Browsing by Author "Prodromakis, Themistoklis"

Now showing 1 - 9 of 9
  • Thumbnail Image
    ItemOpen Access
    Coexistence of memory resistance and memory capacitance in TiO2 solid state devices
    (Springer, 2014-09-23) Salaoru, Iulia; Li, Qingjiang; Khiat, Ali; Prodromakis, Themistoklis
    This work exploits the coexistence of both resistance and capacitance memory effects in TiO2-based two-terminal cells. Our Pt/TiO2/TiOx/Pt devices exhibit an interesting combination of hysteresis and non-zero crossing in their current-voltage (I-V) characteristic that indicates the presence of capacitive states. Our experimental results demonstrate that both resistance and capacitance states can be simultaneously set via either voltage cycling and/or voltage pulses. We argue that these state modulations occur due to bias-induced reduction of the TiOx active layer via the displacement of ionic species.
  • No Thumbnail Available
    ItemOpen Access
    Correlated Resistive/Capacitive State Variability in Solid TiO2 based Memory Devices
    (Springer, 2017-04-25) Li., Qingjiang; Salaoru, Iulia; Ali, Khiat; Prodromakis, Themistoklis; Hui, Xu
    In this work, we experimentally demonstrated the correlated resistive/capacitive switching and state variability in practical TiO2 based memory devices. Based on filamentary functional mechanism, we argue that the impedance state variability stems from the randomly distributed defects inside the oxide bulk. Finally, our assumption was verified via a current percolation circuit model, by taking into account of random defects distribution and coexistence of memristor and memcapacitor.
  • Thumbnail Image
    ItemOpen Access
    Emulating long-term synaptic dynamics with memristive devices
    (arXiv, 2016-04-22) Wei, Shari Lim; Vasilaki, Eleni; Khiat, Ali; Salaoru, Iulia; Berdan, Radu; Prodromakis, Themistoklis
    The potential of memristive devices is often seeing in implementing neuromorphic architectures for achieving brain-like computation. However, the designing procedures do not allow for extended manipulation of the material, unlike CMOS technology, the properties of the memristive material should be harnessed in the context of such computation, under the view that biological synapses are memristors. Here we demonstrate that single solid-state TiO2 memristors can exhibit associative plasticity phenomena observed in biological cortical synapses, and are captured by a phenomenological plasticity model called “triplet rule”. This rule comprises of a spike-timing dependent plasticity regime and a “classical” hebbian associative regime, and is compatible with a large amount of electrophysiology data. Via a set of experiments with our artificial, memristive, synapses we show that, contrary to conventional uses of solid-state memory, the co-existence of field- and thermally-driven switching mechanisms that could render bipolar and/or unipolar programming modes is a salient feature for capturing long-term potentiation and depression synaptic dynamics. We further demonstrate that the non-linear accumulating nature of memristors promotes long-term potentiating or depressing memory transitions.
  • No Thumbnail Available
    ItemOpen Access
    Investigation of the switching mechanism in TiO2-based RRAM: a two-dimensional EDX approach
    (ACS American Chemical Society, 2016-07-22) Carta, Daniela; Salaoru, Iulia; Khiat, Ali; Regoutz, Anna; Mitterbauer, Christoph; Harrison, Nicholas M.; Prodromakis, Themistoklis
    The next generation of non-volatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multi-state memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and life time of TiO2-based resistive random access memory (RRAM).
  • Thumbnail Image
    ItemOpen Access
    Memory Impedance in TiO2 based Metal-Insulator-Metal Devices
    (Nature, 2014-03-31) Qingjiang, Li; Khiat, Ali; Salaoru, Iulia; Papavassiliou, Christos; Hui, Xu; Prodromakis, Themistoklis
    Large attention has recently been given to a novel technology named memristor, for having the potential of becoming the new electronic device standard. Yet, its manifestation as the fourth missing element is rather controversial among scientists. Here we demonstrate that TiO2-based metal-insulator-metal devices are more than just a memory-resistor. They possess resistive, capacitive and inductive components that can concurrently be programmed; essentially exhibiting a convolution of memristive, memcapacitive and meminductive effects. We show how non-zero crossing current-voltage hysteresis loops can appear and we experimentally demonstrate their frequency response as memcapacitive and meminductive effects become dominant.
  • Thumbnail Image
    ItemEmbargo
    Origin of Stochastic Resistive Switching in Devices with Phenomenologically
    (IEEE, 2014) Li, Qingjiang; Khiat, Ali; Salaoru, Iulia; Xu, H.; Prodromakis, Themistoklis
    Nanoscale resistive switching devices are nowadays widely employed in applications of storage, logic and computing. The switching mechanism of metal oxide based devices is normally assumed to be the filamentary formation and rupture within the devices’ active cores but the origin of filaments growth is still controversial. Previous research has already demonstrated that initial filamentary states could significantly affect the devices’ switching dynamics and final resistance distributions. Here we demonstrate the relation between pristine resistive states and distribution of filaments via modeling the switching dynamics by utilizing a current percolation circuit. We show that devices with identical initial resistive states could attain distinct plausible filamentary distributions and correspondingly manifest very dissimilar switching dynamics even when biased with similar stimuli.
  • Thumbnail Image
    ItemEmbargo
    Origin of the OFF state variability in ReRAM cells
    (IOP Science, 2014-03-20) Salaoru, Iulia; Ali, Khiat; Li., Qingjiang; Berdan, Radu; Papavassiliou, Christos; Prodromakis, Themistoklis
    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 × 5μm2 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.
  • Thumbnail Image
    ItemEmbargo
    Resistive switching characteristics of indium-tin-oxide thin film devices
    (Wiley, 2014-03-06) Khiat, Ali; Salaoru, Iulia; Prodromakis, Themistoklis
    We demonstrate that indium tin oxide (ITO), when used as an active core material in metal–insulator–metal type devices, facilitates resistive switching. We fabricated devices both on silicon as well as quartz wafers, to demonstrate transparent devices. Furthermore, we investigated the influence of active core thickness on the devices' characteristics, showing that their switching threshold scales with the ITO thickness. Unipolar switching was observed for devices comprising thick ITO films while bipolar switching occurred for both thin and thick ITO films at the absence of high voltage forming steps. Our study demonstrates that ITO holds good potential for resistive memory applications.
  • Thumbnail Image
    ItemOpen Access
    Stochastic switching of TiO2 based memristive devices with identical initial memory states
    (Springer, 2014-05-17) Li, Qingjiang; Khiat, Ali; Salaoru, Iulia; Xu, H.; Prodromakis, Themistoklis
    In this work, we show that identical TiO2-based memristive devices that possess the same initial resistive states are only phenomenologically similar as their internal structures may vary significantly, which could render quite dissimilar switching dynamics. We experimentally demonstrated that the resistive switching of practical devices with similar initial states could occur at different programming stimuli cycles. We argue that similar memory states can be transcribed via numerous distinct active core states through the dissimilar reduced TiO2-x filamentary distributions. Our hypothesis was finally verified via simulated results of the memory state evolution, by taking into account dissimilar initial filamentary distribution.