All-Oxide-Based Highly Transparent Photonic Synapse for

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Functional Inorganic Materials and Devices

All-Oxide-based Highly Transparent Photonic Synapse for Neuromorphic Computing Mohit Kumar, Sohail Abbas, and Joondong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10870 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Applied Materials & Interfaces

All-Oxide-based Highly Transparent Photonic Synapse for Neuromorphic Computing Mohit Kumar,a,b Sohail Abbas, a,b and Joondong Kim a,b,* a

Department of Electrical Engineering, Incheon National University, 119 Academy Rd.

Yeonsu, Incheon, 22012, Republic of Korea b

Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute

for Future Energies (MCIFE), Incheon National University, 119 Academy Rd. Yeonsu, Incheon, 22012, Republic of Korea

KEYWORDS: photonic synapse, transparent, all-oxide, charge trapping/detrapping, photoresponse

ABSTRACT Neuromorphic system processes enormous information even with very low energy consumption, which practically can be achieved with photonic artificial synapse. Herein, a photonic artificial synapse is demonstrated based on an all-oxide highly transparent device. The device consists of conformally grown In2O3/ZnO thin films on an FTO/glass substrate. The device showed a loop opening in current-voltage characteristics, which was attributed to charge trapping/detrapping. Ultra-violet illumination-induced versatile features such as shortterm/long-term plasticity and paired-pulse facilitation were truely confirmed. Further, photonic potentiation and electrical habituation were implemented. This study paves the way to develop a device in which current can be modulated under the action of an optical stimuli, serving as a fundamental step towards the realization of low cost synaptic behavior.

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INTRODUCTION Modern digital computers are working on the von Neumann architecture, which has been demonstrated to solve complicated and well-formulated mathematical calculations.1,2 However, in the von Neumann architecture, memory storage and processing are two separate units, and this is not very suitable when performing unformulated problems such face recognition, habituation learning etc.3 Therefore, building a new computing architecture that can comfortably resolve the above challenges is essential. Indeed, the initial development of computing architectures that can mimic brain-like functionality have started by researchers, and this approach is termed neuromorphic computing.3,4 To experimentally realize artificial neuromorphic behavior, different approaches such as memristors,5 transistors,1 and atomic switches6 have been proposed and demonstrated. However, with the memristor, poor sustainability, nonlinear writing, uncontrollable filament dynamics and excessive write noise still pose challenges.1,7–9 Meanwhile, resistance drift as well as the need for high programming power are the main obstacles to phase change memories.10,11 As an alternative approach, floating gate flash memories have recently been proposed, however, they required an additional gate voltage, which unavoidably consumed extra energy.12,13 In contrast to a purely electrically triggered device, a photonically stimulated neuromorphic device offers a noncontact writing method, and could be more beneficial to enhance processing speed because it required low power computation along with low crosstalk.3,14–16,17,18 Recently, Lee et al. have used the IZO/IGZO configuration on SiO2/Si susbtrate and mimic major synaptic functions using photons.3 On the other hand, He et al. have employed monolayer MoS2 and demonstrated photonic potentiation and electric habituation.14 Moreoever, Hu et al. have shown photoelectric plasticity in case of ZnO1−x/AlOy heterostructure.16 However, designing all-oxide-based photonic synapse and understanding the typical charge transport process across it have yet to be accomplished. 2 ACS Paragon Plus Environment

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Meanwhile, transparent electronics are drawing enormous attention for the production of “invisible” electronic circuits and optoelectronic devices.19 Transparent devices also have various applications in transportation, military, civilian, including integrated see-through electronics.19–21 In general, wide bandgap metal-oxides are utilized to design these highly transparent electronics, however, the effective utilization of all-oxide-based materials to realize the highly transaprent photonic synapse has not yet been demonstrated.12,22 In this work, we demonstrated highly transparent synapse for the first time, based on an all-oxide heterostructure, which exhibits photonic potentiation and electric habituation behaviors. The device structure consists of sequential In2O3, ZnO thin films on fluorine doped tin oxide (FTO)-coated glass substrate. The type-II band alignment between the ZnO and In2O3 leads to trapping of the photo-generated charge carriers, which serves as the basis for the optically-mediated charge trapping and electrically-induced charge release. In addition, all synaptic functionalities for instance, short- and long-term plasticity along with paired-pulse facilitation are imitated and explained on the basis of persistent photoconductve behaviour of the device. Our work establishes a novel architecture for integrating photonic neuromorphic computing, which is the building block for artificial computing, and is emulated here at the device-level.

EXPERIMENTAL SECTION Device deposition processes. Prior to device fabrication, the FTO-coated glass substrates were cleaned ultrasonically in acetone, methanol and deionized water, sequentially. A commercially available 99.99% pure ZnO target was used to grow the zinc oxide (ZnO) thin film. The working pressure of 5 mTorr was maintained with the flow of ultra-pure (99.999%) argon gas with a rate of 50 sccm. Thw working power was 300 W (RF) and the substrates rotation was fixed to 5 rpm, to achieved uniform film growth. To form the In2O3 film, the reactive sputtering was performed with the flow of Ar/O2 gases (15/5 sccm). A pure In target 3 ACS Paragon Plus Environment


(99.99%) was used to grow its oxide films, and sputtering was done with a RF power (100 W). All used oxides were grown at room temperature. Device characterization. The planar-view, cross-section morphology and energy-dispersive x-ray spectroscopy of the device were carried out by employing a field emission electron microscope (FESEM, JEOL, JSM_7800F). Optical measurements was obtained using an Shimadzu-made (UV-2600) ultraviolet-visible-near-IR spectrophotometer. Chemical analysis of the ZnO and In2O3 were carried out using x-ray photoemission spectroscopy (XPS, PHI 5000 VersaProbe-II, ULVAC). Electrical characterization and photoresponse. An ultraviolet (UV) LED light source (365±10 nm) source was used for the photo-induced measurements. The Au-coated probe and FTO were used as top and bottom electrodes, respectively. The chronoamperometry method under pulsed monochromatic light by varying the light intensity and applied voltage was applied to study the optical response of the device. The UV pulses were generated by a function generator (MFG-3013A, MCH Instruments). UV intensity was calibrated by a power meter (KUSAM-MECO, KM-SPM-11).

RESULTS AND DISCUSSION Figure 1a depicts the planar-view scanning electron microscopy (SEM) image of the In2O3 thin film grown on ZnO/FTO/glass, showing granular distribution with an average grain size of 450±10 nm. Following that, the cross-sectional SEM image of the device is presented in Figure 1b, consisting of sputter grown sequential In2O3 (thickness ~150 nm) and ZnO (thickness ~350 nm) thin films over a fluorine-doped tin oxide (FTO) substrate. Note that no clear interface among these layers can be observed, indicating conformally grown layers. In addition, an elemental distribution was revealed by energy-dispersive x-ray spectroscopy (EDS) line profile and spectra, and the respective results are displayed in Figure 1c and 1d, respectively. The line profile shows that the top layer has indium (In) content while the zinc 4 ACS Paragon Plus Environment

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(Zn) is sandwiched between the In2O3 and FTO layers. In addition, profile of Sn is detecting from the bottom FTO. The distribution of the indium (In), zinc (Zn), and oxygen (O) are presented in different color maps in Figures 1e-g, respectively, revealing that all elements are uniformly distributed. The oxidation states and chemical compositions of as-grown ZnO and In2O3 thin films were investigated quantitatively by XPS spectra. Figures 2a and b depict the core-level spectra of Zn 2p and O 1s from ZnO film, respectively. One can note that the Zn 2p3/2 and Zn 2p1/2 level peaks are around 1021.50 eV and 1044.68 eV respectively, which is due to the orbitals of divalent zinc ions (Zn2+) in ZnO lattice.23,24 In addition, note that the difference between Zn 2p3/2 and Zn 2p1/2 peak position was found 23.2 eV, which matches well to the characteristic value of ZnO.25 Further, the O 1s peak corresponding to the ZnO thin film is deconvoluted into two separate peaks centered at ~530.29 and ~531.73 eV, as shown in Figure 2b.23,25 The binding energy peak at 530.29 eV indicates the presence of O atoms in Zn-O bonds (OL). In fact, this peak intensity indicates the presence of stoichiometric of ZnO. Another peak at 531.73 eV is due to oxygen deficiency within the ZnO; indeed the intensity of this component may be due to oxygen vacancies (OV).25 Figure 2c presents the XPS spectra of In 3d, which depicts two separate peaks at 445.6 eV and 453.14 eV, which correspond to the trivalent indium ions (In3+) of 3d5/2 and 3d3/2, respectively.26,27 In addition, the separation between these two peaks are 7.5 eV, which can be attributed to the spin–orbit splitting. Further, the observed two broad O 1s peaks in the XPS spectra at about 529.9 eV and 531.6 eV originate from the oxygen bond of In–O–In and the presence of oxygen defects in In2O3 [Figure 2d].26,27 As reported, all of the used oxides have band gaps near the ultra-violet range (Eg~3.0 eV), therefore, it is expected that they must show good transmittance in the entire visible region (400–800 nm).20,28,29 To ensure transparency, the optical transmittance was measured. The transmittance is found more than 70% over the measured visible range (400–750 nm), see 5 ACS Paragon Plus Environment


Figure 2e. In addition, an excellent transmittance (~76%) corresponding to 550 nm in noticed at which maximum luminosity curve is noticed for human eyes. In addition, the transmittance decreases sharply around 400 nm, indicating the fundamental absorption of the oxides. The photograph of the full device is depicted in Figure 2f, which is transparent enough to preserves the colors underneath, fulfilling the two major criteria for see-through devices. The typical current-voltage (I-V) characteristics of In2O3/ZnO/FTO heterostructure collected for four repeatable cycles from 0 2 0 -2 0 V under dark are depicted in Figure 3a. A schematic diagram for electrical measurments is shown in the inset of Figure 3a. The device shows hysteresis in the forward to reverse positive voltage scans (e.g. 0 2 0 V) as well as upward shifts with the number of cycles. Under dark, the I-V exhibits a rectification characteristic with a rectification ratio of ≈33 at a bias of 2 V, indicating the formation of a type-II heterojunction For clarity, I-V characteristics are plotted in semi-log scale, which verifies that the device does not show any loop opening during the negative voltage scan range (0 -2 0 V), as presented in Figure 3b. Since the current across the device changes gradually, the loop opening in the present device is attributed to charge trapping/detrapping.30–32 To reveal the underlying governing dynamics across the device, the logarithmic scale I-V characteristics of the device are also plotted. From Figure 3c, the slope close to one for lower positive and negative biases (0 to ±0.5 V) confirms Ohmic-like conductance. The current scales as V3.2 for higher applied positive biases, indicating that in this voltage range the condition is dominated by trap-filled limited conduction mechanisms.33 From the first loop, the trap-filled limit voltage (VTH) shifts from 0.75 to 0.46 V after repeated four cycles, confirming that charge can be trapped across the device. It should be mentioned here that the slope for all the negative scan voltages remained close to linear, and no anomalous behavior was detected. Further, ln(I/V2) versus 1/V plot describes a direct tunneling (low bias) to Fowler–Nordheim (F-N) tunneling (high bias) transition with an inflection point [Figure 6 ACS Paragon Plus Environment

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3d].34,35 In fact, a transition from direct to F-N tunneling is noticed with a small barrier height and/or width,.33 It is worth to mention that the curves clearly show two distinct transport regimes for the positive applied biases, while showing only one regime corresponding to the negative biases. In addition, the voltage inflection points (Vt =1/V) for the device was 1.18 V for the first cycles, which shifted after each cycle, as marked by red arrow on Figure 3d. The above analysis provides an overview for the band alignment of the In2O3/ZnO interface. By applying the positive bias on the bottom ZnO electrode, electrons are trapped in the interface. Meanwhile, electrons are free under the negative bias. Details of charge trapping/detrapping are depicted in the last section. Further, this indicates that at positive biases the trapping sites inside the device are gradually filled. This charge trapping behavior of the device can be used to design an optoelectronic neuromorphic device. Inside a human brain, pre- and post-neurons are connected by a synapse, as depicted schematically in Figure 4a.36,37 In fact, the synapse is a connecting channel between pre- and post-neurons. The strength (e.g., amplitude) of communication between these two are defined as synaptic weight.6,37 Fundamentally, depending on the strength of a signal, the synaptic weight remained unchanged (which is called elasticity) or increase/decrease (called potentiation/depression).6,37,38 Indeed, a key element for learning, forgetting, and memory in the brain is the activity-dependent synaptic connectivity (e.g. synaptic plasticity).39 Our proposed two-terminal highly transparent In2O3/ZnO/FTO/glass device, shown in Figure 4b, is very similar to a synapse, and like a photo-diode, in Figure 4c.40 Figure 4d shows the temporal current response of the device measured at 0.6 V, after being illuminated with a single optical pulse (intensity: 2 mW cm-2, duration: 2 s) and applying 10 electric pulses of 1V (duration and width: 20 ms) for 30 s of measurement. One can see that with UV illumination (grey color area in Figure 4c), the current increases gradually and shifts to 6.5 µA in comparison to the dark (2.5 µA) level due to photogenerated electrons and holes. Once 7 ACS Paragon Plus Environment


the UV is off, the current is followed by a gradual decay (point X, Figure 4d), however, the current does not return to its dark state and remains at the 3.32 level, indicating a persistent photoconductivity. In the meanwhile, with 10 negative voltage pulses (amplitude: -1 V, width and separation: 20 ms), the current level can be reset to its initial value, which is indicated by point Y in Figure 4d. Herein, the current of the device and photonic stimuli are corresponding to synaptic weight and synaptic spikes, respectively. In this way, the observed behavior is similar to the synaptic signal inside the human brain, including memory storage.41 Fundamentally, the synaptic weight also decays gradually over time, and based on its lifetime, it is categorized as short-term plasticity (STP) and long-term plasticity (LTP). The time duration between STP and LTP are not precisely defined, however, STP and LTP are corresponding to a time frame from millisecond to second and seconds to years, respectively.42 To realize the plasticity behavior, different illuminating intensities from 0.4 to 4 mW cm-2 optical pulses were implemented. This was used to ascertain the change in the current signal from the device and the results are portrayed in Figure 4e. One can note that for a low intensity single pulse (0.4 mW cm-2, width: 100 ms) the current increased and returned to its initial state in a short time span of 10 s, which is very similar to the STP. On the other hand, when the UV intensity of the applied single pulse was increased, the current level shifted upward, and for the higher intensity (4 mW cm-2, width: 100 ms) it persisted to a higher level, confirming a transition from STP to LTP, as can see from horizental dotted blue lines in Figure 4e. Further, trains of identical optical pulses (intensity: 0.4 mW cm-2, width and separation: 50 ms) were applied to the device and its conductance was monitored continuously, and the results are presented in Figure 4f. The current in our artificial synaptic device increases, when a pair of successive optical simulated are applied. The paired pulse facilitation (PPF) ratio is defined by the ratio of A2 and A1, where A1 and A2 are the peak amplitudes of current from the first and second optical pulses, respectively.37,42,43 As 8 ACS Paragon Plus Environment

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illustrated in Figure 4g, the facilitation ratio decreases gradually when the pulse interval increases, and 1 s interval generates the value of PPF index close to 180%. In addition, the energy consumption per synaptic event is calculated by V×t×I, where V is the measuring voltage, t is the duration of optical pulse, I is the current across the device. The inset of Figure 4g depicted the calculated energy per synaptic event. Note that the energy consumption in our gate-free two-terminal device increases with increasing pulse duration and a minimum energy of ~0.2 nJ is consumed across it for the duration of 5 ms. Further, the Figure 4h demonstrated the photonic potentiation and electric habituation behavior of the device. The current across the device increased gradually when illuminated with consecutive photonic pulses with an intensity of 0.4 mW cm-2 (width and separation: 1 s), which in fact mimic the potentiation behaviour. On the other hand, the habituation behaviour, e.g. the conductance of the device decreased gradually, is observed with increasing number of -1V electric pulses (width and separation: 20 ms). In addition, it is worth to mention that two optical source configurations is needed to achieved the spike-timing-dependent plasticity (STDP), where these optical inputs will work as pre- and post-neurons.3 Since, the device is working with optical signal and thus, to avoid any interruption from other nearby light source, a proper packing is needed. Based on our experimental observations, we therefore propose a plausible operating mechanism for our photonic synapse, shown in Figure 5. Figure 5a shows the band alignment at the ZnO/In2O3 interface under thermal equilibrium. Since, ZnO has high electron affinity (4.35 eV) than In2O3 (3.5 eV)26,27, on contact significant electron transfer from In2O3 to ZnO can take place, resulting in band bending, as depicted in Figure 5a.44,45 The ZnO and In2O3 shows type-II band alignment, which leads to the charge trapping/detrapping in the positive and negative biases, respectively. For positive biases, the electrons transport from ZnO to In2O3 by F-N tunneling due to the presence of interfacial barrier height. On the other hand, for negative polarities, electron can transfer from In2O3 to ZnO via direct tunneling. This 9 ACS Paragon Plus Environment


interfcae also serves as the basis for the optically-mediated charge trapping and electricallyinduced charge releasing. As depicted, after UV illumination, electron-hole pairs are generated across the device and under the action of an applied electric field, the hole drifts towards the In2O3, while the photogenerated electrons move in the opposite direction, which effectively changes the current conduction level, as shown in Figure 5b. On the other hand, after the UV is switched off, electrons near the ZnO and holes around the In2O3 layer remain trapped [Figure 5c], which in turn reduces the effective barrier height, as observed from the cyclic dark I-V characteristics in Figure 3c and d. Thus, the current will not return to its initial state after removing the UV illumination (Figure 4d and e). If the intensity of the illumination UV is high enough, it generates sufficient trapped charge carriers which will remain highly stable and maintain the memory state for a long time, very similar to LTP. These trapped charges can be erased by a negative bias, which inject the hole from In2O3 to ZnO, as shown in Figure 5d. In fact, the holes in In2O3 are driven to the interface and neutralized by electrons, resulting in a gradual decay of the device conductance with increasing number of electric pulses. This is known as habituation behaviour of the synaptic strength. Overall, the band alingment between In2O3/ZnO provides us an opporchunity to design photonic triggred artificial synapse. These findings serve as a novel architecture for highly transparent emulated synaptic functionality for neuromorphic computing.

CONCLUSIONS In summary, based on persistent photoconductivity and charge trapping/detrapping, we induced and emulated photonic synaptic functionality including STP, LTP, and PPF using a highly transparent all-oxides-based device. The device structure was prepared by the sequential growth of ZnO and In2O3 thin films on a fluorine doped tin oxide (FTO)-coated glass substrate. By employing a type-II heterostructure between the ZnO and In2O3, photogenerated charge carriers could be trapped, and released electrically. The device shows most 10 ACS Paragon Plus Environment

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of the synaptic functionality, including short- and long-term plasticity along with paired-pulse facilitation. Our work provides a novel architecture for highly transparent integrated photonic neuromorphic computing, which are the building blocks for artificial computing.

AUTHOR INFORMATION Corresponding Author E-mail: *J. Kim ([email protected])

Notes The authors declare no conflict of interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support of the Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF-2018R1D1A1B07049871 and NRF- 2018R1D1A1B07045336).

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Figure 1. Device architecture and morphologies: (a) Planar-view scanning electron microscopy (SEM) image of the In2O3 (b) Cross-sectional SEM image of the In2O3/ZnO/FTO device. (c) Cross-sectional EDS elemental line profiles of indium (In), zinc (Zn), oxygen (O) and tin (Sn) across the device, (d) EDS spectra of the device. (e) In- EDS map, (f) Zn-EDS map, (g) O-EDS map. Scale bars on (f) and (g) are same as of (e)



Figure 2. (a) and (b) depict the Zn 2p and O 1s XPS spectra, respectively obtained from the ZnO thin film. (c) and (d) present the In 3d and O 1s XPS spectra, respectively obtained from the In2O3 thin film. (e) Optical transmittance of the In2O3/ZnO-based devices. (f) Original photograph of the device.


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Figure 3. (a) I-V characteristics performed for four cycles; the semi-logarithmic of the same is depicted in (b), taken under dark conditions. (c) Logarithmic scale I-V characteristics for four different cycles. (d) ln(I/V2) versus 1/V plot. The curves show a transition from direct tunneling to Fowler–Nordheim (F-N) tunneling with a voltage inflection point.



Figure 4. (a) and (b) Schematics of a synapse structure within the human brain, and our twoterminal visible light transparent device, respectively. (c) Sketch of a photodetector, which is very similar to the artificial synapse. (d) Top panel shows the transient responses of the device after illumination with a single optical pulse (intensity: 0.4 mW cm-2, duration: 2 s, marked by the gray colored area), showing a gradual decay after the UV is switched off (point X). The bottom panel depicts the applied measuring voltage (0.6 V) and pulses of -1V after 30 S; the inset shows the shape of the applied electric pulses. (e) Transient current-time characteristics for four optical pulses with different intensities from 0.4 to 4 mW cm-2, showing the dynamic behavior of current with the increasing incident light intensity of a single pulse. (f) Change in the current under the action of applied sequential optical pulses. The inset shows the change in current amplitude for two sequential optical pulses. (g) Change in the PPF index with different optical frequencies. The inset shows the energy consumed per synaptic event as a function of optical pulse duration. (h) Gradual current change across the synaptic devices after applying a train of photonic pulses (intensity: 0.4 mW cm-2, duration and separation: 1 s) and negative electrical pulses (-1 V, duration and separation: 20 ms). Top and bottom insets show the shape of the applied pulses.


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Figure 5. Schematic illustrations of the working mechanism of the photonic synapse: (a) Band alignment at the ZnO/In2O3 interface under thermal equilibrium (b) Photo-induced electron-hole pair generation (G) and their respective trapping on the ZnO and In2O3 sides. (c) recombination (R) of charge carriers under dark, and the remaining trapped charges. (d) Electric filed stimulated reset of the device.



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All-Oxide-Based Highly Transparent Photonic Synapse for

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