Biodegradable resistive switching memory based on magnesium difluoride.

Emerging memory technologies show the potentials of new memory devices to store more data at a lower cost compared to the industry-standard silicon-based devices. Among these, resistive switching random access memory (RRAM) devices, which are based on the metal-insulator-metal (MIM) structure, have shown potential in non-volatile memory applications due to their characteristics of low-power consumption, superior switching speed, excellent reliability, ultimate scaling potential with the simple fabrication process and adaptive aptitudes for the neuromorphic applications [1, 2]. The advantage of the biodegradable materials is non-toxic and eco-friendly but the essential material properties remain the major issues in the scaling of biomass-based RRAMs [3]. Biodegradable magnesium fluoride (MgFx) is an insulator with a large bandgap (11.3 eV) which makes it a favorable candidate for biodegradable RRAM devices. Recently, a couple of reports on MgFx based RRAM devices have shown excellent resistive switching and biodegradable properties [2, 4]. Nevertheless, the conduction mechanism and resistive switching mechanism in the MgFx based RRAM devices are not fully understood. The electrodes in RRAM devices may significantly affect the resistive switching (RS) behavior. The effects of electrode materials and operating environment including cryogenic temperature on the device performance have not been studied yet. In this study, we report resistive memory devices employing MgFx as the resistive switching layer that is inserted between the platinum and titanium metal electrodes. The MgFx based RRAM devices exhibit electroforming free bipolar resistive switching characteristics at room temperature and open-air environment. We also report the effects of electrode materials (Top: Pt, Au/Ni, ITO, and Bottom: p+ Si, n+ Si, ITO) on MgFx based RRAM devices. It is found that the difference in workfunctions affects the performances of the devices. We have also analyzed the effect of the operating environment as well as temperature from room temperature down to cryogenic temperature. The detailed performance of the devices, their conduction and switching mechanisms, the effects of the electrode materials, and the operating environment including cryogenic temperature will be presented at the virtual conference.

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.

The traditional von Neumann architecture limits the increase in computing efficiency and results in massive power consumption in modern computers due to the separation of storage and processing units. The novel neuromorphic computation system, an in-memory computing architecture with low power consumption, is aimed to break the bottleneck and meet the needs of the next generation of artificial intelligence (AI) systems. Thus, it is urgent to find a memory technology to implement the neuromorphic computing nanosystem. Nowadays, the silicon-based flash memory dominates non-volatile memory market, however, it is facing challenging issues to achieve the requirements of future data storage device development due to the drawbacks, such as scaling issue, relatively slow operation speed, and high voltage for program/erase operations. The emerging resistive random-access memory (RRAM) has prompted extensive research as its simple two-terminal structure, including top electrode (TE) layer, bottom electrode (BE) layer, and an intermediate resistive switching (RS) layer. It can utilize a temporary and reversible dielectric breakdown to cause the RS phenomenon between the high resistance state (HRS) and the low resistance state (LRS). RRAM is expected to outperform conventional memory device with the advantages, notably its low-voltage operation, short programming time, great cyclic stability, and good scalability. Among the materials for RS layer, indium gallium zinc oxide (IGZO) has shown attractive prospects in abundance and high atomic diffusion property of oxygen atoms, transparency. Additionally, its electrical properties can be easily modulated by controlling the stoichiometric ratio of indium and gallium as well as oxygen potential in the sputter gas. Moreover, since the IGZO can be applied to both the thin-film transistor (TFT) channel and RS layer, it has a great potential for fully integrated transparent electronics application.In this work, we proposed amorphous transparent IGZO-based RRAMs and investigated switching behaviors of the memory cells prepared with different top electrodes. First, ITO was choosing to serve as both TE and BE to achieve high transmittance. A multi-target magnetron sputtering system was employed to deposit all three layers (TE, RS, BE layers) on glass substrate. I-V characteristics were evaluated by a semiconductor parameter analyzer, and the bipolar RS feature of our RRAM devices was demonstrated by typical butterfly curves. The optical transmission analysis was carried out via a UV-Vis spectrometer and the average transmittance was around 80% out of entire devices in the visible-light wavelength range, implying high transparency. We adjusted the oxygen partial pressure during the sputtering of IGZO to optimize the property because the oxygen vacancy concentration governs the RS performance. Electrode selection is crucial and can impact the performance of the whole device. Thus, Cu TE was chosen for our second type of device because the diffusion of Cu ions can be beneficial for the formation of the conductive filament (CF). A ~5 nm SiO2 barrier layer was employed between TE and RS layers to confine the diffusion of Cu into the RS layer. At the same time, this SiO2 inserting layer can provide an additional interfacial series resistance in the device to lower the off current, consequently, improve the on/off ratio and whole performance. Finally, an oxygen affinity metal Ti was selected as the TE for our third type of device because the concentration of the oxygen atoms can be shifted towards the Ti electrode, which provides an oxygengettering activity near the Ti metal. This process may in turn lead to the formation of a sub-stoichiometric region in the neighboring oxide that is believed to be the origin of better performance.In conclusion, the transparent amorphous IGZO-based RRAMs were established. To tune the property of RS layer, the sputtering conditions of RS were varied. To investigate the influence of TE selections on switching performance of RRAMs, we integrated a set of TE materials, and a barrier layer on IGZO-based RRAM and compared the switch characteristics. Our encouraging results clearly demonstrate that IGZO is a promising material in RRAM applications and breaking the bottleneck of current memory technologies.

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.

Flexible resistive switching memory devices based on graphene oxide (GO) polymer nanocomposite were prepared on flexible substrate to research the influence of bending on resistive switching behavior. The devices showed evident response in resistive switching memory characteristics to flexible bending. The 2000 cycles flexible bending leads to the switch of resistive switching memory characteristic from write-once-read-many time memory (WORM) to static random access memory (SRAM). Both WORM and SRAM memory properties are all repeatable, and the threshold switching voltage also showed good consistency. The resistive switching mechanism is attributed to the formation of carbon-rich conductive filaments for nonvolatile WORM characteristics. The bending-induced micro-crack may be responsible for the partial broken of the electrical channels, and may lead to the volatile SRAM characteristics.

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

Resistive switching memory (ReRAM) or Memristor attracted considerable attention due to its technological potential for non-volatile memory and various applications. However, the mechanism is still not resolved clearly because of its difficulties of identifying a conducting channel particularly at oxide systems. Objective of this paper is elucidation of resistive switching mechanism by identifying and analyzing behavior of filaments in-situ in SrTiO3 (STO) and NiO/TiO2 oxides systems. ReRAM can be classified according to different criteria; it can be categorized into unipolar and bipolar systems according to current-voltage (I-V) characteristics, or Electrochemical metallization (ECM), Valence change memory (VCM), and Thermochemical memory (TCM) in terms of different aspects in participation of electrodes in operation. Most of the models that have been presented to explain resistive switching phenomena are based on the formation of filaments, no matter which category their system belongs to. Thus, in order to unravel the mechanism of resistive switching, identifying structure of the filament has its great significance. It has been readily observed in case of the ECM because the filaments, injected metal ion from electrode, are distinguished from electrolyte or insulator. On the contrary, filaments of VCM and TCM have yet to be understood thoroughly. In this paper, our focus will be on the system that could be classified into VCM/TCM. Though grain boundary, defect percolation were suggested as promising candidates of filament at these systems, they have not been clearly verified yet due to their unknown structure, limited size of nanometer scale, and geometry covered by matrix. As the structure and behavior of filament is still controversial, substantial amount of papers reported to unveil the mechanism of resistive switching employed many hypotheses. Here, we identified and analyzed the filament structure at oxide resistive switching system of Pt/SrTiO3/Pt and Pt/NiO/TiO2/Pt, employing in-situ scanning tunneling microscope (STM) / TEM holder. STM tip was in contact with oxide thin film in the TEM and scanned over the region of interest to probe filaments. The filaments could have been detected because of their high conductivity. Consequently, new conducting crystal structure, 2nd phase, is observed in the STO, NiO/TiO2 thin film. Similar as we reported before that 2nd phases of metallic crystals, Magneli TinO2n-1 (n>=4), are generated in the TiO2 thin film as conducting channels. After probing filaments, high-resolution (HR) images and electron-energy loss spectroscopy (EELS) were acquired at corresponding area for determining its crystal and electronic structure respectively. HR images were further analyzed to investigate orientation relationship between the filaments and matrix. EEL spectrum were compared with a simulation data obtained from first principle calculation using Vienna Ab initio simulation package. Furthermore, the sample was turned ‘on’ and ‘off’ repeatedly in-situ and behavior of filament is tracked. The resistance state of the sample was correlated to the structure transformation of filaments. SrTiO3 thin film of 60 nm was deposited by pulsed laser deposition process under oxygen atmosphere and NiO/TiO2 thin film of 50nm was deposited by atomic layer deposition. Platinum pad was exploited for both top and bottom electrodes. Cross-section TEM sample was prepared by focused ion beam. Gatan image filter equipped TEM, F20, was used for in-situ STM/TEM, HR images and EELS.

Hybrid Perovskites have emerged as a class of highly versatile functional materials with applications in solar cells, photodetectors, transistors, and lasers. Recently, there have also been reports on perovskite‐based resistive switching (RS) memories, but there remain open questions regarding device stability and switching mechanism. Here, an RS memory based on a high‐quality capacitor structure made of an MAPbBr3 (CH3NH3PbBr3) perovskite layer sandwiched between Au and indium tin oxide (ITO) electrodes is reported. Such perovskite devices exhibit reliable RS with an ON/OFF ratio greater than 103, endurance over 103 cycles, and a retention time of 104 s. The analysis suggests that the RS operation hinges on the migration of charged ions, most likely MA vacancies, which reversibly modifies the perovskite bulk transport and the Schottky barrier at the MAPbBr3/ITO interface. Such perovskite memory devices can also be fabricated on flexible polyethylene terephthalate substrates with high bendability and reliability. Furthermore, it is found that reference devices made of another hybrid perovskite MAPbI3 consistently exhibit filament‐type switching behavior. This work elucidates the important role of processing‐dependent defects in the charge transport of hybrid perovskites and provides insights on the ion‐redistribution‐based RS in perovskite memory devices.

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.

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.

The impact of a TaOx nanolayer at the GeSex/W interface on the performance of resistive switching memory in an Al/Cu/GeSex/TaOx/W structure has been examined. All materials and the memory structure have been investigated using high-resolution transmission electron microscopy, energy dispersive x ray spectroscopy, and x ray photo-electron spectroscopy analyses. A conically shaped crystalline Cu (111) nanofilament with a diameter of around 17 nm in the TaOx nanolayer after a current compliance (CC) of 500 μA has been observed, and this has been also characterized by fast Fourier transform. The low resistance state (LRS) decreases as the current compliances (CCs) increased from 1 nA to 1 mA, since the nanofilament diameter increased from 0.04 to 23.4 nm. This is also estimated by bipolar resistive switching characteristics. The resistivity of this crystalline Cu nanofilament is approximately 2300 μΩ.cm. The nanofilament has a cylindrical shape, with CCs ranging from 1 nA to 10 μA and a conical shape with CCs ranging from 50 μA-1 mA. The resistive switching mechanism has been explained successfully under SET and RESET operations. Improved resistive switching parameters, such as SET voltage, LRS, and high resistance state with consecutive switching cycles are obtained and compared to those of pure GeSex and TaOx materials. Extrapolated, long program/erase endurance of > 106 cycles, attributed to the Al/Cu/GeSex/TaOx/W structure design, is observed. This resistive switching memory structure shows extrapolated 10 years data retention with a resistance ratio of > 10 at a low CC of 0.1 μA at 50 °C. A large memory size of ∼ 6 Pbit/sq. in. is obtained, considering the nanofilament diameter at a low CC of 0.1 μA. This study is important not only for improving the performance of low-power resistive switching memory, but also helpful for designing other nonvolatile memory devices.

Zinc oxide (ZnO) is an amorphous oxide semiconductor (AOS) material valued for its high carrier mobility, transparency to visible light, and low-temperature manufacturing process compared to silicon-based semiconductors. These desired features make ZnO-based AOSs widely popular as the channel layer in TFTs and as key dielectric layers in electronic devices like light sensors, gas detectors, resistive random-access memory (RRAM), and more. Among them, Conductive Bridge Random Access Memory (CBRAM) with a dielectric switching layer sandwiched between the electrochemically active metal (Ag, Cu) top electrode and the inert bottom electrode, is of a class of RRAM that offers low-voltage operation, high-density storage device, non-volatile, and compatibility with standard CMOS processes [1]. The CBRAM operates based on the principle of conductive filament formation and rupture. During operation, conductive filaments will be formed in switching layer by connecting metal ions which diffuse from the electrode after applying a positive bias. The device will switch to a low resistance state (LRS) after the conductive filaments were formed. After this, applying a negative bias will break the conductive filaments and switch the device back to a high resistance state (HRS). The HRS and LRS are the 0 or1 state of the memory operation. Studies and works have shown that controlling the distribution and diffusion of electrode metal ions in the switching layer is challenging because the conductive filaments formed can vary significantly in strength, ranging from robust to fragile [2]. This leads to instability in the operation of memory devices and encountering difficulties in the integration with TFT pixel array for memory-in-pixel (MiP) power-saving displays [3] or CMOS-integrated compute-in-memory applications [4].In this work, we proposed a novel CBRAM device featuring ZnO nanostructures as the resistive switching layer. The combination of a sputter-deposited seed layer with the water bath method enables the production of high-quality ZnO nanostructures at temperatures below 90°C. These nanostructures play a crucial role in constraining metal ion diffusion and filament formation, leading to the formation of uniform conductive filaments. This improvement significantly enhances the operational stability of the CBRAM device. The experimental results have demonstrated superior memory characteristics with a highly uniform distribution achieved successfully in the ZnO nanostructure-embedded CBRAM, compared to its counterpart lacking ZnO nanostructures. Furthermore, post-treatments are implemented in this study to further promote electrical performance of the nanostructure-embedded CBRAM, like operation voltage, memory window, retention, and endurance. Reference: [1] C. Gopalan, Y. Ma, T. Gallo, J. Wang, E. Runnion, J. Saenz, F. Koushan, P. Blanchard, S. Hollmer, “Demonstration of Conductive Bridging Random Access Memory (CBRAM) in logic CMOS process”, Solid-State Electronics, vol. 58 (1), p. 54-61, 2011.[2] C. C. Hsu, P. T. Liu, K. J. Gan, D. B. Ruan, Simon M. Sze, “Investigation of deposition technique and thickness effect of HfO2 film in bilayer InWZnO-based conductive bridge random access memory”, Vacuum, vol. 201, 111123, 2022.[3] S. H. Lee, B. C. Yu, H. J. Chung, and S. W. Lee, “Memory-in-pixel circuit for low-power liquid crystal displays comprising oxide thin-filmtransistors,” IEEE Electron Device Lett., vol. 11 (11), pp. 1551–1554, 2017.[4] W. Wan, R. Kubendran, C. Schaefer, et al., “A compute-in-memory chip based on resistive random-access memory”, Nature, vol. 608, p. 504–512, 2022.

A nonlinear resistive switching behaviors of Ni/HfO2/TiN memory structures for self-rectifying resistive switching memory

In this paper, the bipolar resistive switching characteristic is reported in Ti/ZrO2/Pt resistive switching memory devices. The dominant mechanism of resistive switching is the formation and rupture of the conductive filament composed of oxygen vacancies. The conduction mechanisms for low and high resistance states are dominated by the ohmic conduction and the trap-controlled space charge limited current (SCLC) mechanism, respectively. The effect of a set compliance current on the switching parameters is also studied: the low resistance and reset current are linearly dependent on the set compliance current in the log—log scale coordinate; and the set and reset voltage increase slightly with the increase of the set compliance current. A series circuit model is proposed to explain the effect of the set compliance current on the resistive switching behaviors.

All-inorganic perovskite CsPbX3 (X = Cl, Br, or I) is widely used in a variety of photoelectric devices such as solar cells, light-emitting diodes, lasers, and photodetectors. However, studies to understand the flexible CsPbX3 electrical application are relatively scarce, mainly due to the limitations of the low-temperature fabricating process. In this study, all-inorganic perovskite CsPbBr3 films were successfully fabricated at 75 °C through a two-step method. The highly crystallized films were first employed as a resistive switching layer in the Al/CsPbBr3/PEDOT:PSS/ITO/PET structure for flexible nonvolatile memory application. The resistive switching operations and endurance performance demonstrated the as-prepared flexible resistive random access memory devices possess reproducible and reliable memory characteristics. Electrical reliability and mechanical stability of the nonvolatile device were further tested by the robust current-voltage curves under different bending angles and consecutive flexing cycles. Moreover, a model of the formation and rupture of filaments through the CsPbBr3 layer was proposed to explain the resistive switching effect. It is believed that this study will offer a new setting to understand and design all-inorganic perovskite materials for future stable flexible electronic devices.