Complementary Hybrid Semiconducting Superlattices with Multiple Channels and Mutual Stabilization

Saturation Mobility of 100 cm2 V-1 s-1 in ZnO Thin-Film Transistors through Quantum Confinement by a Nanoscale In2O3 Interlayer Using Spray Pyrolysis

In this study, we present a comprehensive study on the fabrication and characterization of heterojunction In2O3/ZnO thin-film transistors (TFTs) aimed at exploiting the quantum confinement effect to enhance device performance. By systematically optimizing the thickness of the crystalline In2O3 (c-In2O3) layer to create a narrow quantum well, we observed a significant increase in saturation mobility (μSAT) from 12.76 to 97.37 cm2 V-1 s-1. This enhancement, attributed to quantum confinement, was achieved through the deposition of a 3 nm c-In2O3 semiconductor via spray pyrolysis. Various In2O3 layer thicknesses (2-5 nm) were obtained by adjusting precursor solution concentration, flow rate, and number of spray cycles. Post annealing treatments were employed to reduce the defects at the interface and within the oxide film, enhancing device stability and performance. Transmission electron microscopy (TEM) confirmed the uniformity of the c-In2O3 film thickness, while variations in thickness significantly influenced TFT performance, particularly the turn-on voltage (VGS) due to changes in the carrier concentration. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) supported the formation of a potential well with a two-dimensional electron gas (2DEG). The study of single and multiple superlattice structures of consecutive c-In2O3 and c-ZnO layers provided insights into the effects of multiple quantum wells on the TFT performance. This research presents an advanced approach to TFT optimization, highlighting high reliability, and environmental and bias stabilities. These lead to enhanced mobility and performance uniformity through the precise control of c-In2O3 layer thickness for the quantum confinement effect.

Progress, Challenges, and Opportunities in Oxide Semiconductor Devices: A Key Building Block for Applications Ranging from Display Backplanes to 3D Integrated Semiconductor Chips

As Si has faced physical limits on further scaling down, novel semiconducting materials such as 2D transition metal dichalcogenides and oxide semiconductors (OSs) have gained tremendous attention to continue the ever-demanding downscaling represented by Moore's law. Among them, OS is considered to be the most promising alternative material because it has intriguing features such as modest mobility, extremely low off-current, great uniformity, and low-temperature processibility with conventional complementary-metal-oxide-semiconductor-compatible methods. In practice, OS has successfully replaced hydrogenated amorphous Si in high-end liquid crystal display devices and has now become a standard backplane electronic for organic light-emitting diode displays despite the short time since their invention in 2004. For OS to be implemented in next-generation electronics such as back-end-of-line transistor applications in monolithic 3D integration beyond the display applications, however, there is still much room for further study, such as high mobility, immune short-channel effects, low electrical contact properties, etc. This study reviews the brief history of OS and recent progress in device applications from a material science and device physics point of view. Simultaneously, remaining challenges and opportunities in OS for use in next-generation electronics are discussed.

High-Performance Indium-Based Oxide Transistors with Multiple Channels Through Nanolaminate Structure Fabricated by Plasma-Enhanced Atomic Layer Deposition

An atomic-layer-deposited oxide nanolaminate (NL) structure with 3 dyads where a single dyad consists of a 2-nm-thick confinement layer (CL) (In_0.84Ga_0.16O or In_0.75Zn_0.25O), and a barrier layer (BL) (Ga₂O₃) was designed to obtain superior electrical performance in thin-film transistors (TFTs). Within the oxide NL structure, multiple-channel formation was demonstrated by a pile-up of free charge carriers near CL/BL heterointerfaces in the form of the so-called quasi-two-dimensional electron gas (q2DEG), which leads to an outstanding carrier mobility (μ_FE) with band-like transport, steep gate swing (SS), and positive threshold voltage (V_TH) behavior. Furthermore, reduced trap densities in oxide NL compared to those of conventional oxide single-layer TFTs ensures excellent stabilities. The optimized device with the In_0.75Zn_0.25O/Ga₂O₃ NL TFT showed remarkable electrical performance: μ_FE of 77.1 ± 0.67 cm²/(V s), V_TH of 0.70 ± 0.25 V, SS of 100 ± 10 mV/dec, and I_ON/OFF of 8.9 × 10⁹ with a low operation voltage range of ≤2 V and excellent stabilities (ΔV_TH of +0.27, -0.55, and +0.04 V for PBTS, NBIS, and CCS, respectively). Based on in-depth analyses, the enhanced electrical performance is attributed to the presence of q2DEG formed at carefully engineered CL/BL heterointerfaces. Technological computer-aided design (TCAD) simulation was performed theoretically to confirm the formation of multiple channels in an oxide NL structure where the formation of a q2DEG was verified in the vicinity of CL/BL heterointerfaces. These results clearly demonstrate that introducing a heterojunction or NL structure concept into this atomic layer deposition (ALD)-derived oxide semiconductor system is a very effective strategy to boost the carrier-transporting properties and improve the photobias stability in the resulting TFTs.

Hydrogen-Doping-Enabled Boosting of the Carrier Mobility and Stability in Amorphous IGZTO Transistors

This study investigated the effect of hydrogen (H) on the performance of amorphous In-Ga-Zn-Sn oxide (a-In_0.29Ga_0.35Zn_0.11Sn_0.25O) thin-film transistors (TFTs). Ample H in plasma-enhanced atomic layer deposition (PEALD)-derived SiO₂ can diffuse into the underlying a-IGZTO film during the postdeposition annealing (PDA) process, which affects the electrical properties of the resulting TFTs due to its donor behavior in the a-IGZTO. The a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs at the PDA temperature of 400 °C exhibited a remarkably higher field-effect mobility (μ_FE) of 85.9 cm²/Vs, a subthreshold gate swing (SS) of 0.33 V/decade, a threshold voltage (V_TH) of -0.49 V, and an I_ON/OFF ratio of ∼10⁸; these values are superior compared to those of unpassivated a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs (μ_FE = 23.3 cm²/Vs, SS = 0.36 V/decade, and V_TH = -3.33 V). In addition, the passivated a-In_0.29Ga_0.35Zn_0.11Sn_0.25O TFTs had good stability against the external gate bias duration. This performance change can be attributed to the substitutional H doping into oxygen sites (H_O) leading to a boost in n_e and μ_FE. In contrast, the beneficial H_O effect was barely observed for amorphous indium gallium zinc oxide (a-IGZO) TFTs, suggesting that the hydrogen-doping-enabled boosting of a-IGZTO TFTs is strongly related to the existence of Sn cations. Electronic calculations of V_O and H_O using density functional theory (DFT) were performed to explain this disparity. The introduction of SnO₂ in a-IGZO is predicted to cause a conversion from shallow V_O to deep V_O due to the lower formation energy of deep V_O, which is effectively created around Sn cations. The formation of H_O by H doping in the IGZTO facilitates the efficient connection of atomic states forming the conduction band more smoothly. This reduces the effective mass and enhances the carrier mobility.

Comparative Study of Atomic Layer Deposited Indium-Based Oxide Transistors with a Fermi Energy Level-Engineered Heterojunction Structure Channel through a Cation Combinatorial Approach

Amorphous indium-gallium-zinc oxide (a-IGZO) has become a standard channel ingredient of switching/driving transistors in active-matrix organic light-emitting diode (AMOLED) televisions. However, mobile AMOLED displays with a high pixel density (≥500 pixels per inch) and good form factor do not often employ a-IGZO transistors due to their modest mobility (10-20 cm²/(V s)). Hybrid low-temperature polycrystalline silicon and oxide transistor (LTPO) technology is being adapted in high-end mobile AMOLED devices due to its ultralow power consumption and excellent current drivability. The critical issues of LTPO (including a complicated structure and high fabrication costs) require a search for alternative all-oxide thin-film transistors (TFTs) with low-cost processability and simple device architecture. The atomic layer deposition (ALD) method is a promising route for high-performance all-oxide TFTs due to its unique features, such as in situ cation composition tailoring ability, precise nanoscale thickness controllability, and excellent step coverage. Here, we report an in-depth comparative investigation of TFTs with indium-gallium oxide (IGO)/gallium-zinc oxide (GZO) and indium-zinc oxide (IZO)/GZO heterojunction stacks using an ALD method. IGO and IZO layers with different compositions were tested as a confinement layer (CL), whereas the GZO layer was used as a barrier layer (BL). Optimal IGO/GZO and IZO/GZO channels were carefully designed on the basis of their energy band properties, where the formation of a quasi-two-dimensional electron gas (q2DEG) near the CL/BL interface is realized by rational design of the band gaps and work-functions of the IGO, IZO, and GZO thin films. To verify the effect of q2DEG formation, the device performances and stabilities of TFTs with CL/BL oxide heterojunction stacks were examined and compared to those of TFTs with a single CL layer. The optimized device with the In_0.75Zn_0.25O/Ga_0.80Zn_0.20O stack showed remarkable electrical performance: μ_FE of 76.7 ± 0.51 cm²/(V s), V_TH of -0.37 ± 0.19 V, SS of 0.13 ± 0.01 V/dec, and I_ON/OFF of 2.5 × 10¹⁰ with low operation voltage range of ≥2 V and excellent stabilities (ΔV_TH of +0.35, -0.67, and +0.08 V for PBTS, NBIS, and CCS, respectively). This study suggests the feasibility of using high-performance ALD-derived oxide TFTs (which can compete with the performance of LTPO transistors) for high-end mobile AMOLED displays.

A Covalent Organic-Inorganic Hybrid Superlattice Covered with Organic Functional Groups for Highly Sensitive and Selective Gas Sensing

Organic-inorganic hybrid superlattices (OIHSLs) hold attractive physical and chemical properties, while the construction of single-crystal covalent OIHSLs has not been achieved. Herein a coordination assembly strategy was proposed to create a single-crystal covalent OIHSL PbBDT (BDT=1,4-benzenedithiolate), where layered [PbS₂ ] sublattice covalently connects with benzene sublattice. The covalent bonding offers better thermo-/chemi-stability, inter-sublattice electron transport, and unique organic-group-functionalized surface, which may enable better performances in chemical applications than non-covalent OIHSL. These features endow PbBDT with the highest sensitivity, the lowest detection limit and excellent selectivity towards NO₂ at room temperature among all chemiresistive gas-sensing materials with reported response time less than 2 min without the need of light assistance.

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.