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 review aims to provide a technical roadmap and progress update for backplane thin film transistors (TFTs) used in organic light emitting diodes flat panel displays and next-generation flexible...

https://www.tandfonline.com/doi/pdf/10.1080/15980316.2020.1818641

Recent progress in the development of backplane thin film transistors for information displays

In this letter, we report solution-processed, high-performance Indium-Zinc-Oxide (IZO) thin-film transistors (TFTs). The annealing temperature of IZO films are studied and found that devices annealed at 350 °C exhibit the best performance. With the use of a thin AlxOy layer as the gate dielectric, one-volt IZO TFTs are demonstrated, showing a high current on/off ratio of > 105, a high mobility over 10 cm2/Vs, and a low subthreshold swing (SS) of 83 mV/dec, which is fairly close to the theoretical limit of SS at 300 K. Such a high device performance is also found comparable to those deposited using vacuum-based methods. As a result, the presented devices might possess a great potential in low-cost, low-power electronics.

One-Volt, Solution-Processed InZnO Thin-Film Transistors

Solution processing of metal oxide-based semiconductors is an attractive route for low-cost fabrication of thin films devices. ZnO thin films were synthesized from one-step spin coating-pyrolysis technique using zinc neodecanoate precursor. X-ray diffraction (XRD), UV–visible optical transmission spectrometry and photoluminescence spectroscopy suggested conversion to polycrystalline ZnO phase for decomposition temperatures higher than 400 °C. A 15 % precursor concentration was found to produce optimal TFT performance on annealing at 500 °C, due to generation of sufficient charge percolation pathways. The device performance was found to improve upon increasing the annealing temperature and the optimal saturation mobility of 0.1 cm2 V−1 s−1 with ION/IOFF ratio ~ 107 was achieved at 700 °C annealing temperature. The analysis of experimental results based on theoretical models to understand charge transport envisaged that the grain boundary depletion region is major source of deep level traps and their effective removal at increased annealing temperature leads to evolution of transistor performance. Single-step spin coating-pyrolysis synthesis of ZnO thin films from non-aqueous precursor zinc neodecanoate has been investigated for transistor applications.

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Optimization of Transistor Characteristics and Charge Transport in Solution Processed ZnO Thin Films Grown from Zinc Neodecanoate

A facile low-cost preparation of high-k ZrO2 dielectric films for superior thin-film transistors

The high-performance ZnO thin-film transistors (TFTs) were fabricated on indium tin oxide glass with high-capacitance atomic layer deposition (ALD)-processed ZrO2 as the gate dielectric. The 5-nm ultrathin ZrO2 film showed a very high areal capacitance of 820 nF/cm2 at 20 Hz, a relatively high breakdown field of 14 MV/cm, and low surface root-mean-square (rms) roughness of 0.22 nm, making it possible for ZnO/ZrO2 TFT to not only be operated by an ultralow operating voltage of 1 V but also present a near theoretical limit subthreshold swing of 69 mV/dec. Furthermore, the ZnO TFT with a 5-nm ZrO2 gate dielectric exhibited excellent performance, such as a high Ion/Ioff of 107, large field effect mobility of 36.8 cm2/Vs, low-density of trapping states ( ${N}_{{\text {trap}}}$ ) of $1.6\times 10^{{11}}$ eV−1cm−2, and negligible hysteresis. In addition, the electron transport mode was built to explain the high mobility of nanocrystalline ZnO TFT. As a result, the ultralow operating voltage TFTs exhibited great potential for low-powered electronics applications.

High-Performance 1-V ZnO Thin-Film Transistors With Ultrathin, ALD-Processed ZrO 2 Gate Dielectric

This work reports an alternative atomic layer deposition (ALD) method to fabricate ZnO thin-film transistors (TFTs). The ZnO film is deposited with temperature naturally-cooling process from 200 to 100°C, called a “temperature gradient ZnO (TG-ZnO)”). After optimized annealing treatment at 300°C, the TG-ZnO TFT shows an excellent performance compared to those fabricated with traditional constant temperature deposition, including a high saturation mobility (μ
 sat
) of 11.8 cm
 2
/Vs, which is 5 times higher than the ZnO TFT, a good on/off-state current ratio (I
 on
/I
 off
) of 1.9 × 10
 7
, a small subthreshold swing (SS) of 175 mV/decade and a threshold voltage (V
 th
) of 1.1 V. Meanwhile, the TGZnO TFT has better crystallization than 100°C-ZnO and lower oxygen vacancies than 200°C-ZnO. These characters enable the TG-ZnO TFT not only to maintain a high mobility, but also to present a satisfactory I
 on
/I
 off
 ratio. This promising deposition technique provides a new idea for fabricating TFTs with high mobility.

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Temperature Gradient ZnO Deposited via ALD for High-Performance Transistor Applications

In this work, Y2O3–Al2O3 dielectrics were prepared and used in ZnO thin film transistor as gate insulators. The Y2O3 film prepared by the sol–gel method has many surface defects, resulting in a high density of interface states with the active layer in TFT, which then leads to poor stability of the devices. We modified it by atomic layer deposition (ALD) technology that deposited a thin Al2O3 film on the surface of a Y2O3 dielectric layer, and finally fabricated a TFT device with ZnO as the active layer by ALD. The electrical performance and bias stability of the ZnO TFT with a Y2O3–Al2O3 laminated dielectric layer were greatly improved, the subthreshold swing was reduced from 147 to 88 mV/decade, the on/off-state current ratio was increased from 4.24 × 106 to 4.16 × 108, and the threshold voltage shift was reduced from 1.4 to 0.7 V after a 5-V gate was is applied for 800 s.

A Study on Solution-Processed Y2O3 Films Modified by Atomic Layer Deposition Al2O3 as Dielectrics in ZnO Thin Film Transistor

Solution-processed thin film transistors (TFTs) used in flexible electronics require them to be fabricated under low temperature. Ultraviolet (UV) treatment is an effective method to transform the solution precursors into dense semiconductor films. In our work, high-quality indium zinc oxide (IZO) thin films were prepared from nitrate-based precursors after UV treatment at room temperature. After UV treatment, the structure of IZO thin films was gradually rearranged, resulting in good M–O–M network formation and bonds. TFTs using IZO as a channel layer were also fabricated on Si and Polyimide (PI) substrate. The field effect mobility, threshold voltage (Vth), and subthreshold swing (SS) for rigid and flexible IZO TFTs are 14.3 and 9.5 cm2/Vs, 1.1 and 1.7 V, and 0.13 and 0.15 V/dec., respectively. This low-temperature processed route will definitely contribute to flexible electronics fabrication.

Jie Qi

Papers

High-Performance 1-V ZnO Thin-Film Transistors With Ultrathin, ALD-Processed ZrO 2 Gate Dielectric

Temperature Gradient ZnO Deposited via ALD for High-Performance Transistor Applications

A Study on Solution-Processed Y2O3 Films Modified by Atomic Layer Deposition Al2O3 as Dielectrics in ZnO Thin Film Transistor

Low-Temperature Fabrication of IZO Thin Film for Flexible Transistors.