Mitsuru Nakata

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

A new nucleation-site-control method has been proposed for excimer-laser crystallization, where a cross-coupled phase shifter is placed on the sample surface. High thermal conductivity of molten Si flattens its temperature distribution shortly after laser-light irradiation, and, in turn, confines a low-temperature Si region to a very narrow area. Since nucleation can take place only in this narrow area of unmolten Si, not only is the number of nuclei limited to unity, but also the position of a nucleus can be determined. The usefulness of the proposed method is investigated theoretically and experimentally. A single grain as long as 5 µm in diameter was formed at the desired position.

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.

A New Nucleation-Site-Control Excimer-Laser-Crystallization Method

We have developed InGaZnO4 TFTs with polymer gate insulators that can be formed by spin-coating on plastic substrates at temperatures below 130 °C. A 5-inch QVGA flexible OLED display has been fabricated by means of ink-jet printing on a TFT backplane, and it has successfully displayed clear color video images while in a bent state.

Effects of Excimer Laser Annealing on InGaZnO4 Thin-Film Transistors Having Different Active-Layer Thicknesses Compared with Those on Polycrystalline Silicon

In this paper, we present the effects of thermal annealing and excimer laser annealing (ELA) on the characteristics of zinc-oxide (ZnO) thin-film transistors (TFTs) fabricated by magnetron sputtering at room temperature. The transfer characteristics of the ZnO-TFTs are found to improve with increasing thermal annealing temperature. Results of in situ high-temperature X-ray diffraction and electron back scattered diffraction pattern analyses indicate that this phenomenon arises from the crystallization of a low-crystallinity region present in as-deposited ZnO films. Almost all low-crystallinity regions crystallize at temperatures above 500 °C. Furthermore, to achieve such high-performance TFTs on flexible plastic substrates, ZnO-TFTs on a plastic substrate were irradiated with an excimer laser to raise the temperature of the ZnO films. Because of the extremely short pulse width of the laser (25 ns), it is possible to selectively increase the temperature of the ZnO with negligible thermal damage to the plastic substrate. The ELA process has enabled us to produce flexible high-performance ZnO-TFTs with a field-effect mobility of 12 cm2 V-1 s-1.