PDF An Artificial Optoelectronic Synapse Based on a Photoelectric Memcapacitor

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An Artificial Optoelectronic Synapse Based on a Photoelectric Memcapacitor

Lei Zhao, Zhen Fan, Shengliang Cheng, Lanqing Hong, Yongqiang Li, Guo Tian,* Deyang Chen, Zhipeng Hou, Minghui Qin, Min Zeng, Xubing Lu, Guofu Zhou, Xingsen Gao, and Jun-Ming Liu

DOI: 10.1002/aelm.201900858

away quickly when the light stimuli are removed.^[3–5] In other words, traditional photodetectors can detect the images like a retina, but they lack the memory function owned by the visual cortex (see schematic illustration of human visual system in Figure 1a). To realize both detection and memory functions so as to better imi- tate the human visual system, researchers have attempted to integrate the photodetectors with the nonvolatile memory devices.^[6–9] For example, a bioinspired visual system comprising an In₂O₃ nano- wire photodetector connected in series with an Al₂O₃ memristor was fabricated recently, which could capture a butterfly- shaped image and store it for more than 1 week.^[9] In such integrated devices, however, the units responsible for detection, processing, and storage of optical information are physically separated, resulting in high power consumption for data transfer between different units (just as the Von Neumann bottleneck^[10,11]).

A simple yet effective humanoid optoelectronic device emerging recently is the artificial optoelectronic synapse based on the photoelectric memristor, which can co-locate the detection and memory functions in a single unit. As the name sug- gests, a photoelectric memristor can continuously change its resistance upon light stimuli, resembling the physiological The rapid development of artificial intelligence technology has led to the

urge for artificial optoelectronic synapses with visual perception and memory capabilities. A new type of artificial optoelectronic synapse, namely a photo

electric memcapacitor, is proposed and demonstrated. This photoelectric memcapacitor, with a planar Au/La1.875Sr0.125NiO4/Au metal–semiconductor–

metal structure, displays a complementary optical and electrical modulation of capacitance, which can be attributed to the charge trapping/detrapping

induced Schottky barrier variation. It further exhibits versatile synaptic functions, such as photonic potentiation/electric depression, pairedpulse facilitation, short/longterm memory, and “learningexperience” behavior.

Moreover, the photoplasticity of the memcapacitor can be modulated by varying the frequency of applied AC voltage, thus enabling selfadaptive optical signal detection and mimicry of interestmodulated human visual memory. Therefore, it represents a new paradigm for artificial optoelectronic synapses and opens up opportunities for developing lowpower humanoid optoelectronic devices.

L. Zhao, Prof. Z. Fan, S. Cheng, Dr. G. Tian, Dr. D. Chen, Dr. Z. Hou, Prof. M. Qin, Prof. M. Zeng, Prof. X. Lu, Prof. X. Gao, Prof. J.-M. Liu Institute for Advanced Materials

South China Academy of Advanced Optoelectronics South China Normal University

Guangzhou 510006, China E-mail: fanzhen@m.scnu.edu.cn L. Zhao, Prof. Z. Fan, Prof. G. Zhou

Guangdong Provincial Key Laboratory of Optical Information Materials and Technology

South China Academy of Advanced Optoelectronics South China Normal University

Guangzhou 510006, China

Most of the information assimilated by human beings from the world is through visual perception.^[1] Mimicry of human visual system is thus a critical step toward artificial intelligence. In this context, humanoid optoelectronic devices capable of detecting and memorizing images are very appealing, which may find wide applications in smartphones, drones, robots, and visual prostheses.^[2] While traditional photodetectors can convert light stimuli into electrical signals, the optical information will fade

Dr. L. Hong

Department of Industrial Systems Engineering and Management National University of Singapore

Singapore 117576, Singapore Y. Li, Prof. J.-M. Liu

Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures

Nanjing University Nanjing 210093, China Prof. G. Zhou

National Center for International Research on Green Optoelectronics South China Normal University

Guangzhou 510006, China The ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/aelm.201900858.

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behavior of a biological synapse in the human visual system (Figure 1a). So far, a variety of photoelectric memristors based on Si nanocrystals,^[12] ZnO_1−x/AlO_y,^[13] In₂O₃/ZnO,^[14] CeO_2−x/ AlO_y,^[15,16] and many other materials,^[17,18] showing versatile synaptic functions, have been developed successfully. Particularly, Gao et al.^[19] designed and fabricated a photoelectric memristor based on an In₂O₃-SnO₂/Nb:SrTiO₃ (ITO/Nb:STO) Schottky junction, which exhibited photoresponses over the entire visible spectrum with neuromorphic characteristics, including paired-pulse facilitation (PPF), short-/long-term memory (STM/

LTM), “learning-experience” behavior, and voltage-modulated photoplasticity. Although these neuromorphic characteristics can endow photoelectric memristors with capabilities of self- adaptive detecting, in situ processing, and memorizing optical information, there are still some drawbacks to the photoelectric memristors. First, the photoelectric memristors rely on the persistent photoconductivity effect,^[20–25] so the DC currents always exist during the device operation. This inevitably leads to excessive power dissipation. Additionally, most photoelectric memristors are composed of vertically stacked heterojunctions with limited scalability because the electrode area needs to be small (typically in the sub-mm² range) in order to reduce the leakage current. This limits the area for collecting the photons, which in turn affects the photo–dark current ratio (i.e., sensitivity).

To circumvent the above drawbacks of the photoelectric memristors, we propose a new type of artificial optoelectronic synapses, namely, photoelectric memcapacitors, whose

capacitance can be continuously altered with light stimuli.

In principle, the photoelectric memcapacitors can reproduce all the synaptic functions of the memristor counterparts, since the capacitance can replace the conductance as the synaptic weight (Figure S1, Supporting Information). Besides, the photo electric memcapacitors require only small AC volt- ages for the capacitance detection. Negligible DC currents are thus formed, lowering the power dissipation. In addition, the photo electric memcapacitors can be made with a planar metal/semiconductor/metal (MSM) structure, which has many advantages, including simple device structure, large photosensing area, ultralow leakage current, and high sensitivity.^[26,27]

Therefore, the photoelectric memristors emerge as a suitable candidate for low-power, highly scalable, and highly sensitive artificial optoelectronic synapses. Note that the proposed photoelectric memcapacitors are conceptually new, while the electrical memcapacitors whose capacitance is tuned by electrical stimuli have already been studied recently.^[28–30]

The functioning of photoelectric memcapacitors relies on the persistent photocapacitance (PPC) effect. A significant PPC effect can be achieved if the charge carriers in either the bulk or interface region of an MSM structure are modulat- able by light stimuli via slow charge trapping/detrapping processes.^[31–35] With this consideration, we design a planar Au/

La_1.875Sr_0.125NiO₄ (LSNO)/Au MSM structure (Figure 1b) for the demonstration of photoelectric memcapacitors. This particular structure is chosen because the LSNO bulk contains abundant and stripe-ordered holes,^[36–39] and a Schottky barrier can be Figure 1. a) Schematic illustration of a human visual system, in which the eye and the synapse are highlighted. b) Schematic drawing of the device structure of an Au/LSNO/Au photoelectric memcapacitor. c) Schematic band diagrams of the Au/LSNO Schottky barrier in the initial dark state (left), under illumination (middle), and after illumination (right).

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formed at the interface between LSNO and Au.^[40,41] Therefore, the Au/LSNO/Au structure is expected to yield large dielectric response and significant PPC effect. Then, we experimentally demonstrate that the Au/LSNO/Au structure behaves as a photo electric memcapacitor with several basic synaptic functions, including photonic potentiation/electric depression, PPF, STM/LTM, and “learning-experience” behavior. Moreover, the

photoplasticity can be controlled by a single knob, that is, the AC voltage frequency, analog to the function of iris and pupil in a human eye. This enables the self-adaptive optical signal detection under different illumination conditions and the mimicry of interest-modulated human visual memory.

Figure 2a depicts the atomic force microscopy (AFM) topography image of an LSNO epitaxial film grown on a LaAlO₃ Figure 2. a) AFM topography image of the LSNO film grown on the LAO substrate. b) XRR profile of an ≈32 nm LSNO film, which is three times thinner than those used for all other measurements. c) XRD θ-2θ scan pattern and d) RSM around the LAO (103) reflection for the LSNO/LAO heterostructure.

e) Capacitance–frequency characteristics of an Au/LSNO/Au planar capacitor measured in dark, and f) corresponding impedance data plotted in the Nyquist plot. In f, the boundary between two semicircular arcs (@ ≈10⁵ Hz) is specifically indicated. Inset in (f) shows the equivalent circuit model used for fitting, where the two parallel RC elements connected in series represent the interface and bulk responses, respectively.

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(LAO) (001) substrate. The LSNO film surface is quite flat with a small roughness of ≈0.3 nm. The surface and interface qualities can also be studied by using the X-ray reflectivity (XRR) technique. To achieve higher signal-to-noise ratios in XRR, an LSNO film, which was three times thinner than those used for all other measurements, was used. The XRR profile, as shown in Figure 2b, presents clear periodic interference fringes, suggesting good qualities of both LSNO surface and LSNO/LAO interface. The film thickness can be calculated to be ≈32 nm, based on which one can deduce that the LSNO films used for all other measurements are ≈96 nm thick. Figure 2c shows the X-ray diffraction (XRD) θ-2θ scan pattern of the LSNO/LAO heterostructure. Only (00l) diffraction peaks from LSNO and LAO are observed, while no peaks from any secondary phases exist, indicating the phase purity. Reciprocal space mapping (RSM) taken around the (103) reflection of LAO is shown in Figure 2d. Besides the LAO (103) diffraction spot located at L = 3, a distinct diffraction spot located at L = ≈3.258 is also observed, which corresponds to the (1011) reflection of LSNO.

This observation confirms the following epitaxial relationship:

(001)[100]_LSNO||(001)[100]LAO. Additionally, the LSNO (1011) diffraction spot has a slightly smaller H-coordinate compared with the LAO (103) diffraction spot, suggesting that the strain in the LSNO film is partially relaxed. Based on the XRD and RSM results, the phase in the LSNO epitaxial film is determined to a tetragonal phase with lattice parameters: a = ≈3.803 Å and c = ≈12.796 Å, which is in good agreement with the reported values.^[41]

With the high-quality LSNO epitaxial films, the Au/LSNO (≈96 nm)/Au planar capacitors were constructed and their dielectric behavior was first studied. Figure 2e shows the typical capacitance–frequency (C-f_AC) characteristics of an Au/LSNO/

Au capacitor in the frequency range of 10² to 10⁶ Hz. The capacitance at the frequency of 10⁶ Hz is ≈20 pF, and it starts to increase as the frequency decreases to ≈10⁵ Hz and further reaches a plateau of ≈750 pF at 10⁴ Hz. By contrast, the Au/

LAO/Au capacitor, where the LSNO layer is removed, shows almost zero capacitance, confirming that the LAO substrate has negligible contribution to the capacitance of the Au/LSNO/Au capacitor (Figure S2, Supporting Information).

While the Au/LSNO/Au capacitor exhibits large dielectric response, whether it can also exhibit a photocapacitance effect remains a question. Prior to investigating it, one needs to understand the origin for the dielectric response so as to know the feasibility to realize the photocapacitance effect. It is known that the dipoles in the bulk can follow an AC field with high frequency, while the interfacial polarization (formed by space charges) can only respond to an AC field with low frequency.

Therefore, in Figure 2e, the plateaus in the frequency ranges of 10²–10⁴ and 10⁵–10⁶ Hz may correspond to the interface and bulk responses, respectively. The increase of capacitance below ≈10⁵ Hz (i.e., the transition from the high-frequency plateau to the low-frequency one) is due to the gradual activation of interfacial polarization as the frequency decreases. To further distinguish between the interface and bulk responses, the dielectric data were plotted in the Nyquist plot, that is, imagi- nary component of impedance (Z′′) versus real component of impedance (Z′), as shown in Figure 2f. There are two semicircular arcs in the Nyquist plot: the low-frequency (<10⁵ Hz) arc

corresponding to the interface response and the high-frequency one (>10⁵ Hz) representing the bulk response, which are con- firmed by the equivalent circuit fitting. Therefore, the large capacitance at frequencies below ≈10⁵ Hz is mainly contributed by the interfacial polarization.

The interfacial polarization is naturally associated with the Au/LSNO Schottky barrier(s). Note that LSNO is a p-type semiconductor with an electron affinity of ≈3.5 eV and a bandgap of ≈2.0 eV,^[42,43] and its Fermi level is ≈0.1 eV higher than its valence band edge.^[43] The work function of LSNO (≈5.4 eV) is thus larger than that of Au (≈5.1 eV^[44,45]) (Figure S3, Sup- porting Information); therefore, a p-type Schottky barrier will be formed at the Au/LSNO interface. To verify the dielectric response from the Schottky barrier(s); the capacitance–voltage (C–V) measurement was conducted for the Au/LSNO/Au planar capacitor. As shown in Figure S4, Supporting Informa- tion, the capacitance (@ 60 kHz) decreases with increasing voltage magnitude, which is typical behavior of a reverse-biased Schottky barrier. Additionally, the decrease of capacitance with voltage magnitude is observed at both positive and negative voltage polarities, suggesting that two Schottky barriers in back- to-back configuration are formed at the two Au/LSNO interfaces. These results confirm that the large dielectric response (at low frequencies) mainly comes from the interfacial Schottky barriers, and the capacitance of the whole Au/LSNO/Au capacitor (C_total) may be expressed as

C C A

W

≈ =ε ε

2 2

total S 0 r

D

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where C_S is the Schottky barrier capacitance, ε0 is the vacuum permittivity, εr is the relative dielectric constant of LSNO, A is the effective electrode area, and W_D is the depletion width. If WD can be modulated by certain external stimuli, for example, light stimuli, C_total will undergo a change. Therefore, it is feasible for the Au/LSNO/Au capacitor to realize the photocapacitance effect.

With the above understandings, the photocapacitance effect in the Au/LSNO/Au capacitor was then experimentally inves- tigated. Figure 3a shows the temporal evolution of capacitance (@ 60 kHz) for the Au/LSNO/Au capacitor during and after illumination (light wavelength: 365 nm; intensity: 220 mW cm⁻²).

Unless otherwise specified, the AC voltage frequency for the photocapacitance measurement is fixed at 60 kHz hereafter.

The capacitance gradually increases from ≈300 pF in the initial dark state to ≈393 pF upon illumination for 1000 s (a quanti- tative description of the capacitance increase under illumination is provided in Figure S5, Supporting Information). After the light is turned OFF, the capacitance decays very slowly, indicating a PPC behavior. Figure 3b further displays the mag- nified capacitance decay curve, which can be well fitted to the stretched-exponential function.

C C= 0×exp[ ( / ) ]−t τ ^β +Cb (2) where C₀ is the pre-exponential factor, τ is the relaxation time constant, β is the stretching exponent, and Cb is the background capacitance. Because the stretched-exponential function commonly describes charge trapping mediated

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phenomena,^[46,47] the fitting result therefore implies that the PPC behavior is caused by the charge trapping/deptrapping with slow dynamics.^[31–35] Specifically, when the illumination is ON, the initially trapped holes may be detrapped, leaving behind some empty negatively charged traps (e.g., Sr_La′; see evi- dence in Figure S6, Supporting Information) that can provide an additional potential and consequently reduce the Schottky barrier height and depletion width (middle panel in Figure 1c).

On the contrary, after switching OFF the illumination, the free holes may be recaptured by the traps, leading to enhancements

in Schottky barrier height and depletion width (right panel in Figure 1c). According to Equation (1), the capacitance will therefore increase (decrease) upon (after) illumination, and the increasing (decreasing) rate depends on the dynamics of charge detrapping (trapping).

To further affirm the above mechanism, the effects of light intensity and wavelength on the photocapacitance were investi- gated. Figure 3c,d shows the capacitance increase (ΔC, obtained by using the initial dark-state capacitance as the reference) with various light intensities and colors, respectively, for the Figure 3. a) Transient capacitance responses (@ 60 kHz) of the Au/LSNO/Au memcapacitor during and after illumination (light wavelength: 365 nm;

intensity: 220 mW cm⁻²). b) Experimental capacitance decay curve along with a fitting curve, where the moment t = 0 corresponds to t = 1000 s in (a).

Capacitance changes of the memcapacitor c) under the 365 nm UV lights with different intensities and d) under the lights with different colors but the same intensity (220 mW cm⁻²). f) Suppression of capacitance induced by applying electrical pulses (pulse amplitude: 40 V; width: 10 s).

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Au/LSNO/Au capacitor. Apparently, a higher light intensity and a shorter light wavelength can produce a larger ΔC. This can be well explained by the fact that incident photons with larger amounts or higher energy can cause more holes to be detrapped upon illumination.^[48,49]

While the light stimuli can modulate the detrapping process, an alternative type of stimuli is needed to modulate the trapping process so as to manipulate the capacitance decay after switching OFF the illumination. Figure 3e shows the ΔC evolution of the Au/LSNO/Au capacitor, which was illuminated for 1000 s beforehand, under the pulsed electrical stimuli (pulse amplitude: 40 V, width: 10 s) in dark. Applying an electrical pulse significantly reduces the capacitance, and the initial dark- state capacitance can be recovered after applying about five electrical pulses. The comparison between Figures 3b and 3e reveals that the capacitance decay under electrical stimuli is much faster than that in the absence of electrical stimuli. The large voltage applied to the Au/LSNO/Au capacitor can cause significant charge injection and trapping,^[50,51] thus accelerating the capacitance decay (see detailed illustration in Figure S7, Supporting Information). Figure 3e also indicates that the optically written information can be completely erased via an electrical manner, allowing us to reset the capacitance to the initial dark-state value if needed.

The complementary optical and electrical modulation of capacitance, based on the charge trapping/detrapping-induced Schottky barrier variation, enables our Au/LSNO/Au capacitor to become a photoelectric memcapacitor which can realize various synaptic functions. The capacitance of the memcapacitor is considered as the synaptic weight, while the optical and electrical stimuli are viewed as the synaptic spikes. In this way, the observed optically induced capacitance increase (Figure 3a) and electrically induced capacitance reduction (Figure 3e) correspond to the synaptic potentiation and depression, respectively.

The Au/LSNO/Au memcapacitor can further emulate the PPF, a key short-term synaptic plasticity for decoding provi- sional information in visual and auditory signals.^[52] Specifi- cally, the PPF manifests itself as a phenomenon that the second pulse triggers a larger postsynaptic current than the first one when two successive pulses are applied to the presynaptic neuron. Figure 4a shows the capacitance variation in response to two consecutive optical pulses (light wavelength: 365 nm, intensity: 220 mW cm⁻², and pulse width: 5 s) at a time interval of 1 s. The ΔC evoked by the second optical pulse is larger than that evoked by the first one, which is a PPF-like behavior if the light-induced ΔC (i.e., photocapacitance) in a memcapacitor is considered as the postsynaptic current in a biological synapse. The ratio between the ΔC value immediately after the second pulse (A₂) and that after the first one (A₁) is defined as the PPF index. As shown in Figure 4b, the PPF index gradually decreases as the interval between two pulses (Δt) increases.

Moreover, the dependence of the PPF index on Δt follows a double-exponential function.

PPF index 1= +C1×exp(−∆t/ )τ1 +C2×exp(−∆t/ )τ2 (3) where the first and second exponential terms describe the rapid and slow decay processes, respectively, and C_i and τi (i = 1, 2) are the initial facilitation magnitude and characteristic relaxation

time, respectively. By fitting, τ1 = 1 s and τ2 = 10 s are obtained.

The τ2 value is one order of magnitude larger than the τ1 value, consistent with the rule found in biological synapses.^[53]

Besides the PPF, another two manifestations of the synaptic plasticity, that is, STM and LTM, can also be realized by the Au/

LSNO/Au memcapacitor. STM refers to the temporal holding of incoming information in the hippocampus which persists for only a few seconds.^[54,55] After repeated training and rehearsal (i.e., consolidation), the information is transferred to and stored in the cerebral cortex, which can last for hours and even years (i.e., STM transforms into LTM).^[56,57] To emulate STM, LTM, and the transition from STM to LTM, the Au/LSNO/Au memcapacitor was stimulated by multiple optical pulses with various numbers and frequencies. As shown in Figure 4c, after applying a single pulse (n_p = 1), the evoked ΔC drops from

≈20 pF to ≈4 pF in a short time period (≈10 s), indicating the STM behavior. As the pulse number increases, the maximum ΔC value increases, and the stable ΔC value retained after the stimulation also increases. At n_p = 50, the ΔC value after the stimulation can remain in a relative high level of ≈35 pF for at least ≈200 s, attesting to the LTM behavior. Similar STM-to-LTM transition behavior is observed when the frequency of applied optical pulses (f_p) is increased (Figure 4d).

With the STM and LTM functions, the mimicry of “learning- experience” behavior becomes feasible, as illustrated in Figure 4e. The first stimulation of the memcapacitor by an optical pulse train (50 pulses) leads to a gradual increase in ΔC, which is called a learning process. After the first stimulation, ΔC decreases to an intermediate value of ≈40 pF within ≈20 seconds, implying that partial information has been forgotten.

Recovering ΔC to its maximum value achieved by the first stimulation needs only 5 pulses, much fewer than the 37 pulses needed for producing the identical increment of ΔC in the first stimulation. This appears like the phenomenon that relearning the forgotten information is much easier than learning it for the first time.^[58] In addition, the decay of ΔC after the second stimulation is smaller than that after the first stimulation, akin to the strengthening of memory stability by relearning.^[55,59]

Therefore, the “learning-experience” behavior is demonstrated in our Au/LSNO/Au memcapacitor.

Given the above versatile synaptic functions and good photo- responsivity in wide light wavelength and intensity ranges, we propose that the Au/LSNO/Au memcapacitors can be used to recognize and memorize the images. Figure 5a,b schematically shows a prototype system consisting of nine memcapacitors for the detection and memorizing of images (3 × 3 pixels) encoded by light intensity and color, respectively (note: the nine memcapacitors were arranged in an array format but they were indeed not interconnected with each other by the electrical wires). Each of the memcapacitor was stimulated by an optical pulse train with pre-specified pixel color or intensity, and its capacitance change ΔC was measured experimentally. The ΔC values of the nine memcapacitors were plotted to generate an output image.

As shown in Figure 5a, an input image with different pixel intensities produces a distinct ΔC contrast among the nine memcapacitors and the contrast can persist for at least 100 s, indicating that the image encoded by light intensity can be rec- ognized and memorized for a relatively long time. In addition, these memcapacitors also work for the input image encoded

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by light color, as illustrated by Figure 5b. Therefore, the Au/

LSNO/Au memcapacitor seems to be a viable building block for constructing an artificial visual system.

Moreover, the photoplasticity of the Au/LSNO/Au memcapacitor is found to be controlled by the AC voltage frequency (f_AC) used for the photocapacitance measurement. Specifically, when the Au/LSNO/Au memcapacitor is subjected to a fixed

light stimulation, increasing (decreasing) f_AC will give rise to a smaller (larger) output ΔC, as presented in Figure 6a. This may be because as f_AC decreases, an increasing number of deep traps in the depletion region become activated (see Figure 2e)^[60] and thus contribute to the photocapacitance effect. It is also note- worthy that by adopting appropriate f_AC values, the ΔC values induced by different illumination conditions can be modulated Figure 4. a) Capacitance changes evoked by two successive optical pulses. b) PPF index as a function of the interval between two optical pulses. Transi- tion from STM to LTM realized by varying c) the number (np) and d) the frequency (fp) of applied optical pulses. e) Emulation of “learning-experience”

behavior. The capacitance increasing with number of optical pulses corresponds to the learning or relearning process, while the capacitance decay after the light stimulation represents the forgetting process. The optical pulses used in (a)–(e) have the following parameters: light wavelength: 365 nm; intensity: 220 mW cm⁻²; pulse width: 5 s for (a) while 10 s for (c), (d), and (e); interval: 10 s for (c) and (e) while varied values for (a) and (d).

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to be almost the same, as demonstrated in Figure 6b. This is reminiscent of the iris in human eyes, which can adapt to the light environment variation via controlling the pupil size.^[2]

Therefore, the Au/LSNO/Au memcapacitor exhibits the function of self-adaptive optical signal detection.

The photoplasticity tunable by f_AC also enables the mimicry of interest-modulated visual memory. It is known that the real visual memory is strongly interest-dependent; namely, if a person has a high (low) interest in an item when seeing it, he will obtain a strong (weak) memory for it. To emulate this behavior, we assumed that a person’s interest levels in the letters “S”, “C”, “N”, and “U” were from high to low, and assigned these letters with four f_AC values 20, 40, 80, and 100 kHz, respectively. Then, some of the memcapacitors were stimulated optically, constituting a pattern of the desired letter. For the different letters, all the parameters of light stimulation were the same, representing that these letters were viewed in the same light environment for the same time. The ΔC values generated in the memcapacitors were measured experimentally with the f_AC values assigned for different letters. As shown in Figure 6c, the recorded image intensities, that is, the measured ΔC values, of the letters “S”, “C”, “N”, and “U” decrease from high to low, indicating strong to weak memories for the four letters. Hence, the mimicry of interest-modulated visual memory has been demonstrated.

To highlight the merits of our Au/LSNO/Au photoelectric memcapacitor, its structure and performance are compared in detail with those of previous photoelectric memristive devices, as summarized in Table 1. Apparently, our photo electric memcapacitor has a simple planar structure as well as wide photo- responsive wavelength and intensity ranges, and it can also exhibit comprehensive synaptic functions. In addition, our

memcapacitor is shown to have good endurance and anti- interference performance (Figures S8–S10, Supporting Infor- mation). More importantly, the interest-modulated visual memory in previous photoelectric memristors was realized by the modulation of photoplasticity by voltage stressing,^[19] which would, however, cause excess leakage current and thus a limited modulation range of photoplasticity. By contrast, no such limitation exists in our photoelectric memcapacitor because the modulation of photoplasticity relies on the adjustment of f_AC. Furthermore, as described in the Introduction section, the photoelectric memcapacitor has some additional advantages over the memristor counterpart, such as less energy consumption (in principle), larger photosensing area, and potentially higher sensitivity. For the present Au/LSNO/Au memcapacitor, however, the actual energy consumption is relatively high, which is mainly contributed from the optical writing process.

The energy consumption for optical writing (dE_ow) can be calculated by

E = ×S I × t

d ow light d (4)

where S is the photosensing area of the device, I_light is the light intensity, and dt is the duration of the optical pulse. By sub- stituting I_light = 20–250 mW cm⁻² together with S = 0.09 mm² and dt = 5–10 s into Equation (4), dEow in the memcapacitor is calculated as 9–225 × 10⁴ nJ, which is higher than those of most memristor counterparts listed in Table 1. Further investigation is therefore warranted on reducing dE_ow in the memcapacitor.

Some viable approaches are suggested as follows, from the per- spectives of both material and device. Because the operation mechanism of the Au/LSNO/Au photoelectric memcapacitor is based on the charge trapping/detrapping in the Schottky Figure 5. Detecting and memorizing the images (3 × 3 pixels) encoded by a) light intensity and b) light color, using nine Au/LSNO/Au memcapacitors.

For the light stimulation, 50 optical pulses with a width of 10 s and an interval of 10 s are applied. The light colors and intensities (unit: mW cm⁻²) are indicated in (a) and (b). The colors of UV, blue, green, and red correspond to the wavelengths of 365, 450, 520, and 730 nm, respectively.

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barrier, engineering the Schottky barrier by optimizing the doping concentration of Sr²⁺ in LSNO and replacing Au with other metals with lower work functions may lead to stronger and faster photocapacitance response, thereby reducing the required I_light and dt. In addition, the device can be scaled down to directly reduce S, and simultaneously all relevant device geo- metrical parameters can be optimized to enhance the photocapacitance response.

In summary, a photoelectric memcapacitor, based on the light-induced Schottky barrier variation via charge trapping/detrapping, has been proposed and demonstrated with a simple Au/LSNO/Au planar capacitor. The Au/LSNO/Au memcapacitor shows photocapacitance responses to both UV and visible light in a wide intensity range. Besides, the memcapacitor exhibits a series of synaptic functions, including photonic potentiation/electric depression, PPF, STM/LTM, and

“learning-experience” behavior, thus co-locating the detection,

processing, and memory functions for optical signals. More interestingly, the photoplasticity of the memcapacitor is tunable by varying f_AC, which can be used to mimic the self-adaptive optical signal detection and interest-modulated human visual memory. The photoelectric memcapacitor, as demonstrated here, may serve as a promising building block for artificial elec- tronic eyes and photonic neuromorphic computing systems.

Experimental Section

Device Fabrication: A ≈96 nm thick La1.875Sr0.125NiO4 (LSNO) thin film was grown on the single-crystalline LaAlO₃ (001) substrate, using the pulsed laser deposition (PLD) with a KrF excimer laser (λ = 248 nm) operated at a laser energy of ≈1.2 J cm⁻² and a repetition rate of 5 Hz.

The growth temperature for the LSNO film was 903 K and the oxygen pressure was kept at 10 Pa. Then, the LSNO film was cooled to room temperature at 10 K min⁻¹ under 1 atm oxygen pressure. After that, Figure 6. a) Capacitance changes evoked by the same light stimulation but measured with different AC voltage frequencies (fAC). b) Modulation of different capacitance changes evoked by different light stimulations (different light wavelengths or different light intensities) into almost the same value by using appropriate f_AC values. The f_AC values are indicated by the numbers above the data bars. c) Mimicry of interest-modulated visual memory, where the interest levels from high to low correspond to the fAC values from low to high. The optical pulses used in (a)–(c) have the following parameters:

light wavelength: 365 nm for (a) and (c) while varied values for (b); intensity: 220 mW cm⁻² for (a) and (c), while varied values for (b); pulse width:

10 s; interval: 10 s; number: 50.

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planar electrodes consisting of two Au bars were deposited on the film using sputtering through a shadow mask. The distance between the two Au bars was ≈125 µm, and each bar was ≈720 µm long, ≈130 µm wide, and ≈10 nm thick.

Structural and Morphological Characterizations: The crystalline structure, epitaxial relationship, and thickness of the LSNO film were examined by using XRD, RSM, and XRR, respectively (X’Pert PRO, PANalytical, The Netherlands). The film surface morphology was characterized using AFM (Cypher, Asylum Research, UK).

Photocapacitance Measurement: The capacitances in the frequency range of 100 Hz to 1 MHz were measured using an inductance–

capacitance–resistance (LCR) meter (Agilent E4980A) with an AC voltage of 0.1 V. To illuminate the sample, light-emitting diodes with different wavelengths and adjustable light intensities were used.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

The authors would like to thank the National Key Research Program of China (No. 2016YFA0201002), the State Key Program for Basic Researches of China (No. 2015CB921202), the National Natural Science Foundation of China (Nos. 51602110, 11674108, 51431006, and 51561135014), the Science and Technology Program of Guangzhou (No. 2019050001), the Science and Technology Project of Guangdong Province (Nos. 2016B090918083 and 2017B030301007), the Natural Science Foundation of Guangdong Province (No. 2016A030308019), and the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007). X.G., X.L., and Z.F.

acknowledge the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2014, 2016, and 2018, respectively.

Conflict of Interest

The authors declare no conflict of interest.

Keywords

optoelectronic synapses, persistent photocapacitance, photoelectric memcapacitors, Schottky barrier, visual memory

Received: August 9, 2019 Revised: November 8, 2019 Published online: December 15, 2019

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Table 1. Comparison of device structure and performance between the photoelectric memcapacitor in this work and the memristor counterparts in previous works.

Device structure Light source Light intensity

[mW cm⁻²]

PPF STM-to-LTM transition Learning-experience behavior

Modulation of photoplasticity

IZO/IGZO/IZO^[61] UV–vis 0.6 – √ – –

Au/In2O3/ZnO/FTO^[14] UV 0.4-4 √ √ – –

W/MoS2/SiO2/p-Si^[62] UV 0.11 √ √ – –

Au/TiO2/Au^[63] Violet 0.5 – √ – –

ITO/MWCNTs/CdS^[64] Blue – – √ – –

Pt/HfO2/p-Si^[65] Blue 0.01-5 – √ – –

ITO/Si-NC/Al^[12] UV–NIR 18 √ √ – –

ITO/ZnO/Si-NCs/CBP/MoO3/Au^[66] UV – √ √ – –

Al/ZnO/PDR1A/ITO^[67] Green 180 – √ – –

ITO/ZnO1−x/AlOy/Al^[12] UV 0.072 √ √ – –

ITO/Nb:SrTiO3[19] Vis 10-30 √ √ √ √

Au/LSNO/Au (this work) UV–vis 20-250 √ √ √ √

(11)

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