6 research outputs found
Leakage current and resistive switching mechanisms in SrTiO3
PhD ThesisResistive switching random access memory devices have attracted considerable attention due to exhibiting fast programming, non-destructive readout, low power-consumption, high-density integration, and low fabrication-cost. Resistive switching has been observed in a wide range of materials but the underpinning mechanisms still have not been understood completely.
This thesis presents a study of the leakage current and resistive switching mechanisms of SrTiO3 metal-insulator-metal devices fabricated using atomic layer deposition and pulse laser deposition techniques. First, the conduction mechanisms in SrTiO3 are investigated. The leakage current characteristics are highly sensitive to the polarity and magnitude of applied voltage bias, punctuated by sharp increases at high field. The characteristics are also asymmetric with bias and the negative to positive current crossover point always occurs at a negative voltage bias. A model comprising thermionic field emission and tunnelling phenomena is proposed to explain
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the dependence of leakage current upon the device parameters quantitatively. SrTiO3 also demonstrates bipolar switching behaviour where the current-density versus voltage (J-V) characteristics show asymmetry at all temperatures examined, with resistive switching behaviour observed at elevated temperatures. The asymmetry is explained by the relative lack of electron traps at one electrode, which is determined from the symmetric J-V curve obtained at room temperature due to the redistribution of the dominant electrical defects in the film. Evidence is presented for a model of resistive switching that originates from defect diffusion (possibly oxygen vacancies) at high temperatures. Finally, a peculiar resistive switching behaviour was observed in pulse laser deposited SrTiO3. This switching depends on both the amplitude and polarity of the applied voltage, and cannot be described as either bipolar or unipolar resistive switching. This behaviour is termed antipolar due to the opposite polarity of the set voltage relative to the previous reset voltage. The proposed model based on electron injection by tunnelling at interfaces and a Poole-Frenkel mechanism through the bulk is extended to explain the antipolar resistive switching behaviour. This model is quantified by use of a simple mathematical equation to simulate the experimental results
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
Evaluation des performances des mĂ©moires CBRAM (Conductive bridge memory) afin dâoptimiser les empilements technologiques et les solutions dâintĂ©gration
The constant evolution of the data storage needs over the last decades have led the technological landscape to completely change and reinvent itself. From the early stage of magnetic storage to the most recent solid state devices, the bit density keeps increasing toward what seems from a consumer point of view infinite storage capacity and performances. However, behind each storage technology transition stand density and performances limitations that required strong research work to overcome. This manuscript revolves around one of the promising emerging technology aiming to revolutionize data storage landscape: the Conductive Bridge Random Access Memory (CBRAM). This technology based on the reversible formation and dissolution of a conductive path in a solid electrolyte matrix offers great advantages in term of power consumption, performances, density and the possibility to be integrated in the back end of line. However, for this technology to be competitive some roadblocks still have to be overcome especially regarding the technology variability, reliability and thermal stability. This manuscript proposes a comprehensive understanding of the CBRAM operations based on experimental results and a specially developed Kinetic Monte Carlo model. This understanding creates bridges between the physical properties of the materials involved in the devices and the devices performances (Forming, SET and RESET time and voltage, retention, endurance, variability). A strong emphasis is placed on the current limitations of the technology previously stated and how to overcome these limitations. Improvement of the thermal stability and device reliability are demonstrated with optimized operating conditions and proper devices engineering.Ces derniĂšres dĂ©cennies, la constante Ă©volution des besoins de stockage de donnĂ©es a menĂ© Ă un bouleversement du paysage technologique qui sâest complĂštement mĂ©tamorphosĂ© et rĂ©inventĂ©. Depuis les dĂ©buts du stockage magnĂ©tique jusquâaux plus rĂ©cents dispositifs fondĂ©s sur lâĂ©lectronique dit dâĂ©tat solide, la densitĂ© de bits stockĂ©s continue dâaugmenter vers ce qui semble du point de vue du consommateur comme des capacitĂ©s de stockage et des performances infinies. Cependant, derriĂšre chaque transition et Ă©volution des technologies de stockage se cachent des limitations en termes de densitĂ© et performances qui nĂ©cessitent de lourds travaux de recherche afin dâĂȘtre surmontĂ©es et repoussĂ©es. Ce manuscrit sâarticule autour dâune technologie Ă©mergeante prometteuse ayant pour vocation de rĂ©volutionner le paysage du stockage de donnĂ©es : la mĂ©moire Ă pont conducteur ou Conductive Bridge Random Access Memory (CBRAM). Cette technologie est fondĂ©e sur la formation et dissolution rĂ©versible dâun chemin Ă©lectriquement conducteur dans un Ă©lectrolyte solide. Elle offre de nombreux avantages face aux technologies actuelles tels quâune faible consommation Ă©lectrique, de trĂšs bonnes performances dâĂ©criture et de lecture et la capacitĂ© dâĂȘtre intĂ©grĂ© aux seins des interconnexions mĂ©talliques dâune puce afin dâaugmenter la densitĂ© de stockage. MalgrĂ© tout, pour que cette technologie soit compĂ©titive certaines limitations ont besoin dâĂȘtre surmontĂ©es et particuliĂšrement sa variabilitĂ© et sa stabilitĂ© thermique qui posent encore problĂšme. Ce manuscrit propose une comprĂ©hension physique globale du fonctionnement de la technologie CBRAM fondĂ©e sur une Ă©tude expĂ©rimentale approfondie couplĂ©e Ă un modĂšle Monte Carlo cinĂ©tique spĂ©cialement dĂ©veloppĂ©. Cette comprĂ©hension fait le lien entre les propriĂ©tĂ©s physiques des matĂ©riaux composant la mĂ©moire CBRAM et ses performances (Tension et temps dâĂ©criture et dâeffacement, rĂ©tention de donnĂ©e, endurance et variabilitĂ©). Un fort accent est mis la comprĂ©hension des limites actuelle de la technologie et comment les repousser. GrĂące Ă une optimisation des conditions dâopĂ©rations ainsi quâĂ un travail dâingĂ©nierie des dispositifs mĂ©moire, il est dĂ©montrĂ© dans ce manuscrit une forte amĂ©lioration de la stabilitĂ© thermique ainsi que de la variabilitĂ© des Ă©tats Ă©crits et effacĂ©s
OnâDemand Reconfiguration of Nanomaterials: When Electronics Meets Ionics
Rapid advances in the semiconductor industry, driven largely by device scaling, are now approaching fundamental physical limits and face severe power, performance, and cost constraints. Multifunctional materials and devices may lead to a paradigm shift toward new, intelligent, and efficient computing systems, and are being extensively studied. Herein examines how, by controlling the internal ion distribution in a solidâstate film, a materialâs chemical composition and physical properties can be reversibly reconfigured using an applied electric field, at room temperature and after device fabrication. Reconfigurability is observed in a wide range of materials, including commonly used dielectric films, and has led to the development of new device concepts such as resistive randomâaccess memory. Physical reconfigurability further allows memory and logic operations to be merged in the same device for efficient inâmemory computing and neuromorphic computing systems. By directly changing the chemical composition of the material, coupled electrical, optical, and magnetic effects can also be obtained. A survey of recent fundamental material and device studies that reveal the dynamic ionic processes is included, along with discussions on systematic modeling efforts, device and material challenges, and future research directions.By controlling the internal ion distribution in a solidâstate film, the materialâs chemical composition and physical (i.e., electrical, optical, and magnetic) properties can be reversibly reconfigured, in situ, using an applied electric field. The reconfigurability is achieved in a wide range of materials, and can lead to the development of new memory, logic, and multifunctional devices and systems.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141225/1/adma201702770.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141225/2/adma201702770_am.pd
Chalcogenide and metal-oxide memristive devices for advanced neuromorphic computing
Energy-intensive artificial intelligence (AI) is prevailing and changing the world, which requires energy-efficient computing technology. However, traditional AI driven by von Neumann computing systems suffers from the penalties of high-energy consumption and time delay due to frequent data shuttling. To tackle the issue, brain-inspired neuromorphic computing that performs data processing in memory is developed, reducing energy consumption and processing time. Particularly, some advanced neuromorphic systems perceive environmental variations and internalize sensory signals for localized in-senor computing. This methodology can further improve data processing efficiency and develop multifunctional AI products. Memristive devices are one of the promising candidates for neuromorphic systems due to their non-volatility, small size, fast speed, low-energy consumption, etc.
In this thesis, memristive devices based on chalcogenide and metal-oxide materials are fabricated for neuromorphic computing systems. Firstly, a versatile memristive device (Ag/CuInSe2/Mo) is demonstrated based on filamentary switching. Non-volatile and volatile features are coexistent, which play multiple roles of non-volatile memory, selectors, artificial neurons, and artificial synapses. The conductive filamentsâ lifetime was controlled to present both volatile and non-volatile behaviours. Secondly, the sensing functions (temperature and humidity) are explored based on Ag conductive filaments. An intelligent matter (Ag/Cu(In, Ga)Se2/Mo) endowing reconfigurable temperature and humidity sensations is developed for sensory neuromorphic systems. The device reversibly switches between two states with differentiable semiconductive and metallic features, demonstrating different responses to temperature and humidity variations. Integrated devices can be employed for intelligent electronic skin and in-sensor computing. Thirdly, the memristive-based sensing function of light was investigated. An optoelectronic synapse (ITO/ZnO/MoO3/Mo) enabling multi-spectrum sensitivity for machine vision systems is developed. For the first time, this optoelectronic synapse is practical for front-end retinomorphic image sensing, convolution processing, and back-end neuromorphic computing. This thesis will benefit the development of advanced neuromorphic systems pushing forward AI technology