191 research outputs found

    Transparent conductive oxide TCO buffer layer effect on the resistive switching process in metal TCO TiO2 metal assemblies

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    The effect of transparent conductive oxide TCO buffer layer on the insulator matrix and on the resistive switching process in the metal TiO2 TCO metal assembly was studied depending on the material of TCO ITO In2O3 0.9 SnO2 0.1 or SnO2 or ZnO . First time electro physical studies and near edge x ray absorption fine structure NEXAFS studies were carried out jointly and in the same point of the sample providing the direct experimental evidence that switching process influences strongly the lowest unoccupied bands and local atomic structure of the TiO2 layers. It was established that TCO layer in metal TiO2 TCO metal assembly is an additional source of oxygen vacancies for TiO2 film. The RL RH states are achieved presumably with formation rupture of electrically conductive path of oxygen vacancies. The inserting the Al2O3 thin layer between TiO2 and TCO layers restricts to some extent processes of migration of oxygen ions and vacancies and does not permit to realize the anti clockwise bipolar resistive switching in Au TiO2 Al2O3 ITO Au assembly. The greatest value of the ratio RH RL is observed for assembly with SnO2 buffer layer that will provide to implement the maximum set of intermediate states recording analog data and increases the density of information recording in this cas

    Oxide Memristive Devices

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    Resistive switching in metal oxide materials has recently renewed the interest of many researchers due to the many application in non-volatile memory and neuromorphic computing. A memristor or a memristive device in general, is a device behaving as nonlinear resistor with memory which depends on the amount of charges that passes through it. A novel idea of combining the physical resistive switching phenomenon and the circuit-theoretic formalism of memristors was proposed in 2008. The physical mechanism on how resistive switching occurs is still under debate. A physical understanding of the switching phenomenon is of much importance in order to tailor specific properties for memory applications. To investigate the resistive switching in oxide materials, memristive devices were fabricated starting from materials processing: low-pressure chemical vapor deposition of ZnO nanowires (NWs), low-temperature atomic layer deposition (ALD) of TiO2 thin films and micro-pulse ALD of Fe2O3 thin films. The distinct geometry of ZnO NWs makes it possible to investigate the effect of the electrode material, surface states and compliance to the memristive properties. A simpler method of fabricating TiO2-based devices was explored using low-temperature atomic layer deposition. This approach is very promising for device application using photoresist and polymeric substrates without thermal degradation during and after device fabrication. ALD of pure phase Fe2O3 thin films was demonstrated using cyclic micro-pulses. Based on the performance of the fabricated devices, the oxide materials under this study have promising properties for the next-generation memory devices

    Memristive Anodic Oxides: Production, Properties and Applications in Neuromorphic Computing

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    Memristive devices generally consist of metal oxide elements with specific structure and chemical composition, which are crucial to obtain the required variability in resistance. This makes the control of oxide properties vital. While CMOS compatible production technologies for metal oxides deposition generally involve physical or chemical deposition pathways, we here describe the possibility of using an electrochemical technique, anodic oxidation, as an alternative route to produce memristive oxides. In fact, anodization allows to form a very large range of oxides on the surface of valve metals, such as titanium, hafnium, niobium and tantalum, whose thickness, structure and functional properties depend on process parameters imposed. These oxides may be of interest to build neural networks based on memristive elements produced by anodic oxidation

    Filamentary Threshold Switching In Niobium Oxides

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    Two-terminal metal/oxide/metal (MOM) structures exhibit characteristic resistance changes, including non-volatile memory and volatile threshold switching responses when subjected to electrical stress (i.e., voltage or current stimuli), which are of interest as active elements in non-volatile memory arrays and neuromorphic computing. Recently, the threshold switching response in MOM devices based on vanadium oxides and niobium oxides have attracted particular attention due to their simple structure and reliability. Interestingly, specific phases of these oxides (e.g., VO2, NbO2 etc.) exhibit a metal-insulator transition (MIT) which causes dramatic changes in their intrinsic properties, including electrical and thermal conductivities, and often arguably reported as the dominant cause of the observed threshold switching response. While this response has been extensively studied for VO2, but the low transition temperature (~ 340K) limits their use only to low temperature microelectronics applications. In contrast, NbO2 has a much higher transition temperature ~ 1070 K, and NbO2 and other NbOx phases have drawn recent attention due to their reliable threshold switching characteristics. The resistance changes in MOM structures are often initiated by a one-step electroforming process that forms a filamentary conduction path. Knowledge about the structure, composition and spatial distribution of these filaments is essential for a full understanding of filamentary resistive/threshold-switching and for effective modelling and optimisation of associated devices. Additionally, NbOx-based devices exhibit a wide range of resistive and threshold switching responses that critically depend on operating condition, composition and device geometry. Thus, a proper understanding of these factors is important for achieving reliable switching with desired characteristics. This thesis focuses on understanding the electroforming process and subsequent threshold switching responses in NbOx by employing different techniques, including electrical testing, and thermo-reflectance imaging. At first, a simple means of detecting and spatially mapping conductive filaments in metal/oxide/metal cross-point devices is introduced and the utility of this technique is demonstrated to identify distinct modes of electroforming in low- and high-conductivity NbOx films. After that, the role of metal/oxide interface reactions on the post-forming characteristics of reactive-metal/Nb2O5/Pt devices is demonstrated. Specifically, devices are shown to exhibit stable threshold switching under negative bias but the response under positive bias depends on the choice of metal. Then, the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 are compared. In particular it is shown that doping offers an effective means of engineering the device response. Based on temperature dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature. Finally, the physical origin of the discontinuous 'snapback' NDR is investigated. Specifically, it is shown that the snapback response is a direct consequence of current localisation and redistribution within the oxide film. Furthermore, it is demonstrated that material and device dependencies are consistent with predictions of a two-zone parallel memristor model of NDR which is based on a non-uniform current distribution after electroforming. These results advance the current understanding of threshold switching response in amorphous NbOx films, and provide a strong basis for engineering devices with specific NDR characteristics. Significantly, these results also resolve a long-standing controversy about the origin of the snapback response which has been a subject of considerable debate

    Resistive switching in ferroelectric polycrystalline Yttrium Manganese Oxide thin films

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    A memristor is a two-terminal device which exhibits a hysteresis loop in the current-voltage characteristics. Resistive switching refers to reversible non-volatile change in state of the resistance. There exists a wide range of materials which show resistive switching i.e, phase change materials are used in today’s technology which are a main component of the resistive random access memory. In actual research, mostly metal oxides are investigated regarding their resistive switching which is based on migration of anions and cations. Additionally, in hexagonal manganites, h-RMnO3 (R = Y, In, Sc, Ho,...,Lu), the multiferroic properties and nano-sized conducting domain walls introduce further interesting aspects in this material class which may contribute to additional features in resistive switching. This dissertation investigates the resistive switching in yttrium manganite thin film (Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3 and Y0.94Mn1.05Ti0.01O3) based metal-insulator-metal structures with different top electrodes (Au or Al) and bottom electrodes (Pt or Pt/Ti or Pt/Cr) in 2-point DC probe measurements. Yttrium manganite thin films have been deposited by pulsed laser deposition on metal coated SiO2/Si substrates. Electrical characterization of yttrium manganite thin films in a metal-insulator-metal structure exhibit electroforming-free unipolar resistive switching. High voltages and currents are required for SET (V_ ) and RESET (I_ ), respectively. The observed resistive switching is assigned to the formation (low resistance state) and rupture (high resistance state) of conductive, metallic-like filaments induced by a thermo-chemical phenomena. Observed unipolar RS is classified as the thermo-chemical memory (TCM) resistive switching phenomena related to the locally increased temperature. The stability of conductive filaments leads to good retention of the programmed states with large memory window (OFF to ON resistance in the order of 10^4 - 10^6, depends on electrodes, electrode size and composition of yttrium manganite thin films). The endurance or number of loading cycles of the resistive switching devices are improved and is in the order of 10^3 for Y1Mn1O3 and Y0.95Mn1.05O3 composition with Al-top electrodes and Pt-bottom electrode. The maximum number of loading cycles is observed for an applied negative bias, a preferential negative polarity for switching the yttrium manganite thin film devices with Au or Al top electrodes and Pt or Pt/Ti bottom electrodes. Whereas, yttrium manganite thin film devices with Pt/Cr-bottom electrode and Al-top electrodes a preferential positive bias is required for switching the devices. Temperature-dependent measurements of yttrium manganite thin films deposited on Pt/SiO2/Si show semiconducting and metallic-like conduction in high resistance state and low resistance state, respectively. The activation energy () extracted in the ohmic region for hopping of holes localized at Mn4+ is in the range of 0.36 eV - 0.43 eV. Scanning electron microscopy in secondary electron emission mode with an in-lens detector and a small acceleration voltage of 1.0 kV is used to characterize the ferroelectric charged domain network formation in polycrystalline hexagonal yttrium manganite thin film. The observed bright regions correspond to local polarization vector with upward polarization components (+P ) and dark regions to local polarization vector with downward polarization components (-P ). A dense domain network is observed for Mn-rich samples (Y0.95Mn1.05O3 and Y0.94Mn1.05Ti0.01O3) in comparison to Y1Mn1O3 and Y1Mn0.99Ti0.01O3 with smaller grains show isolated charged domains. The observed dependency of different compositions to the charged domain density network in yttrium manganite thin films may influenced by different factors: stoichiometry gradient, oxygen, dopant concentration and the resulting grain structure.Ein Memristor ist ein Bauelement, welches eine Hysterese beim Vermessen seiner IU-Kennlinie aufweist. Dieses als „Widerstandsschalten“ bezeichnete PhĂ€nomen beruht auf der nichtflĂŒchtigen VerĂ€nderung des Widerstandes. Es existiert eine breite Auswahl an Materialien, welche Widerstandsschalten zeigen, z.B. sind Phasenwechselmaterialien die Hauptkomponenten in aktuellen RRAMs. Aktuelle werden hauptsĂ€chlich Metalloxide untersucht, welche durch Migration von Anionen und Kationen Widerstandsschalten hervorrufen. Weitere Materialien wie hexagonale Manganoxidverbindungen RMnO3 (R = Y, In, Sc, Ho,...,Lu), besitzen zusĂ€tzliche multiferroische Eigenschaften, bei denen geladene DomĂ€nengrenzen weitere interessante Aspekte in dieser Materialklasse einfĂŒhren und das Widerstandsschalten beeinflussen können. Die vorliegende Dissertation untersucht das Widerstandsschalten in Yttriummanganoxid-DĂŒnnfilmen mit unterschiedlichen Kompositionen und unterschiedlichen Elektrodenmaterialien. Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3 und Y0.94Mn1.05Ti0.01O3, wurden mittels gepulster Laserdeposition auf metallisierte Si/SiO2 Substrate abgeschieden. Die elektrische Charakterisierung von Yttriummanganoxid-DĂŒnnfilmen in einer Metall-Isolator-Metall Sandwichstruktur weist auf elektroformierungsfreies, unipolares Widerstandsschalten hin. Das beobachtete Widerstandsschalten wird auf die Formierung (niederohmiger Zustand) und Zerstörung (hochohmiger Zustand) des leitfĂ€higen, metallischen Filaments (geladenen DomĂ€nengrenzen oder auch Vortices), verursacht durch thermisch-chemische VorgĂ€nge, zurĂŒckgefĂŒhrt. Die geladenen DomĂ€nengrenzen und/oder Vortices in Yttriummanganoxid-DĂŒnnfilmen beeinflussen unter UmstĂ€nden als nanoskalige Objekte die Formierung der leitfĂ€higen Filamente. Die StabilitĂ€t der leitfĂ€higen Filamente fĂŒhrt zu einer guten Langzeitspeicherung der programmierten ZustĂ€nde, welche auch ein sehr großes Speicherfenster (WiderstandsverhĂ€ltnis zwischen Aus/An-Zustand von 10^5) aufweisen. Die großen WiderstandsverhĂ€ltnisse sind z.B. fĂŒr die Herstellung von Auswahlschaltern (selektoren) in Crossbar-Strukturen notwendig, um die möglicherweise auftretenden Kriechströme in Crossbar-Strukturen zu unterdrĂŒcken, welche sonst Lesefehler der adressierten Zellen hervorrufen wĂŒrden. Die Wiederbeschreibbarkeit ist in der GrĂ¶ĂŸenordnung von ca. 10^3, abhĂ€ngig von der chemischen Zusammensetzung des Yttriummanganoxide-DĂŒnnfilmes und vom verwendeten Elektrodenmaterial. Resultate der Charakterisierung mittels Rasterelektronenmikroskopie im SekundĂ€relektronenmodus mit einer kleinen Beschleunigungsspannung von 1.0 kV weisen auf geladene ferroelektrische DomĂ€nen in polykristallinem hexagonalen YMnO3 DĂŒnnfilmen hin. Deswegen muß der Einfluss von geladenen DomĂ€nengrenzen und multiferroischen Vortices auf das beobachtete Widerstandsschalten in hexagonalem YMnO3 berĂŒcksichtigt werden

    Memristor based on amorphous zinc-tin oxide Schottky diodes

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    This work reports the fabrication and characterization of metal-insulator-metal (MIM) structures based on Schottky junction between platinum and zinc-tin oxide (ZTO) for memristor application. The devices were produced exclusively by physical vapor deposition methods at room temperature (RT). Before evaluating memory performance, these devices were first characterized in their pristine state as diodes. It was studied how the addition of an intermediate step of oxygen plasma treatment on the platinum surface, as well as the influence of oxide film thickness impact device performance. The produced memristors exhibited a very distinct operation dynamic on the reset process depending on the voltage signal applied in the electroforming process. When the voltage coincides with the forward direction of the diode the reset process runs in gradual analogic way; otherwise the reset process runs in a threshold manner, like the set process in both cases. Oxygen plasma treatment tends to improve the analog mechanism in terms of operation window, regardless of thickness. However, the stability of resistive RAM operation is improved with thicker devices

    Solution-based IGZO nanoparticles memristor

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    This work aims to characterize Indium-Gallium-Zinc-Oxide nanoparticles (IGZOnp) as a resistive switching matrix in metal-insulator-metal (MIM) structures for memristor application. IGZOnp was produced by low cost solution-based process and deposited by spin-coating technique. Several top and bottom electrodes combinations, including IZO, Pt, Au, Ti, Ag were investigated to evaluate memory performance, yield and switching properties. The effect of ambient and annealing temperature using 350 ÂșC and 200 ÂșC was also analysed in order to get more insight into resistive switching mechanism. The Ag/IGZOnp/Ti memristor structure annealed at 200 ÂșC exhibits the best results with a large yield. The device shows a self-compliant bipolar resistive switching behavior. The switching event is achieved by the set/reset voltages of -1 V/+1 V respectively with an operating window of 10, and it can be programmed for more than 100 endurance cycles. The retention time of on and off-states is up to 104 s. The obtained results suggest that Ag/IGZOnp/Ti structure could be applied in system on a panel (SoP) as a viable device

    Electrical Characterization of Spherical Copper Oxide Memristive Array Sensors

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    A new System Protection (SP) technology is explored by using electrical and mechanical interference-sensing devices that are implemented with granular memristive material. The granular materials consist of oxide-coated copper spheres with radii of about 700 ”m that are placed in contact to produce thin oxide junctions which exhibit memristive behavior. Processes for etching, which compared acetic acid and nitric acid etches, and thermal oxidation at 100°C are performed and compared to produce copper spheres with a copper oxide layer over the sphere surface. Oxidized copper spheres are tested as sensor arrays by loading into a capillary tube in an aligned arrangement. The spheres are held in contact to characterize current-voltage behavior for various oxide thicknesses with typical ROFF values in the megaohm range. Electrical characterization of the oxidized copper spheres reveal directly proportional changes to current-voltage hyseteresis in ”W under compressive forces. The thinnest oxide exhibited changes of 8.3 to 21.2 ”W over 9 mN while the thickest had a response from 0.4 to 2.5 ”W over 22.3 mN
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