Takahiro Hiramatsu

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.

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.

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.

Effect of Energetic Particle Bombardment on Microstructure of Zinc Oxide Films Deposited by RF Magnetron Sputtering

Zinc oxide (ZnO) films have been deposited on glass substrates by rf magnetron sputtering with Ar/O2 working gases. The crystallite size of the films was characterized by X-ray diffraction (XRD) analysis. At a deposition pressure of 1.0 Pa, the average crystallite size decreased with increasing O2 flow rate. Optical emission spectroscopy (OES) results showed that the average crystallite size can be controlled by the emission intensity ratio of O*/Ar. The XRD results of the ZnO deposited in both Ar/O2 and O2 working gases revealed that the bombardment of the energetic oxygen particles to the growing surface suppresses the surface migration of the sputtered adatoms and that of energetic argon particles enhances the surface migration of the adatoms. The crystallite size and total volume of the crystallite fraction in the film are affected by the flux of both energetic oxygen and argon particles present in the plasma, and are determined by the competition processes of the bombardment of energetic oxygen and argon under a constant deposition pressure.

Effect of Surface Treatment of Gate-Insulator on Uniformity of Bottom-Gate ZnO Thin Film Transistors

The effect of interface treatment of ZnO/gate insulator on the variations in both the electrical properties and the trap density of bottom-gate ZnO thin film transistors (TFTs) were investigated. Two kinds of interface treatments, as a diluted HF wet treatment and a nitrous oxide (N 2 O) plasma treatment, are individually applied to the gate-insulator surface of the ZnO TFTs. The threshold voltage uniformity of ZnO TFTs with the N 2 O plasma-treated gate insulator was drastically improved. The trap densities extracted from the ZnO TFTs revealed that the variation in the trap density in deep energy level is reduced significantly by the N 2 O plasma treatment.

Photo-Leakage Current of Zinc Oxide Thin-Film Transistors

The origin of photo-leakage current of zinc oxide thin-film transistors (ZnO TFTs) under light irradiation was investigated using a light shield technique. The irradiation position dependence revealed that the effect of light irradiation is much stronger near the source region in the channel than near the drain region. This can be explained by the enhanced carrier injection from the source electrode. The irradiation near the drain region, on the other hand, simply induced photocurrent, which is much smaller than the carrier injection on the source side. Therefore, completely transparent ZnO TFTs under visible light irradiation will be obtained, if the carrier injection from the source electrode is successfully suppressed.