Organic Memristors Based on Polymer Blends PI: Michael Chabinyc Graduate Student: Andrew Jacobs University of California Santa Barbara Organic Memristive Devices Goal: organic ferroelectric diode Develop spin-coatable, organic memristive R F F element with high retention & yield at the F F S S S S n H H n F H m nanoscale R - - - Materials system: Organic semiconductor:ferroelectric blends + + + h+ h+ Milestones: • Functional memristive diodes with sub-100nm polymer domain sizes by nanoimprint or nanoimprinted poly(3-hexylthiophene) spontaneous phase separation • Improved retention time through physical understanding of operational mechanisms • Development of CMOS compatible process e.g. electrodes 100 nm Charge Carrier Mobility and Polymers R DPP-TTs S S μ > 5 cm2/Vs S S n R Ordering ? R PBTTT S S n semicrystalline 2 R μ ~ 0.5 cm /Vs S S P3HT n R R semicrystalline F8T2 μ ~ 0.05 cm2/Vs n N liquid crystalline disordered films μ ~ 0.02 cm2/Vs increasing crystallinity in thin films triarylamine amorphous μ ~ 0.005 cm2/Vs Mobility values from TFTs Active Matrix Arrays and ICs printed OTFT pixel gate line gate line C TFT P TFT data line Mobilities and reproducibility C adequate for displays, diodes, pixel ST electrode circuits pixel electrode data line PARC ISSCC 2011 PAPER CONTINUATIONS ISSCC 2011 / February 23, 2011 / 8:30 AM 8 bit microprocessor (4000 TFTs) Figure 18.1.1: Architecture of the 8b organic microprocessor foil. The connec- tor comprises 30 pins: 18 input pins [opcode(8,0); in(7,0); clk], 9 output pins Figure 18.1.2: Hardware testbench measuring all individual instructions of the [out(7,0), overflow] and power, ground and backgate voltage. microprocessor foil at a clock frequency of 6Hz (VBack = 50V and VDD = 15V). K. Myny, et. al ISSCC 322s,2011 Figure 18.1.7: (a) die picture of the 8bit microprocessor foil; (b) foil comprising several microprocessor circuits laminated on a 6-inch wafer carrier during processing; (c) instruction generator foil for the running averager. Figure 18.1.4: Architecture of instruction foil used in the implementation of 18 Figure 18.1.3: Shmoo plots of the microprocessor foil as a function of the the running averager. The connector uses 14 pins: 2 input pins [reset; clk], 9 clock frequency and backgate voltage (left) or power voltage (right). output pins [opcode(8,0)] and power, ground and backgate voltage. Figure 18.1.5: Measured output of the microprocessor foil connected to the running averager instruction foil. The switches at the input pins are manually Figure 18.1.6: (a) instruction table with corresponding opcodes. RR in the changed from 0 to 7 (00000111). The output reaches 7.0 (00001110), with one opcode represents the selected register RegSel (1,0). X is a don’t care. (b) additional bit of precision, after 3 program cycles. Program code for the running averager. DIGEST OF TECHNICAL PAPERS • 323 • 2011 IEEE International Solid-State Circuits Conference 978-1-61284-302-5/11/$26.00 ©2011 IEEE Organic Diodes Contacts tend to show vacuum level alignment - carrier type controlled by injection barrier J-V characteristics are space-charge limited - most materials are nearly intrinsic - Egap ranges ~1.5-4 eV for organics - Injection barrier changes with interface dipoles Voltage for Metal Filament-Based Polymer Memory J. Phys. Chem. B, Vol. 111, No. 27, 2007 7757 observed at -2 V as a result of the metal filament formation between two metal electrodes. The higher conductive state (set state) showed the typical metal characteristics confirmed by low- temperature experiment where the resistance linearly increased with the temperature from 160 to 300 K.13 The set state could be retained for several months and the application of 4 V switches back to the initial low conductive state (reset state) due to the cleavage of filament by joule heating. In this memory behavior, it was notable that high positive voltage application over 7 V was necessary for reproducible formation of the metal filament. The high positive voltage is believed to play important roles of ionizing the copper electrode and to inject the ions into polymer layer. Assisted by coordina- tion to the heteroatom (S) of the P3HT, copper ions are distributed uniformly throughout the polymer layer. Then, copper ions are metallized to form the filament by the injected electrons under negative voltage bias. This concept of memory behavior is schematically represented in Figure 2. Figure 1. Typical current-voltage curve of regiorandom P3HT device According to the concept mentioned above, the density and which shows the nonvolatile memory behavior. the distribution of copper ions throughout P3HT layer are important factors controlled by the positive electric field. For cell size is 0.25 mm2. The current voltage curve was measured this reason, the memory behavior was investigated at various in air with the sourcemeter (model: 2400, Keithley Inc.). positive voltage strengths. In voltage sweep mode with fre- Measurement in nitrogen atmosphere did not give any significant quency of 0.6 Hz, the maximum positive voltage was varied difference in memory phenomenon. from 5 to 12 V with negative voltage set to -3V.Theswitching To obtain the switching probability of the device with various probability was defined as the probability of occurrence of environmental conditions, continuous voltage sweep was applied switching to the set state during a negative voltage sweep. For to the device with the programmable power supply of Yokogawa accuracy, the probability was determined from 60 cycles instrument (model 7631), and the current was measured with obtained after waiting for 180 s under the continuous voltage digital oscilloscope (model TDS3502B). The copper top elec- sweep. For 50 nm thick devices, the switching probability was trode was connected to signal line, and the bottom electrode near zero when the positive voltage below 7 V was applied. Classeswas used as ground. Cu density withinof P3HT Organic layer was However, sudden increase Memory up to unity was observed over 8 V Devices investigated using time-of-flight secondary ions mass spectro- as shown in Figure 3a. As the thickness of polymer layer in the scopy (TOF-SIMS, model ION-TOF IV). Cu depth profiles were devices increased, the threshold voltage also increased from 8 acquired in the dual beam interlaced mode using 500 eV Cs to 11 V. Interestingly, when represented in terms of electric primary ion and 25 keV Bi analysis gun. field, Figure 3b indicates that all devices with different thickness had identical threshold electric field of about 170 MV/m. It was 3. Results and Discussion 346 Chem. Mater.,surprising Vol.that the 23, switching No. probability 3, 2011 increased sharply Heremans et al. The regiorandom P3HT was chosen as an active material around the threshold electric field. TFT-based because the deviceElectrochemical showed highly reproducible metal filament To study the density of copper ions within polymer layer formation based memory behavior in ourcrossbar previous work.13 network.before and To after reduce the positive sneak voltage application currents, over the it is known Typical switching behavior of the P3HT device is represented threshold value, we used the TOF-SIMS analysis to measure in Figure(Filamentary 1. A sudden decrease in the deviceConduction)that resistance a selecting was the depth device, profile ofsuch copper as ions. a To diode, avoid thehas broadening tobe of placed in series with the conductive switching device. Therefore, it can be highly beneficial that the conductive switching device would inherently possess rectifying characteristics, as for example possible with diodes containing a blend of an organic semiconductor and a ferro-electric polymer,45 which will be discussed in section 4. It should also be mentioned that addressing unipolar switching devices is simpler than addressing bipolar memory devices, because for the latter, the switching of the memory devices re- quires polarity reversal of the addressing circuitry. Figure 2. Schematic concept for the formation of metal filament within polymer layer: (a) device structure, (b) ionization and drift processes of copper caused by positive voltage,W.-J. (c) metal Jo, filament et. al formation2.7.. J. Phys. by Summary theChem. reduction 111, of copper and7756 ions, Outlook. and(2007) (d) the breakdown Several of copper filament mechanisms by joule may heating. Practically, the regiorandom P3HT is not aligned and has a random coil conformation in the organic layer. cause the electrical conductivity of molecules to switch. However, in real devices comprising a thin film of such molecules, the switching is mostly due to the formation of Figure 7. SchematicP. Heremans configuration, et. al. Chem. and Mater. operational 23 341 mechanism(2011) of an conductive filaments in an interfacial oxide. organic memory p-type transistor device. Therefore, only in a few cases could a role in the switching be ascribed to the organic semiconductor. This Several types of gate dielectrics enable reversible trapping of Injection Barrieris the Modulation case in solid-state redox cells as well as in semi- charges upon application of a gate field, for instance polymer conductor ferroelectric diodes. Nanotechnology electrets,20 (2009) 025201 dielectrics with embedded metallic or semiconduct- T-W Kim et al Nanotechnology 20 (2009) 025201 T-W Kim et al ing nanoparticles(a) (NPs) or organic conjugated(b) molecules, and (a) 3. Charge-Storage(b) in Transistor Gate Dielectric ferroelectric gate insulators with permanent and/or switch- able electrical dipoles.8 In this section, we introduce the basic Organic nonvolatile memory devices based on organic properties, operational mechanisms, and recent progress in field-effect transistors (OFETs) are especially attractive, memory devices based on OFETs with gate insulators that because these devices can be read without destruction have charge-storage capacity. of their memory state (“non-destructive read-out”) and Current (A) Charge trapping in the gate dielectric causes the thresh- have a manufacturing advantage because of their archi- 48 old voltage (V ) of the transistor to shift.