Anodic Oxides As Electrolytes for Resistive Switching Devices

Resistive switching devices (ReRAM) represent a broad class of two-terminal continuously tunable resistors including memristors, phase change memory (PCM), valence change memory (VCM), and electrochemical metallization cells (ECM). Though these devices, especially PCM, are increasingly being commercialized by industry for use in next generation memories, they are also all actively studied for use as synaptic weights in next generation hardware-accelerated neuromorphic networks. We have previously investigated Electron Beam Induced Current Microscopy as a means of reliably characterizing resistive switches. In that investigation, we observed surprising electronic effects, such as internal secondary electron emission, in addition to more traditional electron-hole pair separation, and we broke those up into constituent currents based on their origin through Monte-Carlo modeling of the electron beam-matter interaction. Now, armed with a new understanding of the physics of EBIC imaging, we study the impact of manufacturing variations on resistive switches by continuously tuning the thickness of an Al2O3 interfacial barrier. Shifts in the apparent ratios of internal secondary electron emission from the top electrode to the bottom electrode and vice versa appear to indicate a continuous tuning of the apparent filament diameter as both a function of the injected current and the interfacial barrier thickness. This yields an apparent reduction in the current density, the primary effect of which is a reduction in the device damage from forming and a suppression of parasitic leakage currents imaged in devices without interfacial barriers. Figure 1

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

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

Electrochemical devices allowing to modulate their resistive state through ionic motion upon electric bias are known as memristors. Currently, memristors are positioned as a major candidate to overcome the bottlenecks in current electronic-based transistors in terms of downscaling capabilities and energy consumption. The vast majority of memristor are based on two types of ions: either oxygen vacancy migration, in the so called Valence Change Memories (VCM), or a metal cation, usually Ag+ and Cu2+, in the so called Electrochemical Metallization Cells (ECM). Despite the excellent performance of both systems, a widespread implementation of oxygen-based memristors in today’s integrated circuits is delayed due to the need to address cycle-to-cycle and device-to-device variabilities while circumventing electroforming, which are inherent issues associated to the filamentary nature of the switching mechanism. Recently, Li-ion is emerging as an alternative, given the higher diffusivity of Li+ when compared to oxygen, and the ability of Li-oxides solid state conductors to accumulate and deplete lithium at the interfaces and bulk. The scarce literature regarding Li-based memristors focuses mainly in a traditional high-voltage cathode material, LixCoO2 2, in a rather slow lithium conductor as LiLaTiO3 3 and in a magnetic spinel LiFe5O8 4. Performance-wise, Li-based oxides have shown promising applications in neuromorphic computing thanks to a close-to-analog switching response. However, it is still remains unclear the defect chemistry leading to the switching behavior of Li-based materials and the impact of lithiation degree on their performance. Among the potential Li-based oxides showing resistive switching effects, Lithium titanates compounds present intrinsic properties that could be beneficial to implement a Li- based memristor: a) the existence of a metal-insulator transition upon lithium intercalation, concomitant with a phase transition and valence change in the Ti cation, b) the spinel to rock-salt phase transition occurs with a neglectable volume change and c) there is a large difference in electronic conductivity and diffusivity between the two phases. In this work, we exemplify the resistive switching capabilities of these systems for different lithium stoichiometries. To exemplify this, we will report for the first time the non-volatile, non-filamentary bipolar resistive switching characteristics of lithium titanates compounds, Li4+3xTi5O12, as a function of the lithiation degree. We have employed a recently proposed strategy to overcome lithium loss during thin film deposition and finely control the final lithiation degree of the films5 to create a stoichiometrically lithiated Li4Ti5O12 spinel phase and a highly lithiated Li7Ti5O12 rock- salt phase memristive devices. Changing the Li-content from a stoichiometrically lithiated spinel phase to a highly lithiated rock-salt phase results in the capability to tune the performance in a wide range in terms of accessible resistance window (from ratios of 102 to 106 at low voltage operation, respectively), symmetry (from highly asymmetric to symmetric behavior, respectively) and retention (from few minutes up to 105 s at room temperature, respectively), among others. In other words, controlling the lithiation degree might offer a suitable path to reduce stochasticity from which current filamentary memristive devices inherently suffer, mainly due to the difficulties to control the amount of vacancies generated, and paves the way to further control of ionic migration for novel nanoelectronic devices.

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.

In valence change memory (VCM) cells, the conductance of an insulating switching layer is reversibly modulated by creating and redistributing point defects under an external field. Accurately simulating the switching dynamics of these devices can be difficult due to their typically disordered atomic structures and inhomogeneous arrangements of defects. To address this, we introduce an atomistic framework for modeling VCM cells. It combines a stochastic kinetic Monte Carlo approach for atomic rearrangement with a quantum transport scheme, both parametrized at the ab initio level by using inputs from density functional theory. Each of these steps operates directly on the underlying atomic structure. The model thus directly relates the energy landscape and electronic structure of the device to its switching characteristics. We apply this model to simulate field-induced nonvolatile switching between high- and low-resistance states in a TiN/HfO2/Ti/TiN stack and analyze both the kinetics and stochasticity of the conductance transitions. We also resolve the atomic nature of current flow resulting from the valence change mechanism, finding that conductive paths are formed between the undercoordinated Hf atoms neighboring oxygen vacancies. The model developed here can be applied to different material systems to evaluate their resistive switching potential, both for use as conventional memory cells and as neuromorphic computing primitives.

We present 'unusual' resistive switching behaviours in electrochemical metallization (ECM) cells utilizing a dual-layer (SiOx/GeSex: SiOx on GeSex) solid electrolyte (SE). The observed switching behaviour markedly varies with the thickness of the upper SiOx layer and compliance current: (i) monostable switching, (ii) counter-eightwise bipolar switching, and (iii) combination of monostable and eightwise bipolar switching behaviours. Focusing on cases (i) and (iii), electrical and chemical analyses on these chameleonic cells were performed in an attempt to gain clues to the understanding of the observed complexity. The chemical analysis indicated the upper SiOx layer as a chemical potential well for Cu ions-Cu ions were largely confined in the well. This non-uniform distribution of Cu across the SE perhaps hints at the mechanism for the complex behaviour; it may be a 'zero-sum game' between SiOx and GeSex layers, in which the two layers fight over the limited number of Cu atoms/ions.

We evaluated the resistive switching effect of the polycrystalline oxide (Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Cr-SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) fabricated by reactive sputtering and PLD. It shows a well-developed resistive switching behavior. The reproducible resistance switching cycles were observed and the resistance ratio was as high as 50~100 times. The resistance switching was observed under voltage pulse as short as 10 nsec. The estimated retention lifetime at 85degC was sufficiently longer than 10 years. However, we observed a significant non-uniformity of resistance switching behavior. To improve uniformity of resistance switching, we have investigated single crystal Nb-doped SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in terms of its potential utility in nonvolatile memory applications. Compared with polycrystalline oxide (Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and Cr-doped SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ), the Pt/single crystal Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky junction exhibits excellent memory characteristics including uniformity of set/reset bias, die-to-die reproducibility, data retention at high temperature, reliability under cycle stress, and multi-bit operation characteristics. Considering the area dependent resistance value and low temperature I-V characteristics, resistance switching might be explained by modulation of the Schottky barrier height by charge trapping and detrapping at the interface. In order to introduce Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> into CMOS process and reserve its excellent characteristics of single crystal, we deposited epitaxial Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> film on Si substrate using conducting TiN as a buffer layer. Based on XRD, RBS and TEM measurements, epitaxial growth of TiN and Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films were confirmed. Moreover, the electrical and reliability properties of Pt/Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /TiN/Si structure are similar with that of Pt/single crystal Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> junction. Considering excellent uniformity and reproducibility of Pt/Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky junction, it shows good promise for future nonvolatile memory applications.

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

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

Resistance change Random Access Memory (RRAM) devices in which at least two resistance states are used are a top candidate for future nonvolatile data storage. Simple Metal-Insulator-Metal (MIM) structures form the memory element which can be easily incorporated in large arrays. In particular, in the so-called Valence Change Memories (VCM) the drift of anions, typically oxygen, is considered as the key step to explain the bistable resistive switching behavior. A first-order classification of the observed material changes is related to the geometrical location. In “filamentary” type switching the formation and rupture of a thin filament is responsible for the resistance change. In the “distributed” systems the switching can be traced back to modifications at interfaces. Oxygen ion migration into thin tunnel oxides in high electric fields and Schottky barrier engineering with metals and complex perovskites are two mechanisms under discussion for the distributed systems. In the filamentary type of switching fast oxygen ion transport along extended defects is demonstrated to be the key step for the formation of the conducting filaments. The bistable resistance characteristics with switching induced by voltage pulses is a promising approach for future nonvolatile memory technologies. Excellent scaling behavior to sizes below 20 nm has been demonstrated.

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 &amp;gt; 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 &amp;gt; 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.

Dynamic formation/rupture processes of metallic filament have been clarified in solid electrolyte- and oxide-based resistive memory devices, whereas they remain exclusive in organic ones. Here we report these dynamic processes in Cu/poly (3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester/indium–tin oxide (ITO) structure, which exhibits a typical bipolar resistive switching effect. Under illumination, an open circuit voltage of −0.15 V exists in high-resistance state, yet it vanishes in low-resistance state owing to the emergence of Cu filament. By combining the symmetry of current–voltage curves with corresponding energy band diagrams in different resistance states, it is demonstrated that the Cu filament grows from Cu/organics interface, ends at organics/ITO interface, and ruptures near organics/ITO interface. This work might advance the insight into resistive switching mechanisms in organic-based resistive memories.

We present a numerical drift-diffusion model of electronic-ionic transport combined with a Schottky contact barrier model to study resistive switching phenomena in ReRAM devices. Capturing the transition between Schottky and ohmic contact resistances upon temperature-accelerated ion migration, our model correctly describes the quasi-static I-V switching characteristics as well as dynamic set and reset events. It is shown to account for a transition between bipolar resistive switching and complementary switching when reducing the asymmetry between the contact barriers. Further, it is used to characterize the abrupt and gradual behavior of the set and the reset process, respectively.

Memristors, based on inherent memory effects in simple two-terminal structures, have attracted tremendous interest recently for applications ranging from nonvolatile data storage to neuromorphic computing based on non-von Neumann architectures. In a memristor, the ability to modulate and retain the state of an internal variable leads to experimentally observed resistive switching (RS) effects. Such phenomena originate from internal, microscopic ionic migration and associated electrochemical processes that modify the materials' electrical and other physical properties. To optimize the device performance for practical applications with large-size arrays, controlling the internal ionic transport and redox reaction processes thus becomes a necessity, ideally at the atomic scale. Here we show that the RS characteristics in tantalum-oxide-based memristors can be systematically tuned by inserting a graphene film with engineered nanopores. Graphene, with its atomic thickness and excellent impermeability and chemical stability, can be effectively integrated into the device stack and can offer unprecedented capabilities for the control of ionic dynamics at the nanoscale. In this device structure, the graphene film effectively blocks ionic transport and redox reactions; thereby the oxygen vacancies required during the RS process are allowed to transport only through the engineered nanosized openings in the graphene layer, leading to effective modulation of the device performance by controlling the nanopore size in graphene. The roles of graphene as an ion-blocking layer in the device structure were further supported by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and atomistic simulations based on first-principles calculations.