Amorphous InGaZnOx (a-IGZO) thin-film transistors (TFTs) are currently used in flat-panel displays due to their beneficial properties. However, the mobility of ∼10 cm2/(V s) for the a-IGZO TFTs used in commercial organic light-emitting diode TVs is not satisfactory for high-resolution display applications such as virtual and augmented reality applications. In general, the electrical properties of amorphous oxide semiconductors are strongly dependent on their chemical composition; the indium (In)-rich IGZO achieves a high mobility of 50 cm2/(V s). However, the In-rich IGZO TFTs possess another issue of negative threshold voltage owing to intrinsically high carrier density. Therefore, the development of an effective way of carrier density suppression in In-rich IGZO will be a key strategy to the realization of practical high-mobility a-IGZO TFTs. In this study, we report that In-rich IGZO TFTs with vertically stacked InOx, ZnOx, and GaOx atomic layers exhibit excellent performances such as saturation mobilities of ∼74 cm2/(V s), threshold voltage of -1.3 V, on/off ratio of 8.9 × 108, subthreshold swing of 0.26 V/decade, and hysteresis of 0.2 V, while keeping a reasonable carrier density of ∼1017 cm-3. We found that the vertical dimension control of IGZO active layers is critical to TFT performance parameters such as mobility and threshold voltage. This study illustrates the potential advantages of atomic layer deposition processes for fabricating ultrahigh-mobility oxide TFTs.

Amorphous IGZO TFT with High Mobility of ∼70 cm2/(V s) via Vertical Dimension Control Using PEALD

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 ION/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 VO concentration, and HfO2-induced high effective capacitance.

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

This article reports the fabrication of high-performance amorphous indium gallium zinc tin oxide ( ${a}$ -IGZTO) thin-film transistors (TFTs) with superior bias stability. For comparison, amorphous indium gallium zinc oxide ( ${a}$ -IGZO) TFTs were also investigated to clarify the origin of the superior performance of IGZTO TFTs. It was found that the simultaneous heavy loading of In and Sn into the IGZTO system facilitated an effective mass densification, leading to a reduction in tail states and deep states. The fabricated ${a}$ -IGZTO TFTs exhibited a high electron mobility ( $\boldsymbol {\mu }_{\text {FE}}$ ) of 46.7 cm2/Vs, a subthreshold swing (SS) gate of 0.15 V/decade, and an ${I}_{\text {ON/OFF}}$ ratio $> {1} \times {10}^{{8}}$ . Furthermore, greater gate-bias stress stability was observed for the IGZTO TFTs compared with the IGZO TFTs.

Achieving High Mobility and Excellent Stability in Amorphous In–Ga–Zn–Sn–O Thin-Film Transistors

The effect of gallium (Ga) concentration on the structural evolution of atomic-layer-deposited indium gallium oxide (IGO) (In1-xGaxO) films as high-mobility n-channel semiconducting layers was investigated. Different Ga concentrations in 10-13 nm thick In1-xGaxO 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 (VTH) of -0.80 ± 0.05 V, and an ION/OFF ratio of 5.1 × 109 in field-effect transistors (FETs). In contrast, the FETs having polycrystalline In1-xGaxO 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, VTH of -0.64 ± 0.40 and -0.43 ± 0.06 V, and ION/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.

High-Performance Thin-Film Transistors with an Atomic-Layer-Deposited Indium Gallium Oxide Channel: A Cation Combinatorial Approach.

Amorphous metal-oxide semiconductors (AOSs) such as indium-gallium-zinc-oxide (IGZO) as an active channel have attracted substantial interests with regard to high-performance thin-film transistors (TFTs). Recently, intensive and extensive studies of flexible and/or wearable AOS-based TFTs fabricated by solution-process have been reported for emerging approaches based on device configuration and fabrication process. However, several challenges pertaining to practical and effective solution-process technologies remain to be resolved before low-power consuming AOS-based TFTs for wearable electronics can be realized. In this paper, we investigate the non-thermal annealing processes for sol-gel based metal-oxide semiconductor and dielectric films fabricated by deep ultraviolet (DUV) photo and microwave annealing at low temperature, compared to the conventional thermal annealing at high temperature. A comprehensive investigation including a comparative analysis of the effects of DUV photo and microwave annealing on the degree of metal-oxide-metal networks in amorphous IGZO and high-dielectric-constant (high-k) aluminum oxide (Al2O3) films and device performance of IGZO-TFTs in a comparison with conventional thermal annealing at 400 °C was conducted. We also demonstrate the feasibility of wearable IGZO-TFTs with Al2O3 dielectrics on solution-processed polyimide films exhibiting a high on/off current ratio of 5 × 104 and field effect mobility up to 1.5 cm2/V-s operating at 1 V. In order to reduce the health risk and power consumption during the operation of wearable electronics, the operating voltage of IGZO-TFTs fabricated by non-thermal annealing at low temperature was set below ~1 V. The mechanical stability of wearable IGZO-TFTs fabricated by an all-solution-process except metal electrodes, against cyclic bending tests with diverse radius of curvatures in real-time was investigated. Highly stable and robust flexible IGZO-TFTs without passivation films were achieved even under continuous flexing with a curvature radius of 12 mm.

/pdf/wearable-1-v-operating-thin-film-transistors-with-solution-3sa22tqv9g.pdf

Wearable 1 V operating thin-film transistors with solution-processed metal-oxide semiconductor and dielectric films fabricated by deep ultra-violet photo annealing at low temperature.

This paper reports the effect of the cation composition on the electrical properties of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) where atomic layer deposition (ALD)...

https://www.tandfonline.com/doi/pdf/10.1080/15980316.2018.1540365

Impact of cation compositions on the performance of thin-film transistors with amorphous indium gallium zinc oxide grown through atomic layer deposition

Hyunjoo Seul

Papers

Impact of cation compositions on the performance of thin-film transistors with amorphous indium gallium zinc oxide grown through atomic layer deposition