4-3 Results and Discussions

We measured the electrical characteristic of Al/PS-b-P4VP/ITO, Al/PS/ITO, and Al/P4VP/ITO devices by semiconductor parameter analyzer (HP4156C, Agilent) performed on probe station (VFTTP4, Lakeshore) under 10^-5 torr at room temperature, semi-logarithmic current–voltage (I–V) plots from them as displays in Figure 4-2. For the Al/PS/ITO device featuring a homogenous PS surface, the I–V characteristics display a low current that increased slowly upon increasing the applied voltage, indicating that PS is a good insulator that has no memory effect. In contrast, the Al/PS-b-P4VP/ITO and Al/P4VP/ITO devices displayed electrical bistability with a sufficiently large sensing margin between the ON and OFF states.

The I–V characteristics of the Al/P4VP/ITO device initially exhibited a high-resistance state (OFF state); after performing a negative-voltage sweep to ca. –0.5 V, the device was switched to a low-resistance state (ON state). When a positive-voltage sweep was applied to ca. +1.2 V, the device switched back to its initial OFF state. The maximum ON/OFF current ratio for the P4VP device was 2 × 10³ at –0.1 V. For the Al/PS-b-P4VP/ITO device, the turn-on and turn-off threshold voltages were –0.5 and +0.75 V, respectively; the maximum ON/OFF current ratio was 2 × 10⁵ at –0.1 V. The AFM topographic image of the PS-b-P4VP thin film (inset to Figure 4-2) reveals the micro phase separation of quasi-hexagonal structures that resulted from the micellization of PS-b-P4VP; the light regions (size: ca. 30 nm) represent the P4VP domains and the dark areas represent the PS matrix.

For both the Al/PS-b-P4VP/ITO and Al/P4VP/ITO devices, after removing the compliance current limit, the magnitudes of the turn-off currents were larger than their turn-on compliance currents.^[115] In addition, the ON state of each device could be converted into the OFF state by applying the sweeping voltage in the same direction.

This behavior can be explained by considering a metallic filament theory.^[157]

To prove that the metallic filamentary mechanism operated, we used a cryostat system operated under 10^–5 torr to measure the temperature dependence of the resistance of the ON state of the Al/PS-b-P4VP/ITO memory devices. Figure 4-3 reveals that the conduction mechanism of the ON state is dominated by ohmic conduction (I ∝ V). The resistance of the ON state increased linearly upon increasing the temperature—typical of metallic behavior. The equation R = R0[1 + α(T – T0)]

represents the variation of the resistance with respect to the temperature, where α is the temperature coefficient and R0 is the resistance measured at a reference temperature T0, usually 293 K.^[158] The temperature coefficient of 0.0031 that we obtained after fitting the results in the inset to Figure 4-3 is slightly smaller than the acknowledged value of 0.0043.^[159] This discrepancy may be due to thermal losses occurring at the contact between the Al/PS-b-P4VP/ITO device and the cryoprobe.^[160]

P4VP, which contains pyridyl groups, interacts strongly with Al clusters or atoms.

The diffusion of these Al clusters or atoms is, however, negligible because PS has a low affinity toward this metal.^[161] We suspect that Al atoms migrated into the P4VP zones to form metallic filaments and that these Al filaments grew during the writing progress. The dimensions of the P4VP nanodomains in the PS-b-P4VP thin film would limit the growth and size of these Al filaments, whereas a homopolymeric P4VP thin film might have no limitation to the extent of growth of similar Al filaments. In general, filaments having larger size would be more difficult to break.

Thus, scaling down the size of the organic memory materials to the nanometer scale resulting from the self-assembled diblock copolymer provides the organic memory device with a higher ON/OFF ratio and lower erasing and writing voltages.

Figure 4-4a displays the time-resolved I–V characteristics of the Al/PS-b- P4VP/ITO organic memory device. The write/read/erase/read cycle was recorded using an oscilloscope (Keithley, 4200-SCP2HR) equipped with an arbitrary waveform

generator (Keithley, 4205-PG2) and a programming current amplifier (Keithley, 428).

In general, the switching speed depends on the amplitude of the write/erase signal.

Our read/write/erase cycle sequence consisted of a 5 μs write pulse at –1.5 V, a 10 μs erase pulse at 5 V, and a 0.1 V read voltage. Applying the write and erase pulses switched the device to its ON and OFF states, respectively. Although the switching speed of this Al/PS-b-P4VP/ITO memory device was acceptable for its use in most consumer products, there is much room for improvement.

Our Al/PS-b-P4VP/ITO organic memory device was nonvolatile: it was stable and could be read many times as long as the probing voltage remained below the threshold voltage. Figure 4-4b displays the retention characteristics of our Al/PS-b-P4VP/ITO organic memory device, measured by monitoring the current at -0.1 V after programming and erasing at –1.5 and 5 V, respectively, with a 100 μs pulse bias. The currents in the ON and OFF states differed by five orders of magnitude, with no significant changes after 10⁴ s. We extrapolated a 10-year memory window for our Al/PS-b-P4VP/ITO organic memory device; the memory window of ca. 10⁵ is sufficient for its use in practical nonvolatile memory devices.

4-4 Conclusions

We have demonstrated that a device incorporating a micellar thin film of a diblock copolymer, consisting of metal-coordinated cores and insulating shells, operates through the formation of metallic filaments. This Al/PS-b-P4VP/ITO organic nonvolatile memory device is switchable with a long retention time and an acceptable programming speed; notably, it exhibits a lower erase threshold voltage and a higher ON/OFF ratio than those of the corresponding Al/P4VP/ITO device. This fabrication approach opens up new possibilities for improving the memory performance of polymeric materials prepared at low cost using simple processes.

Figure 4-1. (a) Morphology of a cylindrical PS-b-P4VP diblock copolymer in the bulk state. (b) Micellar structure of a cylindrical PS-b-P4VP diblock copolymer in a selective solvent. (c) Nonvolatile memory device comprising an active PS-b-P4VP diblock copolymer film and Al and ITO electrodes. (d) SEM image of the cross-section of the memory device in (c).

Figure 4-2. I–V characteristics of PS (

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Figure 4-3. Logarithmic I–V plots measured at various temperatures for the ON state at potentials ranging from 0.01 to 0.2 V. The inset displays the resistance of the ON state plotted with respect to the temperature.

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Figure 4-4. (a) Write/read/erase test of the ITO/PS-b-P4VP/Al organic memory device. The bottom and top curves represent the applied voltage pulse and the corresponding current response, respectively. (b) Retention times of the ON and OFF states of the ITO/PS-b-P4VP/Al organic memory device probed in terms of the device current after stressing.