Jae Won Na

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.

Interface location-controlled indium gallium zinc oxide thin-film transistors using a solution process

The role of an interface as an electron-trapping layer in double-stacked indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) was investigated and interface location-controlled (ILC) IGZO TFTs were introduced. In the ILC TFTs, the thickness of the top and bottom IGZO layers is controlled to change the location of the interface layer. The system exhibited improved electrical characteristics as the location of the interface layer moved further from the gate insulator: field-effect mobility increased from 0.36 to 2.17 cm2 V−1 s−1, and the on-current increased from 2.43 × 10−5 to 1.33 × 10−4 A. The enhanced electrical characteristics are attributed to the absence of an electron-trapping interface layer in the effective channel layer where electrons are accumulated under positive gate bias voltage.

Effect of Static and Rotating Magnetic Fields on Low-Temperature Fabrication of InGaZnO Thin-Film Transistors

We suggest thermal treatment with static magnetic fields (SMFs) or rotating magnetic fields (RMFs) as a new technique for the activation of indium-gallium-zinc oxide thin-film transistors (IGZO TFTs). Magnetic interactions between metal atoms in IGZO films and oxygen atoms in air by SMFs or RMFs can be expected to enhance metal-oxide (M-O) bonds, even at low temperature (150 °C), through attraction of metal and oxygen atoms having their magnetic moments aligned in the same direction. Compared to IGZO TFTs with only thermal treatment at 300 °C, IGZO TFTs under an RMF (1150 rpm) at 150 °C show superior or comparable characteristics: field-effect mobility of 12.68 cm2 V-1 s-1, subthreshold swing of 0.37 V dec-1, and on/off ratio of 1.86 × 108. Although IGZO TFTs under an SMF (0 rpm) can be activated at 150 °C, the electrical performance is further improved in IGZO TFTs under an RMF (1150 rpm). These improvements of IGZO TFTs under an RMF (1150 rpm) are induced by increases in the number of M-O bonds due to enhancement of the magnetic interaction per unit time as the rpm value increases. We suggest that this new process of activating IGZO TFTs at low temperature widens the choice of substrates in flexible or transparent devices.

Improvement in Electrical Characteristics of Eco-friendly Indium Zinc Oxide Thin-Film Transistors by Photocatalytic Reaction

Eco-friendly solution-processed oxide thin-film transistors (TFTs) were fabricated through photocatalytic reaction of titanium dioxide (PRT). The titanium dioxide (TiO2) surface reacts with H2O under ultraviolet (UV) light irradiation and generates hydroxyl radicals (OH•). These hydroxyl radicals accelerate the decomposition of large organic compounds such as 2-methoxyethanol (2ME; one of the representative solvents for solution-processed metal oxides), creating smaller organic molecular structures compared with 2ME. The decomposed small organic materials have low molar masses and low boiling points, which help improving electrical properties via diminishing defect sites in oxide channel layers and fabricating low-temperature solution-processed oxide TFTs. As a result, the field-effect mobility improved from 4.29 to 10.24 cm2/V·s for IGZO TFTs and from 2.78 to 7.82 cm2/V·s for IZO TFTs, and the Vth shift caused by positive bias stress and negative bias illumination stress over 1000 s under 5700 lux decreased from 6.2 to 2.9 V and from 15.3 to 2.8 V, respectively. In theory, TiO2 has a permanent photocatalytic reaction; as such, hydroxyl radicals are generated continuously under UV irradiation, improving the electrical characteristics of solution-processed IZO TFTs even after four iterations of TiO2 recycling in this study. Thus, the PRT method provides an eco-friendly approach for high-performance solution-processed oxide TFTs.

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.

Various approaches for high performance and stable oxide thin-film transistors

We investigated various approaches to enhance the electrical performance and stability of oxide thin-film transistors (TFTs) fabricated with vacuum- and solution-process: vertically graded oxygen vacancy active layer (VGA) by control of oxygen partial pressure, sequential pressure annealing (SPA), and hydrogen peroxide activation (HPA) using ultraviolet irradiation. By adopting these techniques, we could effectively control the defect densities in active layer which resulted in high performance and stable oxide TFTs.