Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 10(17) m(-2). We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching. (C) 2015 Author(s).Dutch Polymer Institute (DPI), BISTABLE [704]; Fundacao para Ciencia e Tecnologia (FCT) through the research Instituto de Telecommunicacoes (IT-Lx); project Memristor based Adaptive Neuronal Networks (MemBrAiNN) [PTDC/CTM-NAN/122868/2010]; European Community Seventh Framework Programme FP7', ONE-P [212311]; Dutch Ministry of Education, Culture and Science (Gravity Program) [024.001.035]info:eu-repo/semantics/publishedVersio

Resistive Switching in Metal Oxide/Organic Semiconductor Nonvolatile Memories

Diodes incorporating a bilayer of a metal oxide and an organic semiconductor can show unipolar, nonvolatile memory behavior after electroforming. Electroforming involves dielectric breakdown induced by prolonged bias voltage stress. When the power dissipated during breakdown is limited, electroforming is reversible and involves formation of defects at the organic-oxide interface that can heal spontaneously. When the power dissipation during breakdown exceeds a certain threshold, electroforming becomes irreversible. The fully electroformed diodes show electrical bistability, featuring (meta)stable states with low and high conduction that can be programmed by voltage pulses. The high conduction results from current flowing via filamentary paths. The bistability is explained by the coexistence of two thermodynamically stable phases at the interface between semiconductor and oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with low work function give rise to current filaments. In the filaments, Joule heating will raise temperature locally. When the temperature exceeds the critical temperature, the filament will switch off. The switching involves a collective recombination of charge carriers trapped at the defects as evidenced by bursts of electroluminescence

SiOx-based resistive switching memory integrated in nanopillar structure fabricated by nanosphere lithography

textA highly compact, one diode-one resistor (1D-1R) SiOx-based resistive switching memory device with nano-pillar architecture has been achieved for the first time using nano-sphere lithography. The average nano-pillar height and diameter are 1.3 μm and 130 nm, respectively. Low-voltage electroforming using DC bias and AC pulse response in the 50ns regime demonstrate good potential for high-speed, low-energy nonvolatile memory. Nano-sphere deposition, oxygen-plasma isolation, and nano-pillar formation by deep-Si-etching are studied and optimized for the 1D-1R configurations. Excellent electrical performance, data retention and the potential for wafer-scale integration are promising for future non-volatile memory applications.Materials Science and Engineerin

Oxide Memristive Devices

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

Memristors using solution-based IGZO nanoparticles

Solution-based indium-gallium-zinc oldde (IGZO) nanoparticles deposited by spin coating have been investigated as a resistive switching layer in metal-insulator-metal structures for nonvolatile memory applications. Optimized devices show a bipolar resistive switching behavior, low programming voltages of +/- 1 V, on/off ratios higher than 10, high endurance, and a retention time of up to 104 s. The better performing devices were achieved with annealing temperatures of 200 degrees C and using asymmetric electrode materials of titanium and silver. The physics behind the improved switching properties of the devices is discussed in terms of the oxygen deficiency of IGZO. Temperature analysis of the conductance states revealed a nonmetallic filamentary conduction. The presented devices are potential candidates for the integration of memory functionality into low-cost System-on-Panel technology.National Funds through FCT - Portuguese Foundation for Science and Technology [UID/CTM/50025/2013, SFRH/BDP/99136/2013]; FEDER [POCI-01-0145-FEDER-007688]info:eu-repo/semantics/publishedVersio

Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticles

Resistive switching is investigated in thin-film planar diodes using silver oxide nanoparticles capped in a polymer. The conduction channel is directly exposed to the ambient atmosphere. Two types of switching are observed. In air, the hysteresis loop in the current–voltage characteristics is S-shaped. The high conductance state is volatile and unreliable. The switching is mediated by moisture and electrochemistry. In vacuum, the hysteresis loops are symmetric, N-shaped and exhibit a negative differential resistance region. The conductance states are non-volatile with good data retention, programming cycling endurance and large current modulation ratio. The switching is attributed to electroforming of silver oxide clusters

Resistive Switching in Silicon-rich Silicon Oxide

Over the recent decade, many different concepts of new emerging memories have been proposed. Examples of such include ferroelectric random access memories (FeRAMs), phase-change RAMs (PRAMs), resistive RAMs (RRAMs), magnetic RAMs (MRAMs), nano-crystal floating-gate flash memories, among others. The ultimate goal for any of these memories is to overcome the limitations of dynamic random access memories (DRAM) and flash memories. Non-volatile memories exploiting resistive switching – resistive RAM (RRAM) devices – offer the possibility of low programming energy per bit, rapid switching, and very high levels of integration – potentially in 3D. Resistive switching in a silicon-based material offers a compelling alternative to existing metal oxide-based devices, both in terms of ease of fabrication, but also in enhanced device performance. In this thesis I demonstrate a redox-based resistive switch exploiting the formation of conductive filaments in a bulk silicon-rich silicon oxide. My devices exhibit multi-level switching and analogue modulation of resistance as well as standard two-level switching. I demonstrate different operational modes (bipolar and unipolar switching modes) that make it possible to dynamically adjust device properties, in particular two highly desirable properties: non-linearity and self-rectification. Scanning tunnelling microscopy (STM), atomic force microscopy (AFM), and conductive atomic force microscopy (C-AFM) measurements provide a more detailed insight into both the location and the dimensions of the conductive filaments. I discuss aspects of conduction and switching mechanisms and we propose a physical model of resistive switching. I demonstrate room temperature quantisation of conductance in silicon oxide resistive switches, implying ballistic transport of electrons through a quantum constriction, associated with an individual silicon filament in the SiOx bulk. I develop a stochastic method to simulate microscopic formation and rupture of conductive filaments inside an oxide matrix. I use the model to discuss switching properties – endurance and switching uniformity

Fabrication and characterization of memory devices based on nanoparticles

Tese de doutoramento, Engenharia Electrónica e Telecomunicações, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2013The objective of this study is to understand the electrical properties of non-volatile memories based on metal oxide nanoparticles embedded into an insulating polymer matrix. These memories are classified as resistive random access memories (RRAM), as they undergo resistive switching between well-defined conductance states when submitted to a voltage pulse. A number of memory devices were fabricated and studied using electrical techniques. Current-voltage characteristics were studied as a function of the ambient atmosphere and temperature. The dynamic electrical behaviour was probed using triangular voltage profiles with different scan rates, transient techniques and electrical noise techniques. Electrical measurements were complemented with morphological characterization. Important outcomes of this thesis are the following: It was shown that adsorbed moisture on the surface of the devices causes resistive switching. This type of resistive switching can lead to very high on/off ratios, and therefore it is not reliable. Silver oxide nanoparticles undergo an electroforming process similar to a soft-breakdown mechanism as reported for binary oxides. A model that explains the basic features of the electroforming mechanism was proposed. After the electroforming, the devices show resistance switching properties with a high on/off ratio (> 104), good retention time, and programming endurance. A resistive switching mechanism was proposed. The model assumes that during electroforming a percolation network of micro conducting paths (filaments) is established between the electrodes. The creation and rupture of these micro-paths is responsible for the changes in conductance. Results from this study indicate that nanostructured thin films made of silver oxide nanoparticles embedded in an insulating polymer show an electrical behaviour like the bulk oxide based memory structures. The planar structures present the advantage of being programmed in multi-resistance levels suggesting a very interesting finding that may pave the way to achieve a multi-bit memory deviceO objetivo desta tese foi estudar as propriedades elétricas de componentes electrónicos fabricados com nanopartículas de metálicas. Este tipo de memoria é designado por memorias resistivas porque mudam a sua resistência elétrica através da aplicação de um tensão elétrica. Este componente é conhecido por “memristor”. Um conjunto de memorias resistivas foi fabricado e caracterizado. Nomeadamente foram realizadas um conjunto de medidas elétricas em diferentes ambientes (vácuo e atmosfera ambiente) e em função da temperatura para obter informação sobre os mecanismos de transporte electrónico e sobre a comutação elétrica da resistência. As memorias fabricadas tem um elevado hiato entre os estados resistivos (> 104), são não-voláteis e robustas, tendo sido testadas com mais de mil ciclos de programação entre os estados resistivos. Esta tese propõe um modelo para explicar as variações de resistência elétrica. O modelo assume que as partículas de prata oxidam e formam um óxido de prata. Durante o processo de formação da memoria, o elevado campo elétrico aplicado leva a ruptura dielétrica controlada do óxido e forma defeitos eletricamente ativos. Esta rede de defeitos gera micro-caminhos para a condução elétrica ou filamentos. As mudanças de resistência elétrica são causadas pela criação/ruptura deste filamentos. Os resultados desta tese indicam que as mudanças de resistência elétrica em filmes nanoestruturados com nanopartículas metálicas são semelhantes as observadas em estruturas resistivas com base em filmes finos óxidos como o dióxido de titânio (TiO2) e o óxido de alumínio (Al2O3) entre outros. Os “memristors” fabricadas neste tese são estruturas planares. O objectivo inicial foi ter um instrumento de caracterização mais simples que a estrutura convencional em sanduiche. No entanto a estrutura planar permite também obter vários níveis de resistência elétrica sugerindo que pode funcionar como memorias “multi-bit”

New electronic memory device concepts based on metal oxide-polymer nanostructures planer diodes

Nanostructure silver oxide thin films diodes can exhibit resistive switching effects. After an electroforming process the device can be programmed between a low conductance (off-state) and high conductance (on- state) with a voltage pulse and they are already being considered for non-volatile memory applications. However, the origin of programmable resistivity changes in a network of nanostructure silver oxide embedded in polymer is still a matter of debate. This work provides some results on a planer diode which may help to elucidate resistive switching phenomena in nanostructure metal oxide diodes. The XRD pattern after switching appears with different crystalline planes, plus temperature dependent studies reveal that conduction of both on and off states is weak thermal activated. Intriguing the carrier transport is the same for both on and off-states. Difference between states comes from the dramatic changes in the carrier density. The main mechanism of charge transport for on-state is tunneling. The charge transport leads to SCLC in higher voltages pulse for the off state. The mechanism will be explained based on percolation concepts