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

Abstract

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 cm2/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), respectively. 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 (ΔVTH) 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 ΔVTH 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.