Kazushige Takechi

ELECTRICAL INSTABILITY OF HYDROGENATED AMORPHOUS SILICON THIN-FILM TRANSISTORS FOR ACTIVE-MATRIX LIQUID-CRYSTAL DISPLAYS

We investigated the threshold voltage shifts (ΔVT) of inverted-staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) induced by steady-state (dc) and pulsed (ac) gate bias-temperature-stress (BTS) conditions. Our study showed that, for an equivalent effective-stress-time, ΔVT has an apparent pulse-width dependence under negative BTS conditions–the narrower the pulse width, the smaller the ΔVT. This gate-bias pulse-width dependence is explained by an effective-carrier-concentration model, which relates ΔVT for negative pulsed gate-bias stress to the concentration of mobile carriers accumulated in the conduction channel along the a-Si:H/gate insulator interface. In addition, our investigation of the methodology of a-Si:H TFT electrical reliability evaluation indicates that, instead of steady-state BTS, pulsed BTS should be used to build the database needed to extrapolate ΔVT induced by a long-term display operation. Using these experimental results, we have shown that a-Si:H TFTs have a satisfactory electrical reliability for a long-term active-matrix liquid-crystal display (AMLCD) operation.

Temperature-Dependent Transfer Characteristics of Amorphous InGaZnO4 Thin-Film Transistors

The transfer characteristics of amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) were measured at temperatures ranging from 298 to 523 K in order to analyze the behavior of the above-threshold (ON state) and subthreshold regions. For comparison, the transfer characteristics of a hydrogenated amorphous silicon TFT (a-Si:H TFT) were measured in the same temperature range. We developed a simple analytical model that relates the threshold voltage (Vt) decrease due to increasing temperature to the formation of point defects in a-IGZO. It is well known that the formation of point defects results in the generation of free carriers in oxide semiconductors. Incorporating the analytical model with the experimental transfer characteristics data taken at high temperatures over 423 K, we estimated the formation energy to be approximately 1.05 eV. The Vt decrease because of the generation of point defects is peculiar to a-IGZO TFTs, which is not observed in a-Si:H TFTs. The results for the ON-current activation energy suggested that the density of tail states for a-IGZO is much lower than that for a-Si:H.

Comparison of Ultraviolet Photo-Field Effects between Hydrogenated Amorphous Silicon and Amorphous InGaZnO4 Thin-Film Transistors

We discuss the ultraviolet (UV) photo-field effects in amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) compared with those in hydrogenated amorphous silicon (a-Si:H) TFTs. It is shown that the UV illumination induces a much more significant threshold voltage (Vt) decrease and OFF-current increase for the a-IGZO TFTs than for the a-Si:H TFTs. The significant Vt decrease is found to take several tens of min to return to the initial state after switching off the UV light. A qualitative model is introduced to explain the photoresponse unique to the a-IGZO TFTs.

Dual-Gate Characteristics of Amorphous

We compare the mutual interactions between the top-and bottom-gate fields in a dual-gate structure for amorphous InGaZnO4 (a-IGZO) and hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). We find that a-IGZO TFTs show more significant mutual interactions than a-Si:H TFTs. By using a wide variety of a-IGZO TFTs with varying thicknesses of a-IGZO and gate-insulator layers, we have investigated, on the basis of a conventional fully depleted n-channel silicon-on-insulator model, the mechanism behind parallel shifts in transfer characteristics caused by negative top-gate voltages. Experimental results, even for relatively thick (over 150 nm) a-IGZO layers, are in good agreement with the fully depleted model. This fact may be related to the nonexistence of hole accumulation in a-IGZO, which is an intrinsic property of oxide semiconductors. In contrast, for the a-Si:H TFTs, there is a considerable discrepancy between the experimental results and the model. This discrepancy probably results from the limited penetration of band bending into the a-Si:H layer, as well as from the hole accumulation at the top interface. Such analysis of a-IGZO TFTs is of practical importance in device applications because many issues related to the fabrication and structure of a-IGZO TFTs may be resolved with a better understanding of dual-gate performance.

\hbox{InGaZnO}_{4}

This paper describes a method for utilizing an excimer laser to improve the characteristics of InGaZnO4 (IGZO) thin-film transistors (TFTs). IGZO-TFTs fabricated at room temperature are irradiated with an excimer laser to raise the temperature of the IGZO films for only a very short time, some tens of ns. The ON current of an irradiated IGZO-TFT is more than one order of magnitude higher than that of an un-irradiated TFT. This method is promising for achieving high performance IGZO-TFTs on plastic substrates because the thermal damage to substrates will be much less than that which would result from furnace annealing.

Thin-Film Transistors as Compared to Those of Hydrogenated Amorphous Silicon Thin-Film Transistors

Self-heating, a degradation mechanism of n-channel poly-Si thin-film transistors (TFTs) due to bias stress, has been investigated. The aim of this work is to study this effect in depth to be able to propose a device structure designed to reduce it. The variation of the threshold voltage (V/sub t/) shift with the stress-pulsewidth is related to the temperature rise due to the self-heating effect that depends on the stress-pulsewidth. Electron trapping in the oxide caused by the bias stress is considered to be enhanced by the TFT temperature rise owing to the self-heating. We show that copper-film-based TFTs, which have a substrate made of an extremely thin glass layer and a copper film exhibit much reduced self-heating and thus a decrease of V/sub t/ shift caused by the bias stress. These observations are interpreted using numerical simulations to estimate the temperature rise in the poly-Si channel region due to Joule heating.

Improvement of InGaZnO4 Thin Film Transistors Characteristics Utilizing Excimer Laser Annealing

We have investigated an excimer laser annealing (ELA) process for use in fabricating high-performance amorphous InGaZnO4 (IGZO) thin-film transistors (TFTs) on flexible plastic substrates. We numerically estimate the temperature increase of the IGZO film and substrate as a function of laser energy density. This is one of the most important measures for optimizing ELA conditions in order to apply plastic-based TFT fabrication. Because the optical absorption coefficient of IGZO film is three orders of magnitude higher than that of plastic substrates with respect to 308-nm laser light, it is possible to selectively increase the temperature of the IGZO film. The temperature of the IGZO film is estimated to increase to approximately 1500 °C at typical laser energy levels. Furthermore, incorporating a SiO2 buffer layer (some hundreds of nm) between the IGZO film and the plastic substrate is found to effectively suppress thermal damage to the substrate. We have experimentally investigated the properties of IGZO films irradiated with various excimer-laser energy densities. X-ray diffraction patterns and carrier densities of the IGZO films are found to significantly vary with laser energy density. We have used calculations and experimental results to optimize the ELA process; this has enabled us to produce high-performance IGZO-TFTs having a field-effect mobility of 15.6 cm2 V-1 s-1.

Dependence of self-heating effects on operation conditions and device structures for polycrystalline silicon TFTs

Through use of an atomic force microscope (AFM), surface morphologies for SiNx and a-Si:H films were investigated. By controlling deposition conditions, very smooth films have been obtained. The thin film transistor (TFT) with smooth a-Si:H on smooth SiNx has both high mobility (1.0 cm2V-1s-1) and high stability at the same time. This high-performance TFT will make an important impact in application to high-pixel-density liquid crystal displays (LCDs), such as for use in workstations and high-definition television (HDTV).

Flexible High-Performance Amorphous InGaZnO4 Thin-Film Transistors Utilizing Excimer Laser Annealing

Excimer laser annealing (ELA), which can raise the temperature of InGaZnO4 (IGZO) films for a desired very short time, is effective for obtaining good transfer characteristics in IGZO thin-film transistors (TFTs) on plastic substrates. In this study, we investigate the dependence of the effects of ELA on IGZO-TFTs in comparison with that of its effects on low-temperature polycrystalline silicon (LTPS) for various film thicknesses. We show that the optimum laser energy density with respect to TFT performance decreases with increasing IGZO thickness. Results for IGZO film properties such as carrier density and Hall mobility show the same tendency. This is contrary to the tendency for the ELA of LTPS, in which the threshold energy density for micro crystallization increases with increasing film thickness. In order to gain an insight into the mechanisms at work here, we have numerically estimated the rise in temperature of the IGZO and Si films on the basis of a heat-flow equation. Unlike the case for LTPS, the calculated maximum temperature in IGZO films increases with increasing film thickness. We show that this discrepancy is influenced significantly by the difference in penetration depth between the IGZO film (roughly 70 nm) and Si film (roughly 6 nm) for excimer laser light. To improve the IGZO-TFT characteristics, it is necessary to take into account IGZO penetration depth when determining a suitable IGZO thickness and laser energy density.

High-mobility and high-stability a-Si:H thin film transistors with smooth SiNx/a-Si interface

We report on a process technology that makes possible the transfer of polycrystalline silicon thin-film transistor (poly-Si TFT) arrays from an original rigid glass substrate to another flexible plastic substrate. The transfer technology is characterized by its high-rate glass etching process, using a newly developed apparatus. After a description of the transfer sequence free from adhesive contamination, we present experimental observations for high-rate glass etching in hydrofluoric (HF) and hydrochloric (HCl) acid solution mixtures. The etching apparatus provides jets of the solution mixtures to the glass surface in order to achieve good circulation of the solutions in the bath, as well as to remove etch products effectively from the surface. We successfully achieved etch rates as high as 6 /spl mu/m/min with the etched surfaces almost as smooth as the original glass. In order to gain insight into the chemical mechanism of the etching, we developed a simplified kinetic etching model based on a Langmuir isotherm. The model and experimental etch-rate data are generally in good agreement, indicating that the basic modeling approach captures much of the essential chemistry for the high-rate glass etching. The transfer technology allows us to obtain TFT flexible substrates with good electrical characteristics and flexibility even after an annealing process at as high as 150/spl deg/C. These results demonstrate that the transfer technology is a promising candidate for achieving entirely new lightweight electronic devices such as flexible displays and radio frequency identification tags based on TFT flexible substrates.