Yoshihiro Ueoka

Thermal analysis of amorphous oxide thin-film transistor degraded by combination of joule heating and hot carrier effect

Stability is the most crucial issue in the fabrication of oxide thin-film transistors (TFTs) for next-generation displays. We have investigated the thermal distribution of an InSnZnO TFT under various gate and drain voltages by using an infrared imaging system. An asymmetrical thermal distribution was observed at a local drain region in a TFT depending on bias stress. These phenomena were decelerated or accelerated with stress time. We discussed the degradation mechanism by analyzing the electrical properties and thermal distribution. We concluded that the degradation phenomena are caused by a combination of Joule heating and the hot carrier effect.

Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy

The electronic structures of amorphous indium gallium zinc oxide (a-IGZO) on a SiO2 layers before and after annealing were observed by constant final state X-ray photoelectron spectroscopy (CFS-XPS) and X-ray adsorption near-edge structure spectroscopy (XANES). From the results of angle-resolved CFS-XPS, the change in the electronic state was clearly observed in the a-IGZO bulk rather than in the a-IGZO/SiO2 interface. This suggests that the electronic structures of the a-IGZO bulk strongly affected the thin-film transistor characteristics. The results of XANES indicated an increase in the number of tail states upon atmospheric annealing (AT). We consider that the increase in the number of tail states decreased the channel mobility of AT samples.

Effect of contact material on amorphous InGaZnO thin-film transistor characteristics

Amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) having several metals, namely Ag, Ti, and Mo, as the source and drain electrodes were characterized. TFTs with Ti and Mo electrodes showed drain current–gate voltage characteristics without fluctuation. However, TFTs with Ag electrodes indicated a low noisy on-state current at a large channel length under a low drain–source voltage condition. The source and drain resistances [Rs/d (Ω)] of the TFTs with each of the three metals were calculated from the IDS–VGS characteristics. The Rs/d values of the Ag, Ti, and Mo samples reached 4 × 104, 2 × 104, and 1 × 104 Ω, respectively. This implies that a spatial potential barrier exists at the a-IGZO/Ag interface and that the resistance of the potential barrier changes with the application of gate voltage.

Analysis of printed silver electrode on amorphous indium gallium zinc oxide

Silver (Ag) electrodes printed on amorphous indium gallium zinc oxide (a-InGaZnO) were investigated to analyze their effect on thin-film transistor (TFT) characteristics. The TFT characteristics of samples annealed at 250 °C deteriorated seriously owing to an aggregation of Ag. In addition, the TFT characteristics deteriorated even for 200 °C annealing with oxygen and nitrogen atmospheres. From measurements of secondary ion mass spectrometry and Fourier transform infrared spectroscopy, we found that carbon and hydrogen contained in printed Ag electrodes seriously affect the TFT characteristics. The reduction and protection against these impurities in Ag ink is the key point in the application of printed Ag electrodes to a-InGaZnO TFTs.

Origin of Anisotropic Electrical Properties of 4H-SiC Trench Metal--Oxide--Semiconductor Field-Effect Transistors on Off-Axis Substrates

The origin of anisotropic electrical properties on 4H-SiC trench metal–oxide–semiconductor field-effect transistors (MOSFETs) with large off-axis angles has been discussed from the viewpoints of charge pumping current and surface roughness. The channel mobility (µch) on (110) tilted toward [000] was high and that on (20) tilted toward [0001] was low. The roughness at the SiC/SiO2 interfaces was measured after the removal of the oxide. A rough surface was observed on (20). A locally thick gate oxide results in a nonuniform inversion layer, which is the main origin of the anisotropic electrical properties of the trench MOSFETs on off-axis substrates.

Analysis of electron traps in SiO 2 /IGZO interface by cyclic capacitance-voltage method

SiO2/IGZO interface was investigated by cyclic capacitance-voltage method. High pressure water vapor annealing improved interface states. We consider that this measurement method is useful for evaluating interface characteristics.

Highly reliable a-InGaZnO thin film transistors with new SiN x gate insulators

We fabricated highly reliable a-InGaZnO thin film transistors (TFTs) with new silicon nitride (SiNx) gate insulator (GI) fabricated at low temperature (150°C). This new SiNx layer has low hydrogen content which is controlled by the source gases. Hydrogen gas flow rate ratio to SiF4 was changed as 0%, 1%, and 8%, but the bias-stress-induced threshold voltage instabilities on the three kinds of TFTs kept quite low value. We found that the improvement for threshold voltage instabilities was not due to the effect of the hydrogen content. It is assumed that fluorine in the film, which originates in the source gas, has possibility to improve the interface between the channel and GI.

Hydrogen behavior from ALD Al 2 O 3 passivation layer for amorphous InGaZnO TFTs

The characteristics of amorphous indium gallium zinc oxide (a-InGaZnO) thin-film transistors (TFTs) fabricated using atomic layer deposition (ALD) Al2O3 passivation layer and hydrogen distribution in a-InGaZnO were investigated by comparison with plasma enhanced chemical vapor deposition (PECVD) passivation layer. TFTs fabricated using PECVD passivation layer showed conductive or hump behavior, while that fabricated using ALD passivation layer showed enhancement type characteristics. According to secondary ion mass spectroscopy analysis, hydrogen was introduced into a-InGaZnO during PECVD, while it was hardly introduced during ALD regardless of considerable hydrogen in Al2O3. Thus, the behavior of hydrogen in a-InGaZnO is one possible cause of the difference in TFT characteristics between PECVD and ALD passivation.

Reliability of bottom gate amorphous InGaZnO thin-film transistors with siloxane passivation layer

We investigate the influence of polymer passivation on amorphous Indium Gallium Zinc Oxide (a-IGZO) TFT reliability. We made photosensitive siloxane passivation layer on bottom-gate type a-IGZO TFTs with SiO2/Si substrate. For photosensitive materials, contact holes can be formed by UV photolithography technique without plasma etching process, leading to serious damage to the channel layer. The samples with and without passivation layer were tested in terms of the stability during the positive bias stress (PBS; Vgs = 20 V) for 1000 sec. The stability of a-IGZO TFTs was improved by using the photo-sensitive siloxane passivation layer.

Analysis of heating phenomenon in oxide thin-film transistor under pulse voltage stress

Degradation by Joule heating of the thin-film transistors (TFTs) is one of the important issues for realizing next-generation displays. We have investigated the thermal distribution on the channel region of transparent amorphous oxide semiconductor TFT in pulse operation using an infrared imaging system. We also discussed the relationship between the self-heating temperature and the degradation.