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

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. Using a novel precursor HAC3, HfO2, which is a high‐κ material grown by plasma‐enhanced atomic layer deposition, is employed as the dielectric in the optical synaptic device. The double dielectric stacked optical synaptic devices show various synaptic properties. Especially, during the potentiation, a highly linear 8‐bit state is successfully implemented, and the lifetimes are dependent on the wavelengths of illuminated light.