TaO(x)-based memristors have recently demonstrated both subnanosecond resistance switching speeds and very high write/erase switching endurance. Here we show that the physical state variable that enables these properties is the oxygen concentration in a conduction channel, based on the measurement of the thermal coefficient of resistance of different TaO(x) memristor states and a set of reference Ta-O films of known composition. The continuous electrical tunability of the oxygen concentration in the channel, with a resolution of a few percent, was demonstrated by controlling the write currents with a one transistor-one memristor (1T1M) circuit. This study demonstrates that solid-state chemical kinetics is important for the determination of the electrical characteristics of this relatively new class of device.
"To achieve this, various materials have been exploited, with multilevel resistive switching being evident in organic materials , manganites , perovskites  and carbon materials  as well as metal oxides   . In all of these reports it can be observed, although not explicitly identified, that the attainable state-variability is more pronounced for high resistive states (HRS)    , as the device is consecutively programmed between distinct states. This appears to be a common feature of all ReRAM cells. "
[Show abstract][Hide abstract] ABSTRACT: This work exploits the switching dynamics of nanoscale resistive random access memory (ReRAM) cells with particular emphasis on the origin of the observed variability when cells are consecutively cycled/programmed at distinct memory states. It is demonstrated that this variance is a common feature of all ReRAM elements and is ascribed to the formation and rupture of conductive filaments that expand across the active core, independently of the material employed as the active switching core, the causal physical switching mechanism, the switching mode (bipolar/unipolar) or even the unit cells' dimensions. Our hypothesis is supported through both experimental and theoretical studies on TiO2 and In2O3 : SnO2 (ITO) based ReRAM cells programmed at three distinct resistive states. Our prototypes employed TiO2 or ITO active cores over 5 × 5 µm2 and 100 × 100 µm2 cell areas, with all tested devices demonstrating both unipolar and bipolar switching modalities. In the case of TiO2-based cells, the underlying switching mechanism is based on the non-uniform displacement of ionic species that foster the formation of conductive filaments. On the other hand, the resistive switching observed in the ITO-based devices is considered to be due to a phase change mechanism. The selected experimental parameters allowed us to demonstrate that the observed programming variance is a common feature of all ReRAM devices, proving that its origin is dependent upon randomly oriented local disorders within the active core that have a substantial impact on the overall state variance, particularly for high-resistive states.
Journal of Physics D Applied Physics 04/2014; DOI:10.1088/0022-3727/47/14/145102 · 2.72 Impact Factor
"In the extreme cases, the full thickness is either stoichiometric Ta 2 O 5 or Ta with dissolved O (referred to as Ta(O), with solubility up to 20%), the two thermodynamically stable phases in the Ta-O system, to give an insulating or metallic channel, respectively. By either combining these two phases in parallel, or considering Ta(O) with different concentrations of O, there is a range of stable intermediate conductance states  available to the device. "
[Show abstract][Hide abstract] ABSTRACT: A key requirement for using memristors in circuits is a predictive model for device behavior that can be used in simulations and to guide designs. We analyze one of the most promising materials, tantalum oxide, for high density, low power, and high-speed memory. We perform an ensemble of measurements, including time dynamics across nine decades, to deduce the underlying state equations describing the switching in Pt/TaOx/Ta memristors. A predictive, compact model is found in good agreement with the measured data. The resulting model, compatible with SPICE, is then used to understand trends in terms of switching times and energy consumption, which in turn are important for choosing device operating points and handling interactions with other circuit elements.
IEEE Transactions on Electron Devices 07/2013; 60(7):2194-2202. DOI:10.1109/TED.2013.2264476 · 2.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Moisture invasion into memory devices can result in data loss and malfunctions in write/erase switching. Deteriorated uniformity and retention characteristics, and distorted switching hysteresis loops, are observed in moisture-attacked Pt-dispersed SiO2 nanometallic thin-film devices, and can be effectively prevented by coating the device with a nanoscale Al2O3 barrier layer grown by an atomic layer deposition method. The moisture-attacked devices exhibit evidence of cumulating ion current and ion potential with repeated switching. In contrast, a capped device with an extremely uniform and reproducible resistive switching behavior features a completely symmetric current–voltage curve expected for a purely electronic device.
Applied Physics A 08/2013; 112(2). DOI:10.1007/s00339-013-7776-2 · 1.70 Impact Factor
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