Oxygen flow effects on electrical properties, stability, and density of states of amorphous In–Ga–Zn–O thin-film transistors

To investigate the origin of threshold voltage (Vth) shift of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs), a combination of bias-temperature stress (BTS) and multi-frequency capacitance–voltage (C–V) measurements were used to evaluate the impact of oxygen partial pressure (PO2) during a-IGZO deposition on TFT electrical properties, electrical stability, and density of states (DOS). The extracted sub-gap DOS was decomposed into exponential bandtail states and Gaussian-like deep-gap states. The peak density of Gaussian-like states is larger for higher PO2. We conclude that the Gaussian-like states are excess/weakly-bonded oxygen in the form of O0 or O1− ions acting as acceptor-like states and are at the origin of TFT threshold voltage shift during positive BTS.