Papers by Author: T. Umeda

Abstract: We present an electrically detected electron-spin-resonance (ESR) study on SiO2-SiC interface regions of n-channel lateral 4H-SiC MOSFETs with hydrogen annealing. This characterization technique can reveal electrically active defects that interact with channel currents of the MOSFETs. The defects were observed at 20 K, and were labeled “PH0” and “PH1”, one of which (PH1) exhibited a 1H hyperfine splitting of 5.3 mT.

Abstract: We identify the negatively charged dicarbon antisite defect (C2 core at silicon site) in electron irradiated n-type 4H-SiC by means of combined electron paramagnetic resonance (EPR) studies and first principles calculations. The pair of HEI5 and HEI6 EPR centers (S = 1/2, C1h symmetry) are associated with the cubic and hexagonal dicarbon antisite defects, respectively. This assignment is based on the comparison of the measured and calculated hyperfine tensors of 13C and 29Si atoms. We investigated the creation and annihilation of this defect as a function of electron-dose and annealing temperature.

Abstract: The Tv2a center in 4H-SiC irradiated by electrons at room temperature has been studied by pulsed EPR. Various techniques such as pulsed ELDOR (electron-electron double resonance), 2-pulse echo decay, 3-pulse inversion recovery, pulsed ENDOR (electron nuclear double resonance), and 3-pulse ESEEM (electron spin echo envelope modulation) have been applied to perform the detailed structure determination and to exploit applicability for the coherent spin control experiments.

Abstract: We report photo-induced electron paramagnetic resonance (photo-EPR) data for irradiated n-type 4H-SiC. Energy levels and associated photo-induced transitions are discussed for silicon vacancy (VSi), carbon vacancy (VC), carbon antisite-vacancy pair (CSiVC), and divacancy (VSiVC).

Abstract: In EPR (electron paramagnetic resonance) identification of point defects, hyperfine (HF) interaction is decisive information not only for chemical identity but also for the local geometry and the electronic state. In some intrinsic defects in SiC, the wave function of the unpaired electron extends quite unevenly among major atoms comprising the defects. In such a case, the determination of the number of equivalent atoms and the chemical identity (Si or C) of those atoms even with weak HF splitting are useful to compare with HF parameters obtained theoretically. For vacancy-related defects of relatively deep levels, the sum of the spin densities on the nearest-neighbor shell is found to be 60-68%.

Abstract: Carbon antisite-vacancy pair (CSiVC) is a fundamental defect in SiC, and is theoretically predicted to be very stable in p-type materials. However, this pair was found only in the form of a negatively charged state (i.e., the SI5 center = CSiVC −) in n-type and semi-insulating 4H-SiC, and yet, its presence has not been shown in p-type SiC. In this report, we present the first EPR observation on positively charged CSiVC pairs in p-type 4H-SiC. By carefully examining p-type samples after electron irradiation, we found a pair of new defects with C3v and C1h symmetries. They correspond to “c-axial” pairs (C3v) and “basal” pairs (C1h) of CSiVC +, respectively. The positively charged pairs are characterized by a strong 13C hyperfine interaction due to a dangling bond on a carbon antisite (CSi), which is successfully resolved for the c-axial pairs.

Abstract: The SI-5 electron-paramagnetic-resonance (EPR) centre is a dominant defect in some high-purity semi-insulating (HPSI) SiC substrates and has recently been shown to originate from the negatively charged carbon vacancy-carbon antisite pair (VC − Si C ). In this work, photoexcitation EPR (photo-EPR) was used for determination of the energy position of deep acceptor levels of VCCSi in 4H-SiC. Our photo-EPR measurements in slightly n-type material show an increase of the EPR signal of VC − Si C for photon energies from ~0.8 eV to ~1.3 eV. Combining the data from EPR, deep level transient spectroscopy and supercell calculations we suggest that the (1–|2–) levels of the different configurations of the defect are located in the range ~0.8-1.1 eV below the conduction band.

Abstract: The effect of Fe content on hot tearing of the high-strength Al-Mg-Si alloy was systematically investigated. In this study, a thermodynamic calculation software Thermo-Calc was used to calculate the solidification path under the non-equilibrium condition, and the mechanical properties of this alloy have also been investigated during solidification using an electromagnetic induction heating tensile machine. In order to confirm the calculation results of solidification path, a quenching test also was carried out. By using the Thermo-Calc, the sequence of crystallization, crystallization temperature of formed phases and their crystallized amount were systematically investigated for each alloy in which Fe content was changed. Furthermore, by comparing the fracture surfaces of the tensile testing sample and DC billet, the temperature range of crack initiation of the alloy was examined. Comparing the temperature range of crack initiation with the crystallization phase and its crystallization order, Fe content of high-strength Al-Mg-Si alloy influenced hot tearing significantly owing to the crystallization behavior of α(AlFeMn).

Abstract: EPR spectra originating from phosphorus shallow donors occupying silicon sites in 3C-, 4H-, and 6H-SiC are identified by using CVD grown films in which the interference from the signals from the nitrogen shallow donors is practically absent. Phosphorus donors occupying both silicon and carbon sites are observed in high-energy phosphorus ion implanted semi-insulating 6H-SiC which was also free from the interference from the signals from the nitrogen shallow donors.

Abstract: We present new electron-paramagnetic-resonance (EPR) data on the HEI4/SI5 center in 4H-SiC. So far, the SI5 (SI-5) center has been observed only in as-grown SiC substrates; however, we found that it can be created by electron irradiation to commercial n-type 4H-SiC. The artificially created SI5 center, which we had preliminary called HEI4, was found to be identical with the SI5 center in as-grown SiC. A high-intensity HEI4/SI5 spectrum of irradiated SiC revealed clear hyperfine structures of 29Si and 13C, which enabled us to identify the origin of this center as a carbon antisite-vacancy pair in the negative charge state (CSi-VC –). We assessed its electronic levels using photo-EPR.