Papers by Author: Shinichi Nakashima

Abstract: We have investigated Raman spectra of p-type 4H-SiC crystals with hole concentration ranging from 3×10¹⁶ to 1×10²¹ cm^-3. For folded transverse acoustic (FTA) doublet modes, Fano interference profiles were analyzed, and the frequency shift due to Fano interference was obtained as a function of hole concentration. We demonstrated that the shift of the FTA doublet modes is a quantitative measure for evaluating of hole concentration of p-type 4H-SiC. Spectral features of transverse optical (TO) and longitudinal optical phonon-plasmon coupled (LOPC) modes were also studied for various carrier concentrations. The results show that the full width at half maximum of the LOPC and relative intensity of TO and the LOPC can be used as other calibration measures for hole concentration.

Abstract: There is still little consensus regarding why low contact resistivity is achieved when Ni on n-type 4H- and 6H-SiC is annealed at temperatures of more than above 950°C. The objective of this paper is to provide an answer concerning to this question. It is has been reported that even Ni-based contacts formed in the n++ region exhibited a steep reduction of contact resistivity in an annealing temperature range > 900°C. This effect reduction cannot be explained by the carbon vacancy induced donor model (Vc model) proposed by Han and his coworkers [Appl. Phys. Lett., Vol. 79, p. 1816 (2001)]. And, it is clarified that It was observed that the surface of substrates annealed at 1000°C was not covered with not Ni2Si but with a thin layer of NiSi. Finally, a plausible model is proposed that as the result of annealing at higher temperatures, results in the formation of the a NiSi/SiC system is builtat the substrate interface, resulting in significant reduction in low causing contact resistivity to be reduced significantly.

Abstract: We characterized the HVPE grown freestanding GaN crystals with various shallow impurity concentrations by Raman scattering spectroscopy. Electrical properties such as free carrier density were examined for the GaN crystals through Raman measurements of the A1 LO-phonon plasmon coupled (LOPC) mode and its line shape analysis. The asymmetric broadening and the up-shift of the LOPC band were clearly observed as the carrier density increased from 1017 to 1019 cm-3. The line shape analysis revealed that the carrier density in the GaN crystals can be simply estimated from measured frequency shift of LOPC mode. The variations of the width for TO phonon bands were also observed in this carrier density range. The band widths were slightly increased by 0.2 cm-1 as the impurity concentration increased from 1018 to 1019 cm-3 in samples with low dislocation density.

Abstract: We characterized the 4H- and 6H-SiC bulk crystals with graded doping and epitaxial films with various carrier densities by Raman scattering spectroscopy. Electrical properties such as free carrier density were examined for the SiC crystals through Raman measurements of the A1 LO-phonon plasmon coupled (LOPC) mode and its line shape analysis. The peak frequency and band width of LOPC mode varied with carrier density in the range from 1016 to 1019 cm-3. The line shape analysis revealed that the carrier density in the SiC crystals can be simply estimated from measured frequency shift of LOPC mode for 4H- and 6H-SiC crystals.

Abstract: AlGaN/GaN HFETs were fabricated around micropipes and on a domain boundary in a semi-insulating silicon carbide (SI-SiC) substrate and the DC characteristics of the fabricated devices were measured. Devices around micropipe showed no pinch-off or large gate leakage. The devices on the domain boundaries showed no degradation in the performances, even though an X-ray topographic analysis indicated that crystal imperfections, due to the defects, propagated to the GaN layer across the hetero interface. Based on these results, we concluded that micropipe degrades the DC characteristics and that the domain boundary does not affect the DC characteristics. From Raman analysis on the devices around the micropipes, these degradations could be attributed to the free carriers introduced into the GaN crystal by the micropipes.

Abstract: Visible and deep UV Raman measurements have been applied to investigate the structural and electrical properties of stacking disordered 3C-SiC crystals. It is found that free-carrier density shows the significant dependence on the density of stacking faults in 3C-SiC. The density of stacking faults has been estimated from the comparison between experimentally obtained Raman spectra and Raman intensity profiles simulated using one-dimensional lattice models considering the disorder in bond polarizability arrangement.

Abstract: Deep-ultraviolet (DUV) micro-Raman spectroscopy was applied to study the micro structures of surface defects in a 4H-SiC homoepitaxially grown film. From DUV Raman spectrum, inclusions of 3C-SiC was found in comet defects. The shape of 3C-structure in comets was investigated and it was found that 3C inclusions in comets can be classified into two types. In addition, spectrum broadening due to the coupling of nonfolded longitudinal optical phonon mode and the photo-excited carriers was also found. The formation mechanisms of 3C inclusion in comets were discussed.

Abstract: Raman spectroscopy using deep UV (DUV) light excitation has been applied to characterizing process-induced defects in surface layers in SiC. Raman spectra of P+-ion implanted and post annealed SiC have been measured as a function of dose level and annealing temperature. The recovery of the crystallinity and electrical activity have been evaluated. Precipitation of excess phosphorus was found in heavily doped specimens. High dose implanted and post annealed samples show uneven distribution of residual defects, which is demonstrated by mapping of Raman bandwidth. Damage in 4H-SiC surfaces, which were mechanically polished with various sizes of abrasives, has been evaluated from DUV micro-Raman measurements. The Raman analysis demonstrates that bandwidth and peak frequency can be used as monitors of the polish–induced damage. It is found that localized defects reducing free carrier density remain even after polishing with small sized abrasives.