Combination of Gate-Stack Process and Cationic Composition Control for Boosting the Performance of Thin-Film Transistors Using In–Ga–Zn–O Active Channels Prepared by Atomic Layer Deposition

Combination impacts of controlling the gate-stack process conditions and the indium contents of In–Ga–Zn–O (IGZO) channels prepared by atomic layer deposition (ALD) were investigated to strategically enhance the device performance including carrier mobility for ALD IGZO thin-film transistors (TFTs). The ALD cyclic ratios (triethyl indium/In–Ga precursor/diethyl zinc) were varied to 0:2:2, 2:2:2, and 4:2:2 for the formation of IGZO channels, which correspond to the In/Ga ratios of 0.3, 1.0, and 2.0. While the device using the IGZO channel with an In/Ga ratio of 0.3 did not show any marked difference with the variations in the type of oxidants during the gate-stack formation processes, the carrier concentration and relative amounts of oxygen vacancy within the IGZO channels were found to significantly exhibit an increasing trend with increasing In/Ga ratio, when the Al2O3 protection layer (PL) was prepared with the oxidants incorporating a larger amount of hydrogen-related species. Thus, the physical properties of Al2O3 PL and gate insulator (GI) were controlled by changing the type of oxidants during the ALD processes. As a result, the carrier mobility (9.9 cm2/V s) of the device using an IGZO film with an In/Ga ratio of 0.3 could be enhanced to 23.5 cm2/V s by increasing the In/Ga ratio to 2.0 with the combination of the PL formation process designed with O2-plasma. Furthermore, the carrier mobility additionally improved to 27.6 cm2/V s when the In/Ga ratio increased to 1.4 with simultaneously employing the process conditions using the O3 oxidant for the formations of PL and GI layers. The ALD IGZO TFTs fabricated with the proposed optimum conditions also exhibited excellent bias stabilities.