PROBING LOCAL VACANCY-DRIVEN RESISTIVE SWITCHING IN METAL OXIDE NANOSTRUCTURES

Novel nonvolatile memory technologies garner intense research interest as conventional ash devices approach their physical limit. Memristors, often comprising an insulating thin film between two metal electrodes to constitute a class of two-terminal devices, enable a variety of important large data storage and data-driven computing applications. In addition to nonvolatile behavior, other features such as high scalability, low power consumption, and sub-nanosecond response times make memristors among the most attractive candidate systems. Their strength in electronic storage relies on the unique properties of the tunable variations in resistance induced from the accumulation of charged defects based on the applied bias history. Metal oxides serve as the most common storage materials, demonstrating advantages including simple fabrication, high reliability, and fast operation speeds. While the basic working concepts and the underlying conduction mechanisms have been established through combined experimental and simulation studies, the role of metal insulator interface, which acts as the crux of coupled electronic-ionic interactions, has not been fully understood. Continuous scaling, for the purpose of high density memories, also requires a detailed understanding of the switching behavior and transport mechanism. Other technical challenges include the development of innovative, low-cost fabrication methods that effectively enable high-performance structures as an alternative to complicated process modules. Stable retention and endurance of the switching characteristics, as well as uniformity of the switching parameters to ensure a valid program/read operation also represent significant challenges. Studies in device and materials optimization remain in the formative stages, and thus motivate this work to drive progress in the most attractive areas, including size dependent behavior and switching performance of memristors. This collection of work aims to correlate resistive switching within metal oxide based memristors with the fundamental physical mechanisms and material properties on a highly localized scale. Chapter 3 relates the device size and the resulting performance matrix of memory cells in the first step towards fully understanding the scaling projection and reliability issues that affect nanoscale architectures. Chapter 4 demonstrates a convective self-assembly, transferable approach that enables the fabrication of highly-controlled nanoribbon comprising solution-processed nanocrystals, providing multiple degrees of freedom for understanding the interfacial memristive behavior of functional oxide nanostructures. As a powerful tool in the study of resistive switching, conductive AFM probes the homogeneity of the charge transport properties, thus offering electrical information by locally applied bias when it is placed in direct contact with desired regime. Finally we also focus on the improving the cycle-to-cycle uniformity by embedding nanostructure into conventional metal-insulator-metal (MIM) geometry in Chapter 5. This improvement is attributed to the concentration of electric field when metal nanoislands are inserted into the oxide film matrix. The details of this work will highlight the tunable and optimizable template-driven method that can be applied on any memristive systems, yielding a superior uniformity of operating voltage and resistance states. In summary, this thesis promotes the development of novel, high-performance metal oxide based memristors enabled by the availability of new, nanostructured materials and innovations in device structure engineering. The switching performance, underlying mechanisms, area/defect concentration effects, development of solution-processed nanocrystals assemblies and chemistries, and highly enhanced uniformity in memristors are addressed by combining systematic deposition approaches with the advanced nanoscopic observation of the conducting filament, leading to the strongest competitor among future nonvolatile memory solution

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Size-Dependent Metal-insulator Transition in Pt-Dispersed Sio2 Thin Film: A Candidate for Future Non-Volatile Memory

Non-volatile random access memories (NVRAM) are promising data storage and processing devices. Various NVRAM, such as FeRAM and MRAM, have been studied in the past. But resistance switching random access memory (RRAM) has demonstrated the most potential for replacing flash memory in use today. In this dissertation, a novel RRAM material design that relies upon an electronic transition, rather than a phase change (as in chalcogenide Ovonic RRAM) or a structural change (such in oxide and halide filamentary RRAM), is investigated. Since the design is not limited to a single material but applicable to general combinations of metals and insulators, the goal of this study is to use a model material to delineate the intrinsic features of the electronic metal/insulator transition in random systems and to demonstrate their relevance to reliable memory storage and retrieval. We fabricated amorphous SiO2 thin films embedded with randomly dispersed Pt atoms. Macroscopically, this random material exhibits a percolation transition in electric conductivity similar to the one found in various insulator/metal granular materials. However, at Pt concentrations well below the bulk percolation limit, a distinct insulator to metal transition occurs in the thickness direction as the film thickness falls below electron’s “diffusion” distance, which is the tunneling distance at 0K. The thickness-triggered metal-to-insulator transition (MIT) can be similarly triggered by other conditions: (a) a changing Pt concentration (a concentration-triggered MIT), (b) a changing voltage/polarity (voltage-triggered MIT), and (c) an UV irradiation (photon-triggered MIT). The resistance switching characteristics of this random material were further investigated in several device configurations under various test conditions. These include: materials for the top and bottom electrodes, fast pulsing, impedance spectroscopy, static stressing, retention, fatigue and temperature from 10K to 448K. The SiO2-Pt RRAM exhibits fast switching speed (~25 ns), high resistance ratio (\u3e100), long retention time/write time ratio (\u3e1012), multi-bit storage and extraordinary performance reproducibility. The device switches by a purely electronic mechanism: electron trapping makes it an insulator; charge detrapping returns it to a metal. The switching voltages are low, ~ 1 V, and are independent of size, thickness, composition, temperature and write/erase time. The insulator state has a conductance that exponentially decays with the thickness

Memristive Non-Volatile Memory Based on Graphene Materials

Resistive random access memory (RRAM), which is considered as one of the most promising next-generation non-volatile memory (NVM) devices and a representative of memristor technologies, demonstrated great potential in acting as an artificial synapse in the industry of neuromorphic systems and artificial intelligence (AI), due its advantages such as fast operation speed, low power consumption, and high device density. Graphene and related materials (GRMs), especially graphene oxide (GO), acting as active materials for RRAM devices, are considered as a promising alternative to other materials including metal oxides and perovskite materials. Herein, an overview of GRM-based RRAM devices is provided, with discussion about the properties of GRMs, main operation mechanisms for resistive switching (RS) behavior, figure of merit (FoM) summary, and prospect extension of GRM-based RRAM devices. With excellent physical and chemical advantages like intrinsic Young’s modulus (1.0 TPa), good tensile strength (130 GPa), excellent carrier mobility (2.0 × 105 cm2∙V−1∙s−1), and high thermal (5000 Wm−1∙K−1) and superior electrical conductivity (1.0 × 106 S∙m−1), GRMs can act as electrodes and resistive switching media in RRAM devices. In addition, the GRM-based interface between electrode and dielectric can have an effect on atomic diffusion limitation in dielectric and surface effect suppression. Immense amounts of concrete research indicate that GRMs might play a significant role in promoting the large-scale commercialization possibility of RRAM devices

Resistance switching devices based on amorphous insulator-metal thin films

Nanometallic devices based on amorphous insulator-metal thin films are developed to provide a novel non-volatile resistance-switching random-access memory (RRAM). In these devices, data recording is controlled by a bipolar voltage, which tunes electron localization length, thus resistivity, through electron trapping/detrapping. The low-resistance state is a metallic state while the high-resistance state is an insulating state, as established by conductivity studies from 2K to 300K. The material is exemplified by a Si3N4 thin film with randomly dispersed Pt or Cr. It has been extended to other materials, spanning a large library of oxide and nitride insulator films, dispersed with transition and main-group metal atoms. Nanometallic RRAMs have superior properties that set them apart from other RRAMs. The critical switching voltage is independent of the film thickness/device area/temperature/switching speed. Trapped electrons are relaxed by electron-phonon interaction, adding stability which enables long-term memory retention. As electron-phonon interaction is mechanically altered, trapped electron can be destabilized, and sub-picosecond switching has been demonstrated using an electromagnetically generated stress pulse. AC impedance spectroscopy confirms the resistance state is spatially uniform, providing a capacitance that linearly scales with area and inversely scales with thickness. The spatial uniformity is also manifested in outstanding uniformity of switching properties. Device degradation, due to moisture, electrode oxidation and dielectrophoresis, is minimal when dense thin films are used or when a hermetic seal is provided. The potential for low power operation, multi-bit storage and complementary stacking have been demonstrated in various RRAM configurations.Comment: 523 pages, 215 figures, 10 chapter

Nanoionic Resistive‐Switching Devices

Advances in the understanding of nanoscale ionic processes in solid‐state thin films have led to the rapid development of devices based on coupled ionic–electronic effects. For example, ion‐driven resistive‐switching (RS) devices have been extensively studied for future memory applications due to their excellent performance in terms of switching speed, endurance, retention, and scalability. Recent studies further suggest that RS devices are more than just resistors with tunable resistance; instead, they exhibit rich and complex internal ionic dynamics that equip them with native information‐processing capabilities, particularly in the temporal domain. RS effects induced by the migration of different types of ions, often driven by an electric field, are discussed. It is shown that, by taking advantage of the different state variables controlled by the ionic processes, important synaptic functions can be faithfully implemented in solid‐state devices and networks. Recent efforts on improving the controllability of ionic processes to optimize device performance are also discussed, along with new opportunities for material design and engineering enabled by the ability to control ionic processes at the atomic scale.Solid‐state resistive‐switching devices driven by nanoscale ionic processes are reviewed, with the focus on the rich ionic dynamics that enable natural implementation of a range of biological synaptic and neuron functions. Efforts to control ion redistribution at the atomic scale have led to improved device performance, and enabled applications based on reconfigurable nanostructures and materials through controlled ionic processes in solid‐state devices.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151267/1/aelm201900184_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151267/2/aelm201900184.pd

Thin-film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity

A design concept of phase-separated amorphous nanocomposite thin films is presented that realizes interfacial resistive switching (RS) in hafnium oxide-based devices. The films are formed by incorporating an average of 7% Ba into hafnium oxide during pulsed laser deposition at temperatures ≤400°C. The added Ba prevents the films from crystallizing and leads to ∼20-nm-thin films consisting of an amorphous HfOx host matrix interspersed with ∼2-nm-wide, ∼5-to-10-nm-pitch Ba-rich amorphous nanocolumns penetrating approximately two-thirds through the films. This restricts the RS to an interfacial Schottky-like energy barrier whose magnitude is tuned by ionic migration under an applied electric field. Resulting devices achieve stable cycle-to-cycle, device-to-device, and sample-to-sample reproducibility with a measured switching endurance of ≥104 cycles for a memory window ≥10 at switching voltages of ±2 V. Each device can be set to multiple intermediate resistance states, which enables synaptic spike-timing-dependent plasticity. The presented concept unlocks additional design variables for RS devices

Colloidal inorganic nanocrystal based nanocomposites: Functional materials for micro and nanofabrication

The unique size- and shape-dependent electronic properties of nanocrystals (NCs) make them extremely attractive as novel structural building blocks for constructing a new generation of innovative materials and solid-state devices. Recent advances in material chemistry has allowed the synthesis of colloidal NCs with a wide range of compositions, with a precise control on size, shape and uniformity as well as specific surface chemistry. By incorporating such nanostructures in polymers, mesoscopic materials can be achieved and their properties engineered by choosing NCs differing in size and/or composition, properly tuning the interaction between NCs and surrounding environment. In this contribution, different approaches will be presented as effective opportunities for conveying colloidal NC properties to nanocomposite materials for micro and nanofabrication. Patterning of such nanocomposites either by conventional lithographic techniques and emerging patterning tools, such as ink jet printing and nanoimprint lithography, will be illustrated, pointing out their technological impact on developing new optoelectronic and sensing devices. © 2010 by the authors

Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications

Transition metal oxides, TiO2 and Ta2O5, are two of the most extensively studied wide bandgap semiconductor materials (with high work functions). Due to their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation, their application in heterogeneous photocatalysis has attracted a lot of attention. These oxides are also great components in the field of electronic devices such as field effect transistors, solar cells, and more recently advanced memory devices. Here, we focus on ultrasmall nanoclusters (< 5 nm) and nanocrystalline thin films of defect-rich TiO2 and Ta2O5 and their applications as high-performance photocatalysts in photoelectrochemical water splitting reactions and as resistive switching materials in memory applications. The present work is divided into two main parts. In the first part, ultrasmall nanoclusters (below 10 nm) of defect-rich TiO2 and Ta2O5 are synthesized using a gas phase aggregation technique in a nanocluster generation source based on DC magnetron sputtering. With a careful optimization of the deposition parameters such as aggregation zone length (condensation volume), Ar gas flow rate, deposition temperature and source power, we are able to produce metal/metal oxide nanoclusters with a narrow size distribution. As most of these as-grown nanoclusters are negatively charged, it is possible to conduct size-selection according to their mass-to-charge ratio. Using a quadrupole mass filter (directly coupled to the magnetron source), we achieve precise size-selection of nanoclusters, with the size distribution reduced to below 2% mass resolution. The nearly monosized nanoclusters so produced are deposited onto appropriate substrates to serve as the photoanodes for photoelectrochemical water splitting application. We demonstrate, for the first time, that the precisely size-selected TiO2 nanoclusters can be deposited on H-terminated Si(001) in a soft-landing condition and they can be used as highperformance photocatalysts for solar harvesting, with greater enhancement in the photoconversion efficiency. Three different sizes of TiO2 nanoclusters (4, 6 and 8 nm) are synthesized with appropriate combinations of aggregation length and Ar flow rate. Despite the low amount of material loading (of ~20% of substrate coverage), these supported TiO2 nanoclusters exhibit remarkable photocatalytic activities during photoelectrochemical water splitting reaction under simulated sunlight (50 mW/cm^2). Higher photocurrent densities (up to 0.8 mA/cm^2) and photoconversion efficiencies (up to 1%) with decreasing nanocluster size (at the applied voltage of –0.22 V vs Ag/AgCl) are observed. We attribute this enhancement to the presence of surface defects, providing a large amount of active surface sites, in the amorphous TiO2 nanoclusters as-grown at room temperature. We have further shown that the incorporation of metallic nanoclusters with the semiconductor photocatalysts can enhance the photoconversion efficiency. In this work, we have co-deposited surface oxygen deficient Ta2O5 or TaOx nanoclusters along with Pt nanoclusters of similar nanocluster size (~5 nm), the latter used as a promoter. The electron-hole pairs generated in the water splitting reaction can be effectively separated and stored with the presence of Pt nanoclusters, while the increase in Pt loading as a promoter can enhance the reaction by providing a large number of electrons for H2 evolution. However, loading too much Pt nanoclusters could actually reduce the photoresponse, which is due to blocking of photosensitive TaOx surface by excess Pt nanoclusters. In both cases, the photoconversion efficiency could potentially be enhanced at least 5 times by increasing the amount of nanocluster loading from 20% coverage to a monolayer coverage (e.g., by increasing the amount of deposition time for TiO2 and TaOx nanoclusters). Even higher photoconversion efficiency can be obtained with multiple layers of nanoclusters and by employing nanoclusters with even smaller size and/or with modification by chemical functionalization. These potential improvements could dramatically increase the photoconversion efficiency, making these nanocluster samples to be among the top photoelectrochemical catalysis performers. In the second part of the present work, we employ defect-rich nanocrystalline TiOx and TaOx thin films as active materials for resistive switching for memory application. Based on resistive switching principle, memristive devices (or memristors) provide the unique capability of multistep information storage. The development of memristors has often been hailed as the next evolution in non-volatile memories, low-power remote sensing, and adaptive intelligent prototypes including neuromorphic and biological systems. One major obstacle in achieving high switching performance is the irreversible electroforming step that is required to create oxygen vacancies for resistive switching. Using magnetron sputtering film deposition technique, we have fabricated the heterojunction memristor devices based on nanocrystalline TiOx and TaOx thin films (10-60 nm thick) with a high density of built-in oxygen vacancies, sandwiched between a pair of metallic Pt electrodes (30 nm thick). To avoid the destructive electroforming process and to achieve a high switching performance in the memristor device, we carefully manipulate the chamber pressure and ambient in deposition chamber during deposition to generate the required highly oxygen deficient semiconducting films. The films, as-deposited at room temperature, exhibit a crystallite size of 4-5 nm. In the fabricated Pt/TiOx/Pt memristors, a high electric field gradient can be generated in the TiOx film at a much lower electroforming voltage of +1.5 V, due in part to the nanocrystalline nature, which causes localization of this electric field and consequently enhanced reproducibility and repeatability in the device performance. After the first switching, consecutive 250 switching cycles can be achieved with a low programing voltage of ±1.0 V, along with a high ON/OFF current ratio, and long retention (up to 10^5 s). We further improve this TiOx memristor device by totally removing the electroforming step by fabricating an electroforming-free memristive device based on a heterojunction interface of TiOx and TaOx layers. In the Pt/TiOx/TaOx/Pt architecture structure (with Pt serving as the top and bottom electrodes), a high-κ dielectric TaOx layer is used to facilitate trapping and release of the electronic carriers, while a TiOx layer provides low-bias rectification as an additional oxygen vacancy source. With the incorporation of TaOx layer, the need for the electroforming step can be eliminated. More importantly, the resistance states of the device can be tuned such that switching between the high resistance state and the low resistance state can be achieved even smaller programming voltage of +0.8 V. With the low leakage current properties of TaOx, the high endurance (10^4 repeated cycles) and high retention capabilities (up to 10^8 s) can be enhanced manifold with highly stable ON/OFF current ratio. In both memristor devices, four different junction sizes (5×5, 10×10, 20×20 and 50×50 μm^2) have been evaluated according to their ON/OFF current ratio. We observe that the smaller is the junction size is, the higher is the current ratio. For the Pt/TiOx/TaOx/Pt memristor, we have also analyzed the thickness dependent effect of the switching behavior of devices with four different TaOx layer thicknesses (10, 20, 40 and 60 nm) and a TiOx layer thickness constant at 10 nm. The device with 10 nm thick TaOx (being amorphous in nature) shows unipolar switching with two SETs and two RESETs in one sweep cycle. This is in contrast to the bipolar resistive switching found in devices with the thicker TaOx films with a SET in the positive sweep and a RESET during the negative sweep. We further demonstrate that resistive switching can also occur at very low programming voltage (~50 mV), thus qualifying it as an ultralow power consumption device (~nW). The stable non-volatile bipolar switching characteristics with high ON/OFF current ratio and low power consumption make our devices best suitable for various analog and discrete programmable electric pulses. With the simplicity in the construction, the performance achieved for our memristors represents the best reported to date. This new class of defect-rich metal oxides nanomaterials with an ultrananocrystalline nature shows solid promises for various catalytic and electronic applications and, also, the simple, scalable roomtemperature device fabrication process makes this approach easily migratable further to transparent and/or flexible devices