at 2 V is ∼103 Ω for the radius of 50 μm) by applying a voltage pulse (e.g., 10 V/1 μs). The WORM device has good data-retention and reading-endurance capabilities.

Interfacial dipole in organic p–n junction to realize write-once–read-many-times memory

A new approach is exploited to realize nonvolatile organic write-once–read-many-times (WORM) memory based on copper phthalocyanine (CuPc)/hexadecafluoro-copper-phthalocyanine (F16CuPc) p–n junction. The as-fabricated device is found to be at its ON state and can be programmed irreversibly to the OFF state by applying a negative bias. The WORM device exhibits a high ON/OFF current ratio of up to 2.6×104. An interfacial dipole layer is testified to be formed and destructed at the p–n junction interface for the ON and OFF states, respectively. The ON state at positive voltage region is attributed to the efficient hole and electron injection from the respective electrodes and then recombination at the CuPc/F16CuPc interface, and the transition of the device to the OFF state results from the destruction of the interfacial dipole layer and formation of an insulating layer which restricts charge carrier recombination at the interface.

Effect of Switching on Metal-Organic Interface Adhesion Relevant to Organic Electronic Devices

Considerable efforts are currently being devoted to investigation of metal-organic, organic-organic and organic-inorganic interfaces relevant to organic electronic devices such as organic light emitting diode (OLEDs), organic photovoltaic solar cells, organic field effect transistors (OFETs), organic spintronic devices and organic-based Write Once Read Many times (WORM) memory devices on both rigid and flexible substrates in laboratories around the world. The multilayer structure of these devices makes interfaces between dissimilar materials in contact and plays a prominent role in charge transport and injection efficiency which inevitably affect device performance. This paper presents results of an initial study on how switching between voltage thresholds and chemical surface treatment affects adhesion properties of a metal-organic (Au-PEDOT:PSS) contact interface in a WORM device. Contact and Tapping-mode Atomic Force Microscopy (AFM) gave surface topography, phase imaging and interface adhesion properties in addition to SEM/EDX imaging which showed that surface treatment, switching and surface roughness all appeared to be key factors in increasing interface adhesion with implications for increased device performance.

Flexible Write-Once–Read-Many-Times Memory Device Based on a Nickel Oxide Thin Film

A write-once-read-many-times (WORM) memory device based on conduction switching of a NiO thin film in a metal-insulator-metal structure is fabricated on a flexible substrate. The device can be switched from a low-conductance state (unprogrammed state) to a high-conductance state (programmed state) with the formation of conductive filament(s) in the NiO layer. The two memory states can be easily distinguished at a very low reading voltage. For example, at the reading voltage of 0.1 V, the current ratio of the state programmed at 3 V for 1 μs to the unprogrammed state is larger than 104. The WORM device exhibits good reading-endurance and data-retention characteristics. The flexible device is promising for low-cost and low-power archival storage applications.

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.

Entirely solution-processed write-once-read-many-times memory devices and their operation mechanism

We investigate the mechanism of operation of low-power write-once-read-many-times (WORM) memory devices based on injection of electrons from ZnO into PEDOT:PSS (polydioxythiophene doped with polystyrenesulfonic acid). Using Raman spectroscopy and in situ absorbance measurements, we directly observe the change of doping level of PEDOT during the device switching. Our results clearly show that the change of device conductance is due to the dedoping of p-doped PEDOT by injected electrons. Based on this understanding, we further demonstrate an entirely solution-processed low-power WORM device by inkjet printing metal electrodes onto arbitrary substrates.

Two-Terminal Write-Once-Read-Many-Times Memory Device Based on Charging-Controlled Current Modulation in Al/Al-Rich $\hbox{Al}_{2}\hbox{O}_{3}$/p-Si Diode

A write-once-read-many-times (WORM) memory device was realized based on the charging-controlled modulation in the current conduction of Al/Al-rich <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Al}_{2}\hbox{O}_{3}$</tex></formula> /p-type Si diode. A large increase in the reverse current of the diode could be achieved with a negative charging voltage, e.g., charging at <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex></formula> 25 V for 1 ms results in a current increase by about four orders. Memory states of the WORM device could be altered by changing the current conduction with charge trapping in the Al-rich <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Al}_{2}\hbox{O}_{3}$</tex></formula> layer. The memory exhibited good reading endurance and retention characteristics.

Electrical bistabilities and carrier transport mechanisms of write-once-read-many-times memory devices fabricated utilizing ZnO nanoparticles embedded in a polystyrene layer

High-resolution transmission electron microscopy images showed that ZnO nanoparticles were randomly distributed in a polystyrene (PS) layer. Current-voltage (I-V) curves at 300 K for Al/ZnO nanoparticles embedded in PS layer/indium tin oxide devices showed a current bistability with a large ON/OFF ratio of 103 for write-once-read-many-times (WORM) memory devices. The estimated retention time of the ON state for the WORM device was more than 10 years. The carrier transport mechanisms for the WORM memory device are described on the basis of the I-V results.

Electrical Conductance Tuning and Bistable Switching in Poly(N-vinylcarbazole)−Carbon Nanotube Composite Films

By varying the carbon nanotube (CNT) content in poly(N-vinylcarbazole) (PVK) composite thin films, the electrical conductance behavior of an indium-tin oxide/PVK-CNT/aluminum (ITO/PVK-CNT/Al) sandwich structure can be tuned in a controlled manner. Distinctly different electrical conductance behaviors, such as (i) insulator behavior, (ii) bistable electrical conductance switching effects (write-once read-many-times (WORM) memory effect and rewritable memory effect), and (iii) conductor behavior, are discernible from the current density-voltage characteristics of the composite films. The turn-on voltage of the two bistable conductance switching devices decreases and the ON/OFF state current ratio of the WORM device increases with the increase in CNT content of the composite film. Both the WORM and rewritable devices are stable under a constant voltage stress or a continuous pulse voltage stress, with an ON/OFF state current ratio in excess of 10(3). The conductance switching effects of the composite films have been attributed to electron trapping in the CNTs of the electron-donating/hole-transporting PVK matrix.

Non-volatile WORM memory device based on an acrylate polymer with electron donating carbazole pendant groups

Abstract A write-once read-many-times (WORM) memory device based on an acrylate polymer containing electron donating carbazole pendant groups, or poly(2-(9H-carbazol-9-yl)ethyl methacrylate) (PCz), was demonstrated. The as-fabricated device was found to be at its OFF state and could be programmed irreversibly to the ON state. The WORM device exhibited a high ON/OFF current ratio of up to 10 6 , a long retention time in both ON and OFF states, a switching time of 1 ms, and number of read cycles up to ∼10 7 . Compared to that of poly( N -vinylcarbazole) (PVK), the well-defined memory property of PCz can be attributed to the presence of the spacer between the pendant carbazole group and the polymer backbone. The molecular spacer has allowed the transition of the pendant carbazole groups from the disoriented state to the ordered face-to-face conformation at the threshold voltage.

Bi-stable State for WORM Application Based on Carbazole-containing Polymer

ABSTRACTRecently, several studies have been done by various groups to understand the memory effects behind organic materials, which include understanding in terms of conformation changes, conjugation modification and oxidation-reduction process. In this paper, a WORM (write-once read-many times) memory device using a new polymer material 2-(9H-carbazol-9-yl)ethyl methacrylate (PCz) containing carbazole donor group has been demonstrated. The device uses a MIM (metal-insulator-metal) structure with ITO coated glass as bottom electrode, the synthesized polymer material PCz as the active layer and Al as the top electrode. The toluene solution of PCz was spin-coated on the ITO, followed by solvent removal in a vacuum chamber. Finally, Al was thermally evaporated through shadow mask onto the PCz film.The memory effect of PCz was observed in the I-V characteristic of the MIM structure. The as-fabricated device is found to be in its OFF state, and can be programmed to ON state which is not reversible. The WORM device exhibits a high ON/OFF current ratio of up to 106, and shows a good retention time for both the ON and OFF states which can be sustained within a 24 h timeframe, and extrapolated to sustain for another 10 years. The effect of continuous read pulse on the ON and OFF states was evaluated and no resistance degradation is observed for read cycles up to 107 times. By comparing the electrical characteristics of PCz and PVK as well as their optimized geometry simulation corresponding to their minimized energy states, the memory effect or bi-stable states of PCz can be attributed to the long linker between the carbazole groups and backbone present in PCz which play a part in the conduction mechanism.

Fullerene-based bistable devices and associated negative differential resistance effect

We have observed bistability in single layer devices made from fullerenes (C60) mixed with polystyrene (PS) and sandwiched between two Al electrodes. By merely changing the concentration of C60 in PS we found three distinctly different device properties, namely a true insulator, a bistable device switching between an OFF and an ON state having ON–OFF current ratios larger than 104 and a write-once, read-many times (WORM) device. An additional negative differential resistance (NDR) was observed in the ON state of both the bistable and WORM devices leading to multilevel switching capability of the devices. This opens up a wide range of application possibilities of such devices in disposable printable electronics.