Zn₂SnO₄ Thin Film Based Nonvolatile Positive Optoelectronic Memory for Neuromorphic Computing

Abstract

An optoelectronic synaptic device based on the ITO/ Zn 2 SnO 4 (ZTO)/ITO structure is fabricated which integrates the electronic memory and optical sensing properties along with synaptic functions. The fabricated device shows over 80% optical transparency for the entire visible region (400−800 nm). Post-oxide annealing treatment is performed in a nitrogen environment at 200 °C. Significant improvements in bipolar resistive switching properties of the device with low SET voltage (+0.93 V) and long DC endurance cycles (∼12000) are observed in the annealed device. The linearity of such memristive synapse is improved for 350 training epochs with a total number of 175000 pulses. The spike time dependent plasticity learning rule for the annealed device is demonstrated through the electric field. The optical sensing capabilities of this device including photonic potentiation (responsivity: 0.52 μA/W), photonic paired pulse facilitation by adjusting time interval between two identical light pulses, learning experience behavior, and multilevel memory feature by the repetition of optical pulse for ∼10 3 s are demonstrated under the blue light (wavelength "λ" = 405 nm) illumination at 50 mW/cm 2 . Photonic potentiation and electric depression behavior of the device mimic its nonvolatile synaptic plasticity. The linear fitting of I−V curve illustrates the dominance of Schottky emission and Poole−Frenkel conduction mechanisms at high and low resistance states, respectively. The electric response of the device is explained by the oxygen vacancy based filamentary model. The trapping and detrapping of electrons during the adsorption and desorption processes of atmospheric oxygen molecules on the ITO surface are responsible for the photoconduction phenomenon. To train the Hopfield neural network (HNN) model for image processing of 28 × 28 pixels, the normalized experimental data of long-term potentiation/depression are employed to mimic the learning behavior of the human brain. The convergence of electronic data storage and optical sensor has high potential which provides a path toward the future smart invisible optoelectronics for artificial intelligence.

Efficient UV-Sensitive Si-In-ZnO-Based Photo-TFT and Its Behavior as an Optically Stimulated Artificial Synapse

“…The observation of PPC is common in this kind of oxide material-based wide band gap semiconductors. The PPC effect in oxide materials is widely explained by trapping of the photo-generated electrons in the ionized oxygen vacancies in the oxide semiconductors or in the interfacial trap states between the active channel, electrodes, and gate oxide. ,, These trap states will also have a significant effect on the spectral photoresponse of the photo-TFT, as discussed later in the report.…”

“…Using this transmittance spectrum, the optical band gap has been estimated by formulating the Tauc plot, as shown in Figure f. The steep region in the Tauc plot signifies an increase in optical absorption with photon energy . The linear portion of the curve was extrapolated and the position where it intersects the x -axis provides the optical band gap of the material.…”

“…The advancement of optogenetics in neuroscience, along with the fact that a human being acquires maximum information through a visual cortex system, has been the inspiration behind the present research and development of optoelectronic synaptic devices that can successfully imitate the behavior of biological synapses reacting to optical stimulus. − Successful integration of optoelectronic synaptic devices plays a key role in the achievement of artificial neural networks in a high-performance neuromorphic system that can handle large data with ease, similar to a human brain. − Aiming for large-scale fabrication of neuromorphic systems, the development of amorphous oxide semiconductors (AOSs)-based phototransistors for the integration of optoelectronic artificial synapses will be quite encouraging. ,,− …”

“…The three-terminal configuration of TFTs allows signal processing (learning) and transmission to occur simultaneously, making it a desirable choice for synaptic devices. ,,− However, reported TFT-based synaptic devices are mostly electrically driven and have bandwidth limitations and interconnection issues. ,− ,− Optically stimulated artificial synapses, with their wide optical bandwidth and ultrafast signal transmission, were proposed to be one of the encouraging solutions to this issue. ,− ,− Moreover, the optoelectronic synapses have the inherent advantage of simulating the human ocular system. ,,,, Therefore, it is extremely desirable to develop efficient photo-TFT that can be utilized as optically stimulated artificial synapses, coupling optical and electrical stimuli together. The persistent photoconductivity (PPC) unique to AOSs plays a crucial role in this context.…”

“…The persistent photoconductivity (PPC) unique to AOSs plays a crucial role in this context. The PPC is a phenomenon where the photocurrent is retained for a longer duration even after the removal of the optical signal due to the subgap trap state-mediated slow recombination process of the photogenerated carriers. ,, Consequently, this phenomenon permits various synaptic functions such as the excited postsynaptic current (EPSC), short-term plasticity (STP), long-term plasticity (LTP), synaptic weight, paired pulsed facilitation (PPF), etc. ,,, Hence, researchers worldwide are extensively investigating AOS-based photo-TFTs and their utilization as functional optically stimulated artificial synapses.…”