Papers by Author: Mitsuo Okamoto

Abstract: In this study, 4H-SiC bonded substrates (bonded-SiC) with an average resistivity of 2.4–31.5 mΩ·cm were prepared, and attention has been directed toward the relationship between the resistivity of bonded-SiC and the contact resistance at the backside where metal Ti/Ni was applied. A circular transmission line model (cTLM) was used to accurately measure the backside contact resistance. A linear correlation was found between and the resistivity of bonded-SiCs at room temperature (RT). This result indicates the existence of a threshold resistivity at which the specific contact resistance in the range of 2.2 × 10⁻⁶ to 1.5 × 10⁻⁵ Ω·cm² can be achieved without contact annealing; it also indicates that the temperature dependence of between 17.4 and 34.4 mΩ·cm is eliminated. This phenomenon can occur because is dominated by tunneling current above the nitrogen concentration at the threshold resistivity, which is driven by the high nitrogen concentration and sufficient carrier activation in the polycrystalline portion (polycrystalline layer) of bonded-SiCs. These are important properties resulting from a polycrystalline layer with a 3C structure in bonded-SiC.

Abstract: A novel substrate of 4H-SiC bonded substrate is expected to solve issues such as decreasing the on-resistance, which has attracted much attention. Therefore, several studies have been conducted on the use of bonded substrates. In this study, we fabricated a DMOSFET on a bonded substrate and compared its static and dynamic characteristics with those on a single-crystal substrate. Consequently, the on-resistance of the DMOSFET fabricated on a bonded substrate was lower than that on a single-crystal substrate owing to the low resistivity of the polycrystalline substrate. Also, reverse recovery loss of the DMOSFET fabricated on a bonded substrate was lower than that on single-crystal substrate at high temperature due to low carrier lifetime in a drift layer. Additionally, we observed that the DMOSFET fabricated on a bonded substrate did not generate bipolar degradation despite the application of a forward-current stress of over 1500 A cm^-2. According to these results, we expected that the carrier lifetime in both drift layer and transfer layer was decreased on a bonded substrate.

Abstract: In this study, the simultaneous realization of high-speed and high-temperature switching operations is demonstrated using a custom-made high-speed and high-temperature power module installed with a silicon carbide (SiC) CMOS gate driver, which can reduce gate loop inductance and operate at high temperatures. Approximate switching speeds of 70 and 60 V/ns are achieved during the turn-on and turn-off operations, respectively, at 300°C, 600 V DC bus voltage, and 20 A load current using the developed module. The switching speed remained above 50 V/ns in the temperature range from room temperature to 300°C. Numerical calculations based on the static properties of the SiC power MOSFET and CMOS gate driver can predict the actual switching properties over a wide temperature range when the developed module incorporating the fabricated SiC CMOS gate driver is used.

Abstract: We investigated the effects of γ-ray irradiation to single photon sources (SPSs) embedded in 4H-SiC metal-oxide-semiconductors field-effect transistors (MOSFETs). After the γ-ray irradiation, the number of SPSs was temporarily increased. However, the ratio of unstable SPSs was increased with increasing the radiation dose, and such unstable ones gradually disappeared. Finally, the density of the SPSs nearly recovered that before the irradiation. We discuss a possible explanation on these phenomena in terms of interactions between mobile hydrogen atoms and interface defects.

Abstract: The SiO₂/SiC interface quality has a significant effect on the performance of 4H-SiC MOS devices. The introduction of nitrogen to the SiO₂/SiC interface is a well-known method for reducing the interface state density (D_it). In this study, we introduced nitrogen to the SiO₂/SiC interface by forming SiN_x films using atomic layer deposition (ALD) and thus improved the interface quality. O₂ annealing with a SiN_x interface layer of optimal thickness enhanced the field effect mobility.

Abstract: We investigated single photon sources (SPSs) in 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) by means of confocal microscope techniques. We found SPSs only in 4H-SiC/SiO₂ interface regions of wet-oxide C-face MOSFETs. The other regions of MOSFETs such as source, drain and well did not exhibit SPSs. The luminescent intensity of the SPSs at room temperature was at least twice larger than that of the most famous SPSs, the nitrogen-vacancy center, in diamond. We examined four types of C-face and Si-face 4H-SiC MOSFETs with different oxidation processes, and found that the formation of the SPSs strongly depended on the preparation of SiC/SiO₂ interfaces.

Abstract: The threshold voltage (V_th) instability of 4H-SiC MOSFETs was investigated using high-speed IV measurement instrument. DC stress measurement of wide time span ranging from 10^-6 to 10³ s without relaxation effect was conducted. The high-speed measurement allowed of dynamic ΔV_th measurement under pulsed AC gate bias stress. We investigated effects of NO POA in gate oxidation process on the V_th instabilities.

Abstract: We studied interface defects of C-face 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) by means of electrically-detected-magnetic-resonance (EDMR) spectroscopy. EDMR measurements were carried out on opposite types of C-face MOSFETs, which were prepared by dry oxidation and wet oxidation, and we found EDMR signals of interface defects from both the MOSFETs. Judging from their spectroscopic features, the interface signals of the two MOSFETs are assigned to be the same type, and we call them “C-face defects.” The density of C-face defects was found to be larger in the dry-oxide MOSFETs than in the wet-oxide MOSFETs. It is also revealed that part of C-face defects in wet-oxide MOSFETs are coupled with hydrogen atoms.

Abstract: We present EDMR (electrically detected magnetic resonance) observations on “C-face defects” in C-face 4H-SiC MOSFETs. We found that negative threshold-voltage shifts of C-face MOSFETs are increased in association with EDMR signals of C-face defects as well as with the dissociation of hydrogen atoms induced by gamma-ray irradiation.

Abstract: 4H-SiC(000-1) C-face was oxidized in H₂O and H₂ mixture gas (H₂ rich wet ambient) for the first time. H₂ rich wet ambient was formed by the catalytic water vapor generator (WVG) system, where the catalytic action instantaneously enhances the reactivity between H₂ and O₂ to produce H₂O. The dependence of SiC oxidation rate on the H₂O partial pressure was investigated. We fabricated 4H-SiC C-face MOS capacitor and MOSFET by the H₂ rich wet re-oxidation following the dry O₂ oxidation. The density of interface traps was reduced and the channel mobility was improved in comparison with the conventional O₂ rich wet oxidation.