Yousuke Morita

Effects of gamma‐ray irradiation on cubic silicon carbide metal‐oxide‐semiconductor structure

Radiation effects on cubic silicon carbide (3C‐SiC) metal‐oxide‐semiconductor (MOS) structures have been studied with high‐frequency capacitance‐voltage measurements. It was found that interface traps are generated at the 3C‐SiC/SiO2 interface and oxide‐trapped charges are built up in the oxide by 60Co gamma‐ray irradiation. The generation of the interface traps and the oxide‐trapped charges are affected by bias polarity applied to the gate electrode during the irradiation. Absorbed dose dependencies of their generation are discussed comparing with those of Si MOS structures.

Fine SiC fiber synthesized from organosilicon polymers: relationship between spinning temperature and melt viscosity of precursor polymers

A very fine silicon carbide (SiC) fiber with diameter of 6 μm, about a half of that of a commercially available SiC fiber, was synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS). The fine SiC fiber was obtained by optimizing the composition and the spinning temperature of PCS-PVS polymer blends. In order to determine these optimum conditions, the relationship between temperature and melt viscosities of the polymer blends was investigated. As a result, it was found that the optimum spinning temperature range was within a temperature range where the melt viscosity is 5–10 Pa · s. Moreover, by blending PVS with PCS, the spinning temperature of the polymer blends was lowered, the spinnability of polymer system was improved, and finer polymer fiber was obtained compared with PCS. The optimum content of PVS in the polymer blend was 15–20 wt%.

ESR studies of incorporation of phosphorus into high-pressure synthetic diamond

Abstract Electron spin resonance (ESR) signals associated with phosphorus, which should give microscopic evidence of incorporation of phosphorus into the lattice of diamond, have been searched in high-pressure synthetic diamond crystals. In a crystal grown from phosphorus catalyst, a new ESR spectrum labelled NIRIM-8 which exhibits both 14N and 31P hyperfine interactions has been found. In high-pressure synthetic type-IIa crystals implanted with high-energy (from 9 to 21 MeV) phosphorus ions at room temperature, ESR signals from point defects have been observed, after searching the implantation conditions which considerably lower the density of lattice damages.

Mechanism of anomalous degradation of silicon solar cells subjected to high-fluence irradiation

We have found anomalous degradation of electrical performance in silicon solar cells designed for space use due to high-fluence irradiation of charged particles, e.g., 1 MeV-electrons of /spl sim/10/sup 17/ e/cm/sup 2/ and 10 MeV-protons of /spl sim/10/sup 14/ p/cm/sup 2/. This anomalous degradation has two typical features, i.e., sudden-drop-down of electrical performances and slight recovery of the short circuit current I/sub SC/ just before the sudden-drop-down, which cannot be explained by a conventional model based on decrease of the minority-carrier life-time. In order to account for these features, we propose a new model, in which decreases of the majority-carrier concentration and the minority-carrier mobility are considered in addition to that of the minority-carrier life-time. The anomalous degradation observed is well represented by this model.

Anomalous degradation in silicon solar cells subjected to high-fluence proton and electron irradiations

Distinct from the well-known logarithmic degradation in electrical performances of a crystalline silicon solar cell, an anomalous degradation of short-circuit current density (Isc) was observed in a cell irradiated by energetic protons and electrons at high fluence. From results of proton irradiations with various energies (0.4–10 MeV) and high frequency (1 MHz) capacitance measurements, the anomalous drop of Isc is found to be caused by (1) the p-type substrate changes into the intrinsiclike layer (Fermi level shift) by the irradiations, followed by an extension of the depletion layer, and (2) the drift length of the minority carrier becomes shorter than the depletion layer.

Fine silicon carbide fibers synthesized from polycarbosilane-polyvinylsilane polymer blend using electron beam curing

A fine SiC fiber is synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS) with electron beam curing under vacuum. The obtained SiC fiber from the PCS-PVS blend polymer has smaller average diameter of 8.5 μm than that of 11.8 μm from PCS, and shows higher average tensile strength of 3.2 GPa than that of 2.8 GPa from PCS after heat treatment at 1673 K in Ar gas atmosphere. However, the SiC fiber from the polymer blend decreases in tensile strength after heat treatment above 1773 K due to β-SiC crystal growth near the fiber surface, because of a small amount of oxygen incorporated in the fiber.

Radiation processing for carbon fiber-reinforced polytetrafluoroethylene composite materials

Abstract The present work is an attempt to evaluate the performance of the fiber composites with crosslinked polytetrafluoroethylene (PTFE) as a polymer matrix by radiation. The uni-directional carbon fiber-reinforced composites were fabricated with PTFE fine powder impregnation method and then crosslinked by electron beams irradiation under selective conditions. The carbon fiber-reinforced crosslinked PTFE composites show good mechanical properties compared with crosslinked PTFE. The radiation resistance of crosslinked PTFE composites is improved more than that of crosslinked resin without fiber.

ESR studies of high-energy phosphorus-ion implanted synthetic diamond crystals

Abstract Synthetic diamond crystals of type IIa irradiated with 9–18 MeV phosphorus ions at room temperature to fluences 4 × 1012−4 × 1015 ions cm−2 were studied by using electron spin resonance (ESR) technique. The degree of amorphization increasing with the fluence is manifested in the variation of the temperature dependence of the linewidth of the ESR signal of the dangling bonds. Both the ESR signal intensity and the annealing at 1800°C under 6 GPa have indicated presence of a critical fluence initiating irreversible amorphization.

Application of radiation-crosslinked polytetrafluoroethylene to fiber-reinforced composite materials

Abstract Plain-woven carbon fiber-filled polytetrafluoroethylene (PTFE) composites were fabricated by radiation-crosslinking under selective conditions. High mechanical and frictional properties are found in the composite materials compared with crosslinked PTFE without fiber. The composite materials with optional shapes, which are laminated after electron beam (EB) crosslinking treatment of each mono-layer could also be fabricated.

Gamma-Ray Irradiation Effects on Cubic Silicon Carbide Metal-Oxide-Semiconductor Structure

Gamma-ray irradiation effects on cubic silicon carbide (3C-SiC) metal-oxide-semiconductor (MOS) structures have been studied with high frequency capacitance-voltage measurements. Using Termann analysis to determine the densities of interface traps and oxide traps, we found a 2/3 power law dependence of the radiation-induced traps on the absorbed dose. The generation of these traps was found to depend on bias during irradiation, similar to those of Si MOS devices. We also compared the radiation response of dry and pyrogenic oxides on 3C-SiC with that of dry oxides of Si, and showed less trapping in pyrogenic 3C-SiC oxides.