Papers by Author: Masanobu Yoshikawa

Abstract: For precise investigation of distribution for impurity or composition at SiO₂/SiC interface, dual-beam Time-of-flight Secondary ion mass spectrometry (TOF-SIMS) with low energy sputtering beam was available. In addition to the experimental profiles, simulation using MRI model, in which Mixing, Roughness and Information depth were employed as parameters, enabled to acquire a more authentic distribution at the SiO₂/SiC interface. Slight discrepancy on depth profiles between samples with different surface roughness was duplicated on the convoluted profiles in the simulation. Moreover, reconstructed profile of nitrogen indicated a real distribution with less impact of mixing and roughness, although that may contain uncertainty due to incompletion in the simulation model or variation of the distribution owing to detection species in the experiment. From the result of carbon profiles of both experimental and convoluted profiles, the relative discrepancy on the carbon distribution between samples was clarified, which suggested the possibility that a carbon thin layer at the SiO₂/SiC interface would be found in the future.

Abstract: Although the nitridation by N₂O or NO oxidation with annealing has been used in order to reduce interface trap density, the atomic location or coordination environment of nitrogen atom have been still unclear. In this study, we have investigated atomic coordination environment of nitrogen atom in both SiO₂/SiC interface and SiO₂ layer after nitridation by N₂O annealing using XAFS measurements and ab initio multiple scattering calculations. Nitrogen in SiO₂ layer was suggested to have the same atomic coordination environments as general SiON thin film. On the other hand, in SiO₂/SiC region, nitrogen atoms were isolated and occupied the C-site of SiC structure, and it was confirmed that nitrogen atoms were surrounded by silicon atoms and strong Si-N bonds were formed in the interface region that was not eliminated by HF etching.

Abstract: We measured Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of SiO₂ films with a various thickness, grown on 4H-SiC substrates. The peak frequency of the transverse optical (TO) phonon mode was blue-shifted by about 5 cm⁻¹ as the oxide-layer thickness decreased from 50-60 nm to 10 nm. The blue shift of the TO mode is considerd to be caused by interfacial compressive stresses in the oxide-layer. On the other hand, the TO phonon mode was found to dramatically decrease as the oxide-layer thickness decreased from 10 nm to 1.7 nm. The CL measurement indicates that the intensity of the CL peaks at about 460 and 490 nm attributed to oxygen vacancy centers (OVCs) for No.2 become stronger than that for No.1. From a comparison between FT-IR and CL measurements, we concluded that the red-shift of the TO phonon with decreasing the oxide-layer thickness can mainly be attributed to an increase in inhomogeneity at the SiO₂/SiC interface with decreasing oxide-layer thickness.

Abstract: The Raman spectra of boron-doped single-crystalline diamond were measured at excitation wavelengths between 364.0 and 1064.0 nm and found that the first-order Raman band at 1332 cm^-1 shifts to the low-frequency side, broadens, and develops a derivative-like lineshape as the boron concentration increases. The derivative-like lineshape can be explained by Fano interference. Furthermore, I found that the asymmetric lineshape changes between excitation wavelengths of 514.5 and 785.0 nm. From a comparison of the normalized relative Raman intensity as a function of the excitation energy and the density of states (DOS) in the valence band in the B-doped diamond calculated previously by the coherent potential approximation, the abnormal change in the Raman lineshape is attributed to a change in the DOS in the valence band at approximately 2.0 eV. Raman spectroscopy provides us with extensive information on carrier concentrations, and electronic band structures of B-doped diamond.

Abstract: We measured Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of SiO₂ films with various thicknesses, grown on 4H-SiC substrates. The appearance of broad phonon modes at ~1150–1250 cm^-1 in p-polarized light and their disappearance in s-polarized light confirmed that the phonon modes at ~1150–1250 cm^-1 originated from surface polaritons (SPPs). For the thin SiO₂ film (8-nm thick), the peak frequency of the transverse optical (TO) phonon in the SiO₂ film on the 4H-SiC substrate was observed at ~1080 cm^-1 and was higher than that in SiO₂ films on the Si substrate (1074 cm^-1). This suggested that the thin SiO₂ film (8-nm thick) is under compressive stresses at the interface between the SiO₂ film and SiC substrate. On the other hand, for the thick SiO₂ films (85 and 130-nm thick), the TO phonon peak frequency tended to shift toward lower frequencies with increasing oxide layer thickness. The CL measurement indicated that the CL peak intensity at ~640 nm, attributed to non-bridging oxidation hole centers (NBOHCs), became stronger with increasing oxide layer thickness, relative to that of the CL peaks at ~460 and 490 nm due to oxygen vacancy centers (OVCs). By comparing the FT-IR and CL measurements, we concluded that the TO phonon red-shift with increasing oxide layer thickness can mainly be attributed to an increase in inhomogeneity with increasing oxide layer thickness for the thick SiO₂ films.

Abstract: Depth profiles of ion-implantation induced defect centers have been investigated by cross-sectional CL measurements in the energy range from visible to near infrared. CL observation has shown that point defects diffused out from implanted region to ~10 µm depth during activation annealing. Annealing temperature dependence of the depth distribution of CL intensity of these defects has suggested that structural transformation of point defects proceeds as “silicon vacancy (VSi) → carbon vacancy-antisite pair (VC-CSi ; UD2) → antisite pair (CSi-SiC ; DI)”.

Abstract: We have studied InGaN single-quantum-well (SQW) films using atomic force microscopy (AFM) and cathodoluminescence (CL) spectroscopy. It has been found that a screw dislocations (SDs) distribution in the height image by AFM is well correlated with images of the CL spectra at about 440nm assigned to the spontaneous emission from the InGaN SQW. These results at least mean an existence of non-radiative recombination centers within the InGaN SQW films. It has been also found that the average period of the peak-intensity and the FWHM change is smaller than that of the peak-wavelength change assigned to InN mole fluctuations. These results suggest that the exciton diffusion length of the spontaneous emission at about 440nm is not larger than the average period of InN mole fluctuations in the InGaN SQW.

Abstract: We have measured cathodoluminescence (CL) spectra in the vicinity of V-defects in InGaN single-quantum-well(SQW) films at nanometer level, using newly developed CL apparatus (SE-SEM-CL). From spectroscopic CL measurement, it has been found that the spectra change dramatically in the vicinity of V-defects in the region of £50nm. The SE-SEM-CL has a potential to detect the CL spectral variation at spatial resolution with £50nm.

Abstract: Cross-sectional CL measurements have been performed on the cleaved surface of the Al-ion implanted 4H-SiC. The strong L1 luminescence that originates from the DI defect has been observed even in the deep region (~10 μm) where implanted ions do not penetrate. In the implanted layer, CL results show that high-density non-radiative defects remain even after activation annealing. Generation of the DI defect in the deep region is presumably attributed to the diffusion of point defects from the implanted layer.