a-InGaZnO Thin-Film Transistors With Novel Atomic Layer-Deposited HfO₂ Gate Insulator Using Two Types of Reactant Gases

Plasma-enhanced chemical vapor deposition is often utilized to fabricate amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) with mobility of approximately 7 cm2/( $\text{V}\cdot \text{s}$ ) using a SiOx gate insulator. For use in high-resolution organic light-emitting diode displays, this value must be markedly improved. Therefore, we used various reactants to create a HfO2 bilayer via atomic layer deposition (ALD) and examined the electrical properties of IGZO TFTs, such as their mobility and subthreshold swing (SS). By adjusting the thickness of HfO2 with H2O reactant gas, the amount of hydrogen that diffused into the IGZO channel was controlled. The IGZO TFT with a specific HfO2 bilayer gate insulator exhibited high saturation mobility of 16.75 cm2/( $\text{V}\cdot \text{s}$ ) and an improved SS of 159 mV/dec compared to a conventional device with a HfO2 gate insulator formed using only O3 reactant gas. Furthermore, the fabricated HfO2 bilayer devices presented excellent reliability under positive bias stress and were more robust against the short-channel effect. Thus, based on the findings of this study, to improve the electrical properties of a-IGZO TFTs, the ALD process is suggested to be used to deposit a high- ${k}$ gate insulator.