Variation in the threshold voltage of amorphous-In2Ga2ZnO7 thin-film transistors by ultrathin Al2O3 passivation layer

The variations in the performance of amorphous In2Ga2ZnO7 thin-film transistors with ultrathin Al2O3 passivation layers deposited by atomic layer deposition (ALD) were examined. As the ALD Al2O3 deposition cycle number increased, the threshold voltage shifted to the negative voltage direction, while the saturation mobility was invariant. These variations are attributed to the removal of electronegative species such as OH− groups on back channel surface, while the bulk properties of the channel were hardly affected during the ALD. The ALD may not influence the oxygen vacancy concentration in the amorphous In2Ga2ZnO7 channel. The OH− groups on the Al2O3 surface further influenced the threshold voltage through capacitive coupling. The shifted properties recover the initial values after long-term exposure to air (100 days), by diffusion of OH− to the Al2O3/In2Ga2ZnO7 interface. These findings were further confirmed by spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and electrical characterization using a p++-Si/In2Ga2ZnO7 junction diode.