Ferroelectric-Like Charge Trapping Thin-Film Transistors and Their Evaluation as Memories and Synaptic Devices

This work presents a defect charging mechanism in 5-nm-thick amorphous Al2O3 thin-films fabricated on plastic, which leads to multistate memory effects, and thus the realization of synaptic thin-film transistors (TFTs) for neuromorphic applications. First, the Al2O3 thin-films are characterized in metal-insulator-metal stacks. These devices exhibit ferroelectric-like behavior, which is visible in the small-signal capacitance and the surface charge density. Furthermore, the quantum-mechanical simulation of the current-voltage characteristic leads to a physical model with trap charges close to the anode interface where deep-level traps are identified by fitting the experimentally obtained resonant tunneling peaks. The trap charge lifetime and frequency behavior is evaluated in InGaZnO4 TFTs, where the 5-nm-thick Al2O3 layer is employed as gate dielectric. At an operating voltage as low as +/- 2 V, a charge trapping retention up to approximate to 3 h and a discernable ON/OFF read-out with a factor >3 at 2 kHz are achieved. When subjected to a train of gate-source voltage pulses, the TFTs show charge integration properties which emulate facilitating and depressing behaviors of biological synapses. These results indicate that thin low-temperature defect-rich metal-oxide dielectrics may be candidates for low-voltage memory applications and neuromorphic circuits on unconventional substrates.