Investigation on Effect of Defective Interface Using Solution Process for IGZO Optical Synaptic Transistor

Recently, brain-inspired artificial neuromorphic computing system has widely been researched to replace conventional von Neumann-based computation because of their advantages such as low power consumption and efficient space usage. The artificial neuromorphic computing system is typically divided into transistor and mersister type according to the type of unit element, and optical and electrical type according to the type of input signal. Among them, optical synaptic transistor type is favorable option with high bandwidth, ultrafast operation speed, and low crosstalk. Oxide semiconductor is powerful candidate for active materials of optical synaptic transistor with low off-current, capability of large-are deposition, and high photosensitivity. Additionally, oxygen vacancies in oxide semiconductor materials are ionized with light absorption resulting in slow recovery of photo induced currents and persist photoconductance (PPC). The optical synaptic behavior can be originated from these oxygen vacancy and PPC. However, oxide semiconductor materials have limitation of detection wavelength range because of their wide bandgap (&gt; 3eV) to be driven with visible light region. Furthermore, there is tradeoff between synaptic behavior and switching characteristic when oxygen vacancies in channel layer increase.<br/>In this paper, we suggested Indium-Gallium-Zinc-Oxide (IGZO) optical synaptic transistor with solution processed IGZO layer with quasi 2-dimensional thickness between sputtered IGZO layer and gate insulator and absorption layer above channel layer. Inserted defective interface caused interfacial trapping & de-trapping resulting in increase of PPC and accumulated current. Furthermore, additional solution processed absorption layer (SAL) could increase photo generated carrier. IGZO optical synaptic transistor with defective interface shows the peak of photo induced postsynaptic current (PSC) of 1171 pA and maximum gain of 24.67 under red the light illumination of 635 nm wavelength.<br/>We compared parameter (on/off current ratio and S.S) derived from transfer characteristics of IGZO TFT with each condition (concentration of IGZO precursor: 0.2, 0.1, and 0.05 / spin coating rotation speed: 6000 and 3000 rpm) for defective interface and pristine IGZO TFT and optimized concentration of precursor solution to 0.1 M. The gain was improved from 2.53 to 9.61 in the red light region with the addition of defective interface compared to pristine IGZO TFT and increased from 9.61 to 24.67 under red light illumination (635 nm) with additional absorption layer. Short term plasticity (STP)-to-long term plasticity (LTP) transition is important factor which showed learning and decision-making functions for a biological neural system, which can be well simulated by varying the number of pulses, pulse frequency, pulse width, and even light power density. A series of light pulses with different frequencies are applied with wavelength of 635 nm. We showed that the peak PSC increased from 58 to 1171 pA with the increase of the pulse frequency from 0.02 Hz to 1 Hz. We compared the gain (A25/A1) of the IGZO optical synaptic transistor with defective interface with various the pulse frequency. As the pulse frequency increases from 0.2 to 1 Hz, the gain increased from 2.62 to 27.46, which means the STP-to-LTP transition can be caused by increase of frequency. Additionally, we suggest Pavlov’s associative learning mimicked by using photonic spike and electric spikes and their computation of IGZO optical synaptic transistor with defective interface.