Recently, artificial synapses involving an electrochemical reaction of Li-ion have been attributed to have remarkable synaptic properties. Three-terminal synaptic transistors utilizing Li-ion intercalation exhibits reliable synaptic characteristics by exploiting the advantage of non-distributed weight updates owing to stable ion migrations. However, the three-terminal configurations with large and complex structures impede the crossbar array implementation required for hardware neuromorphic systems. Meanwhile, achieving adequate synaptic performances through effective Li-ion intercalation in vertical two-terminal synaptic devices for array integration remains challenging. Here, two-terminal Au/LixCoO2/Pt artificial synapses are proposed with the potential for practical implementation of hardware neural networks. The Au/LixCoO2/Pt devices demonstrated extraordinary neuromorphic behaviors based on a progressive dearth of Li in LixCoO2 films. The intercalation and deintercalation of Li-ion inside the films are precisely controlled over the weight control spike, resulting in improved weight control functionality. Various types of synaptic plasticity were imitated and assessed in terms of key factors such as nonlinearity, symmetricity, and dynamic range. Notably, the LixCoO2-based neuromorphic system outperformed three-terminal synaptic transistors in simulations of convolutional neural networks and multilayer perceptrons due to the high linearity and low programming error. These impressive performances suggest the vertical two-terminal Au/LixCoO2/Pt artificial synapses as promising candidates for hardware neural networks
Ferroelectric thin films have recently received unprecedented attention due to the need to miniaturize electronic circuit devices. Synthesis and deposition processes along with theoretical calculations are improved remarkably to realize stable ferroelectric thin films up to nanometer thickness. However, even with technological advances, it is still difficult to overcome the size effect of ferroelectrics, so research is being conducted to achieve stable ferroelectricity in unit‐cell thicknesses thinner than the typical critical thicknesses. In this review, the size effects in ferroelectric thin films are described, and their importance and fundamental limitations are discussed. First, intrinsic and extrinsic factors affecting ferroelectricity are introduced based on the theoretical background of the size effects in ferroelectricity. Then, on understanding the size effects by considering complex interacting factors, the recent works showing ferroelectricity below the commonly known critical thicknesses in perovskite, fluorite oxides, and two‐dimensional (2D) ferroelectrics are introduced. Finally, the results of research efforts in scaling ferroelectric thin films with a future perspective are summarized.
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