Cationic compositional effects on the bias-stress stabilities of thin film transistors using In-Ga-Zn-O channels prepared by atomic layer deposition

Cationic compositional effects of amorphous In-Ga-Zn-O (a-IGZO) prepared by atomic layer deposition (ALD) were strategically investigated for thin film transistor applications. The atomic compositions (In:Ga:Zn) of ALD-IGZO films were varied to 1:1:1, 1:1:3, and 1:1:5 by controlling the ALD cyclic ratios. The relative content of oxygen vacancies and temperature-dependent electrical conductivities among the films markedly varied with the In/Zn ratio. The device employing the 1:1:5 composition exhibited inferior characteristics owing to the excessive Zn content in the IGZO channel. With increasing In/Zn ratio, the density of subgap states near both the conduction and valence bands increased, resulting in a higher degree of bias-stress instability. The device employing the 1:1:3 composition exhibited the most promising device characteristics including excellent stabilities under positive bias-stress at 60 degrees C and a negative bias-illumination-stress condition using a green wavelength, in which the threshold voltage shifts were estimated to be as low as +0.8 and -1.5 V, respectively.