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- Electroforming process in metal-oxide-polymer resistive switching memoriesPublication . 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 memoriesPublication . 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 semiconductorPublication . 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.
- Non-volatile memory device using a polymer modified nanocrystalPublication . 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 diodesPublication . 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.
- Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticlesPublication . 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.
- Anomalous temperature dependence of the current in a metal-oxide-polymer resistive switching diodePublication . Gomes, Henrique L.; Rocha, Paulo R. F.; Kiazadeh, Asal; De Leeuw, Dago M.; Meskers, S. C. J.Metal-oxide polymer diodes exhibit non-volatile resistive switching. The current–voltage characteristics have been studied as a function of temperature. The low-conductance state follows a thermally activated behaviour. The high-conductance state shows a multistep-like behaviour and below 300K an enormous positive temperature coefficient. This anomalous behaviour contradicts the widely held view that switching is due to filaments that are formed reversibly by the diffusion of metal atoms. Instead, these findings together with small-signal impedance measurements indicate that creation and annihilation of filaments is controlled by filling of shallow traps localized in the oxide or at the oxide/polymer interface.
- Planar non-volatile memory based on metal nanoparticlesPublication . Kiazadeh, Asal; Gomes, Henrique L.; Costa, Ana M. Rosa da; Rocha, P. R. F.; Chen, Q.; Moreira, José; De Leeuw, Dago M.; Meskers, S. C. J.Resistive switching properties of silver nanoparticles hosted in an insulating polymer matrix (poly(N-vinyl-2-pyrrolidone) are reported. Planar devices structures using interdigitated gold electrodes were fabricated. These devices have on/off resistance ratio as high as 103 , retention times reaching to months and good endurance cycles. Temperature-dependent measurements show that the charge transport is weakly thermal activated (73 meV) for both states suggesting that nanoparticles will not aggregate into a metallic filament.
- Low-frequency diffusion noise in resistive-switching memories based on metal-oxide polymer structurePublication . Rocha, P. R. F.; Gomes, Henrique L.; Vandamme, L. K. J.; Chen, Q.; Kiazadeh, Asal; De Leeuw, Dago M.; Meskers, S. C. J.Low-frequency noise is studied in resistive-switching memories based on metal–oxide polymer diodes. The noise spectral power follows a 1/fγ behavior, with γ = 1 in the ohmic region and with γ = 3/2 at high bias beyond the ohmic region. The exponent γ = 3/2 is explained as noise caused by Brownian motion or diffusion of defects which induce fluctuations in diode current. The figure of merit to classify 1/f noise in thin films has an estimated value of 10−21 cm2/Ω, which is typical for metals or doped semiconductors. This value in combination with the low diode current indicates that the 1/f noise is generated in the narrow localized regions in the polymer between the contacts. The analysis unambiguously shows that the current in bistable nonvolatile memories is filamentary.