Futoshi Utsuno

High-Performance Thin Film Transistor with Amorphous In2O3–SnO2–ZnO Channel Layer

We have developed a high-mobility and high-processability oxide semiconductor using amorphous In2O3–SnO2–ZnO (a-ITZO) as the channel material. An a-ITZO thin-film transistor (TFT) was fabricated by a back-channel-etch process. Its field effect mobility was more than 20 cm2 V-1 s-1 and its subthreshold swing was 0.4 V s-1, which makes it a promising candidate for next-generation TFTs.

Oxygen Close-Packed Structure in Amorphous Indium Zinc Oxide Thin Films

Amorphous indium zinc oxide (IZO) thin film structures of varying amounts of Zn content were investigated using X-ray diffraction measurements and molecular dynamics (MD) simulations. The characteristic amorphous structure having high oxygen coordination number and edge-shared polyhedra were confirmed using both techniques. Detailed analysis of the structural model revealed that the oxygen close-packed structure was almost realized in the nanometer range. It was also found that the number of Zn ions occupying the tetrahedral site of the oxygen close-packed structure increased with increasing ZnO content although In ions occupied the octahedral site. We conclude that the amorphous structure stability of the indium zinc oxide thin films is enhanced by the existence of Zn ions in the tetrahedral site, which block In ions in the octahedral site ordering similar to that in an In(2)O(3) crystal.

Amorphous indium-tin-zinc oxide films deposited by magnetron sputtering with various reactive gases: Spatial distribution of thin film transistor performance

This work presents the spatial distribution of electrical characteristics of amorphous indium-tin-zinc oxide film (a-ITZO), and how they depend on the magnetron sputtering conditions using O2, H2O, and N2O as the reactive gases. Experimental results show that the electrical properties of the N2O incorporated a-ITZO film has a weak dependence on the deposition location, which cannot be explained by the bombardment effect of high energy particles, and may be attributed to the difference in the spatial distribution of both the amount and the activity of the reactive gas reaching the substrate surface. The measurement for the performance of a-ITZO thin film transistor (TFT) also suggests that the electrical performance and device uniformity of a-ITZO TFTs can be improved significantly by the N2O introduction into the deposition process, where the field mobility reach to 30.8 cm2 V–1 s–1, which is approximately two times higher than that of the amorphous indium-gallium-zinc oxide TFT.

The structural characteristics of VOx films prepared by He-introduced reactive RF unbalanced magnetron sputtering

Abstract Vanadium oxide (VOx, x=0∼2.5) films with several different structures were deposited using reactive unbalanced magnetron sputtering; a metallic vanadium target was used. Helium gas was introduced to the deposition process to cause Penning ionization, which was enhanced by the ‘unbalanced’ magnetic field arrangement at the cathode. The transition region corresponding to the oxidation of a target surface was observed at the lower reactive gas (oxygen) partial pressure. A thermochromic film, rutile VO2, and its metastable phase, VO2(B), were successfully deposited at 400°C by controlling the oxygen and helium partial pressures. The plasma diagnostics, examined by optical emission mcitoring, revealed that the relative electron temperature was also increased by the Helium introduction, which should result in an increasing plasma potential close to the substrate surface and hence an increase in the ion-bombardment energy during the film growth.

Electron-phonon scattering in amorphous In2O3–ZnO films

For amorphous transparent conductive oxide In2O3–ZnO films over a wide range of resistivities ρ, the temperature dependences of ρ and Hall coefficient RH have been measured in the temperature range of 2.0–300 K. The low-resistivity films show a metallic characteristic (dρ/dT>0), although high-resistivity films show an insulating behavior (dρ/dT<0). Even in metallic films, however, the resistivity slightly increases with decreasing temperature below 20 K because of the term ρquanta(T) due to quantum effects. Through a careful analysis, we have found that the ρ(T) of metallic films changes in the form of ρ(T)−ρ0−ρquanta∝ρ0T2 at temperatures below ∼100 K. This temperature dependence can be explained by the interference term ρel-imp-ph between the impurity scattering and the electron-phonon scattering. At temperatures of 20–300 K, it has been found that ρ(T) agrees well with the sum of the Gruneisen–Bloch term ρel-ph(T)=βel-phF(T,Θd) and the term ρel-imp-ph(T)=Bel-imp-phG(T,Θd). From analyses, with the coeffi...

Crystallization behavior of amorphous indium–gallium–zinc-oxide films and its effects on thin-film transistor performance

Amorphous indium–gallium–zinc oxide (a-IGZO) films were deposited by DC magnetron sputtering and post-annealed in air at 300–1000 °C for 1 h to investigate the crystallization behavior in detail. X-ray diffraction, electron beam diffraction, and high-resolution electron microscopy revealed that the IGZO films showed an amorphous structure after post-annealing at 300 °C. At 600 °C, the films started to crystallize from the surface with c-axis preferred orientation. At 700–1000 °C, the films totally crystallized into polycrystalline structures, wherein the grains showed c-axis preferred orientation close to the surface and random orientation inside the films. The current–gate voltage (Id–Vg) characteristics of the IGZO thin-film transistor (TFT) showed that the threshold voltage (Vth) and subthreshold swing decreased markedly after the post-annealing at 300 °C. The TFT using the totally crystallized films also showed the decrease in Vth, whereas the field-effect mobility decreased considerably.

Smoothing of surface of silica glass by heat treatment in wet atmosphere

The effect of heat treatment on the surface morphology of fused silica glass substrates was investigated. It was found that the water vapor pressure during heat treatment had a strong influence on the flattening of the silica glass surface. The surface of the frosted glass changed into a transparent and lustrous surface after heat treatment with water vapor at 1200 °C for 48 h, whereas surface irregularities remained for heat treatment under a dry atmosphere. It was suggested that the difference in surface flattening was caused by changes in surface viscosity that depended on the concentration of OH groups on the surface. In order to quantitatively understand the effect of the heat treatment atmosphere, power spectral density (PSD) analysis and a novel peak and valley method were applied to the experimental results. From the PSD analysis, it was found that the Mullins’ model could not explain the smoothing behavior by heat treatment. The peak and valley method, which could separate the surface morphology ...

Sputtering rate studies on different MoSi targets

Abstract Amorphous MoSiO films were prepared by reactive sputtering in O 2 /Ar atmospheres with a target consisting of molybdenum silicide (MoSi 2 ). In addition to the MoSi 2 compound target, three other types of target (a composite, an Mo and an Si target) were used in this study. The composite target was composed of Mo and Si pieces. From an analysis of the relation between sputtering rates of these four targets, the rates of both the MoSi 2 target and the composite target could be calculated by those from the Mo and Si targets taking the surface atomic density into consideration.

Material Design of Glasses Based on Database – INTERGLAD

Owing to the release of a large scale database describing compositions and properties of glasses in the literature, a possibility to construct a universal system for material design of glasses became quite high enough to be considered. In this report, we describe the results of several trials to evaluate the data available in the database. We focused especially on two rather new glass systems, fluorides and chalcogenides, because these two systems are two extremes in view of the relation between compositions and properties, which is called additivity. It was found that in the case of fluoride glasses, a system utilizing the additivity reproduced reliable properties out of glass compositions, and a prototype system has been constructed. In the case of chalcogenide glass, it was found that the additivity held only for the limited numbers of properties, and it was found that a system based on a concept of a model glass should be considered.