Takashi Hirao

Novel top‐gate zinc oxide thin‐film transistors (ZnO TFTs) for AMLCDs

— High-performance top-gate thin-film transistors (TFTs) with a transparent zinc oxide (ZnO) channel have been developed. ZnO thin films used as active channels were deposited by rf magnetron sputtering. The electrical properties and thermal stability of the ZnO films are controlled by the deposition conditions. A gate insulator made of silicon nitride (SiNx) was deposited on the ZnO films by conventional P-CVD. A novel ZnO-TFT process based on photolithography is proposed for AMLCDs. AMLCDs having an aperture ratio and pixel density comparable to those of a-Si:H TFT-LCDs are driven by ZnO TFTs using the same driving scheme of conventional AMLCDs.

Bottom-Gate Zinc Oxide Thin-Film Transistors (ZnO TFTs) for AM-LCDs

In this paper, high-performance bottom-gate thin-film transistors (TFTs) with transparent zinc oxide (ZnO) channels have been developed. The ZnO film for active channels was deposited by RF magnetron sputtering. The crystallinity of the ZnO film drastically improved when it was deposited on a doublelayer SiOx/SiNx gate insulator. In order to achieve a ZnO TFT back-plane for liquid-crystal display (LCD) with the required pattern accuracy, dry etching of the ZnO film in an Ar and CH4 chemistry has been developed. The etching rate and tapered profile of the ZnO film could be controlled by the Ar content in the etching gases of Ar and CH4. The saturation mobility (musat) of the ZnO TFT strongly depended on a gate voltage. A musat of 5.2 & cm2 .(V .s)-1 at VGS = 40 V and VDS = 10 V, and an on/off-current ratio of 2.7 x 107 were obtained. A drain-current uniformity of plusmn7% was achieved within a radius of 20 mm from the substrate center. A 1.46 -in diagonal LCD with 61 600 pixels has been driven by the ZnO-TFT back-plane. A moving picture image was available on fabricated LCD driven by the ZnO TFTs.

Efficient field emission from an individual aligned carbon nanotube bundle enhanced by edge effect

The authors report on the field emission from an aligned carbon nanotube (CNT) bundle grown by thermal chemical vapor deposition. The CNT bundle showed a low-threshold electric field of 2.0V∕μm that produced a current density of 10mA∕cm2, sustainable evolution of current density up to 2.8A∕cm2 at 2.9V∕μm, and good emission stability without degradation for 200h of continuous dc emission. By calculating the electric-field distribution, it was found that the electric field was significantly higher at the edge of the CNT bundle than at the center. The excellent field-emission properties of the aligned CNT bundle were attributed to the edge effect and the high-density structure.

Stacked Image Sensor With Green- and Red-Sensitive Organic Photoconductive Films Applying Zinc Oxide Thin-Film Transistors to a Signal Readout Circuit

A vertically stacked image sensor composed of green (G)- and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) that uses a transparent zinc oxide (ZnO) channel to read out a signal generated in the organic film, was fabricated. The effective number of pixels of the ZnO-TFT circuits was 1410 (47 times 30), and their pitch was 600 mum. The current on/off ratio and turn-on voltage of the ZnO-TFT were over 105 and 1.5 V, respectively. The G- and R-sensitive organic photoconductive films showed excellent wavelength selectivity: the peak wavelength of the G-sensitive film was 540 nm, and that of the R-sensitive one was 700 nm. A color image with a resolution corresponding to the number of pixels was obtained by a shooting experiment with the fabricated image sensor, which clearly demonstrated color separation in the depth direction of the image sensor, using a stacked structure of wavelength-selective organic films with ZnO-TFT readout circuits.

Analysis of Hump Characteristics in Thin-Film Transistors With ZnO Channels Deposited by Sputtering at Various Oxygen Partial Pressures

The electrical properties of thin-film transistors (TFTs) with ZnO channels which were deposited by radio-frequency magnetron sputtering at various oxygen partial pressures [p( O2)] are investigated. A negative shift of the turn-on voltage with a “hump” was observed, and donorlike traps were generated at intermediate energy levels from the conduction band when the ZnO channel was deposited at p(O2) below a critical pressure. Thermal desorption spectroscopy study revealed that the donorlike traps were generated when the ZnO film changed from O- to Zn-rich condition. The Zn-related native defects would be a possible origin of the donorlike traps generated at intermediate energy levels in the ZnO TFTs.

4.1: Distinguished Paper: High Mobility Top-Gate Zinc Oxide Thin-Film Transistors (ZnO-TFTs) for Active-Matrix Liquid Crystal Displays

High-performance top-gate ZnO thin-film transistors (TFTs) for AM-LCDs have been developed. Sputtered ZnO was used as an active channel and silicon nitride (SiNx) deposited by plasma enhanced chemical vapor deposition (P-CVD) was used as a gate insulator. Field effect mobility and threshold voltage of the ZnO-TFT are 50.3 cm2/V⋅sec and 1.1 V, respectively. We first demonstrated a 1.46″ diagonal AM-LCD driven by ZnO-TFTs.

Influence of Thermal Annealing on Microstructures of Zinc Oxide Films Deposited by RF Magnetron Sputtering

The effects of postdeposition annealing at up to 350 °C on the crystallinity and thermal stability of sputter-deposited ZnO films have been investigated in terms of the deposition pressure. The average crystallite size and biaxial film stress of an as-deposited ZnO film is strongly related to the deposition pressure. A crystallization process during postdeposition annealing was only observed when the film was deposited under low pressure. Thermal desorption spectrometry (TDS) measurement revealed that Zn desorption from the ZnO film was suppressed with decreasing deposition pressure. Zn desorption from the films was correlated with rf plasma analysis results. It was found that oxygen was desorbed only from the films deposited at low pressure with annealing temperatures above 250 °C. This desorption of oxygen was strongly related to the crystallization process during postdeposition annealing.

Ultra-Low-Threshold Field Electron Emission from Pillar Array of Aligned Carbon Nanotube Bundles

We observed the field electron emission of the technologically useful current density of 10 mA/cm2 at an extremely low threshold electric field (Eth) of 1.0 V/µm, from an array of pillars of aligned carbon nanotube bundles, which were grown on a Si substrate by thermal chemical vapor deposition. Adjusting the distance between the neighboring pillars (R) and the pillar height (H) to the optimal condition (R/H = 2) can effectually enhance the field concentration, resulting in a highly efficient electron emission. The obtained Eth is 1/2–1/3 times lower than the best values that have been reported to date.

Electrical Properties of the Thin-Film Transistor With an Indium–Gallium–Zinc Oxide Channel and an Aluminium Oxide Gate Dielectric Stack Formed by Solution-Based Atmospheric Pressure Deposition

We developed a thin-film transistor (TFT) with an amorphous-indium-gallium-zinc oxide (IGZO) channel and aluminium oxide (AlO_x) gate dielectric stack that was formed using a solution-based atmospheric pressure chemical vapor deposition. A breakdown electric field of 5.9 MV/cm and a dielectric constant of 6.8 were achieved for the AlO_x gate dielectric. The nonvacuum-processed IGZO TFT gave a field-effect mobility of 4.2 cm² · V^-1 · s^-1 and an on/off current ratio of over 10⁸. Moreover, the proposed deposition method is a powerful tool for material research to explore multicomponent oxide insulators and semiconductors.

A 128×96 Pixel Stack-Type Color Image Sensor: Stack of Individual Blue-, Green-, and Red-Sensitive Organic Photoconductive Films Integrated with a ZnO Thin Film Transistor Readout Circuit

A color image was produced by a vertically stacked image sensor with blue (B)-, green (G)-, and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) array that uses a zinc oxide (ZnO) channel to read out the signal generated in each organic film. The number of the pixels of the fabricated image sensor is 128×96 for each color, and the pixel size is 100×100 µm2. The current on/off ratio of the ZnO TFT is over 106, and the B-, G-, and R-sensitive organic photoconductive films show excellent wavelength selectivity. The stacked image sensor can produce a color image at 10 frames per second with a resolution corresponding to the pixel number. This result clearly shows that color separation is achieved without using any conventional color separation optical system such as a color filter array or a prism.