Seonghwan Hong

Simple method to enhance positive bias stress stability of In-Ga-Zn-O thin-film transistors using a vertically graded oxygen-vacancy active layer.

We proposed a simple method to deposit a vertically graded oxygen-vacancy active layer (VGA) to enhance the positive bias stress (PBS) stability of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). We deposited a-IGZO films by sputtering (target composition; In2O3:Ga2O3:ZnO = 1:1:1 mol %), and the oxygen partial pressure was varied during deposition so that the front channel of the TFTs was fabricated with low oxygen partial pressure and the back channel with high oxygen partial pressure. Using this method, we were able to control the oxygen vacancy concentration of the active layer so that it varied with depth. As a result, the turn-on voltage shift following a 10 000 s PBS of optimized VGA TFT was drastically improved from 12.0 to 5.6 V compared with a conventional a-IGZO TFT, without a significant decrease in the field effect mobility. These results came from the self-passivation effect and decrease in oxygen-vacancy-related trap sites of the VGA TFTs.

A review of multi-stacked active layer structures for solution processed oxide semiconductor thin film transistors

ABSTRACT In this review, the multi-stacked active-layer (MSAL) structures for the solution-processed amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) are summarized to improve their electrical characteristics and stabilities based on the authors’ previous researches. The MSAL structures can overcome an inherent weakness of the solution-processed AOS TFTs, which is the creation of porosities from solvent volatilization. Furthermore, by modifying each layer, the performance and reliability of the solution-processed AOS TFTs could be improved more. Here, the fundamental studies of MSAL structures with homo-stacked active-layer TFTs are presented, and the various modulations of active layers in hetero-stacked active-layer TFTs are covered. In addition, the effect of the interface between the stacked layers is also discussed.

Low-temperature fabrication of an HfO2 passivation layer for amorphous indium-gallium-zinc oxide thin film transistors using a solution process

We report low-temperature solution processing of hafnium oxide (HfO2) passivation layers for amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). At 150 °C, the hafnium chloride (HfCl4) precursor readily hydrolyzed in deionized (DI) water and transformed into an HfO2 film. The fabricated HfO2 passivation layer prevented any interaction between the back surface of an a-IGZO TFT and ambient gas. Moreover, diffused Hf4+ in the back-channel layer of the a-IGZO TFT reduced the oxygen vacancy, which is the origin of the electrical instability in a-IGZO TFTs. Consequently, the a-IGZO TFT with the HfO2 passivation layer exhibited improved stability, showing a decrease in the threshold voltage shift from 4.83 to 1.68 V under a positive bias stress test conducted over 10,000 s.

Improvement of Electrical Characteristics and Stability of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors Using Nitrocellulose Passivation Layer

In this research, nitrocellulose is proposed as a new material for the passivation layers of amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs). The a-IGZO TFTs with nitrocellulose passivation layers (NC-PVLs) demonstrate improved electrical characteristics and stability. The a-IGZO TFTs with NC-PVLs exhibit improvements in field-effect mobility (μFE) from 11.72 ± 1.14 to 20.68 ± 1.94 cm2/(V s), threshold voltage (Vth) from 1.85 ± 1.19 to 0.56 ± 0.35 V, and on/off current ratio (Ion/off) from (5.31 ± 2.19) × 107 to (4.79 ± 1.54) × 108 compared to a-IGZO TFTs without PVLs, respectively. The Vth shifts of a-IGZO TFTs without PVLs, with poly(methyl methacrylate) (PMMA) PVLs, and with NC-PVLs under positive bias stress (PBS) test for 10,000 s represented 5.08, 3.94, and 2.35 V, respectively. These improvements were induced by nitrogen diffusion from NC-PVLs to a-IGZO TFTs. The lone-pair electrons of diffused nitrogen attract weakly bonded oxygen serving as defect sites in a-IGZO TFTs. Consequently, the electrical characteristics are improved by an increase of carrier concentration in a-IGZO TFTs, and a decrease of defects in the back channel layer. Also, NC-PVLs have an excellent property as a barrier against ambient gases. Therefore, the NC-PVL is a promising passivation layer for next-generation display devices that simultaneously can improve electrical characteristics and stability against ambient gases.

Flexible carbon nanofiber electrodes for a lead zirconate titanate nanogenerator

The performance and stability of flexible carbon nanofiber (CNF) electrodes were investigated for a lead zirconate titanate (PZT) nanogenerator. A comparative study was carried out with indium tin oxide coated polyethylene 2,6-naphthalate (ITOP) electrodes. The piezo voltage generated for CNF/PZT/CNF was almost 2 V which is ∼10% higher than ITOP/PZT/ITOP after the bending test. The increase in voltage shows a good contact probability between PZT nanofibers and CNF electrodes and demonstrates the feasibility of CNFs as flexible and stable electrodes for nanogenerators.

Plasma Polymerization Enabled Polymer/Metal–Oxide Hybrid Semiconductors for Wearable Electronics

A facile fabrication of polymer/metal-oxide hybrid semiconductors is introduced to overcome the intrinsically brittle nature of inorganic metal-oxide semiconductors. The fabrication of the hybrid semiconductors was enabled by plasma polymerization of polytetrafluoroethylene (PTFE) via radio frequency magnetron sputtering process which is highly compatible with metal-oxide semiconductor manufacturing facilities. Indium-gallium-zinc oxide (IGZO) and PTFE are cosputtered to fabricate PTFE-incorporated IGZO thin-film transistors (IGZO:PTFE TFTs) and they exhibit a field-effect mobility of 10.27 cm2 V-1 s-1, a subthreshold swing of 0.38 V dec-1, and an on/off ratio of 1.08 × 108. When compared with conventional IGZO TFTs, the IGZO:PTFE TFTs show improved stability results against various electrical, illumination, thermal, and moisture stresses. Furthermore, the IGZO:PTFE TFTs show stable electrical characteristics with a threshold voltage ( Vth) shift of 0.89 V after 10 000 tensile bending cycles at a radius of 5 mm.

The self-activated radical doping effects on the catalyzed surface of amorphous metal oxide films

In this study, we propose a self-activated radical doping (SRD) method on the catalyzed surface of amorphous oxide film that can improve both the electrical characteristics and the stability of amorphous oxide films through oxidizing oxygen vacancy using hydroxyl radical which is a strong oxidizer. This SRD method, which uses UV irradiation and thermal hydrogen peroxide solution treatment, effectively decreased the amount of oxygen vacancies and facilitated self-passivation and doping effect by radical reaction with photo-activated oxygen defects. As a result, the SRD-treated amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) showed superior electrical performances compared with non-treated a-IGZO TFTs. The mobility increased from 9.1 to 17.5 cm2/Vs, on-off ratio increased from 8.9 × 107 to 7.96 × 109, and the threshold voltage shift of negative bias-illumination stress for 3600 secs under 5700 lux of white LED and negative bias-temperature stress at 50 °C decreased from 9.6 V to 4.6 V and from 2.4 V to 0.4 V, respectively.

Silicon Cations Intermixed Indium Zinc Oxide Interface for High-Performance Thin-Film Transistors Using a Solution Process

Solution-processed amorphous metal-oxide thin-film transistors (TFTs) utilizing an intermixed interface between a metal-oxide semiconductor and a dielectric layer are proposed. In-depth physical characterizations are carried out to verify the existence of the intermixed interface that is inevitably formed by interdiffusion of cations originated from a thermal process. In particular, when indium zinc oxide (IZO) semiconductor and silicon dioxide (SiO2) dielectric layer are in contact and thermally processed, a Si4+ intermixed IZO (Si/IZO) interface is created. On the basis of this concept, a high-performance Si/IZO TFT having both a field-effect mobility exceeding 10 cm2 V-1 s-1 and a on/off current ratio over 107 is successfully demonstrated.