High-Performance Homojunction a-IGZO TFTs With Selectively Defined Low-Resistive a-IGZO Source/Drain Electrodes

We report high-performance homojunction amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with low-resistive a-IGZO source/drain (S/D) electrodes. The a-IGZO S/D electrodes are selectively treated with high-power NF3 plasma, which reduces their resistivity from ~16 to 5.5×10^-3 Ω · cm. X-ray photoelectron spectroscopy indicates an increase in weakly bonded oxygen and a substantial amount of indium-fluorine and zinc-fluorine bonds at the a-IGZO top surface (extending to ~7 nm into the bulk) after plasma treatment. Temperature-dependent conductivity measurements show metallic behavior of the a-IGZO after treatment. It is concluded that fluorine atoms substitute for oxygen atoms-generating free electrons in the process and/or occupy oxygen vacancy sites-eliminating electron trap sites. As a result, the homojunction TFTs show good ON-state characteristics with typical field-effect mobility, subthreshold gate-voltage swing, and turn-ON voltage of 19 ± 1 cm²/V·s, 178 ± 30 mV/decade, and -3.2 ± 1.5 V, respectively. Good stability at high temperature and under bias and light stress are also exhibited by the homojunction TFTs, verifying a stable doping effect by the NF3 plasma treatment.