Myung-kwan Ryu

42.4L: Late‐News Paper: 4 inch QVGA AMOLED Driven by the Threshold Voltage Controlled Amorphous GIZO (Ga2O3‐In2O3‐ZnO) TFT

We successfully fabricated GIZO (Ga2O3-In2O3-ZnO) TFTs with high mobility of 2.6 cm2/Vs and threshold voltage standard deviation of 0.7V which is comparable to that of a-Si TFTs. Because conventional 5 mask process and bottom gate TFT structure of back channel etch type with channel length of 5 μm is used, it is expected to be transferred to mass production line in near future. Also we report the dependency of threshold voltage on the post process after the back surface of GIZO is exposed and suggest the effective method for controlling the threshold voltage of amorphous GIZO TFTs. Finally we demonstrate 4 inch QVGA AMOLED display driven by GIZO TFTs.

Investigation of the effects of Mg incorporation into InZnO for high-performance and high-stability solution-processed thin film transistors

We have fabricated high-performance and high-stability sol-gel-processed MgInZnO thin films transistors with varying Mg content. As the Mg content was increased, the turn-on-voltage increased and the off-current decreased. This is because the incorporation of Mg (with low standard electrode potential and high optical band gap, Eopt, when oxidized) causes reduction in the oxygen vacancy, acting as a carrier source, and an increase in Eopt of the film. This results in reduction in carrier concentration of the film. Small grains and smooth morphology by varying the Mg content lead to an improvement of the mobility, on-current, and subthreshold gate swing.

Comparative Study on Light-Induced Bias Stress Instability of IGZO Transistors With

This letter examines the effect of the gate dielectric material on the light-induced bias-temperature instability of an In-Ga-Zn-O (IGZO) thin-film transistor (TFT). After applying positive and negative bias stresses, the SiNx-gated TFT exhibited inferior stability to the SiO2-gated TFT, which was explained by the charge trapping mechanism. However, light illumination under a negative bias stress accelerated the negative displacement of the threshold voltage (Vth) of the SiNx-gated IGZO TFT compared to that of the SiO2-gated TFT. This was attributed to the injection of photocreated hole carriers into the underlying gate dielectric bulk region as well as the hole trapping at the gate/channel interface.

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The effects of adding Hf into a InZnO (IZO) system, particularly the electrical characteristics of their thin film and thin film transistors (TFTs), were investigated as a function of atomic concentration from 0 to 10 at. % of Hf and Ga/Zn. Because Hf has a high affinity for oxygen in IZO system, the Hf suppresses carrier generation more effectively than does Ga. At 10 at. % of Hf/Zn atomic concentration, the HfInZnO TFTs showed wider on-to-off ratios than those of GaInZnO TFTs due to the low standard-electrode-potential of Hf and sharp subthreshold swings due to low trap density.

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Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

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Effects of plasma treatments on the back-channel of amorphous Ga 2 O 3 -In 2 O 3 -ZnO (GIZO) thin film transistors (TFTs) are compared for N 2 and N 2 O plasma. Acceptor-like states originating from the oxygen adsorbed on the back-channel of the GIZO TFTs suppress the back-channel current by capturing the electrons in the GIZO active layer and thus shift the threshold voltage to the positive direction. It is also shown that the oxygen in a silicon oxide passivation layer reduces the back-channel current. An enhancement-mode GIZO TFT has been successfully fabricated by combining the N 2 O plasma treatment and the silicon oxide passivation layer.

Gate Dielectrics

We developed thin-film transistors (TFTs) that use solution-processed amorphous indium zinc oxide for the channels in an all-photolithographic process. The transistors, which operate in depletion mode, have excellent transfer characteristics, including saturation mobility of 6.57 cm2/ V·s, threshold voltages of - 0.30 V, turn-on voltages of -1.50 V, on/off ratios of 109, and inverse subthreshold slopes of 0.15 V/dec. We measured the time, temperature, gate voltage, and drain-voltage dependence of the threshold voltage shift, which was 2.16 V under stress conditions. This is nearly the same as that of conventional amorphous silicon TFTs.

Investigating addition effect of hafnium in InZnO thin film transistors using a solution process

A novel method to design metal oxide thin-film transistor (TFT) devices with high performance and high photostability for next-generation flat-panel displays is reported. Here, we developed bilayer metal oxide TFTs, where the front channel consists of indium-zinc-oxide (IZO) and the back channel material on top of it is hafnium-indium-zinc-oxide (HIZO). Density-of-states (DOS)-based modeling and device simulation were performed in order to determine the optimum thickness ratio within the IZO/HIZO stack that results in the best balance between device performance and stability. As a result, respective values of 5 and 40 nm for the IZO and HIZO layers were determined. The TFT devices that were fabricated accordingly exhibited mobility values up to 48 cm(2)/(V s), which is much elevated compared to pure HIZO TFTs (∼13 cm(2)/(V s)) but comparable to pure IZO TFTs (∼59 cm(2)/(V s)). Also, the stability of the bilayer device (-1.18 V) was significantly enhanced compared to the pure IZO device (-9.08 V). Our methodology based on the subgap DOS model and simulation provides an effective way to enhance the device stability while retaining a relatively high mobility, which makes the corresponding devices suitable for ultradefinition, large-area, and high-frame-rate display applications.

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors

Interest in oxide semiconductors stems from benefits, primarily their ease of process, relatively high mobility (0.3–10 cm2/vs), and wide-bandgap. However, for practical future electronic devices, the channel mobility should be further increased over 50 cm2/vs and wide-bandgap is not suitable for photo/image sensor applications. The incorporation of nitrogen into ZnO semiconductor can be tailored to increase channel mobility, enhance the optical absorption for whole visible light and form uniform micro-structure, satisfying the desirable attributes essential for high performance transistor and visible light photo-sensors on large area platform. Here, we present electronic, optical and microstructural properties of ZnON, a composite of Zn3N2 and ZnO. Well-optimized ZnON material presents high mobility exceeding 100 cm2V−1s−1, the band-gap of 1.3 eV and nanocrystalline structure with multiphase. We found that mobility, microstructure, electronic structure, band-gap and trap properties of ZnON are varied with nitrogen concentration in ZnO. Accordingly, the performance of ZnON-based device can be adjustable to meet the requisite of both switch device and image-sensor potentials. These results demonstrate how device and material attributes of ZnON can be optimized for new device strategies in display technology and we expect the ZnON will be applicable to a wide range of imaging/display devices.

Threshold Voltage Control of Amorphous Gallium Indium Zinc Oxide TFTs by Suppressing Back-Channel Current

In this letter, we proposed solution-processed AlInZnO (AIZO)/InZnO (IZO) dual-channel thin-film transistors to realize both proper switching behavior and competitive device performance at the low annealing temperature of 350°C. A thin IZO layer provides a higher carrier concentration, thereby maximizing the charge accumulation and yielding high saturation mobility μ_sat, whereas a thick AIZO layer controls the charge conductance resulting in suitable threshold voltage <i>V</i>_th. We therefore obtain excellent device characteristics at 350°C with μ_sat of 1.57 cm²/V ·s, <i>V</i>_th of 1.28 V, an on/off ratio of ~1.4 × 10⁷, and a subthreshold gate swing of 0.59 V/dec.