Combination of InZnO/InGaZnO Bi-layered channels prepared by atomic layer deposition and ozone-based gate-stack formation for guaranteeing high field-effect mobility and long-term stability of thin film transistors

Bi-layered channel structures comprising In-Zn-O (IZO) prompt and In-Ga-Zn-O (IGZO) prime layers were introduced to enhance the carrier mobility of oxide thin film transistors (TFTs) by atomic layer deposition (ALD) technique. Considering that hydrogen-related species in the gate stack may convert the semiconducting channel with In-rich composition into a more conducting layer during the ALD process, ozone oxidant was chosen for the formation of Al2O3 protection and gate insulator layers. The TFT using a IZO/IGZO bi-layered channel exhibited the highest carrier mobility of 50.8 cm(2)/Vs when the film thickness and ALD sub-cyclic ratio between the In and Zn precursors for the IZO prompt layer were designed to be 3 nm and 2:1, respectively. The optimum cationic compositions of the IZO and IGZO layers were determined to be 1.3:1 (In:Zn) and 1.4:1:2 (In:Ga:Zn), respec-tively. The turn-on position of the fabricated device was located at near 0 V without additional heat treatment. Furthermore, the device using the optimum IZO/IGZO bi-layered channel exhibited threshold voltage shifts (delta V-TH) of +1.8 V under a positive bias stress (PBS) condition that corresponded to an improvement over a single IGZO channel device (+4.5 V). From the numerical analyses on the sub-gap states below the conduction band edge and trap densities in the mid-gap regions, it was found that the electron trapping sites especially located in deep-level states could be reduced by implementing a bilayer channel configuration. Alternatively, the delta V-TH further decreased to +1.3 V during the PBS at an elevated temperature of 80 ?C because of the migration of hydrogen from the gate stack toward the active channel during the measurement, demonstrating the synergic impact of bi-layered channel structures and gate-stack formation conditions.