Artificial Synapse Based on Oxygen Vacancy Migration in Ferroelectric‐Like C‐Axis‐Aligned Crystalline InGaSnO Semiconductor Thin‐Film Transistors for Highly Integrated Neuromorphic Electronics

Abstract

Artificial synapses are a key component of neuromorphic computing systems. To achieve high‐performance neuromorphic computing ability, a huge number of artificial synapses should be integrated because the human brain has a huge number of synapses (≈1015). In this study, a coplanar synaptic, thin‐film transistor (TFT) made of c‐axis‐aligned crystalline indium gallium tin oxide (CAAC–IGTO) is developed. The electrical characteristics of the biological synapses such as inhibitory postsynaptic current (IPSC), paired‐pulse depression (PPD), short‐term plasticity (STP), and long‐term plasticity at VDS = 0.1 V, are demonstrated. The measured synaptic behavior can be explained by the migration of positively charged oxygen vacancies (Vo+/Vo++) in the CAAC–IGTO layer. The mechanism of implementing synaptic behavior is completely new, compared to previous reports using electrolytes or ferroelectric gate insulators. The advantage of this device is to use conventional gate insulators such as SiO2 for synaptic behavior. Previous studies use chitosan, Ta2O3, SiO2 nanoparticles , Gd2O3, and HfZrOx for gate insulators, which cannot be used for high integration of synaptic devices. The metal–oxide TFTs, widely used in the display industry, can be applied to the synaptic transistors. Therefore, CAAC–IGTO synaptic TFT can be a good candidate for application as an artificial synapse for highly integrated neuromorphic chips.

Multifunctional Crystalline InGaSnO Phototransistor Exhibiting Photosensing and Photosynaptic Behavior Using Oxygen Vacancy Engineering

“…[35,36] The crystallization of amorphous IGTO (a-IGTO) increases the ratio of oxygen vacancy (V o /(M-O + V o + M-OH)) from 13.5% to 32.8%. [8,9,34] It can be confirmed by analyzing the X-ray photoelectron spectroscopy (XPS) O1s intensity spectra deconvoluted into three Gaussian peaks of metal-oxygen bonds (529.9 ± 0.2 eV), oxygen vacancy (530.9 ± 0.2 eV), and oxygen-hydroxyl group (531.6 ± 0.1 eV) respectively as reported previously. [34,37] The optical bandgaps of a-IGTO and CAAC-IGTO thin films by Tauc' plot are 3.78 and 3.81 eV, respectively.…”

“…Further details of the crystallization process are described in the experimental section. According to our previous work, [34] we confirmed that the metal cations are aligned to the c-axis after crystallization by measuring both X-ray diffraction intensity peak at 2𝜃 = 31°and transmission electron microscopy image of the IGTO thin film. [35,36] The crystallization of amorphous IGTO (a-IGTO) increases the ratio of oxygen vacancy (V o /(M-O + V o + M-OH)) from 13.5% to 32.8%.…”

“…That is due to the recombination between V o + and the electrons in the conduction band minimum (CBM) takes place at the front channel during the reset pulse on. Note that previous work [34] confirms that migration of V o + occurs in CAAC-IGTO, not in a-IGTO.…”

“…+ in the IGTO layer as shown in Figure 3d-f. We report V o + migration in CAAC-IGTO layer by applying electric field. [34] The V o + 's in CAAC-IGTO layer can be migrated by applying gate bias. As shown in Figure 3d, there are high density of V o in the CAAC-IGTO, confirmed through XPS O1s spectra.…”

“…Electrical Measurements of the Phototransistor: The fabrication process of phototransistor appeared in ref. [34]. All the electrical characteristics of the phototransistors were measured using an Agilent 4156C at room temperature.…”