Papers by Author: Takeshi Tawara

Abstract: Single Event Gate Rupture (SEGR) is one of the catastrophic failures caused by heavy ions in power MOS devices. In this study, n-type SiC MOS capacitors representing the gate structure generally used in SiC power MOSFETs were used to conduct heavy ion irradiation tests to clarify the SEGR mechanism. The Linear Energy Transfer (LET) dependence of the critical electric field (E_cr) for these capacitors was evaluated with two different oxidation processes in accumulation to confirm whether the oxidation process affects SEGR tolerance. We found that the E_cr value and slopes of the LET dependence for SEGR between DRY samples and DRY + POA samples were approximately consistent. We also simulated SEGR and studied its mechanism. The simulation results suggested that SEGR for SiC MOS capacitors is caused by carriers in electron-hole pairs generated by a heavy ion instead of gate electric field fluctuation.

Abstract: Application of highly N-doped buffer layers or a (N+B)-doped buffer layer to PiN diodes to suppress the expansion of Shockley stacking faults (SSFs) from the epilayer/substrate interface was studied. These buffer layers showed very short minority carrier lifetimes of 30–200 ns at 250°C. The PiN diodes were fabricated with buffer layers of various thicknesses and were then tested under high current injection conditions of 600A/cm². The thicker buffer layers with shorter minority carrier lifetimes demonstrated the suppression of SSFs expansion and thus that of diode degradation.

Abstract: Epitaxial growth of 4H-SiC with intentional V or Ti doping was performed to obtain short minority carrier lifetimes, using VCl₄ or TiCl₄ as the doping sources. The doping efficiencies and quality of the epilayers were compared for H₂+SiH₄+C₃H₈ and H₂+SiH₄+C₃H₈+HCl gas systems. The addition of V or Ti in highly N-doped epilayer demonstrated very short minority carrier lifetimes of 20-30 ns at 250°C.

Abstract: An epitaxial growth technique for 4H-SiC with B doping was developed to control the carrier lifetimes of the epilayers. A linear relationship was observed between the B doping concentration and the flow rate of tri-ethyl-boron, which was used as the B doping source. A room temperature photoluminescence spectrum of a N-and B-doped epilayer showed a broad B-related peak at 2.37 eV instead of a band-edge luminescence, which indicates that the carrier recombination path was changed by the B doping. The minority carrier lifetime decreased (< 30 ns at 250°C) with increasing B doping concentration. The thermal stability of the short carrier lifetime was compared with a conventional carrier lifetime reduction method, namely an electron irradiation technique. After thermal annealing at 1700°C, the carrier lifetime of the electron irradiated epilayer recovered while that of the B-doped epilayer remained, indicating that the carrier lifetime controlled by the B doping technique was more stable against the thermal processes.

Abstract: SiC power module with low loss and high reliability was developed by utilizing IEMOSFET and SBD. The IEMOSFET is the SiC MOSFET with high channel mobility in which the channel region is the p-type carbon-face epitaxial layer with low acceptor concentration. Elemental technologies for the high channel mobility and the high reliability of the gate oxide have been developed to realize the excellent characteristics by the IEMOSFET. The SBD was designed so as to minimize the forward voltage drops and the reverse leakage current. For the fabrication of these SiC power devices, the mass production technology such as gate oxidation, ion implantation and following activation annealing have been also developed.