Achieving a Low-Voltage, High-Mobility IGZO Transistor through an ALD-Derived Bilayer Channel and a Hafnia-Based Gate...

Ultrahigh-resolution displays for augmented reality (AR) and virtual reality (VR) applications require a novel architecture and process. Atomic-layer deposition (ALD) enables the facile fabrication of indium-gallium zinc oxide (IGZO) thin-film transistors (TFTs) on a substrate with a nonplanar surface due to its excellent step coverage and accurate thickness control. Here, we report all-ALD-derived TFTs using IGZO and HfO₂ as the channel layer and gate insulator, respectively. A bilayer IGZO channel structure consisting of a 10 nm base layer (In_0.52Ga_0.29Zn_0.19O) with good stability and a 3 nm boost layer (In_0.82Ga_0.08Zn_0.10O) with extremely high mobility was designed based on a cation combinatorial study of the ALD-derived IGZO system. Reducing the thickness of the HfO₂ dielectric film by the ALD process offers high areal capacitance in field-effect transistors, which allows low-voltage drivability and enhanced carrier transport. The intrinsic inferior stability of the HfO₂ gate insulator was effectively mitigated by the insertion of an ALD-derived 4 nm Al₂O₃ interfacial layer between HfO₂ and the IGZO film. The optimized bilayer IGZO TFTs with HfO₂-based gate insulators exhibited excellent performances with a high field-effect mobility of 74.0 ± 0.91 cm²/(V s), a low subthreshold swing of 0.13 ± 0.01 V/dec, a threshold voltage of 0.20 ± 0.24 V, and an I_ON/OFF of ∼3.2 × 10⁸ in a low-operation-voltage (≤2 V) range. This promising result was due to the synergic effects of a bilayer IGZO channel and HfO₂-based gate insulator with a high permittivity, which were mainly attributed to the effective carrier confinement in the boost layer with high mobility, low free carrier density of the base layer with a low V_O concentration, and HfO₂-induced high effective capacitance.