Emerging nanoionic memristive devices are considered as the memory technology of the future and have been winning a great deal of attention due to their ability to perform fast and at the expense of low-power and -space requirements. Their full potential is envisioned that can be fulfilled through their capacity to store multiple memory states per cell, which however has been constrained so far by issues affecting the long-term stability of independent states. Here, we introduce and evaluate a multitude of metal-oxide bi-layers and demonstrate the benefits from increased memory stability via multibit memory operation. We propose a programming methodology that allows for operating metal-oxide memristive devices as multibit memory elements with highly packed yet clearly discernible memory states. These states were found to correlate with the transport properties of the introduced barrier layers. We are demonstrating memory cells with up to 6.5 bits of information storage as well as excellent retention and power consumption performance. This paves the way for neuromorphic and non-volatile memory applications.
Titanium oxide (TiOx) has attracted a lot of attention as an active material for resistive random access memory (RRAM), due to its versatility and variety of possible crystal phases. Although existing RRAM materials have demonstrated impressive characteristics, like ultra-fast switching and high cycling endurance, this technology still encounters challenges like low yields, large variability of switching characteristics, and ultimately device failure. Electroforming has been often considered responsible for introducing irreversible damage to devices, with high switching voltages contributing to device degradation. In this paper, we have employed Al doping for tuning the resistive switching characteristics of titanium oxide RRAM. The resistive switching threshold voltages of undoped and Al-doped TiOx thin films were first assessed by conductive atomic force microscopy. The thin films were then transferred in RRAM devices and tested with voltage pulse sweeping, demonstrating that the Al-doped devices could on average form at lower potentials compared to the undoped ones and could support both analog and binary switching at potentials as low as 0.9 V. This work demonstrates a potential pathway for implementing low-power RRAM systems.
Resistive RAM (ReRAM) crossbar arrays have become one of the most promising candidates for next-generation non volatile memories (NVMs). To become a mature technology, the sneak path current issue must be solved without compromising all the advantages that crossbars offer in terms of electrical performances and fabrication complexity. Here, we present an highly integrable access device based on nickel and substoichiometric amorphous titanium dioxide (TiO_{2-x}), in a metal insulator metal (MIM) crossbar structure. The high voltage margin of 3 V, amongst the highest reported for monolayer selector devices, and the good current density of 104 A/cm2 make it suitable to sustain ReRAM read and write operations, effectively tackling sneak currents in crossbars without compromising fabrication complexity in a 1 Selector 1 Resistor (1S1R) architecture. Furthermore, the voltage margin is found to be tunable by an annealing step without affecting the device's characteristics.
6Transition metal-oxide resistive random access memory (RRAM) devices have demonstrated excellent performance in switching speed, versatility of switching and low-power operation. However, this technology still faces challenges like poor cycling endurance, degradation due to high electroforming switching voltages and low yields. Engineering of the active layer by doping or addition of thin oxide buffer layers, are approaches that have been often adopted to tackle these problems. Here, we have followed a strategy that combines the two; we have used ultra-thin Al 2 O 3-y buffer layers incorporated between TiO 2-x thin films taking into account both 3+/4+ oxidation states of Al/Ti cations. Our devices were tested by DC and pulsed voltage sweeping and in both cases demonstrated improved switching voltages. We believe that the Al 2 O 3-y layers act as reservoirs of oxygen vacancies which are injected during EF, facilitate a filamentary switching mechanism and provide enhanced filament stability as shown by the cycling endurance measurements.
Emerging memory technologies have sparked great interest in studying a variety of materials that can be employed in metal-insulator-metal topologies to support resistive switching. While the majority of reports focus on identifying appropriate materials that can be used as active core layers, the selection of electrodes also impacts the performance of such memory devices. Here, both the top and the bottom interfaces of symmetric Metal-Al:TiO x -Metal structures have been investigated by the analysis of their current versus voltage characteristics in the temperature range of 300-350 K. Three different metals were utilized as electrodes, Nb, Au, and Pt, for covering a wide range of work function and electronegativity values. Despite their symmetric structure, the devices were found to exhibit asymmetric performance with respect to the applied bias polarity. Clear signature plots indicating thermionic emission over the interface Schottky barriers have been obtained. The asymmetry between the top and the bottom interfaces was further evaluated by the values of the potential barrier heights and by the barrier lowering factors, both calculated from the experimental data. This study highlights the importance of the interface effects and proves that in addition to film doping, proper (top/bottom) metal selection, and interface engineering should also be exploited for developing thin film metal oxide based devices with tailored electrical characteristics.
Pt/TiO x /Pt resistive switching (RS) devices are considered to be amongst the most promising candidates in memristor family and the technology transfer to flexible substrates could open the way to new opportunities for flexible memory implementations. Hence, an important goal is to achieve a fully flexible RS memory technology. Nonetheless, several fabrication challenges are present and must be solved prior to achieving reliable device fabrication and good electronic performances. Here, we propose a fabrication method for the successful transfer of Pt/TiO x /Pt stack onto flexible Parylene-C substrates. The devices were electrically characterised, exhibiting both digital and analogue memory characteristics, which are obtained by proper adjustment of pulsing schemes during tests. This approach could open new application possibilities of these devices in neuromorphic computing, data processing, implantable sensors and bio-compatible neural interfaces.S Online supplementary data available from stacks.iop.org/NANO/0/000000/mmedia
Resistive switching (RS) and Resistive Random Access Memories (ReRAMs) that exploit it have attracted huge interests for next generation non volatile memory (NVM) applications, also thought to be able to overcome flash memories limitations when arranged in crossbar arrays. A cornerstone of their potential success is that the RS between two different resistive states, usually High (HRS, High resistive state) and Low(LRS, Low Resistive State) is an intrinsic non-volatile phenomenon with the two states thermodynamically stable. Titanium Dioxide is one of the most common materials known to show non-volatile RS. In this paper we report the first observed volatile resistive switching (VRS) in a Titanium Dioxide thin film. The aim of this paper is to study and understand the VRS phenomenon to give an extensive picture of its underlying Physics. A possible exploitation of the VRS could be in access devices in ReRAM crossbar arrays.
ReRAM crossbar arrays are known to be susceptible to the presence of the sneak current issue during the readout operations which undermines crossbar scaling. This problem can be solved by the addition of an highly non-linear two-terminal selector device. In this work we present a 5 nm thick TiO2-based selector which exploits a volatile threshold resistive switching, so far unreported for this material. The device shows a current density up to 100 kA/cm 2 , 10 7 current non-linearity and a 4 V voltage margin, the highest reported for TiO2-based selectors and sub 100 pA off current.
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