Hisami Yumoto

Control of preferential orientation of AlN films prepared by the reactive sputtering method

Abstract When aluminum nitride (AlN) thin films are applied to surface acoustic wave devices, (100) orientated films are necessary, because the electromechanical coupling coefficient of the (100) oriented films are superior to that of the (001) oriented films. However, it is hard to prepare the (100) oriented films by sputtering and ion plating. In this study, preferential orientation of AlN films was investigated. It was found that the (100) oriented films were deposited by the reactive DC magnetron sputtering method at a long distance ( L ) between the Al target and substrate (above 120 mm) and at high sputtering pressures (above 0.6 Pa). According to our theory, the relative growth rate of (100)/(001) is varied by changing the deposition unit from atoms to a dimer such as Al–N. Namely, when the mean free path of Al and N is longer than the distance L , Al and N atoms deposit directly on the substrate and the (001) oriented films grow. When it is shorter than the distance L , the collisions of Al and N atoms occur in the space between the target and the substrate and the Al–N dimers are formed and deposit on the substrate. In this case, the (100) orientation is enhanced.

Application of ITO films to photocatalysis

Abstract The concentration of NO 2 gas, an environmental pollutant, can be reduced by photocatalysis. A simple method for measuring the efficiency of photocatalysis is required. In general, the efficiency of photocatalysis is determined by measuring the decrease in the concentration of NO 2 gas during UV light irradiation. In this study, an NO 2 gas sensor made of ITO film was used to detect the photocatalytic oxidation of NO 2 . When ZnO film prepared by electroplating was used as the photocatalyst, the photocatalytic reaction was measured continuously with good reproducibility, showing that this method is useful. Next, the photocatalyst was changed from ZnO to ITO film, and the photocatalytic efficiency was studied. ITO film composed of fine particles (30–60 nm), which was prepared by magnetron sputtering, had a very low efficiency. However, ITO film with ITO whiskers (30 nm in diameter and 500–700 nm in length) prepared by the electron shower method showed a similar high efficiency to ZnO film.

Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel

Abstract316L stainless steel powder was sprayed by a high-pressure high-velocity oxygen fuel (HVOF) process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument, first, at the substrate position. A strong negative correlation between the particle temperature and the diameter was found, whereas the correlation between the velocity and the diameter was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly, whereas the particle temperature remained largely unchanged. In-situ curvature measurement was employed in order to study the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings. X-ray stress measurement revealed that the residual stress on the surface of the HVOF coatings is low and often in tension, but the stress inside the coatings is in a high level of compression.

Young`s modulus of single phase cementite

The Young`s modulus of cementite, Fe{sub 3}C, is one of the physical quantities which are inevitably required to understand the mechanical properties of the high strength steels. Thus, efforts have been devoted to estimate the Young`s modulus of cementite from the mechanical tests on cast iron containing cementite. Very recently, the electron-shower-assisted physical vapor deposition (ES-PVD, hereafter) has been applied to prepare single phase cementite films, and their Young`s modulus and Poisson`s ratio have been evaluated using the surface wave method. On the other hand, the vibrating reed method to measure the Young`s modulus of a thin film deposited on a reed-substrate has been established recently. In the present paper, the authors applied the vibrating reed method to the single phase cementite films deposited on silicon reed-substrates to determine the Young`s modulus of cementite.

Characterization of cementite films prepared by electron-shower-assisted PVD method.

Abstract It is important to understand the mechanical properties of single phase cementite in order to design the microstructure of advanced steels. Several papers have been published on mechanical properties of cementite embedded in steels or extracted from steels by electrolysis. However, single phase cementite is not documented. The purpose of this study is to prepare cementite films by the PVD method and to characterize their properties. We applied the electron-shower-assisted PVD method in order to produce a single phase cementite film (2.5 μm in thickness) in a C 2 H 2 atmosphere at 523 K. The composition of the film was determined to be 70Fe–28C–2O (at.%). ESCA spectra indicated that the film was not contaminated with other metallic elements. The grain size of the film was revealed to be about 50 nm by AFM. The Vickers hardness was 1230 (under a load of 2.5 g) at room temperature, nearly equal to the value of 1270 of the cementite extracted from carbon steel by electrolysis. Temperature dependence of the hardness was measured up to 673 K. Youngs modulus and Poissons ratio at room temperature were 160 GPa and 0.26, respectively.

ITO-film gas sensor for measuring photodecomposition of NO2 gas

Abstract ITO-film NO2 gas sensors (thickness=15 nm) were prepared by DC magnetron sputtering. The highest sensitivity was obtained at the sensor temperature of 523 K. When the NO2 gas flow in a measurement tube was terminated, the as-prepared ITO-film sensor showed a gradual decrease of NO2 concentration without NO2 gas leak. We attribute this phenomenon to the desorption of decomposed NO2 from the sensor surface. By annealing the sensor in NO2 gas at 573 K, the sensor showed no decrease of the concentration and could be used for detecting the photodecomposition of NO2. XPS revealed that annealing in NO2 gas for 0.5 h caused a CO-like bond to be formed on the sensor surface. This may become a stable adsorption site for NO2, thus inhibiting decomposition.

Effect of bias voltage on A1N thin films prepared by electron shower method

Abstract When AlN thin films are applied to surface acoustic wave devices, c-axis- and a-axis-oriented films are necessary for longitudinal and transverse waves respectively. However, it is difficult to prepare a-axis-oriented films by sputtering and ion plating. Nevertheless, a-axis-oriented films have been prepared by the electron shower method, and the films can be changed to c-axis orientation by a negative bias of −200 V. The concentration of O in the films decreases with increasing negative bias voltage. These AlN films contain 20 at.% N, which is independent of the bias voltage.

Growth kinetics of silicon carbide CVD

Abstract In CVD growth of silicon carbide on graphite substrates, the growth rate has been increased by the use of a thick intermediate layer. Before the CVD, the intermediate layer is simply deposited by the partial pressure control of the source gas in the same chamber. The intermediate layer of a few micrometers affects the CVD layer thickness of several tens of micrometers and makes the growth rate approximately double in the case of the source gases, dimethyl-dichloro-silane (DDS) and hydrogen. The temperature dependence of the growth rates indicates that the surface growth kinetics change.

Thin layer VLS growth, its cessation and morphologies of Cd crystals with Bi impurities

Abstract Hexagonal prismatic Cd crystals having a hemispherical tip were grown by means of the thin layer VLS mechanism, in which 30–40 wt% of Bi was localized in an alloy liquid phase in the thin skin layer of the hemispherical tip. As the Bi content in the thin skin layer was decreased to a few tenths of a wt%, the VLS growth ceased, and changed to VS growth, which resulted in a flat basal plane instead of the hemispherical tip. The growth rates were measured during the VLS and VS growth processes in connection with the morphology changes. It was found that the VLS growth rate was higher than the VS growth rate by about one order of magnitude.

Surface morphology of VLS Cd and Zn crystals grown in Helium inert gas atmosphere

Abstract Surface microtopographs were taken from VLS Cd and Zn crystals grown in a helium inert gas atmosphere and were compared with those taken from crystals grown in vacuum. During growth in vacuum, the impurity element caused the {10 1 0} and {10 1 1} fac es to become flat or striated. When these faces are flat during vacuum-growth, they contain a cavity during growth in a helium inert gas atmosphere due to the effect of the diffusion field. However, the striated faces during vacuum-growth exhibited striations also during growth in a helium gas atmosphere but without any cavity. The presence of the cavity is explained by the advancement of the “curved” steps toward the center of the face. A similar movement of the “straight” steps (striations) would not have produced any cavity.