Takeshi Okamoto

Dendritic structure in unidirectionally solidified aluminum, tin, and zinc base binary alloys

Abstract The effects of the cooling rate, and the kind and amount of solute, on the fineness of the dendritic structure are investigated in unidirectionally solidified aluminum, tin, and zinc base binary alloys. The primary arm spacing, Z1 is analysed theoretically and expressed by the following equation: Z 1 = 2e(-m(1−k)DC 0 /V) 1 2 , where Z1 is the primary arm spacing, e a constant which is smaller than unity, m the slope of the liquids, k the distribution coefficient, D the diffusion coefficient of solute in the liquid, C0 the initial solute content, and V the cooling rate. The experimental results can be well represented by the above equation. The dendrite spacing in tin and zinc alloys and the cell spacing in tin alloys are also expressed by this equation. The secondary arm spacing in aluminum alloys is inversely proportional to the fourth root of the cooling rate, and decreases with increasing solute content. The secondary arm spacing is also roughly proportional to the cube root of the local solidification time and decreased slightly with increasing solute content.

Dendritic structure in unidirectionally solidified cyclohexanol

Abstract Cyclohexanol has been grown unidirectionally between glass plates, and the dendritic structure has been directly observed. The effects of the cooling rate, growth velocity and temperature gradient on the primary and secondary arm spacings and other variables representing the dendritic structure have been determined. The effects of liquid flow on the variables of the dendritic structure have been also investigated. Flow causes the primary arm spacing to become markedly larger but does not effect the secondary arm spacing measured near the tip of a primary arm.

Efficient ZnO-SiO 2 -Si Sezawa Wave Convolver

A detailed design theory of the Sezawa wave convolver is developed, and the fabrication of a high-efficiency convolver using a ZnO-Si0,-Si structure is discussed. The important points to improve the efficiency are 1) an optimum choice of SAW propagation direction on the Si substrate, 2) an optimum design of the resistivily of the Si epitaxial layer and ZnO film thickness, and 3) an improvement for low- ering SAW propagation lsos and resistance of output circuit. The ex- periments were carried out for two specifications each with a 20-mm and 40-mm gate length. The highest efficiency (F,) of -35 dBm was obtained in the gate length of 20 mm while the time-bandwidth product (ET) was 107. The highest ET product of 227 was obtained in the gate length of 40 mm, while F, was -47.5 dBm. At the present time, the maximum available ETproduct is less than 320 due to the group velocity dispersion.

Effect of Nitrogen on Electrical and Physical Properties of Polyatomic Layer Chemical Vapor Deposition HfSixOy Gate Dielectrics

The effect of nitrogen incorporation on polyatomic layer chemical vapor deposition (PLCVD) hafnium silicate (HfSixOy) films was investigated. The physical and electrical properties of nitride hafnium silicate (HfSixOyNz) and HfSixOy dielectric films are reported. X-ray photoelectron spectroscopy (XPS) was used to check chemical compositions, nitrogen profile, band gap, and band offset of the HfSixOy and HfSixOyNz films. The nitrogen incorporation results in decreases in the band gap and band offset of the HfSixOy sample. The nitrogen profile obtained by secondary ion mass spectroscopy (SIMS) shows a gradient decrease from the surface to the interface. The prepared HfSixOy and HfSixOyNz films have reasonable electrical performance.

Normal macrosegregation due to natural convection

Abstract The effects of solidification variables and solute content on normal macrosegregation have been investigated in Al-Ag and Al-Mg alloys solidified unidirectionally upward and downward with a dendritic interface. No macrosegregation occured in Al-Ag alloys solidified upward except in low solute alloys solidified at low growth rates, and extensive normal macrosegregation occured in the alloys solidified downward. In Al-Mg alloys, macrosegregation occured in the castings solidified both upward and downward. The intensity of macrosegregation was reduced with increasing growth rate and increasing solute content. Decreasing the temperature gradient also reduced the intensity of macrosegregation.

Improvement of Removal Rate in Abrasive-Free Planarization of 4H-SiC Substrates Using Catalytic Platinum and Hydrofluoric Acid

We used catalyst-referred etching, which is an abrasive-free planarization method, to produce an extremely smooth surface on a 4H-SiC substrate. However, the removal rate was lower than that obtained by chemical mechanical polishing, which is the planarization method generally used for SiC substrates. To improve the removal rate, we investigated its dependence on rotational velocity and processing pressure. We found that the removal rate increases in proportion to both rotational velocity and processing pressure. A lapped 4H-SiC substrate was planarized under conditions that achieved the highest removal rate of approximately 500 nm/h. A smooth surface with a root-mean square roughness of less than 0.1 nm was fabricated within 15 min. Because the surface, which was processed under conditions of high rotational velocity and high processing pressure, consisted of a step–terrace structure, it was well ordered up to the topmost surface.

Planarization of GaN(0001) Surface by Photo-Electrochemical Method with Solid Acidic or Basic Catalyst

A novel planarization technique for the GaN(0001) surface has been developed. In this method, the surface is oxidized by a photo-electrochemical reaction and the resulting oxide is removed using a solid acidic/basic catalyst. Smooth surfaces that are free from scratches and etch pits are obtained. Photoluminescence analysis shows that the intensity of the band-edge luminescence markedly increases after the planarization.

Damage-Free Planarization of 2-Inch 4H-SiC Wafer Using Pt Catalyst Plate and HF Solution

We report a damage-free and efficient planarization process for silicon carbide (SiC) using platinum as a catalyst in hydrofluoric acid (HF) solution. In previous studies, 4H-SiC (0001) on-axis wafers were planarized by this process and an extremely flat surface was obtained. However, electronic device substrates require off-axis wafers. In the present study, 4H-SiC (0001) 8° off-axis Si-face wafers were planarized using a Pt catalyst plate and HF solution. In the first trial using these wafers, the surface roughness worsened and a diagonal pattern was observed by phase-shift interference microscopy. The pattern seemed to have been formed when the Pt plate morphology was transcribed onto the wafer. The removal rate of the 8° off-axis Si-face wafer is much greater than that of the on-axis Si-face wafer. Thus, we concluded that the use of a smoother catalyst plate would be necessary to obtain a smooth 8° off-axis Si-face wafer surface. Improving the Pt plate morphology by hand lapping also improved the surface roughness of the processed wafer as compared with the preprocessed surface. The maximum height of the surface irregularity (peak-to-valley, P-V) and root-mean-square roughness were improved to 0.513 nm and 0.044 nm, respectively, as determined by atomic force microscopy (2×2 μm2).

Defect-Free Planarization of 4H–SiC(0001) Substrate Using Reference Plate

In this paper, a new defect-free planarization technique for 4H–SiC(0001) substrate is described. This technique uses hydroxyl radicals (OH radicals) that are generated on an Fe metal surface in a hydrogen peroxide (H2O2) solution. First, the oxidation of a 4H–SiC substrate by OH radicals is investigated by X-ray photoelectron spectroscopy (XPS) analysis. Next, the planarization of the 4H–SiC substrate is conducted. A very flat and smooth surface without any scratches and etch pits is obtained. The planarized surface has a step-terrace structure.

Reduction of Surface Roughness of 4H-SiC by Catalyst-Referred Etching

Flat and well-ordered surfaces of silicon carbide (SiC) substrates are important for electronic devices. Furthermore, researchers have reported that 4H-SiC surface roughness increases by step-bunching during epitaxial growth and annealing. Degradation of device properties induced by surface roughening is of great concern. Therefore, a method to reduce this surface roughening is requested. We have developed a damage-free planarization method called catalyst-referred etching (CARE). In this paper, we planarized 4H-SiC substrates and evaluated the processed surface before and after the epitaxial growth. Then, we reduced the step-bunching on the epi-wafer surface and determined the electrical properties of the Schottky barrier diodes (SBD) on the processed surface.