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 (In0.52Ga0.29Zn0.19O) with good stability and a 3 nm boost layer (In0.82Ga0.08Zn0.10O) 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.

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The effect of gallium (Ga) concentration on the structural evolution of atomic-layer-deposited indium gallium oxide (IGO) (In1–x Ga x O) films as high-mobility n-channel semiconducting layers was investigated. Different Ga concentrations in 10–13 nm thick In1–x Ga x O films allowed versatile phase structures to be amorphous, highly ordered, and randomly oriented crystalline by thermal annealing at either 400 or 700 °C for 1 h. Heavy Ga concentrations above 34 atom % caused a phase transformation from a polycrystalline bixbyite to an amorphous IGO film at 400 °C, while proper Ga concentration produced a highly ordered bixbyite crystal structure at 700 °C. The resulting highly ordered In0.66Ga0.34O film show unexpectedly high carrier mobility (μFE) values of 60.7 ± 1.0 cm2 V–1 s–1, a threshold voltage (V TH) of −0.80 ± 0.05 V, and an I ON/OFF ratio of 5.1 × 109 in field-effect transistors (FETs). In contrast, the FETs having polycrystalline In1–x Ga x O films with higher In fractions (x = 0.18 and 0.25) showed reasonable μFE values of 40.3 ± 1.6 and 31.5 ± 2.4 cm2 V–1 s–1, V TH of −0.64 ± 0.40 and −0.43 ± 0.06 V, and I ON/OFF ratios of 2.5 × 109 and 1.4 × 109, respectively. The resulting superior performance of the In0.66Ga0.34O-film-based FET was attributed to a morphology having fewer grain boundaries, with higher mass densification and lower oxygen vacancy defect density of the bixbyite crystallites. Also, the In0.66Ga0.34O transistor was found to show the most stable behavior against an external gate bias stress.

Low-temperature (≤400 °C), stackable oxide semiconductors are promising as an upper transistor ingredient for monolithic three-dimensional integration. The atomic layer deposition (ALD) route provides a low-defect, high-quality semiconducting oxide channel layer and enables accurate controllability of the chemical composition and physical thickness as well as excellent step coverage on nanoscale trench structures. Here, we report a high-mobility heterojunction transistor in a ternary indium gallium zinc oxide system using the ALD technique. The heterojunction channel structure consists of a 10 nm thick indium gallium oxide (IGO) layer as an effective transporting layer and a 3 nm thick, wide band gap ZnO layer. The formation of a two-dimensional electron gas was suggested by controlling the band gap of the IGO quantum well through In/Ga ratio tailoring and reducing the physical thickness of the ZnO film. A field-effect transistor (FET) with a ZnO/In0.83Ga0.17O1.5 heterojunction channel exhibited the highest field-effect mobility of 63.2 ± 0.26 cm2/V s, a low subthreshold gate swing of 0.26 ± 0.03 V/dec, a threshold voltage of −0.84 ± 0.85 V, and an I ON/OFF ratio of 9 × 108. This surpasses the performance (carrier mobility of ∼41.7 ± 1.43 cm2/V s) of an FET with a single In0.83Ga0.17O1.5 channel. Furthermore, the gate bias stressing test results indicate that FETs with a ZnO/In1–x Ga x O1.5 (x = 0.25 and 0.17) heterojunction channel are much more stable than those with a single In1–x Ga x O1.5 (x = 0.35, 0.25, and 0.17) channel. Relevant discussion is given in detail on the basis of chemical characterization and technological computer-aided design simulation.

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