A nonvolatile organic resistive switching memory based on lotus leaves

Oxide-based metal-insulator-metal structures are of special interest for future non-volatile resistive switching memory, also known as resistive random-access memory (ReRAM) [1]. In such cells, resistive switching in the oxide layer is widely believed to be caused by the electric‐field‐driven motion and internal redistribution of oxygen ions (or vacancies) with filament region, resulting in valence change on the cation sublattice [1]. There are very special oxide-material groups which undergo a reversible topotactic phase transition between the insulating brownmillerite and conducting perovskite structure upon the redox processes [2]. Recently, we introduced a topotactic phase changing brownmillerite SrMO2.5 (M = Co, Fe, etc.) as a novel material platform to harness exceptional oxygen ion transport properties for resistive switching memory devices for the first time [3,4]. Accordingly, we were able to demonstrate a promising resistive switching performance with high endurance (> 106 cycles), fast switching speed (10 ns), and high uniformity in key switching parameters [5,6]. By using X-ray absorption spectromicroscopy and in site TEM, we could prove that the resistance change is indeed caused by a reversible topotactic phase transition between an insulating SrFeO2.5 brownmillerite and a conductive SrFeO3 perovskite phase [7]. By engineering the ordered oxygen vacancy channels in brownmillerite structure, we further demonstrated complimentary resistive switching and synaptic memory behavior in the SrFeOx memristive devices [8]. Beyond resistive switching, our results demonstrated a clear path to tune the ionic controlled functionalities in the multiphase oxides by electrical bias at the nanoscale level, which is highly interesting in the field of magnetoelectric and spintronic device applications. Reference [1] R. Waser, R. Dittmann, C. Staikov, K. Szot, Adv. Mater. 21 (2009) 2632–2663. [2] H. Jeen, W.S. Choi, M.D. Biegalski, C.M. Folkman, I.C. Tung, D.D. Fong, J.W. Freeland, D. Shin, H. Ohta, M.F. Chisholm, H.N. Lee, Nat. Mater. 12 (2013) 1057–1063. [3] O.T. Tambunan, et. al. Appl. Phys. Lett. 105 (2014) 063507. [4] V.R. Nallagatla, C.U. Jung, Appl. Phys. Lett. 117 (2020). [5] S.K. Acharya, R.V. Nallagatla, O. Togibasa, B.W. Lee, C. Liu, C.U. Jung, B.H. Park, J.Y. Park, Y. Cho, D.W. Kim, J. Jo, D.H. Kwon, M. Kim, C.S. Hwang, S.C. Chae, ACS Appl. Mater. Interfaces 8 (2016) 7902–7911. [6] S.K. Acharya, J. Jo, N.V. Raveendra, U. Dash, M. Kim, H. Baik, S. Lee, B.H. Park, J.S. Lee, S.C. Chae, C.S. Hwang, C.U. Jung, Nanoscale 9 (2017) 10502–10510. [7] V.R. Nallagatla, T. Heisig, C. Baeumer, V. Feyer, M. Jugovac, G. Zamborlini, C.M. Schneider, R. Waser, M. Kim, C.U. Jung, R. Dittmann, Adv. Mater. 31 (2019) 1903391. [8] V. Raveendra Nallagatla, J. Kim, K. Lee, S. Chul Chae, C. Seong Hwang, C. Uk Jung, Cite This ACS Appl. Mater. Interfaces 12 (2020) 41740–41748.

The integration of multiple functional devices to achieve complex functions has become an essential requirement for future wearable biomedical electronic devices and systems. In this paper, we present a flexible multi-functional device composed of a biocompatible organic polymer resistive random-access memory (RRAM) and a photoresistor for wearable image sensing application. The resistive layer of organic polymer RRAM is composed by polychloro-para-xylylene (parylene-C), which is a flexible, transparent, biocompatibility and chemical stability polymer material. What is more, parylene-C is quite safe to be used within human body as it is a Food and Drug Administration (FDA)-approved material. This organic RRAM shows stable switching characteristics, low operation voltages (3.25 V for set voltage and $-$0.55 V for reset voltage), low static power consumption, high storage window and good retention properties ($>$10$^4$ s). A multi-functional device that can detect the light intensity of incident light and simultaneously store the information in the memory devices for wearable image sensing application was proposed and fabricated by integrating the organic resistive memory and a photoresistor. The threshold of incident light intensity can be easily adjust by changing the external voltage. This device is promising for building wearable electronic systems with various multiple functionalities.

Vanadium oxide thin film deposited on Si by atomic layer deposition for non-volatile resistive switching memory devices

Recently, solution-processed resistive switches for wearable electronics have got tremendous attention and are required for different applications due to their easy process and fabrication. Hence, this paper proposes the solution-processed resistive switching memory device based on two polymers, poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4, 8-diyl)] (F8BT) and polyvinylpyrrolidone (PVP) composite, which is fabricated on a flexible indium–tin–oxide (ITO)-coated polyethylene terephthalate (PET) substrate through spin coating technology. The fabricated device demonstrates a perfect non-volatile bipolar resistive switching through small operating voltage sweeping of ± 1.5 V, and its high-resistance state (HRS) and low-resistance state (LRS) are 92678.89 Ω and 337.85 Ω, respectively. To verify the non-volatility and long-term stability, the device is checked for more than 700 endurance cycles. During these cycles, the variations of HRS and LRS are 48 Ω and 37.35 Ω, respectively. The retention time is checked for more than 60 days, and the ROFF/RON ratio is 274.31. The bendability is carried out up to bending diameters < 10 mm, and FESEM is used for the morphological characteristics of the device. Conduction mechanism of the proposed device is supported by space charge-limited conduction (SCLC) which is explained by the log–log I–V slope-fitting curve. The results insure that the F8BT:PVP composite-based resistive switching device is to be a potential candidate for the future flexible and low-power non-volatile resistive switching memory device.

Effect of top electrode material on resistive switching properties of WN based thin films for non volatile memory application

This work focuses on Au/HfO2/TiN nonvolatile resistive memory [resistive random access memories (RRAMs)] stacks, where HfO2 is deposited by the atomic layer deposition technique on TiN electrodes. For as-grown RRAMs, no Reset is observed (the structure remains locked in a low resistive state). It is observed that an NH3 plasma treatment of the HfO2/TiN bilayer can restore a Reset stage. X-ray photoelectron spectroscopy analyses showed that the Reset recovery is related to a modification of the HfO2/TiN interface via transformation of the TiON interfacial layer. Thus, postdeposition plasma treatments of the oxide/electrode interface are identified as a valuable tool to improve the switching properties of oxide-based RRAMs.

We report a novel nonvolatile resistive switching memory with a Cu/Ta 2 O 5 /TiN structure. This memory device has low power (100 μA/1.5 V), large threshold voltage of 0.8V, moderate endurance of 102 cycles, and excellent retention with high resistance ratio of 2.4x102 after 105 s (26 hours). Furthermore, the threshold voltage can be increased by both the thickness of Ta2O5 solid electrolyte and annealing temperatures, which can make easier to design the memory circuit.

The resistive random access memory (RRAM) device has been widely studied due to its excellent memory characteristics and great application potential in different fields. In this paper, resistive switching materials, switching mechanism, and memory characteristics of RRAM are discussed. Recent research progress of RRAM in high-density storage and nonvolatile logic application are addressed. Technological trends are also discussed.

Over the last decade, two-dimensional (2D) materials have drawn much attention for the next-generation electron devices, optoelectronics and bioelectronics [1, 2]. Recently, non-volatile resistance switching (NVRS) behavior has been observed in various 2D materials, including graphene oxide, functionalized MoS2 and partially degraded black phosphorus, etc. [3-5], where the resistance can be modulated between a high-resistance state (HRS) and a low-resistance state (LRS) [6]. In addition, there are several reports about NVRS in multi-layer hexagonal boron-nitride (h-BN) [7]. Although 2D materials are widely regarded as promising candidates to overcome vertical scaling obstacle in NVRS based on conventional metal-insulator-metal (MIM) structure [3], few researchers believe that single-layer 2D material can produce NVRS [8] due to excessive leakage current. Sangwan et al. discovered that grain boundaries in single-layer MoS2 can produce NVRS based on planar (horizontal) structure [9]. However, the planar structure without 3D stacking ability has the limitation of low integration density. Recently, we reported the discovery that NVRS phenomenon is accessible in a variety of single-layer transition metal dichalcogenides (TMDs) in vertical MIM structure [10, 11]. Herein, we report another 2D insulating material, single-layer h-BN, also shows stable and desirable NVRS behavior. Compared with monolayer TMDs with three atomic layers, monolayer h-BN has only one atomic layer and ~0.33 nm in thickness, which is the thinnest active layer in non-volatile resistance memory. These devices can be labelled as “atomristor”, which means the memristor effect in atomically thin nanomaterials.[12] The h-BN atomristors have been studied using a crossbar structure (Au/h-BN/Au) [See Figure a]. Figure b shows a representative I-V curve of h-BN atomristor, demonstrating forming-free switching with large on/off ratio (up to >106) and low switching voltage (down to <1V). In addition, pulse operation proves its fast switching speed (<15 ns), which is comparable to the state-of-the-art in 2D memory. Ab-initio simulation reveals that the metal ion substitution in sulfur vacancies may be related to the switching from HRS to LRS, and area scaling test result is also consistent with this conductive-bridge-like mechanism. The results of this work indicate a potential universal resistive switching behavior in 2D insulating monolayers, which is applicable to memory technology, neuromorphic computing, RF switch and flexible electronics.

Since the introduction of a concept of memristor by Leon Chua in 1971 as the fourth fundamental passive component in electronic circuits, and its functional prototype introduced by HP Lab in 2008, research has increasingly concentrated on exploring its application ability in electronic circuits. First, the memristor principles have been exploited in various memory devices within a classical von Neumann computer architecture. The properties of memristor were found particularly suitable for resistive random-access memories (ReRAM).1 Contrary to the classical von Neumann architecture based on the binary logic, advanced methods like neuromorphic computing require an electronic element with continuously varying states depending on previous input signals. The input signal consists of a sequence of voltage spikes that stimulate the device, similarly as a neural synapse does in biological systems. These trains of spikes change the output state of the device continuously to a lower resistance state, mimicking a learning process. Having these functionalities, memristors are considered to be possible building blocks of brain-inspired neuromorphic computing and artificial neural networks.2 Memristors with neurosynaptic functionality take a continuity of resistance values with synaptic weights modulated by the number and frequency of homogeneous spikes. In order to mimic the neural synapse, memristors must exhibit analog properties including non-abrupt switching transitions, memory loss, continuously variable resistance states, and predictable response. We present such functionality on thin films of two kinds of organic materials:(i) A newly synthesized poly(methacrylamide) derivative with carbazole charge transporting group separated from the polymer backbone by an alkyl chain, was synthesized recently and found to behave as a bistable resistive memory.3 Thin films of the polymer, sandwiched between Al and ITO electrodes, exhibit rewritable flash memory behavior with bistable conductivity, with the set switching voltage ranging from 2 to 4.5 V, and the current ON/OFF ratio exceeding 100. The device demonstrates a remarkable lifetime and remains persistent for more than 104 seconds under the static voltage of 0.5 V. The main physical mechanisms driving the resistive switching have been attributed to the electric field-induced reorientation of heterocycles, which modulates charge transport, and trapping/detrapping of charges in localized states within the bulk of the polymer. Memory persistence is strengthened by the physical crosslinking caused by hydrogen bonds between amide and carbonyl groups in the side chains. Depending on the layer thickness, electrode material and applied voltage range the electrical characteristics can change from bistable to analog behavior showing memristive properties.(ii) Ditopic π-conjugated bis-terpyridine (tpy-) ligands form supramolecular structures in which the ligands are interconnected by complexation with metal ions. These molecules offer a low energy operation with improved reproducibility of memristive characteristics. They possess low-lying molecular energy levels and relatively narrow bandgap enabling the low-power consuming resistive switching and longer stability. We used a newly synthesized bis-terpyridine ligands based on 9-phenyl carbazole, Cbz, and its complex with cobalt Co2+ as an active layer of a memristor device. A two-terminal memristor architecture was employed, consisting of the ligand or its cobalt complex deposited on an ITO substrate, and aluminum, gold, or gallium as a top electrode. We show that Cbz as a memristor active layer exhibits an excellent nonvolatile bistable memory behavior, while Co-Cbz exhibits both electronic memory and synaptic plasticity. The ligand exhibited bi-stable conduction and non-volatile memory effect with persistence over 18 hours, while its cobalt complex displayed a synaptic effect characterized by subsequent potentiation and depression cycles. Voltage-induced modulation of synaptic weight was observed with a low applied voltage below 500 mV and short pulse duration below 20 ms. Pair pulse facilitation and pair pulse depression demonstrated significant synaptic weight changes, showing the cobalt complex ability to emulate biological neurons. The operational mechanism was attributed to a combination of the redox effect in the ligand and complex, and to the voltage-induced migration of perchlorate counter ions and subsequent redox processes, altering the metal-to-ligand charge transfer band and changing the conducting state of the active layer.Acknowledgments: The work was financially supported by the Czech Science Foundation, project 24-10384S, and MEYS of the Czech Republic, project LUAUS24032. Strukov, D. B., Snider, G. S., Stewart, D. R. & Williams, R. S. The missing memristor found. Nature 453, 80–83 (2008).Chang, T., Jo, S. H. & Lu, W. Short-term memory to long-term memory transition in a nanoscale memristor. ACS Nano 5, 7669–7676 (2011).Panthi, Y.R, Pfleger, J., Vyprachticky, D., Pandey, A., Thottappali, M.A., Sedenkova, I., Konefal, M. & Foulger, S.H. Rewritable resistive memory effect in poly[N-(3-(9H-carbazol-9-yl)propyl)-methacrylamide] memristor. Mater. Chem. C 11, 17093 (2023).

Organic–inorganic and polymer-based non-volatile resistive switching memory devices have gained increasing attention because of their excellent data storage capacity, flexibility, low power consumption, durability and widely used in modern electronics. Among the materials, 2D layered nanomaterial and polymer nanocomposites have attracted increasing attention because they can easily be integrated into electronic and optoelectronic devices due to their easy fabrication process, high storage capacity and promising structural properties. Fabrication of bistable memory devices based on organic–inorganic nanohybrid with charge blocking materials and polymer nanocomposite has been broadly elucidated for their extraordinary material properties. Recent enhancement in flexible memory devices based on organic–inorganic semiconductor dispersed in ferroelectric blend polymer has been investigated thoroughly in this review paper. Recent advancement in 2D layered material, ferroelectric polymer and the development of non-volatile memristive devices is extensively explained to understand their importance in flexible devices. Outstanding memory characteristics such as current on/off ratio, Vset, Vreset retention time, endurance of nanohybrid embedded polymer nanocomposite are also discussed meticulously. Next-generation memristor devices based on 2D nanomaterial and ferroelectric polymer with high speed and high density for developing neuromorphic computing system have been demonstrated.

In this work, an engineered submicrometer-scale bilayer stacking in via-type one-time programmable (OTP) memory and self-rectified resistive switching memory [resistive random access memory (ReRAM)] is demonstrated. The current development has achieved that co-existing memory functionality (OTP and ReRAM) with mitigating scaling requirement (fuse voltage trending with via size scaling), low fabrication complexity [via-fuse vs. gate-dielectric anti-fuse (AF)], and match with the current metal fuse technology ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$ </tex-math></inline-formula> 2 V). In addition, an electrode engineered has been proposed to realize low programming voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 1.9 V) in via-fuse OTP featuring by metal–insulator–metal advanced back-end-of-line (BEOL) process with ruthenium materials. The impact of via-size, programming window, stacked structures, and integration capability has been extensively studied. Our results provide a pathfinding of high density, integration capability, low programming voltage, multifunctionality between programmable read-only memory (PROM), and resistive switching memory co-existing in future embedded applications.

Resistive memory (RRAM) based on a solid–electrolyte insulator is a type of critical nanoscale device with promising potential in non-volatile memory, analog circuits and neuromorphic synapse applications.

Among the novel non-volatile memories, the electrochemical metallization (ECM)-type Resistive Random Access Memory (ReRAM) stands out from all the competitors. ECM-type memories use active metals as the electrode, such as copper. However, copper is hard to pattern by using dry etch. Therefore in this study, the copper chemical displacement technique (CDT) was applied to form the electrode of ReRAM device. We used CDT-Cu to solve the etch problem and improve the electric characteristics of device. The obtained results showed that samples using CDT-Cu exhibited lower set electric field than that of sample without CDT-Cu. The current density in the LRS, as well as operation electrical field, was also improved by CDT-Cu method. The retention of CDT-Cu sample can retain over 104 seconds.

This paper presents a novel analog resistive random-access memory (RRAM), named NeuroMem, which consists of Au/GO/Au. The device’s resistance can be tuned to any value within its R OFF to R ON range with high precision. The analog characteristic of NeuroMem mimics the memorization behavior of the brain, which makes it great asset for artificial neural network applications. In this work, NeuroMem-based crossbars are fabricated to hold the synaptic weights needed to perform Iris classification. The weight values are mapped to conductance states within NeuroMem R OFF to R ON range, and then written accurately on the actual devices. Unlike other RRAM-based hardware with limited conductance states, in this work no quantization is needed which enables efficient inmemory-computing without scarifying accuracy. Furthermore, Neuromem device has been demonstrated in crossbars on flexible polymer substrate using standard photolithography process, which facilitates producing low cost flexible electronics. This work opens up great insights towards realizing RRAM-based computing at the edge.