Characteristics of PEALD–Hafnium Dioxide Films and their Application to Gate Insulator Stacks of Photosynaptic Transistors

Abstract

For practical applications of photosynaptic devices in neuromorphic systems, photosynaptic transistors prepared using TiO2 channels and TiO2/Al2O3 deep trap interfaces exhibit high stability and retention. However, there is scope for improving photosynaptic properties such as large numbers of analog states with good linearity and high sensitivity. Herein, HfO2 thin films of high‐κ gate dielectric layers grown by plasma‐enhanced atomic layer deposition with the Hf precursor methyl‐3‐cyclopentadiene propylamine bis(dimethylamino) hafnium are employed. 30‐nm‐thick HfO2 films deposited at 200 °C exhibit a κ of 13.4 and an E‐field of up to 5 MV cm−1. They are grown on a Si substrate and appropriately used in the form of double‐insulator stacks, in addition to Al2O3, in TiO2‐based photosynaptic transistors. The synaptic transistors successfully demonstrate better performance in terms of various parameters relevant to synaptic plasticity. For long‐term plasticity, 256 analog states are observed, which is more than that reported previously, with superior linearity. Moreover, lights of various wavelengths, including visible and UV, are illuminated on the devices, resulting in various lifetimes (wavelength‐dependent) of the trapped carriers. These properties are commercially attractive and suggest the feasibility of employing these devices in applications such as visual information detection and computing in future intelligent networks.

Photo‐Synaptic Oxide Transistors with Al2O3/SiOx Stacked Gate Dielectric Exhibiting 1024 Conduction States with Good Linearity

“…This number of states is significantly higher compared to previously reported synaptic devices using voltage and light stimuli. [9,13,14,19] In particular, the number of conduction states, 1024, is considerably higher than those previously reported for TiO 2 -based transistors that employ only Al 2 O 3 dielectrics. [15] These results are obtained primarily owing to the presence of the CID in the proposed device.…”

“…Also, the EPSCs were fitted using a modified Kohlrausch equation, which is a monotonous exponential decrease function. [9] The relaxation time (τ) can be obtained using the equation…”

“…Still, synaptic changes due to light stimulation are also significant and the demand for light-sensitive synaptic devices is expected to increase. [9] Practically, optical approaches exhibit higher spatiotemporal resolutions than electrical stimulation. [10] Photonic-based neuromorphic systems can enable multimodal plasticity with increased speed, lower power consumption, and higher efficiency.…”

“…Recently, a synaptic device with 265 analog states with excellent linearity during the synaptic potentiation process induced by ultra-violet (UV) light pulses was demonstrated. [9] Complex sensory synaptic devices using optical and electrical stimulation in parallel are also being actively investigated. [14,18] Moreover, for synaptic devices to have an accurate prediction ability, an excellent linear relationship must exist between the number of stimuli and the current during the learning process.…”

Replicating the Bright-Light Therapy of Seasonal Affective Disorder Using Dual-Gate Dielectric Synaptic Transistors: An Exploration of Neuromorphic Computing Approaches

“…Our research group has focused on photosensitive field-effect transistors that feature stable, deep interfacial traps. For instance, our synaptic devices can be programmed using ultraviolet (UV) optical stimulation, harnessing the deep traps at the interface between the channel and gate dielectric layers. ,− Moreover, the developed synaptic transistors have demonstrated CMOS compatibility and used oxide semiconductors and dielectric materials conducive to fabrication using vacuum-based thin-film deposition techniques, favoring mass production.…”