Neuromorphic computing refers to one of the most promising choices to solve the von Neumann bottleneck. The key to develop neuromorphic computing is to make the device able of simulating biological synaptic behavior. Optically stimulated synaptic devices have the advantages of fast speed and low energy consumption. Many materials including carbon group materials, oxide materials and 2D materials have been used to make photoelectronic synaptic devices. However, most of the devices can only respond by violet and/or ultraviolet light stimulation, and very few of them can work in the near-infrared range. Here, we report an optoelectronic synaptic device based on SiOy/a-Si1-xRux bilayer memristive materials. By doping with ruthenium (Ru), the optical bandgap of amorphous silicon (a-Si) film could be engineered, making the doped a-Si1-xRux film infrared sensitive. Therefore, a-Si1-xRux film can effectively absorb light illumination in a wideband range from 450 nm to 905 nm. Many photoelectronic synaptic behaviors including excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF) and short-term plasticity (STP) to long-term plasticity (LTP) transition, have been simulated successfully by using different light spikes at wavelengths of 450 nm, 635 nm and 905 nm, respectively. We refer the obtained synaptic plasticities to originate from the trapping and de-trapping of photogenerated carriers by light-induced defects inside the silicon oxide (SiOy) which was deposited directly on a-Si1- xRux film, and to the generation of electron-hole pairs from the underlying a-Si1-xRux film. Our newly fabricated optoelectronic synaptic device shows a great application potential in neuromorphic computing.
Memristors are emerging and being considered to be used as candidates to realize multiple bio-synaptic plasticities and to act in the developed neuromorphic computing systems. Simulating the human visual neural network is an effective way to build a new generation of artificial visual systems and a realistic method to break the von Neumann bottleneck. In this article, we report for the first time a newly proposed and fabricated oxide-based optoelectronic synaptic device with a structure of ITO/Ag:SrTiO3/CuAlO2/ITO, and demonstrate its diverse synaptic plasticities. It is found that the device can respond light stimulation from visible to near-IR (450 nm-905 nm) wavelengths, and also can exhibit interestingly various synaptic behaviors including short-term plasticity (STP), paired-pulse facilitation (PPF), long-term plasticity (LTP) and the transition from STP to LTP, respectively. More importantly, our optoelectronic synaptic device has successfully simulated several artificial vision properties as image memory, image preprocessing and color recognition. It is worth acknowledging that our optoelectronic synaptic device has a simple structure of ITO/Ag:SrTiO3/CuAlO2/ITO and an excellent synaptic behavior, showing a potential to be used in the artificial vision and neuromorphic computing systems in the near future.
The implementation of biological synapses onto a hardware platform is an important step toward neuromorphic electronics. However, to replicate biological functions, electronic synapses still have to be reliant on the electric field, or the output of synaptic devices is photonic rather than an electronic synapse. Therefore, the development of optoelectronic synapses with optically stimulated fidelity and flexibility for information sensing and memory processes is a big step toward next-generation computing systems. Herein, the implementation of optical sensing and synaptic functionalities combined has been demonstrated in an artificial optoelectronic synapse based on a silicon-tin alloyed film. The synaptic device can mimic versatile light-stimulated synaptic neuromorphic functions, including excitatory postsynaptic current (ESPC), paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP), and learning experience. Furthermore, the mimicry of the human visual system is achieved based on these synaptic neuromorphic functions. In addition, the silicon-tin thin film with increased optical absorption provides photonic memory (PM) function under very low light intensity, showing great potential in light-stimulated neuromorphic applications.
The structural and optical properties of amorphous silicon (a-Si) and Ag-dispersed amorphous silicon (a-Si:Ag) thin films irradiated by femtosecond (fs) laser at various energy densities are investigated comparatively in this article. It is found that at a lower energy density of 100 mg/cm2 , the film microstructure evolves from a completely amorphous phase to an intermediate one containing both amorphous and polycrystalline silicon. During laser irradiation, the formation of nanocrystals in a-Si films begins at lower energy density, but the existing Ag nanoparticles inhibits somehow the crystallization of a-Si in a-Si:Ag films at the same energy density. As the energy density is increased to a moderate value of 200 mj/cm2 , the surface of a-Si:Ag films featuring a vertically aligned pillar-shaped structure is emerging. Both the crystallinity and the root mean square of surface roughness exhibit a monotonic increase with the increase of energy density. The Ag nanoparticles are dispersed uniformly in a silicon matrix, resulting in a resonant light absorption due to so-called localized surface plasmon. The localized surface plasmon resonance (LSPR) wavelengthes of the irradiated aSi:Ag films are increased significantly from 600 nm to about 820 nm, and the bandwidth of the measured absorptance is enhanced in the range of 600~1600 nm. The nanocrystallization mechanism, the formation of pillar-shaped structures and the light absorption enhancement are explained regarding the high electron density and the plasma-surface interactions.
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