Enhancement of In-Sn-Ga-O TFT performance by the synergistic combination of UV + O3 radiation and low temperature annealing

High performance thin film transistors (TFTs) based on amorphous In-Sn-Ga-O (ITGO) semiconductor were fabricated. In order to activate the electrical properties of the oxide semiconductor, different processes were used, involving thermal annealing, UV + O3 (UVO) radiation, and UVO-assisted thermal annealing. While either UV radiation or thermal annealing at 150 °C results in rather poor transfer characteristics, the combination of both allows the fabrication of high performance devices with field effect mobility values exceeding 20 cm2/Vs. X-ray photoelectron spectroscopy (XPS) analyses of ITGO films suggest that the density of defects related to oxygen-deficient sites are reduced upon UVO-assisted annealing. Also, hydroxide bonds are found to increase in the semiconductor material, which is highly likely to increase the free carrier concentration. The reduction of oxygen-related defects results in a decrease in charge trap density near the semiconductor/gate dielectric interface, and the UV-assisted annealed ITGO devices exhibit relatively small shifts in the threshold voltage (Vth) under positive bias stress.