Direct Evaluation of Electron Traps in Amorphous In-Ga-Zn-O Thin Film Transistors Using Transient Capacitance Technique

Electron traps in the channel region of amorphous In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) were evaluated using capacitance transient responses from thin (50 nm) metal-oxide-semiconductor diodes. It was found that the carrier transport was mainly dominated by the conduction band tail states. The trap density of a-IGZO deposited at 5 mTorr was higher than that at 1 mTorr, providing direct evidence that the trap density in the channel layer decreased by decreasing the sputtering pressure during the IGZO deposition. The defect states with activation energies of ∼0.2 eV were observed only a-IGZO deposited at 5 mTorr. The reduction of the corresponding TFT mobility should be originated from the increase in the conduction band tail states. The high pressure sputtering is suggested to enhance the structural fluctuation in a-IGZO films, at least in the structure surrounding defect, resulting in an increase of the concentration of the electron traps.