Resistive switching memory based on chitosan/polyvinylpyrrolidone blend as active layers

Resistive random access memories (RRAMs) have great potential as a candidate for next-generation nonvolatile memories. A 16Gb Cu-based ReRAM chip with 180MB/s write and 900MB/s read performance fabricated at the 27nm node has been demonstrated [1-3]. The remaining issue is to clarify the switching mechanism to understand the degradation mechanisms of switching and to guarantee the reliability. To overcome the problem, we employed in-situ transmission electron microscopy (TEM) and attained reproducible resistive switching. Direct observation of formation and rupture of a Cu filament were also achieved corresponding to repeatable switching of SET (high resistance to low) and RESET (low resistance to high) characteristics, respectively. The most important point is that the achieved electrical characteristics in the in-situ TEM are almost the same with those measured in the real ReRAM devices.In this work, we used two types of Cu-based ReRAM (CBRAM: Conductive Bridge RAM) memories for in-situ TEM observation. One has Cu/MoOx structure, where MoOx is used as a solid electrolyte. The TEM sample was prepared using the ion-shadow method [4], which is an ion milling technique with carbon mask particles. This method is simple and effective to make thin samples for TEM observation because many cone-shaped needles with miniaturized ReRAM devices were formed. The other is Cu-Te based ReRAM that has a structure similar to previously reported cells [1-3]. The 30- or 70-nm Cu-Te ReRAM cell was thinned by conventional FIB techniques to 100 nm thick for TEM observation [5]. The method is suitable to cut up the small ReRAM devices from a patterned memory chip. In-situ TEM observation was performed with a homemade TEM sample holder with a piezo actuator that was used to control a Pt-Ir probe needle so as to contact the needle to the top electrode of the ReRAM to be measured. Near the TEM specimen, a MOSFET was serially connected to control the current compliance. Together with the I-V characteristics, real-time TEM videos were also collected by the CCD Camera. The clear formation and rupture of Cu filament were observed in TEM corresponding to SET and RESET resistive switching for the Cu/MoOx samples. The size of the filament increased as the increase of compliance current. An interesting results is that the positions of the filament changed. The phenomena was thought to be caused by the over-reset where the filament seeds were completely removed. In the Cu-Te based sample, filament formation was not detected at low current compliance though the SET and RESET resistive switching were clearly observed. When the current compliance increased higher than 100 mA, filament formation corresponding to the SET process was detected in the TEM images by enhancing the contrast. In this device, the filament formed almost the same position. In addition, we achieved 100k pulse switching cycles of SET and RESET inside the TEM without any damage. These results clearly show that the in-situ TEM will be a powerful tool to analyze the mechanisms and the reliability of ReRAM. Acknowledgement The authors wish to thank Drs. K. Ohba, M. Shimuta, I. Fujiwara, S. Yasuda, A. Maesaka, S. Kusanagi, and K. Aratani from SONY Corporation, and Drs. T. Fujii, K. Hamada, and N. Sakaguchi from Hokkaido University for their technical supports and meaningful discussions. This work was partly supported by Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS KAKENHI 24360128, 25420279, and 2663014104) and the Nanotechnology Platform Program organized by MEXT. Reference [1] W. Fackenthal, et al.: “A 16Gb ReRAM with 200MB/s Write and 1GB/s Read in 27nm Technology,” ISSCC, p.338 (2014). [2] S. Sills, et al.: “A Copper ReRAM Cell for Storage Class Memory Applications,” VLSI Symposium, p. 80 (2014). [3] J. Zahurak, et al.: “Process Integration of a 27nm, 16Gb Cu ReRAM,” IEDM, p.140 (2014). [4] M. Kudo, M. Arita, Y. Ohno, and Y. Takahashi: “Filament formation and erasure in molybdenum oxide during resistive switching cycles,” Appl. Phys. Lett. 105, 173504 (2014). [5] M. Kudo, M. Arita, Y. Takahashi, K. Ohba, M. Shimuta, and I. Fujiwara: “Visualization of conductive filament during write and erase cycles on nanometer-scale ReRAM achieved by in-situ TEM,” IMW (2015). Figure 1

Due to the outstanding performance of resistance random access memory (RRAM) in the memory field, the study of resistive switching (RS) phenomena has become extremely noticeable in the recent years. The mechanism of metal conductive filamentary RRAM is already clear, but the conditions of the RS are still unclear. Therefore, this paper aims to explore the conditions for the occurrence of resistive, using a new RS structure called Electrolyte-Oxide-Semiconductor (EOS). This structure is based on the formation of metal conductive filament and exhibits the unipolar switching characteristics. Due to the formation or rupture of the conductive filaments, this device exhibits different resistance states. A series model of electrolyte and conductive filaments is used to explain the IV curve of this device. Compared with the device using a metal active electrode, the active electrode of this device is originally ionized. Therefore, it would be a better tool to explore the mechanism of ion migration and the formation of conductive filaments. Materials screening of metal in RRAM would also be more efficient.

This study presents a new type of resistive switching memory device that can be used in biodegradable electronic applications. The biodegradable device features magnesium difluoride as the active layer and iron and magnesium as the corresponding electrodes. This is the first report on magnesium difluoride as a resistive switching layer. With on-off ratios larger than one hundred, the device on silicon switches at voltages less than one volt and requires only sub-mA programming current. AC endurance of 10(3) cycles is demonstrated with ±1 V voltage pulses. The switching mechanism is attributed to the formation and rupture of conductive filaments comprising fluoride vacancies in magnesium difluoride. Devices fabricated on a flexible polyethylene terephthalate substrate are tested for functionality, and degradation is subsequently demonstrated in de-ionized water. An additional layer of magnesium difluoride is used to hinder the degradation and extend the lifetime of the device.

Effect of Ag doping on bipolar switching operation in molybdenum trioxide (MoO3) nanostructures for non-volatile memory

We report the design and fabrication of W:AlOx/WOx bilayer based resistive switching cells in a standard 0.18 μm CMOS process with only one extra mask. The devices show excellent performance with low power consumption. Low operation voltages (SET voltage < 1.5 V, RESET voltage < 1.3 V) are achieved, and specifically, the RESET and SET currents are lower than 1 μA. For the 0.3 μm×0.3 μm active area of the cell, the current density is below 1.1×103A/cm2, which is much smaller than previous reported results. To reveal the resistive switching mechanism, various physical analysis techniques were employed to examine the microstructures, compositions, and chemical states. Current-voltage and capacitance-voltage electrical characterizations were carried out on these devices. Based on the physical and electrical characteristics, a conductive filament formation and rupture mechanism is proposed to explain the W:AlOx/WOx bilayer structure resistive switching phenomena.

The present work demonstrates the non-volatile resistive switching behavior of cellulose-gelatine microcomposite fibers (CGMFs). The CGMFs were synthesized using the electrospinning technique and used as a switching layer for a resistive memory device. The morphological study reveals that the switching layer was composed of good quality of microcomposite fibers and possesses cross-link like morphology. The functional groups present in the switching layer were confirmed by Fourier-transform infrared spectroscopy technique. The CGMFs based device shows the bipolar resistive switching effect and requires low SET (+ 1.33 V) and RESET (− 1.42 V) voltages for the operation. The charge-magnetic flux relation of the device was calculated and memristive device like properties were observed from the results. The charge driving capability of the device was found to be ~ 1.3 × 10−2 C, which is sufficient to get the good resistive switching property. The non-volatile resistive switching behavior of the CGMFs based device suggested that the device has good performance in terms of endurance (500 cycles) and retention (2000 s). The uniformity and reliability of the device were confirmed by statistical calculations (standard deviation and coefficient of variation). The conduction model fitting results suggested that the space charge limited current (SCLC) and Ohmic conduction mechanisms were responsible for device operation during the high resistance state (HRS) and low resistance state (LRS), respectively. A possible resistive switching mechanism is presented by considering the formation and rupture of Ag conductive filament under the influence of external voltage.

With the goal to render the ReRAM memory integrable with CMOS backend and compatible with current BEOL metal choices (Cu, Ta, Ti, W), we have chosen the well-behaved and well-characterized Cu/TaOx/Pt ReRAM cell as a benchmark device and have replaced the active (Cu) and inert (Pt) electrodes with Ta and Ti electrodes. Five derivative devices were manufactured including: Pt/TaOx/Pt, Cu/TaOx/Ti, Cu/TaOx/Ta, Ta/TaOx/Pt, and Ti/TaOx/Pt. Out of these five devices, only the Ti/TaOx/Pt produced reliable set and reset switching characteristics. Nevertheless, the switching behavior of the other derivative devices provides a great deal of insight into the formation, shape and rupture of conductive filaments in ReRAM devices which would be difficult to gain otherwise from only one type of a device. The present findings are compared with results on the same and similar devices reported elsewhere in the literature.

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).

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

HfO<sub><i>x</i></sub> memristors have emerged as one of the most promising candidates for next-generation non-volatile memory due to their low operating voltage, excellent endurance, and cycling characteristics. However, the randomness in the formation and rupture of oxygen vacancy conductive filaments within HfO<sub><i>x</i></sub> thin films leads to a relatively dispersed threshold voltage distribution and poor stability. Therefore, improving the stability of HfO<sub><i>x</i></sub> devices by modulating oxygen vacancies is of significant research importance. In this study, three groups of W/HfO<sub><i>x</i></sub>/Pt devices are prepared using magnetron sputtering with argon-to-oxygen ratios of 30∶20, 40∶10 and 45∶5, respectively. X-ray photoelectron spectroscopy results indicate that the 45∶5 device has the highest oxygen vacancy concentration (25.59%). All of three groups exhibit bipolar resistive switching behavior. Of the three W/HfO<sub><i>x</i></sub>/Pt devices, the device with the argon-to-oxygen ratio of 45∶5 demonstrates the best overall performance: over 200 <i>I</i> -<i>V</i> cycles, a switching ratio of ~10<sup>3</sup>, excellent data retention within 10<sup>4</sup> s, and a concentrated threshold voltage distribution. Analysis of the conduction mechanisms reveals that the device follows a space-charge-limited current (SCLC) mechanism in the high-resistance state and exhibits Ohmic conduction behavior in the low-resistance state. In the initial state, there is a high density of oxygen vacancies near the nucleation region of the conductive filament, which can shorten the effective migration path of oxygen vacancies. Under an applied electric field, negatively charged oxygen ions migrate toward the top electrode, while oxygen vacancies gradually accumulate from the bottom electrode to the top electrode, leading to the formation of continuous conductive filaments. A higher oxygen vacancy concentration facilitates the development of robust and structurally more stable conductive filaments, thereby enhancing the uniformity of resistive switching and device reliability. This study reveals the critical role of oxygen vacancy modulation in the performance of HfO<sub><i>x</i></sub> memristors and provides an effective pathway for developing high-performance and highly reliable resistive random-access memory.

Unipolar resistive switching (URS) as well as bipolar resistive switching (BRS) behaviors in a Cu/TaOx/Pt structure were investigated. Upon increasing the compliance current (Ic), the current-voltage characteristics of the Cu/TaOx/Pt structure showed a URS behavior at Ic = 0.1 mA then experienced a non-reversible transition from the URS to a BRS mode at Ic = 10 mA. Through a detailed analysis of the electrical properties in each resistance state of URS and BRS, we revealed that the permanent transition from the URS to the BRS mode was induced by the formation of stronger Cu metal conductive filaments within the TaOx thin film. More interestingly, both URS and BRS modes were governed by the formation and rupture of conductive filaments, whereas the rupture of these filamentary paths in BRS was proposed due to both Joule heating and electric field effects.

Resistance switching and memory effects have been observed in a heterostructure consisting of BiFeO3 (BFO) on a LaNiO3 (LNO) conducting oxide bottom electrode fabricated by chemical solution deposition on quartz substrates. The phase purity and lattice strain of monodispersed thin film nanostructures were confirmed by XRD. Bipolar switching with a resistance switching ratio of one order of magnitude at small bias voltages of +1.3 V and −0.7 V for positive and negative bias respectively has been estimated from the I–V traces measured over a span of more than 6 h. Excellent retention behavior of the BFO–LNO device is confirmed for its possible use as a non-volatile resistance random access memory device. The modeling of the I–V data suggests that in the low resistance state (on) Ohmic conduction is well fitted, whereas in the high resistance state (off) the trap-controlled space-charge-limited current mechanism dominates. The observed resistive switching (RS) is attributed to formation of a metallic filament and its rupture under electric stress conditions. The valence states of Fe ions in the fabricated device structures indicate that oxygen vacancies could also lead to RS in the device.

A resistive switching (RS) memory has been expected to be a next-generation memory owing to its high density, low power consumption, and high switching speed. The mechanism of the reversible RS phenomenon has been considered in terms of the formation and rupture of conductive filaments (CF). Recently, it has been reported that a ferromagnetic CF was formed in NiO-based RS memory. This suggested the possibility of the new device that has both RS memory and spintronics device functions. The presence of a ferromagnetic CF in the NiO layer was confirmed by measuring the magnetization of the large number of CFs at room temperature using a SQUID magnetometer. Although, it is not yet confirmed that a single ferromagnetic CF is formed by the SET process. The purpose of this study is to understand the physical properties of a single ferromagnetic CF by focusing on magnetoresistance (MR). We prepared Ni/HfO2/Pt memory device, having ferromagnetic Ni electrode and the oxide of a non-magnetic metal. The fabricated RS memory devices have a cross-bar structure. The crossed electrode size is 1×1 mm2. RS characteristic was measured with the bipolar operation mode. When current limit is 5 mA, the anisotropic magnetoresistance (AMR) was observed in the low resistance state. The direction of the external magnetic field is in-plane to the substrate, and the current is perpendicular to the magnetic field. The AMR showed a convex behavior with magnetic field, and the MR ratio was approximately 0.15%. It is strongly suggested that AMR came from a ferromagnetic CF formed in the oxide layer. Further discussions will be shown in the workshop.

ZrO2-based resistive switching memory has attracted much attention according to its possible application in the next-generation nonvolatile memory. The Al/ZrO2/Pt resistive switching memory with bipolar resistive switching behavior is revealed in this work. The thickness of the ZrO2 film is only 20 nm. The device yield improved by the non-lattice oxygen existing in the ZrO2 film deposited at room temperature is firstly proposed. The stable resistive switching behavior and the long retention time with a large current ratio are also observed. Furthermore, it is demonstrated that the resistive switching mechanism agrees with the formation and rupture of a conductive filament in the ZrO2 film. In addition, the Al/ZrO2/Pt resistive switching memory is also possible for application in flexible electronic equipment because it can be fully fabricated at room temperature.

In this letter, an atomistic-level simulation model of bipolar TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> RRAM is addressed, which adopts the typical MIM structure and combines the equation of oxygen vacancy transport, current continuity and Joule heating. Based on the model, the dynamic formation and rupture of conductive filament are described in a 3D geometry model of COMSOL. Besides, the electro-thermal model with a conical shape conductive filament achieved is carried out to actually reflect the dynamic resistive switching processes and increase the understanding of conductive mechanism.