Nobuki Ibaraki

Threshold voltage instability of a-Si:H TFTs in liquid crystal displays

Abstract A method is presented for simulating the threshold voltage (Vth) shift of a-Si:H TFTs in liquid crystal displays. The Vth shift after a certain operation period can be obtained by summing up each Vth shift, which is caused by pulsed voltage stress between the gate and source, step by step along with the pulse sequence. The estimated lifetime of 10 years at 50°C for one of the prepared samples, assuming a life end of ΔVth≦2V, is quite satisfactory for practical application in LCDs.

a-Si TFT Technologies for AM-LCDS

A technical trend for a-Si TFTs is their application to large-size, high-pixel density AM-LCDs such as XGA, EWS, and HDTV. In order to realize these LCDs, the TFT device characteristics must be improved. Future technologies, which will be necessary to fabricate TFTs with improved characteristics are as follows (1) Fully self-aligned TFT technology: A SA-TFT structure reduces the feedthrough voltage caused by parasitic capacitance due to gate/source overlap. This results in an improved picture quality and a higher aperture ratio. Fabrication of such a structure would require ion doping technology. (2) Ion doping technology: This non-Mass-separated implantation technique has large area doping capability and much higher doping speed compared to conventional ion implantation technique. The Major problems with the ion doping technique is the implantation of unwanted species which deteriorate the quality of source/drain and channel regions of TFTs.

13.4: A 20.8-inch WXGA Full Color AMOLED Display by Integrating Scattering Reflector with Micro-Bumps

In order to improve light extraction efficiency from organic light emitting diodes (OLED), we report a novel structure of top-emission OLED by integrating a scattering reflector with micro-bumps into the substrate. The mechanism of light extraction has been investigated by evaluating the dependency of flux on the shape of the micro-bumps. It was confirmed that the scattering effect of the micro-bumps improves the light extraction of OLED. Moreover, the scattering reflector has been applied on a 20.8-inch WXGA full color AMOLED display. This is the worlds first report that actually proves a light extraction technology, integrating scattering reflector, applied on AMOLED display.

Properties of a-SiN0.7:H Films in High Electric Field

To understand the mechanism of transport in a high electric field, various properties of a-SiN0.7:H films deposited by rf glow discharge were investigated. The results are explained by a model in which two kinds of centers with different barrier heights are subject to Poole-Frenkel ionization. Either one of these centers contributes to the current, depending on the temperature. Above room temperature, the center contributing to Poole-Frenkel current is the neutral Si dangling bond, where the Fermi level is located. Below room temperature, the center can be associated with the negatively charged Si dangling bond, which behaves as an electron trap for photoconductivity. Based on this understanding of energy levels subject to Poole-Frenkel ionization, field enhancement of the photocurrent was expected and actually observed for the first time.

Importance of first layer thickness on TFT characteristics using a-Si:H deposited by 2-step process

In order to increase the throughput during thin film transistor-liquid crystal display manufacture, we investigated a 2-step deposition process for hydrogenated amorphous silicon. The first hydrogenated amorphous silicon layer of an inverted staggered type thin film transistor was deposited at a lower deposition rate to improve the interface with the gate insulator layer and the second hydrogenated amorphous silicon layer was deposited at a higher rate to improve the throughput. It was found that the mobility values of thin film transistors increased with increasing first layer thickness and reached a saturation value after a certain first layer thickness. When the first hydrogenated amorphous silicon layer quality was improved by decreasing its deposition rate, a thicker first layer hydrogenated amorphous silicon was needed to reach a mobility saturation.

Switching performance of high rate deposition processing a-Si:H TFTs

Abstract We studied the switching performance of thin film transistors fabricated using high deposition rate hydrogenated amorphous silicon deposited at high temperature. High field effect mobility was obtained with material deposited at rates close to 100 nm/min. The mobility decreased, the output characteristics showed current crowding, and capacitance-voltage ( C - V ) characteristics showed frequency dispersion and did not respond to high frequency when the hydrogenated amorphous silicon deposition rates were further increased. We also studied the performance of thin film transistors made using a 2-step deposition process for the hydrogenated amorphous silicon in an attempt to further increase the effective deposition rate. By using the 2-step process for hydrogenated amorphous silicon deposition, both static and dynamic mobility were improved and the charging of the pixel was not seriously affected for material deposited at close to ∼ 200 nm/min.

Effect of Ion Doping Conditions on Electrical Conductivity of Amorphous Silicon Films and Its Application to Thin Film Transistors

In this paper, we present a systematic study of the effect of phophorus ion doping conditions on electrical properties of amorphous silicon (a-Si) films. A large variation in electrical conductivity for various ion doped amorphous silicon films was found to be related to simultaneous implantation of hydrogen during the P ion doping process. The electrical conductivities of amorphous silicon films were qualitatively related to incorporation of hydrogen during the ion doping process, by measuring optical gaps of ion-doped amorphous silicon films and by simulating the ion doping process. By minimizing hydrogen incorporation during ion doping, a-Si films with conductivity greater than 10-2 S/cm were obtained, which is at least one order of magnitude higher than the best results in literature for ion doping of plasma-enhanced chemical vapor deposited (PECVD) amorphous silicon films. Reasonably good thin film transistor (TFT) characteristics were obtained for ion doping energy of 10 keV (mobility=0.68 cm2 V-1 s-1, threshold voltage=3.8 V), which deteriorated upon increase of the ion doping energy.

A new a-Si TFT with SiO 2 /SiN/sub x/ gate insulator for 10.4-inch LCDs

A novel a-Si TFT (thin-film transistor) with a composite gate insulator of CVD (chemical-vapor-deposited) SiO/sub 2//PE-CVD (plasma-enhanced CVD) SiN/sub x/ has been developed. This TFT has been applied to 10.4-in-diagonal LCDs (liquid crystal displays). Because of the high quality of CVD SiO/sub 2/, Vth drift, which was often observed after prolonged application of gate bias, was remarkably reduced compared to PE-CVD SiO/sub x/N/sub y/. Also, the degradation of subthreshold characteristics on a-Si TFT (often observed after long-term operation of LCDs at high temperature) was improved. Selective etching technologies of SiN/sub x/ against SiO/sub 2/, which is one of key issues in connection with obtaining high production yield, have been developed.<<ETX>>

36.3: An Organic Light-Emitting Diode with Highly Efficient Light Extraction Using Newly Developed Diffraction Layer

In order to improve light extraction from an organic light emitting diode (OLED), a novel structure of a top emission OLED was proposed by applying the diffraction layer. We have achieved 1.65 times higher light extraction efficiency in case of electroluminescence (EL), and 2.37 times in case of photoluminescence (PL).

58.1: Invited Paper: Views on the Present and Future Promise of OLED Displays

Although OLED performance has been explained quasi-quantitatively by using a radar chart analysis, there are few reports of actual quantitative evaluation results. We quantitatively evaluated OLED optical and electrical characteristics and compared them with LCD and CRT performance. OLED optical characteristics are superior to those of LCDs. Furthermore, problems related to OLEDs are discussed in terms of light emitting materials, device structure, and panel components.