Papers by Author: Masashi Kato

Abstract: Bipolar degradation poses a significant concern for the reliability of SiC bipolar power devices. The basic cause for bipolar degradation is expansion of Shockley Stacking Faults SSFs. These glide planes can be pinned and prevented from expansion. This study involves 19 MeV Energy Filtered Ion Implantation of Nitrogen (i.e. resulting in an energy spectrum ranging from 0 MeV to nearly 19 MeV in one shot) to explore the pinning effect of Nitrogen ions that suppresses recombination glide, which minimizes SSF growth, while providing precise doping of the entire drift region by the same Nitrogen implantation. All is performed in one single step. This procedure paves the path to immobilize any nucleation sites in the entire drift layer, this way enhancing the reliability and facilitating mass production of SiC power devices. This study employs UV illumination as an optical stressing method to create e-/h+ pair, which subsequently induce 1SSF expansion. Both, UV induced 1SSF expansion and pining were observed by photoluminescence. Carrier lifetime measurements were employed for understanding the mechanism of pinning defects.

Abstract: The expansion behavior of basal plane dislocations (BPDs) in a 4H-SiC epitaxial layer on the (110) A-plane under electron beam (EB) (//[110]) irradiation was observed. BPD expanded and formed a single Shockley stacking fault (SSSF) between a partial dislocation (PD) pair. The width of the SSSF was proportional to the EB current. The dependence of the expansion velocity on the irradiation position was observed with a fixed EB spot. It was found that the electron-hole pair migration to the PD and/or SSSF can expand the SSSF. The velocity of SSSF expansion by direct SSSF excitation with an EB was much smaller than that by the preferential excitation of a PD with migrated electron-hole pairs.

Abstract: For high voltage SiC bipolar devices, carrier lifetime is an important parameter, and for optimization of device performance, we need to control distribution of the carrier lifetime in a wafer. So far, there have been limited systems for depth-resolved carrier lifetime measurements without cross sectional cut. In this study, we adopted a free carrier absorption technique and made local overlapping of the probe laser light with excitation laser light to develop depth-resolved carrier lifetime measurements. We named the developed system a microscopic FCA system and demonstrated measurement results for samples with and without intentional carrier lifetime distribution.

Abstract: We evaluated the carrier lifetime to estimate surface recombination velocities for 4H-SiC whose surfaces were treated by various processes. We found that the reactive ion etching (RIE) increased the surface recombination velocity, and we considered that point defects introduced by RIE influence the surface recombination velocity.

Abstract: We investigated annealing behavior of the carrier lifetime and the deep levels in electron irradiated n-type and semi-insulating 4H-SiCs. We observed two peaks for each sample by photo induced current transient spectroscopy (PICTS) measurements, and their heights depended on annealing temperature. By comparing the annealing behavior of the peak height with reported temperature dependence of concentrations of various defects, we speculated that the observed peaks originate from either V_SiV_C, C_SiV_C or EI4.

Abstract: The photolytic hydrogen generation using sunlight attracts attention as a next generation energy technology. A key of this technology is a selection of materials for the photolysis and SiC is one of the candidate materials for this application. The conversion efficiency from the solar to the hydrogen energy would be affected by the carrier lifetime in SiC. Therefore, in this study, we measured carrier lifetimes in SiC and compared them with photocurrents in electrolytes that is directly correlated to the conversion efficiency.

Abstract: Carrier lifetime in a high injection condition is a key parameter for design of bipolar devices. Microwave photoconductivity decay (μ-PCD) is a popular method to evaluate the carrier lifetime in silicon carbide (SiC). For accurate evaluation of the carrier lifetime by μ-PCD measurements, the microwave reflectivity needs to be proportional to the excess carrier concentration. In this study, we observed microwave reflectivity from 4H-SiC as a function of injected photon density and suggested a method to keep proportionality of the reflectivity to the excess carrier concentration.

Abstract: Solar-to-hydrogen conversion efficiencies of water photolysis with epitaxially grown p-type 4H-, 6H- and 3C-SiC were estimated in the two electrode system. For all the polytypes, the efficiency with a Pt counter electrode in the two electrode system was very low compared with those in the three electrode system. However, when Ni was used as a counter electrode in the two electrode system, photocurrents were as large as the three electrode system. The estimated efficiencies seem to depend on the bandgap of SiC polytypes, and the highest solar-to-hydrogen conversion efficiency was 0.38% with 3C-SiC.

Abstract: We have characterized deep levels in as-grown and electron irradiated p-type 4H-SiC epitaxial layers by the current deep-level transient spectroscopy (I-DLTS) method. A part of the samples were irradiated with electrons in order to introduce defects. As a result, we found that electron irradiation to p-type 4H-SiC created complex defects including carbon vacancy or interstitial. Moreover, we found that observed deep levels are different between before and after annealing, and thus annealing may change structures of defects.

Abstract: Optical monitoring of diffusivity in wide bandgap semiconductors was performed by using a picosecond light-induced transient grating technique. The bandgap renormalization and carrier-carrier scattering manifested itself at room temperature as two-fold decrease of the ambipolar diffusion coefficient Da in cubic SiC and 5-fold decrease of Da in diamond at excess carrier density N > 10¹⁷ cm^-3, while for GaN the impact was observed only at T 19 cm^-3, the plasma degeneracy led to enhanced D_a values in SiC and GaN and compensated the diffusivity decrease in diamond.