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Artificial visual systems enabled by quasi–two-dimensional electron gases in oxide superlattice nanowires

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Science Advances  11 Nov 2020:
Vol. 6, no. 46, eabc6389
DOI: 10.1126/sciadv.abc6389
  • Fig. 1 Structure and electrical characterizations of InGaO3(ZnO)3 superlattice NWs.

    (A) Scanning electron microscopy (SEM) image of InGaO3(ZnO)3 superlattice NWs fabricated by Au-catalyzed VLS process at ambient pressure. (B) HRTEM image and (C) SAED pattern of a typical individual InGaO3(ZnO)3 superlattice NW. (D) Structural schematic of the InGaO3(ZnO)3 superlattice NW and its corresponding HRTEM image. (E) Qualitative energy-band diagram of quasi-2DEGs in the InGaO3(ZnO)3 superlattice. CB, VB, and EF are the abbreviations of conduction band, valence band, and Fermi level, respectively. (F) Temperature-related transport characterizations of InGaO3(ZnO)3 superlattice NWs (red points) and In1.6Ga0.4O3 crystalline NWs (green points) with temperatures ranging from 78 to 298 K.

  • Fig. 2 Oxygen adsorption-desorption kinetics and strong carrier quantum-confinement effects on InGaO3(ZnO)3 superlattice NWs.

    (A) Schematic illustration and corresponding SEM image of the FET device based on InGaO3(ZnO)3 superlattice NW arrays with SiO2 layer as the gate dielectric. (B) Transfer characteristics of the InGaO3(ZnO)3 NW array FET measured at different ambient and vacuum conditions. The scan rate is 500 mV/s. (C) Transfer characteristics of the InGaO3(ZnO)3 NW array FET before and after Al2O3 passivation measured at ambient atmosphere. The scan rate is 500 mV/s. (D) HRTEM image of a typical InGaO3(ZnO)3 NW with the distinct surface defect layer. (E) Wide scan XPS spectrum and (F) O 1s peak analysis of the InGaO3(ZnO)3 NWs. a.u., arbitrary units. (G) Light spike-induced EPSC generation and decaying characteristics of InGaO3(ZnO)3 NW arrays, demonstrating typical STP and LTP models. (H) EPSC triggered by a low-intensity presynaptic light spike (left) and short-term synaptic enhancement by two consecutive presynaptic light spikes (right) in 2DEG photonic synapses. (I) Spike duration–dependent plasticity of the InGaO3(ZnO)3 NW arrays with tspike ranging from 10 to 1000 ms. (J) Schematics of oxygen adsorption–induced charge trapping and subsequent optically induced charge release on InGaO3(ZnO)3 superlattice NWs.

  • Fig. 3 Brain-like information processing, nonvolatile charge retention, and multibit-level memory functions of flexible quasi-2DEG photonic synapses.

    (A) Photograph of flexible quasi-2DEG photonic synapse devices based on polyimide substrates. Inset shows the corresponding SEM image of the fabricated 6 × 5 device array. Photo credits: You Meng, City University of Hong Kong. (B) Paired-pulse facilitation (PPF) behavior of a typical quasi-2DEG photonic synaptic device. (C) PPF index, defined as the ratio of A2/A1, plotted as a function of interspike time. (D) Light intensity–dependent plasticity of quasi-2DEG photonic synapses at a constant spike time of 10 ms for 10 pulses with light intensity ranging from 0.1 to 0.5 mW/cm2. (E) Pulse number–dependent plasticity of quasi-2DEG photonic synapses at a constant spike time of 10 ms and light intensity of 0.1 mW/cm2 with the pulse number ranging from 3 to 9. (F) Giant PPC phenomenon in InGaO3(ZnO)3 superlattice NW arrays with the nonvolatile charge retention time using logarithm y coordinate and (inset) linear y coordinate. (G to I) Multibit storage properties of InGaO3(ZnO)3 superlattice NW arrays with more than 400 conductance states are continuously programed by laser pulses.

  • Fig. 4 Artificial visual system based on flexible quasi-2DEG photonic synapse arrays.

    (A) Schematic diagram of the human visual system, consisting of retina, visual nerve, and primary visual cortex. (B) Schematic diagram for the experimental setup of the attended orientation selectivity. (C) Attended and unattended visual responses of quasi-2DEG photonic synapses plotted as a function of the orientation angle. The solid line represents the Gaussian fitting curve as a guide to the eye. (D and E) Visual responses of quasi-2DEG photonic synapses with orientation angles of 0° and 72°, respectively. (F) Optical micrograph of the artificial visual system based on quasi-2DEG photonic synapse arrays. A human hair with a diameter of ~80 μm was used to pattern the light. (G to I) Imaging and memorizing behaviors of the artificial visual system after different retention time.

Supplementary Materials

  • Supplementary Materials

    Artificial visual systems enabled by quasi–two-dimensional electron gases in oxide superlattice nanowires

    You Meng, Fangzhou Li, Changyong Lan, Xiuming Bu, Xiaolin Kang, Renjie Wei, SenPo Yip, Dapan Li, Fei Wang, Tsunaki Takahashi, Takuro Hosomi, Kazuki Nagashima, Takeshi Yanagida, Johnny C. Ho

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