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Gomes, Henrique Leonel

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  • Electroforming process in metal-oxide-polymer resistive switching memories
    Publication . Chen, Q.; Gomes, Henrique L.; Kiazadeh, Asal; Rocha, Paulo R. F.; De Leeuw, Dago M.; Meskers, S. C. J.
    Electroforming of an Al/Al2O3/polymer/Al esistive switching diode is reported. Electroforming is a dielectric soft-breakdown mechanism leading to hysteretic current–voltage characteristics and non–volatile memory behavior. Electron trapping occurs at early stages of electroforming. Trapping is physically located at the oxide/polymer interface. The detrapping kinetics is faster under reverse bias and for thicker oxides layers. Thermally detrapping experiments give a trap depth of 0.65 eV and a density of 5x1017 /cm2. It is proposed that the trapped electrons induce a dipole layer across the oxide. The associated electric field triggers breakdown and ultimately dictate the overall memory characteristics.
  • The role of internal structure in the anomalous switching dynamics of metal-oxide/polymer resistive random access memories
    Publication . Rocha, Paulo R. F.; Kiazadeh, Asal; De Leeuw, Dago M.; Meskers, S. C. J.; Verbakel, F.; Taylor, D. M.; Gomes, Henrique L.
    The dynamic response of a non-volatile, bistable resistive memory fabricated in the form of Al2O3/polymer diodes has been probed in both the off- and on-state using triangular and step voltage profiles. The results provide insight into the wide spread in switching times reported in the literature and explain an apparently anomalous behaviour of the on-state, namely the disappearance of the negative differential resistance region at high voltage scan rates which is commonly attributed to a “dead time” phenomenon. The off-state response follows closely the predictions based on a classical, two-layer capacitor description of the device. As voltage scan rates increase, the model predicts that the fraction of the applied voltage, Vox , appearing across the oxide decreases. Device responses to step voltages in both the off- and on-state show that switching events are characterized by a delay time. Coupling such delays to the lower values of Vox attained during fast scan rates, the anomalous observation in the on-state that, device currents decrease with increasing voltage scan rate, is readily explained. Assuming that a critical current is required to turn off a conducting channel in the oxide, a tentative model is suggested to explain the shift in the onset of negative differential resistance to lower voltages as the voltage scan rate increases. The findings also suggest that the fundamental limitations on the speed of operation of a bilayer resistive memory are the time- and voltage-dependences of the switch-on mechanism and not the switch-off process.
  • Switching speed in resistive random access memories (RRAMS) based on plastic semiconductor
    Publication . Rocha, P. F.; Gomes, Henrique L.; Kiazadeh, Asal; Chen, Q.; De Leeuw, D. M.; Meskers, S. C. J.
    This work addresses non-volatile memories based on metal-oxide polymer diodes. We make a thorough investigation into the static and dynamic behavior. Current-voltage characteristics with varying voltage ramp speed demonstrate that the internal capacitive double-layer structure inhibits the switching at high ramp rates (typical 1000 V/s). This behavior is explained in terms of an equivalent circuit. It is also reported that there is not a particular threshold voltage to induce switching. Voltages below a particular threshold can still induce switching when applied for a long period of time. The time to switch is longer the lower is the applied voltage and follows an exponential behavior. This suggests that for a switching event to occur a certain amount of charge is required. © 2011 Materials Research Society.
  • Trapping of electrons in metal oxide-polymer memory diodes in the initial stage of electroforming
    Publication . Bory, Benjamin F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; De Leeuw, Dago M.
    Metal oxide-polymer diodes require electroforming before they act as nonvolatile resistive switching memory diodes. Here we investigate the early stages of the electroforming process in Al/Al2O3 /polyspirofluorene /Ba/Al diodes using quasistatic capacitance-voltage measurements. In the initial stage, electrons are injected into the polymer and then deeply trapped near the polyspirofluorene-Al2O3 interface. For bias voltages below 6 V, the number of trapped electrons is found to be CoxideV/q with Coxide as the geometrical capacitance of the oxide layer. This implies a density of traps for the electrons at the polymer-metal oxide interface larger than 31017 m−2.
  • Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide
    Publication . Bory, B. F.; Wang, J. X.; Gomes, Henrique L.; Janssen, R. A. J.; de Leeuw, D. M.; Meskers, S. C. J.
    Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 10(25)/m(3). The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics. (C) 2014 Author(s).
  • Non-volatile memory device using a polymer modified nanocrystal
    Publication . Kiazadeh, Asal; Gomes, Henrique L.; Costa, Ana M. Rosa da; Moreira, José; De Leeuw, Dago M.; Meskers, S. C. J.
    Thin-film planar structures using AgCl nanocrystals embedded in a polymer blend; exhibit reliable and reproducible switching between different non-volatile conductance states. It is shown that resistive switching in these systems cannot be related with migration diffusion or aggregation of metals to form metallic filaments. This is supported by temperature-dependent measurement showing that the current in the high conductance state is thermal activated (0.6 eV).
  • Opto-electronic characterization of electron traps upon forming polymer oxide memory diodes
    Publication . Chen, Q.; Bory, Benjamin F.; Kiazadeh, Asal; Rocha, Paulo R. F.; Gomes, Henrique L.; Verbakel, F.; De Leeuw, Dago M.; Meskers, S. C. J.
    Metal-insulator-polymer diodes where the insulator is a thin oxide (Al2O3) layer are electroformed by applying a high bias. The initial stage is reversible and involves trapping of electrons near the oxide/polymer interface. The rate of charge trapping is limited by electron transport through the polymer. Detrapping of charge stored can be accomplished by illuminating with light under short-circuit conditions. The amount of stored charge is determined from the optically induced discharging current transient as a function of applied voltage and oxide thickness. When the charge density exceeds 8 1017/m2, an irreversible soft breakdown transition occurs to a non-volatile memory diode.
  • Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electrons
    Publication . Bory, B. F.; Rocha, Paulo R. F.; Janssen, R. A. J.; Gomes, Henrique L.; de Leeuw, D. M.; Meskers, S. C. J.
    Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
  • Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticles
    Publication . Kiazadeh, Asal; Gomes, Henrique L.; Rosa da Costa, Ana; Moreira, José; De Leeuw, Dago M.; Meskers, S. C. J.
    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.
  • Reproducible resistive switching in nonvolatile organic memories
    Publication . Verbakel, F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; Coelle, M.; Buechel, M.; de Leeuw, D. M.
    Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 10(9) V/m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.