Spatial atmospheric atomic layer deposition of InxGayZnzO for thin film transistors

Atmospheric-pressure atomic layer deposition: recent applications and new emerging applications in high-porosity/3D materials

Atomic layer deposition (ALD) is a widely recognized technique for depositing ultrathin conformal films with excellent thickness control at Ångström or (sub)monolayer level. Atmospheric-pressure ALD is an upcoming ALD process with a potentially lower ownership cost of the reactor. In this review, we provide a comprehensive overview of the recent applications and development of ALD approaches emphasizing those based on operation at atmospheric pressure. Each application determines its own specific reactor design. Spatial ALD (s-ALD) has been recently introduced for the commercial production of large-area 2D displays, the surface passivation and encapsulation of solar cells and organic light-emitting diode (OLED) displays. Atmospheric temporal ALD (t-ALD) has opened up new emerging applications such as high-porosity particle coatings, functionalization of capillary columns for gas chromatography, and membrane modification in water treatment and gas purification. The challenges and opportunities for highly conformal coating on porous substrates by atmospheric ALD have been identified. We discuss in particular the pros and cons of both s-ALD and t-ALD in combination with their reactor designs in relation to the coating of 3D and high-porosity materials.

High Mobility IZTO Thin-Film Transistors Based on Spinel Phase Formation at Low Temperature through a Catalytic Chemical Reaction

In this paper, In_0.22 Zn_δ Sn_{0.78- δ} O_{1.89- δ} (δ = 0.55) films with a single spinel phase are successfully grown at the low temperature of 300 °C through careful cation composition design and a catalytic chemical reaction. Thin-film transistors (TFTs) with amorphous In_{0 .22} Zn_δ Sn_{0.78- δ} O_{1.89- δ} (δ = 0.55) channel layers have a reasonable mobility of 41.0 cm² V^-1 s^-1 due to the synergic intercalation of In and Sn ions. In contrast, TFTs with polycrystalline spinel In_{0 .22} Zn_δ Sn_{0.78- δ} O_{1.89- δ} (δ = 0.55) channel layers, achieved through a metal-induced crystallization at 300 °C, exhibit a remarkably high field-effect mobility of ≈83.2 cm² V^-1 s^-1 and excellent stability against external gate bias stress, which is attributed to the uniform formation of the highly ordered spinel phase. The relationships between cation composition, microstructure, and performance for the In₂ O₃ -ZnO-SnO₂ ternary component system are investigated rigorously to attain in-depth understanding of the roles of various crystalline phases, including spinel Zn_{2- y} Sn_{1- y} In_{2 y} O₄ (y = 0.45), bixbyite In_{2-2 x} Zn_x In_x O₄ (x = 0.4), rutile SnO₂ , and a homologous compound of compound (ZnO)_k (In₂ O₃ ) (k = 5). This work concludes that the cubic spinel phase of Zn_{2- y} Sn_{1- y} In_{2 y} O₄ (y = 0.45) film is a strong contender as a substitute for semiconducting polysilicon as a backplane channel ingredient for mobile active-matrix organic light-emitting diode displays.

Atomic Layer Deposition of Metal Oxides and Chalcogenides for High Performance Transistors

Atomic layer deposition (ALD) is a deposition technique well-suited to produce high-quality thin film materials at the nanoscale for applications in transistors. This review comprehensively describes the latest developments in ALD of metal oxides (MOs) and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high-performance field-effect transistors. By ALD various n-type and p-type MOs, including binary and multinary semiconductors, can be deposited and applied as channel materials, transparent electrodes, or electrode interlayers for improving charge-transport and switching properties of transistors. On the other hand, MO insulators by ALD are applied as dielectrics or protecting/encapsulating layers for enhancing device performance and stability. Metal chalcogenide semiconductors and their heterostructures made by ALD have shown great promise as novel building blocks to fabricate single channel or heterojunction materials in transistors. By correlating the device performance to the structural and chemical properties of the ALD materials, clear structure-property relations can be proposed, which can help to design better-performing transistors. Finally, a brief concluding remark on these ALD materials and devices is presented, with insights into upcoming opportunities and challenges for future electronics and integrated applications.

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

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.

Importance of Local Bond Order to Conduction in Amorphous, Transparent, Conducting Oxides: The Case of Amorphous ZnSnOy

In this report, reactive and nonreactive sputtering of amorphous ZnSnO_y (a-ZnSnO_y) was investigated, and extensive composition maps have been measured by X-ray photoelectron spectroscopy. The comprehensive analysis of the ((ZnO)_x(SnO₂)_1-x) composition reveals that the best Zn/Sn ratio for high conductivity of the material can vary depending on the deposition technique utilized. Best conductivities of 225 S/cm were found to occur at x = 0.32 for reactive sputtering of a Sn target and x = 0.27 for nonreactive sputtering of a SnO₂ target. These values correspond to unstable polymorphs of a-ZnSnO_y, ZnSn₂O₅, and ZnSn₃O₇. Distinct local bonding arrangements have been confirmed by Raman spectroscopy.

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

Amorphous InGaZnO_x (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 InO_x, ZnO_x, and GaO_x 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.

Effects of Deposition Temperature on the Device Characteristics of Oxide Thin-Film Transistors Using In-Ga-Zn-O Active Channels Prepared by Atomic-Layer Deposition

We demonstrated the physical and electrical properties of the In-Ga-Zn-O (IGZO) thin films prepared by atomic-layer deposition (ALD) method and investigated the effects of the ALD temperature. The film composition (atomic ratio of In:Ga:Zn) and film density were examined to be 1:1:3 and 5.9 g/cm³, respectively, for all the temperature conditions. The optical band gaps decreased from 3.81 to 3.21 eV when the ALD temperature increased from 130 to 170 °C. The amounts of oxygen-related defects such as oxygen vacancies increased with increasing the ALD temperature. It was found from the in situ temperature-dependent electrical conductivity measurements that the electronic natures including the defect structures and conduction mechanism of the IGZO thin films prepared at different temperatures showed marked variations. The carrier mobilities in the saturation regions (μ_sat's) for the fabricated thin film transistors (TFTs) using the IGZO channel layers were estimated to be 6.1 to 14.8 cm² V^-1 s^-1 with increasing the ALD temperature from 130 to 170 °C. Among the devices, when the ALD temperature was controlled to be 150 °C, the IGZO TFTs showed the best performance, which resulted from the fact that the amounts of oxygen vacancies and interstitial defects could be appropriately modulated at this condition. Consequently, the μ_sat, subthreshold swing, and on/off ratio for the TFT using the IGZO channel prepared at 150 °C showed 10.4 cm² V^-1 s^-1, 90 mV/dec, and 2 × 10⁹, respectively. The threshold voltage shifts of this device could also be effectively reduced to be 0.6 and -3.2 V under the positive-bias and negative-bias-illumination stress conditions. These obtained characteristics can be comparable to those for the sputter-deposited IGZO TFTs.

Rapid open-air deposition of uniform, nanoscale, functional coatings on nanorod arrays

Coating of high-aspect-ratio nanostructures has previously been achieved using batch processes poorly suited for high-throughput manufacturing. It is demonstrated that uniform, nanoscale coatings can be rapidly deposited on zinc oxide nanorod arrays in open-air using an atmospheric pressure spatial deposition system. The morphology of the metal oxide coatings is examined and good electrical contact with the underlying nanorods is observed. The functionality of the coatings is demonstrated in colloidal quantum dot and hybrid solar cells.

Flexible and High-Performance Amorphous Indium Zinc Oxide Thin-Film Transistor Using Low-Temperature Atomic Layer Deposition

Amorphous indium zinc oxide (IZO) thin films were deposited at different temperatures, by atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (INCA-1) as the indium precursor, diethlzinc (DEZ) as the zinc precursor, and hydrogen peroxide (H₂O₂) as the reactant. The ALD process of IZO deposition was carried by repeated supercycles, including one cycle of indium oxide (In₂O₃) and one cycle of zinc oxide (ZnO). The IZO growth rate deviates from the sum of the respective In₂O₃ and ZnO growth rates at ALD growth temperatures of 150, 175, and 200 °C. We propose growth temperature-dependent surface reactions during the In₂O₃ cycle that correspond with the growth-rate results. Thin-film transistors (TFTs) were fabricated with the ALD-grown IZO thin films as the active layer. The amorphous IZO TFTs exhibited high mobility of 42.1 cm² V^-1 s^-1 and good positive bias temperature stress stability. Finally, flexible IZO TFT was successfully fabricated on a polyimide substrate without performance degradation, showing the great potential of ALD-grown TFTs for flexible display applications.

Stable and High-Performance Flexible ZnO Thin-Film Transistors by Atomic Layer Deposition

Passivation is a challenging issue for the oxide thin-film transistor (TFT) technologies because it requires prolonged high-temperature annealing treatments to remedy defects produced in the process, which greatly limits its manufacturability as well as its compatibility with temperature-sensitive materials such as flexible plastic substrates. This study investigates the defect-formation mechanisms incurred by atomic layer deposition (ALD) passivation processes on ZnO TFTs, based on which we demonstrate for the first time degradation-free passivation of ZnO TFTs by a TiO2/Al2O3 nanolaminated (TAO) film deposited by a low-temperature (110 °C) ALD process. By combining the TAO passivation film with ALD dielectric and channel layers into an integrated low-temperature ALD process, we successfully fabricate flexible ZnO TFTs on plastics. Thanks to the exceptional gas-barrier property of the TAO film (water vapor transmission rate (WVTR)<10(-6) g m(-2) day(-1)) as well as the defect-free nature of the ALD dielectric and ZnO channel layers, the TFTs exhibit excellent device performance with high stability and flexibility: field-effect mobility>20 cm2 V(-1) s(-1), subthreshold swing<0.4 V decade(-1) after extended bias-stressing (>10,000 s), air-storage (>1200 h), and bending (1.3 cm radius for 1000 times).

Synthesis and modeling of uniform complex metal oxides by close-proximity atmospheric pressure chemical vapor deposition

A close-proximity atmospheric pressure chemical vapor deposition (AP-CVD) reactor is developed for synthesizing high quality multicomponent metal oxides for electronics. This combines the advantages of a mechanically controllable substrate-manifold spacing and vertical gas flows. As a result, our AP-CVD reactor can rapidly grow uniform crystalline films on a variety of substrate types at low temperatures without requiring plasma enhancements or low pressures. To demonstrate this, we take the zinc magnesium oxide (Zn(1-x)Mg(x)O) system as an example. By introducing the precursor gases vertically and uniformly to the substrate across the gas manifold, we show that films can be produced with only 3% variation in thickness over a 375 mm(2) deposition area. These thicknesses are significantly more uniform than for films from previous AP-CVD reactors. Our films are also compact, pinhole-free, and have a thickness that is linearly controllable by the number of oscillations of the substrate beneath the gas manifold. Using photoluminescence and X-ray diffraction measurements, we show that for Mg contents below 46 at. %, single phase Zn(1-x)Mg(x)O was produced. To further optimize the growth conditions, we developed a model relating the composition of a ternary oxide with the bubbling rates through the metal precursors. We fitted this model to the X-ray photoelectron spectroscopy measured compositions with an error of Δx = 0.0005. This model showed that the incorporation of Mg into ZnO can be maximized by using the maximum bubbling rate through the Mg precursor for each bubbling rate ratio. When applied to poly(3-hexylthiophene-2,5-diyl) hybrid solar cells, our films yielded an open-circuit voltage increase of over 100% by controlling the Mg content. Such films were deposited in short times (under 2 min over 4 cm(2)).