Papers by Author: Naoya Iwamoto

Abstract: Titanium carbide has been attractive for years an engineering ceramics due to its high hardness, high melting point, and good chemical stability. Similarly, titanium dioxide has excellent anti-microbial ceramic material by photon energy. In this study, the anatase type titanium dioxide layer prepared by oxidation of the titanium carbide powder by high temperature oxidation in air atmosphere was investigated in order to determine the possibility of its photocatalyst materials by radiant energy. TiC powder samples of different grain size were gained by ball mill. These samples were oxidized at room temperature to high temperature. The samples exhibited a steady mass gain with increasing oxidation temperature. Based on the results of the X-ray diffraction analysis, anatase type TiO₂ was detected on the titanium carbide samples. It is considered, the titanium carbide showed convension anatase type titanium dioxide layer produced by oxidation.

Abstract: In order to test the response of radiation-induced current with wide range of dose rate, a Silicon Carbide (SiC) dosimeter is exposed to gamma-rays emitted from a ⁶⁰Co source. The SiC dosimeter in this study is made of a high purity semi-insulating 4H-SiC with nickel and aluminum electrodes. We have successfully demonstrated that the radiation-induced currents in the dosimeter show a linear relationship with the dose rate, and are repeatable and stable.

Abstract: Defect levels in high purity semi-insulating 4H silicon carbide substrates are studied by charge transient spectroscopy using 5.5 MeV alpha particles. A shallow defect level with the activation energy around 0.3 eV is found in all samples annealed at temperatures from 1400 to 1600 °C. Some other defect levels lying at deeper in the bandgap are found in samples annealed at 1400 and 1500 °C. As these deep levels are annealed out by 1600 °C, the series resistance of samples is decreased.

Abstract: Peak value degradation of heavy-ion induced transient currents in Metal-Oxide-Semiconductor (MOS) capacitors fabricated on n-type and p-type 6H-SiC was observed. The capacitances of MOS capacitors measured during the ion irradiation suggest that the depletion layer width decreased with increasing number of incident ions and was saturated. Since the number of incident ions obtained at the peak current saturation corresponded to that at the saturation of the capacitance, the decrease in peak current can be interpreted in terms of the decrease in the depletion layer width.

Abstract: Defects in electron-irradiated 6H-SiC diodes have been studied by single alpha particle induced charge transient spectroscopy and deep level transient spectroscopy (DLTS) in order to identify critical defects responsible for the charge collection efficiency (CCE) decreased by high-energy electron irradiation. The defect X2 detected by the charge transient spectroscopy and the electron trap Ei detected by the DLTS had a similar activation energy of around 0.50 eV. In addition, the annealing at 200oC completely removed defects X2 and Ei, and restored the CCE. The defect X2 is attributed to the electron trap Ei, and responsible for the decreased CCE.

Abstract: Charge induced in 6H-SiC nMOS capacitors by 15 MeV oxygen ion microbeams was measured using Transient Ion Beam Induced Current (TIBIC) before and after gamma-ray irradiations. The peak amplitude of TIBIC signals decreases and the fall time increases with increasing number of incident ions. The decrease in the TIBIC peak eventually saturated. The TIBIC signal can be refreshed to its original shape by applying a positive bias of + 1V to gate oxide. Small decrease in both the peak amplitude of TIBIC signal and collected charge was observed due to gamma-ray irradiation.

Abstract: The effects of compensation on the hole concentration and mobility in Al-doped 4H-SiC have been investigated by theoretical calculations using the parameters taken from our experimental results on the less-compensated epilayers. The hole concentrations, hole Hall mobilities and resistivities obtained by experiment and calculations are compared and discussed.

Abstract: The effect of electron irradiation on the charge collection efficiency of a 6H-SiC p+n diode has been studied. The diodes were irradiated with electrons of energies from 100 keV to 1 MeV. The charge collection efficiencies of the samples were measured for alpha particles before and after the electron irradiation. The electron irradiation at 100 keV does not affect the charge collection efficiency, while the electron irradiation at 200 keV or higher decreases the charge collection efficiency. The degree of the degradation of the diodes correlates with the energy of the electron irradiation.

Abstract: We investigated the energy spectra in p+n 6H-SiC diodes by a wide variety of charged particles with energies up to several hundred MeV. Though Pulse Height Defect (PHD) was detected when the samples were irradiated with high energy heavy ions (322MeV-Kr and 454MeV-Xe), linearity between pulse height (peak channel) and ion energy up to 150MeV was observed.

Abstract: Single event transient currents induced in 6H-SiC MOS capacitors are measured by using oxygen ions. Charges collected from the samples are calculated by the transient currents. Applying the drift-diffusion model to the charges, the diffusion length of electron is estimated. Transient currents induced in the gamma ray irradiated MOS capacitors are also investigated. No significant change in the transient currents is observed after gamma ray irradiation.