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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.
- New electronic memory device concepts based on metal oxide-polymer nanostructures planer diodesPublication . Kiazadeh, Asal; Rocha, P. R. F.; Chen, Q.; Gomes, Henrique L.Nanostructure silver oxide thin films diodes can exhibit resistive switching effects. After an electroforming process the device can be programmed between a low conductance (off-state) and high conductance (on- state) with a voltage pulse and they are already being considered for non-volatile memory applications. However, the origin of programmable resistivity changes in a network of nanostructure silver oxide embedded in polymer is still a matter of debate. This work provides some results on a planer diode which may help to elucidate resistive switching phenomena in nanostructure metal oxide diodes. The XRD pattern after switching appears with different crystalline planes, plus temperature dependent studies reveal that conduction of both on and off states is weak thermal activated. Intriguing the carrier transport is the same for both on and off-states. Difference between states comes from the dramatic changes in the carrier density. The main mechanism of charge transport for on-state is tunneling. The charge transport leads to SCLC in higher voltages pulse for the off state. The mechanism will be explained based on percolation concepts.
- 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.
- Dynamic behavior of resistive random access memories (RRAMS) based on plastic semiconductor (RRAMS) based on plastic semiconductorPublication . Rocha, Paulo R. F.; Kiazadeh, Asal; Chen, Q.; Gomes, Henrique L.Resistive Random Access Memories based on metal-oxide polymer diodes are characterized. The dynamic behavior is studied by recording current-voltage characteristics with varying voltage ramp speed. It is demonstrated that these organic memory devices have an internal capacitive double-layer structure, which inhibits the switching at high ramp rates (1000 V/s). This behavior is modeled and explained in terms of an equivalent circuit.
- Improving positive and negative bias illumination stress stability in parylene passivated IGZO transistorsPublication . Kiazadeh, Asal; Gomes, Henrique L.; Barquinha, Pedro; Martins, Jorge; Rovisco, Ana; Pinto, Joana V.; Martins, Rodrigo; Fortunato, ElviraThe impact of a parylene top-coating layer on the illumination and bias stress instabilities of indium-gallium-zinc oxide thin-film transistors (TFTs) is presented and discussed. The parylene coating substantially reduces the threshold voltage shift caused by continuous application of a gate bias and light exposure. The operational stability improves by 75%, and the light induced instability is reduced by 35%. The operational stability is quantified by fitting the threshold voltage shift with a stretched exponential model. Storage time as long as 7 months does not cause any measurable degradation on the electrical performance. It is proposed that parylene plays not only the role of an encapsulation layer but also of a defect passivation on the top semiconductor surface. It is also reported that depletion-mode TFTs are less sensitive to light induced instabilities. This is attributed to a defect neutralization process in the presence of free electrons. Published by AIP Publishing.
- Memristors using solution-based IGZO nanoparticlesPublication . Rosa, Jose; Kiazadeh, Asal; Santos, Lidia; Deuermeier, Jonas; Martins, Rodrigo; Gomes, Henrique L.; Fortunato, ElviraSolution-based indium-gallium-zinc oldde (IGZO) nanoparticles deposited by spin coating have been investigated as a resistive switching layer in metal-insulator-metal structures for nonvolatile memory applications. Optimized devices show a bipolar resistive switching behavior, low programming voltages of +/- 1 V, on/off ratios higher than 10, high endurance, and a retention time of up to 104 s. The better performing devices were achieved with annealing temperatures of 200 degrees C and using asymmetric electrode materials of titanium and silver. The physics behind the improved switching properties of the devices is discussed in terms of the oxygen deficiency of IGZO. Temperature analysis of the conductance states revealed a nonmetallic filamentary conduction. The presented devices are potential candidates for the integration of memory functionality into low-cost System-on-Panel technology.
- 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.