Frank Verbakel

Electronic memory effects in diodes from a zinc oxide nanoparticle-polystyrene hybrid material

Current-voltage characteristics of diode structures with an active layer of a zinc oxide nanoparticle-polystyrene hybrid material (1:2 by weight) deposited by spin coating from solution were investigated. Aluminum and poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate were used as electrodes. After a forming step, the conduction under reversed bias voltage can be raised or lowered in a gradual and reversible manner by applying forward and reverse bias voltages, respectively. Electrically induced switching between states with high and lower conductivities is possible on a time scale of 100ms and the conduction levels remain stable for over 1h.

Reproducible resistive switching in nonvolatile organic memories

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 109V∕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.

Electronic memory effects in diodes of zinc oxide nanoparticles in a matrix of polystyrene or poly(3-hexylthiophene)

Electronic memory effects in metal-insulator-metal devices with aluminum and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) as electrodes and a solution processed active layer consisting of zinc oxide (ZnO) nanoparticles embedded in a matrix of poly(3-hexylthiophene) or polystyrene are investigated. After an initial forming process, the devices show a reversible change in conductivity. The forming process itself is interpreted in terms of desorption of molecular oxygen from the ZnO nanoparticle surface, induced by injection of holes via the PEDOT:PSS contact, leading to a higher n-type conductivity via interparticle ZnO contacts. The forming can also be induced with ultraviolet light and the process is studied with electron paramagnetic resonance, photoinduced absorption spectroscopy, and field effect measurements. Also, the composition of the active layer is varied and the memory effects can by influenced by changing the ZnO content and the polymer, allowing for data storage with lifet...

Opto-electronic characterization of electron traps upon forming polymer oxide memory diodes

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.

The role of internal structure in the anomalous switching dynamics of metal-oxide/polymer resistive random access memories

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.

LOW-FREQUENCY NOISE TO CHARACTERIZE RESISTIVE SWITCHING OF METAL OXIDE ON POLYMER MEMORIES

Resistive switching in aluminum-polymer diodes has been investigated by noise measurements. Quantitative criteria to characterize the diode states are: (i) Pristine state shows I ∝ Vm with m ≈ 6 at higher bias typical for tunneling. The resistance is very high, 1/f noise is very low, but the relative 1/f noise, fSI/I2 ≡ C1/f is very high. (ii) Forming state is a time-dependent soft breakdown in the Al-oxide that results in random telegraph signal noise (RTS) with a Lorentzian spectrum or in multi-level resistive switching (MLS) with a 1/f3/2 or 1/f-like spectrum. (iii) The H- or L-state shows I ∝ Vm with m = 1 for V 1V. Deviations from ohmic behavior are explained by space charge limited current in the polymer. Reliable H- and L-states show pure 1/f noise, a resistance R that changes by at least a factor 30 and 1/f noise that follows the proportionality: C1/f ∝ R with a proportionality factor αμ(cm2/Vs) of the same level as observed in metals, polymers and other semiconductors. C1/f ∝ R is explained by switching of the number of homogeneous conducting paths in parallel. Deviations in C 1/f ∝ R are also explained. In the pristine state and even in the H-state, only a fraction of the device are is carrying current and switching seems to be at spots of the Al/Al2O3/polymer interface.

Electronic memory effects in zinc oxide nanoparticle -polystyrene devices with a calcium top electrode

Diodes with an active layer of solution processed zinc oxide (ZnO) nanoparticles and polystyrene are studied. Poly(3,4-ethylenedioxythiophene)- polystyrenesulfonate (PEDOT:PSS) on indium doped tin oxide (ITO) is used as the bottom electrode and aluminum or calcium are used as top electrode. Pristine devices show diode behavior in their current-voltage characteristics. The conductivity of the device in reverse bias can be raised three orders of magnitude by applying a positive voltage or by illumination with UV light. In this high conductivity state we observe reversible electronic memory effects. The electronic memory effects are attributed to a reversible electrochemical process at the PEDOT:PSS/ZnO interface. Memory effects in diodes with Al and Ca metal electrode are found to be very similar, consistent with the view that the memory effects arise at the PEDOT:PSS/ZnO interface.

Measurement of charge division in pixelated CZT

In medical X-ray imaging using semiconductor detectors, small pixels are employed to increase image resolution and count rate capability. However, the use of small pixels increases the occurrence of imaging signals that are split between neighboring pixels or even lost in the gap regions between pixels. To assess this problem monolithic pixelated CZT blocks were scanned from pixel to gap to pixel using a filtered X-ray beam and a narrow diameter collimator. A novel technique for recording and digitizing time series data from neighboring pixels for analysis offline was employed. Singles and coincidence spectra were compiled for each location and energy. The extent of charge splitting or charge loss was determined. The technique of digital summing of coincidence signals in neighboring pixels to recover split events is demonstrated.