Electric Memristor for Light-Based Logic and

Jan 10, 2019 - Light-based information processing has the potential to increase speed, security, and scalability of electronic devices if issues in th...
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Surfaces, Interfaces, and Applications

A Hybrid Optical/Electric Memristor for Light-Based Logic and Communication Shu-Yi Cai, Chen-Yang Tzou, Yi-Rou Liou, Ding-Rui Chen, Cing-Yu Jiang, Jing-Meng Ma, Chi-Yuan Chang, Chen-Yang Tseng, Yu-Ming Liao, Ya-Ping Hsieh, Mario Hofmann, and Yang-Fang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19424 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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A Hybrid Optical/Electric Memristor for LightBased Logic and Communication Shu-Yi Cai1,+, Chen-Yang Tzou1,+, Yi-Rou Liou1, Ding-Rui Chen2, Cing-Yu Jiang3, Jing-Meng Ma1, Chi-Yuan Chang1, Chen-Yang Tseng4, Yu-Ming Liao1, Ya-Ping Hsieh5, Mario Hofmann1,*, and Yang-Fang Chen1,* 1

National Taiwan University, Department of Physics, Taipei, 10617, Taiwan

2

National Chung Cheng University, Institute of Opto-Mechatronics, Chiayi 621, Taiwan 3 National Taiwan University, Department of Applied Physics, Taipei, 10617, Taiwan 4 National Taiwan Ocean University, Department of Optoelectronic Sciences, Keelung 202, Taiwan 5 Academia Sinica, Institute for Atomic and Molecular Sciences, Taipei, 10617, Taiwan *E-mail: [email protected] *E-mail: [email protected] + these authors contributed equally to this work

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ABSTRACT Light-based information processing has the potential to increase speed, security and scalability of electronics if issues in the device complexity could be resolved. We here demonstrate an integrated nano-electronic device that can combine, store, and manipulate optical and electronic information. Employing a mechanically flexible and multilayered structure, a device is realized that shows memristive behavior. Illumination is shown to control the device operation in several unique ways. First, the device produces photocurrent that allows us to read out the device state in a selfpowered manner. More importantly, a varying light intensity modulates the switching transition in a proportional manner that is akin to a neuron with variable plasticity and which can be taught and queried using either light or electrical inputs. This behavior enables a multi-level light-controlled logic and teaching schemes that can be applied to light-based communication devices and provides a route towards ubiquitous and lowcost sensors for future internet of things applications.

KEYWORDS: memristor; light-based communication; hybrid optoelectronics; light fidelity; memory

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Introduction Optical information processing has been heralded as a novel route to enhancing the performance of current computers due to the large band width and low stray capacitances associated with light-based signal routing for future supercomputers.1-2 Surprisingly, however, light could also solve problems that plague current low cost electronics such as distributed or wearable sensors.3-4 Due to the higher fidelity of light signals compared to radio communication, receivers usually require less complex amplifying stages for signal conditioning which reduces device complexity and cost.57

Moreover, light receivers such as photodetectors can serve a dual purpose as energy

harvesters.8 High data transmission rates of Gigabits per second have already been achieved using readily available and low cost sources such as laser pointers.9 Finally, light cannot pass through walls, offering inherent advantages for security and privacy. These advantages make light-based communication an ideal platform for the internet of things (IoT) where ubiquitous electronic sensors relay data to a central hub and thus form a smart environment.

To realize this vision for ubiquitous light-based electronics, however, effective and scalable schemes for light-based information processing have to be devised. Current approaches require discrete components for receiving, analyzing, powering and storing light-based information resulting in devices that are more complex than traditional 3 ACS Paragon Plus Environment

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network-enabled sensors and thus negate the simplification of using light instead of RF signals.

The emerging memory technologies have the huge potential for simplifying circuit and three-dimensional (3D) integrated devices.10-11 Moreover, emerging memory computing can be used to store data and also directly process data. Which enable inmemory computing to solve the von Neumann bottleneck and provide a platform for high-performance computing.12

We here present a low-cost integrated device that can receive, store, and manipulate light-based signals and opens up new possibilities for communication and information processing. Using a scalable, lithography-free fabrication approach a nanometer-thick assembly of functional layers is produced. The resulting device acts as a hybrid memristor that converts and combines light-based and electrical signals. Light illumination was found to produce an analogue-tunable memristive behavior over a wide range akin to biological synaptic plasticity.13-15 This behavior enables multilevel logic applications and novel communication schemes which are well suited for sensor network applications.11,

16-17

Moreover, the device is demonstrated to exhibit

mechanical flexibility and high reliability as well as energy-harvesting capabilities

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making it ideal for wearable remote sensors. The presented hybrid memristor thus opens new routes towards the scalable realization of ubiquitous electronic devices.

Results and Discussion

Figure 1a depicts the structure and materials employed in the hybrid optoelectronic memory integrated in a vertically stacked architecture.18-19 Indium doped tin oxide (ITO) (15 Ω sq-1) was patterned on either glass or polyethylene terephthalate (PET) to serve as the transparent electrode and mechanical support for the devices. Then, ZnO was spin coated as an electron transport layer. A P3HT:PCBM photo-active layer was deposited by spin coating. Then MoO3 (7 nm) was thermally evaporated as a hole transport layer, followed by the thermal evaporation of Ag layer (100 nm). Polymethylmetacrylat (PMMA) was deposited by spin coating and an Au/Ti top electrode was deposited by evaporation. Contact was made to the top and the bottom of the device as shown in Figure 1b. The cross-sectional scanning electron microscope (SEM) images of the vertically stacked cell (Au/PMMA/Ag/MoO3/P3HT:PCBM/ZnO/ITO), is shown in Figure 1c showing that the device exhibits well separated layers with a total thickness of ~ 600 nm.

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When applying voltage to the Ag/Au electrode, we observe a complex IV curve as shown in Figure 2a. In the beginning, the device is in a high resistance state (HRS). If a positive bias in excess of the SET voltage VSET (0.5 V) is applied the device transitions between HRS and low resistance state (LRS) and the current increases until it reaches the limit set by the compliance (IC) of 0.05 mA. The current ratio between high and low resistance state is about 103 at read voltage (0.2 V). A reset from LRS to HRS can be performed if a negative voltage is applied between the top electrode and a temporary electrode between Ag and MoO3 (Supporting Information Figure S1). This behavior is indicative of bipolar switching memristors.11, 13, 20-24 Our device exhibits high endurance as demonstrated by the repeatable switching between HRS and LRS for 100 cycles (Figure 2b). Due to its low thickness the device can be applied to flexible electronics.2535

The bending test of a flexible cell as shown in Figure 2c. The device is shown to

reliably work under different amounts of tensile stain. Moreover, stable on/off current ratio of ~ 103 could be achieved even after 1,000 tensile and compressive bending cycles (Figure 2d). The incorporated photoactive layer (P3HT:PCBM) imparts the device with novel functionality under illumination.36-37 A photovoltaic effect leads to a reliable change in output current after set and reset (Figure 3a) which can be employed to read out the memory state without application of a bias (Figure 3e) in an analogous way as when 6 ACS Paragon Plus Environment

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applying a bias (Figure 3d). Moreover, if the illumination exceeds 1.4 mW/mm2 the light intensity is sufficient to write information in a self-powered manner (Figure 3e).28, 38-40

Figure 3b shows the change of the I-V curves when the cell is illuminated at different light intensities. It can be seen that the set voltage is significantly affected by the illumination condition. Figure 3c shows the relationship between light intensity and set voltage is based on the formula41 𝑉𝑠𝑒𝑡 = 𝑉𝑠𝑒𝑡0 − 𝛼 𝑙𝑛(𝛽 ∙ 𝐼𝐿𝑖𝑔ℎ𝑡 + 1)

(1)

where 𝐼𝐿𝑖𝑔ℎ𝑡 is the light power intensity, 𝛼 and 𝛽 are constant terms that scale with the open circuit voltage and the photovoltaic efficiency, respectively. This proportionality holds for a wide range of light intensities between 0.2 and 1.4 mW/mm2 and imparts the device with an analog tunability that is similar to the variable plasticity of neurons.13-14, 42-45 We can use this novel property to realize hybrid optical/electronic logic. First, the device can serve as an AND gate where electronic and light-based inputs are compared. Only if the combination of input bias and light intensity according to equation 1 exceeds the SET voltage a transition occurs. To demonstrate this application, we apply a train of light pulses and electrical pulses and show that a set-transition only occurs when both pulses coincide (Figure 3f). Different from traditional AND gates the inputs are 7 ACS Paragon Plus Environment

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not independent but their relative values can be adjusted enabling multi-level logic systems which increase the information density.46 Moreover, this approach can be employed to store information about light intensity in binary values by providing a tunable analog-to-digital converter. An even more promising application of the hybrid device is suggested from the time-resolved SET process. Figure 4a-c show the device switching characteristics during pulse train experiments with electric pulse at 1 V, optical pulse at 0.172 mW/mm2 and pulse durations of 1 ms. Even for these short pulses we observe that the SET transition will only occur of the pulses are in phase (Figure 4a) which indicates the high speed of the device. Interestingly, the SET will not occur immediately but only after six cycles (Figure 4b). The delayed switching phenomenon is known as Voltage– Time Dilemma in resistive RAM research and is related to the conductive filament forming process due to mobile ions.14, 20, 47 We observe that the delay can be adjusted by varying the light intensity (Figure 4c) with higher intensity resulting in shorter switching delays. These unique device characteristics open up a new route for light-based communication where a transmitter is sending out a light pulse train towards the device (Figure 4d). The received sequence is converted into an electrical current within the device and transferred into a delayed voltage response. If subsequent light pulses are in 8 ACS Paragon Plus Environment

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phase and complementary to the voltage pulse, switching occurs. This process can be used to identify one specific device since the light intensity at a specific location depends on the distance from the transmitter and thus switching will occur after a fixed number of pulses only if the light intensity is strong enough. Moreover, differences in device dimensions and signal delay can be employed to distinguish devices at the same location or closer locations. Thus, a receiver could be envisioned that first acts as a passive identification tag and then as a power supply for attached devices, such as sensors, microprocessors and light emitters. Since the device relies only on passive components it does not require any lateral patterning steps and can be produced scalably and then cut to suitable dimensions producing extremely cost effective components for IoT applications. Conclusion In summary, we successfully designed and fabricated a novel hybrid optical/electric memristor, which uses light and voltage as inputs. The simple device fabrication is compatible with scalable deposition techniques and yields a robust and flexible device. A memristive behavior is demonstrated whose operation can be controlled by light illumination enabling self-powered operation and hybrid optical/electric logic. Exploiting a continuously tunable dependence of set voltage and light intensity, a novel device is realized that permits multi-level logic and storage as 9 ACS Paragon Plus Environment

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well as time-delayed switching. These novel properties can be applied towards lithography-free light-based communication devices for the realization of distributed sensing and IoT.

Methods Device Fabrication.

The prepatterned ITO-glass substrates (15 Ω sq-1) were cleaned by sequential sonication in water, ethanol, and isopropyl alcohol. A 30 nm thick ZnO film was deposited by spin-coating the prepared nanoparticle solution on the cleaned ITO glass substrates, followed by thermal annealing at 120 °C for 10 min. The active layer was then deposited on top of the ZnO nanoparticles layer by spin-coating from a 40 mg/mL o-dichlorobenzene solution of P3HT:PCBM (Lumtec) (D/A ratio 1:1) at 800 rpm 40 s, in air. Then MoO3 (7 nm) was thermally evaporated as a hole transport layer, followed by the thermal evaporation of Ag (100 nm). For the RRAM part, The PMMA (PMMA 950 A6, MicroChem) was spun on the top of ITO/ZnO/P3HT:PCBM/MoO3/Ag at 4500 rpm for 50 s. Finally, the thermal deposition of Au (80 nm) layers was carried out on the RRAM material layer to complete the device fabrication.

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Device measurements.

White LED (Mightex Systems, LCS-6500-15-22) controlled by Universal LED Controller (Mightex Systems, SLC-AA02-US) serves as the light source. Photovoltaic parameters were measured by the solar simulator (Newport Inc.) with AM 1.5 G filter. The electrical properties were measured by Power supplies (Agilent B2912A and Keithley 2410). In bending test, different curvature radius substrates (5.2 mm, 6.4 mm, 7.35 mm, 12.3 mm, and 18.2 mm) were used to measure the stability.

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Figure 1. Schematic representation of the device. (a) Schematic illustration of the vertically

stacked

architecture

and

the

energy-level

diagram

(Au/PMMA/Ag/MoO3/P3HT:PCBM/ZnO/ITO). (b) Optical images of the device. (c) The cross-sectional SEM images of the vertically stacked cell.

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Figure 2. ON/OFF switching properties of devices. (a) I-V characteristic of the Ag/PMMA/Au memory. (b) ON/OFF switch for 100 cycles of the Ag/PMMA/Au memory, read by 0.2 V. (c) The ON and OFF currents were read by 1 V /0.172 mW/cm2 under different degree of bending. (d) The ON and OFF currents were read by 1 V /0.172 mW/cm2 over 1000 times bending with bending radius 5 mm.

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Figure 3. Optical/electric programming of a hybrid cell. (a) I-T characteristic of a hybrid cell under constant voltage 1 V. (b) I-V characteristics of a hybrid cell under different light intensities with white light LED. (c) SET voltage-light intensity of a hybrid cell with the fitted curve. (d) Electric pulse programming of Ag/PMMA/Au memory. (e) Light pulse programming of a hybrid cell. (f) Light and electric pulse programming of a hybrid cell. The inset figure is the concept of logic.

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Figure 4. Logical operations of the hybrid cell. (a) I-T characteristics of the hybrid cell, when the light pulses (0.172 mW/mm2) do not coincide with the electrical pulses (1 V). (b) I-T characteristics of the hybrid cell, when the light pulses (0.172 mW/mm2) coincides with electric pulses (1 V). (c) The switching behavior of the hybrid cell under the electric and light pulse input cycles. (d) The concept of light-based communication.

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ASSOCIATED CONTENT

Supporting Information. Bipolar resistive switching property, I-V characteristic of Ag/PMMA/Au memory. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions S.Y.C. and C.Y.T. contributed equally. Y.F.C. and M.H. supervised the project and conceived the study. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was financially supported by the “Advanced Research Centre for Green Materials Science and Technology” from The Featured Area Research Centre Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001).

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Notes The authors declare no competing financial interests.

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41. Tress, W.; Marinova, N.; Inganäs, O.; Nazeeruddin, M.; Zakeeruddin, S. M.; Graetzel, M., Predicting the Open‐Circuit Voltage of Ch3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: The Role of Radiative and Non‐Radiative Recombination. Adv. Energy Mater. 2015, 5, 1400812. 42. Wu, C.; Kim, T. W.; Choi, H. Y.; Strukov, D. B.; Yang, J. J., Flexible ThreeDimensional Artificial Synapse Networks with Correlated Learning and Trainable Memory Capability. Nat. Commun. 2017, 8, 752. 43. Lee, J.; Lu, W. D., On‐Demand Reconfiguration of Nanomaterials: When Electronics Meets Ionics. Adv. Mater. 2018, 30, 1702770. 44. Zidan, M. A.; Strachan, J. P.; Lu, W. D., The Future of Electronics Based on Memristive Systems. Nat. Electron. 2018, 1, 22-29. 45. Yu, S., Neuro-Inspired Computing with Emerging Nonvolatile Memorys. P. IEEE 2018, 106, 260-285. 46. Borghetti, J.; Snider, G. S.; Kuekes, P. J.; Yang, J. J.; Stewart, D. R.; Williams, R. S., ‘Memristive’switches Enable ‘Stateful’logic Operations via Material Implication. Nature 2010, 464, 873-876. 47. Menzel, S.; Böttger, U.; Wimmer, M.; Salinga, M., Physics of the Switching Kinetics in Resistive Memories. Adv. Funct. Mater. 2015, 25, 6306-6325.

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