Doping controlled resistive switching dynamics in transition metal oxide thin films

Transition metal oxide thin films have attracted increasing attention due to their potential in non-volatile resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal-insulator-metal (MIM) configurations. Titanium dioxide is one of the most celebrated oxides among the ones that exhibit resistive switching behaviour due to its wide band gap, high thermal stability, and high dielectric constant. RRAM devices with various materials as active layers, have demonstrated very fast switching performance but also huge potential for miniaturisation, which is the bottleneck of FLASH memory. Nevertheless, these devices very often suffer poor endurance, physical degradation, large variability of switching parameters and low yields. In most cases, the physical degradation stems from high electroforming and switching voltages. Doping of the active layer has been often employed to enhance the performance of RRAM devices, like endurance, OFF/ON ratio, forming voltages, etc. In this work, doping in TiO2-x RRAM devices was used to engineer the electroforming and switching thresholds so that device degradation and failure can be delayed or prevented. Al and Nb were selected with basic criteria the ionic radius and the oxidation state. The doped RRAM devices, showed improved switching performance compared to their undoped counterparts. Alternative approaches to doping were also investigated, like multilayer stacks comprising Al2O3-y and TiO2-x thin films. Furthermore, Al:TiO2-x/Nb:TiO2-x bilayer RRAM devices were fabricated, to prove whether a diode behaviour of the p-n interface inside the RRAM was feasible. The latest would be a particularly interesting finding towards active electronics

Data for 'Impact of ultra-thin Al2O3–y layers on TiO2–x ReRAM switching characteristics'

Data accompanying the paper: Prodromakis, Themistoklis; Trapatseli, Maria; Cortese, Simone; Serb, Alexantrou / Improved switching characteristics of TiO2-x ReRAM with embedded ultra-thin 2 Al2O3-y layers. In: Journal of Applied Physics, 28.03.2017. Abstract for accompanying paper: Transition 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 electro-forming 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 Al2O3-y buffer layers incorporated between TiO2-x thin &#12;lms 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 Al2O3-y layers act as reservoirs of oxygen vacancies which are injected during EF, facilitate a &#12;lamentary switching mechanism and provide enhanced &#12;lament stability as shown by the cycling endurance measurements.</span

A TiO2-based volatile threshold switching selector service with 10^7 non linearity and sub 100 pA Off Current

Data for the conference paper: A TiO2-based volatile threshold Switching Selector Device with 10^7 non linearity and sub 100 pA Off Current. The file features the Resistive Switching Characteristics of the volatile selector at both polarities, including electroforming step, showing a non linearity of 10^7 between high and low resistance states. Also, the linearity of the low resistance states is presented, showing that the resistance values are the same at both polarities.</span

Al-doping engineered electroforming and switching dynamics of TiO_x ReRAM devices

TiO2 thin films have drawn a lot of attention for their application in emerging memory devices, such as resistive random access memory (ReRAM). However, TiO2 ReRAM still faces reliability issues, including poor endurance, large device-to-device and cycle-to-cycle variability of switching parameters and low yields. Moreover, high electroforming voltages have been often associated with irreversible damage to devices. Doping of TiO2 has been employed as a strategy for overcoming these issues. Therefore in this work, we used Al as a dopant in TiO2 thin films to investigate its effect on electroforming and switching voltages of ReRAM devices. Conductive atomic force microscopy (C-AFM) measurements on these thin films, suggested that Al doping decreased the switching voltages compared to the undoped thin films. This result was confirmed by pulse voltage sweeping of ReRAM devices employing the same doped thin films. The Al-doped devices were on average electroforming at -5.7 V, compared to -6.4 V for the undoped ones, and they were switching with potentials as low as ±0.9 V. These findings suggest a potential pathway for implementing low-power ReRAM systems<br/

Spike sorting using non-volatile metal-oxide memristors

Electrophysiological techniques have improved substantially over the past years to the point that neuroprosthetics applications are becoming viable. This evolution has been fuelled by the advancement of implantable microelectrode technologies that have followed their own version of Moore's scaling law. Similarly to electronics, however, excessive data-rates and strained power budgets require the development of more efficient computation paradigms for handling neural data in situ; in particular the computationally heavy task of events classification. Here, we demonstrate how the intrinsic analogue programmability of memristive devices can be exploited to perform spike-sorting on single devices. Leveraging the physical properties of nanoscale memristors allows us to demonstrate that these devices can capture enough information in neural signal for performing spike detection (shown previously) and spike sorting at no additional power cost.</p

Field emission properties of low-temperature, hydrothermally grown tungsten oxide

Tungsten oxide layers have been prepared on conductive glass substrates using aqueous chemical growth from a sodium tungstate precursor at low-temperature hydrothermal conditions. The deposits were then tested as cold electron emitters. Traceable layers could be deposited only within a narrow pH range of 1.5-2 at a time length not exceeding 4 h. Transmittance in the visible spectrum was found to decrease with deposition time. The presence of both monoclinic and hexagonal phases was always detected. At the longest deposition times and highest precursor concentrations, morphologies comprise randomly oriented spikes or rods. The overall emission performance is found to improve with growth time and precursor concentration. The role of morphology on the emission properties of the films is discussed. Field Emission Properties of Low-Temperature, Hydrothermally Grown Tungsten Oxide. Available from: https://www.researchgate.net/publication/51234194_Field_Emission_Properties_of_Low-Temperature_Hydrothermally_Grown_Tungsten_Oxide [accessed May 7, 2015]