Papers by Author: Takeshi Ohshima

Abstract: In this study, we conducted in-situ measurements on a SiC JFET operational amplifier operating under gamma-ray irradiation. It shows that the radiation did not affect the output waveform or voltage gain, but shifted the output offset voltage. This shift may result mainly from holes generated by irradiation and trapped in the oxide layer, which modified the I-V characteristics of the level-shifting diodes. It can be compensated by applying bias voltage, and it may also be prevented by optimizing the diode structure and/or circuit topology.

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: We demonstrated the enhancement of the optically detected magnetic resonance (ODMR) contrast of negatively charged silicon vacancy (V_Si^-) in SiC by thermal treatment. To create high density V_Si^-, Proton Beam Writing (PBW) was conducted. After an annealing at 600 °C, ODMR contrast showed the highest value in the investigated temperature range. At a fewer irradiation fluence, despite no significant change was observed in terms of V_Si^- PL intensity, the improvement of the ODMR contrast was observed. Considering defect energy levels and annealing behavior previously reported, it was deduced that the improvement of the ODMR contrast was caused by the reduction of other irradiation induced defect centers, such as EH1/EH3 centers.

Abstract: Silicon carbide junction field effect transistors (SiC JFETs) were irradiated with gamma-rays up to 9 MGy (H₂O). With increasing dose, apparent shift of drain current-gate voltage (I_D-V_G) curves to negative voltage side as observed for SiC metal oxide semiconductor (MOS) FETs did not take place. No significant difference is observed between drain and gate leakage currents of irradiated JFETs. This strongly indicates that defects as leakage paths were introduced into not bulk region but the interface between bulk and the passivation layer of SiO₂. While, the transfer characteristics including threshold voltage and transconductance were slightly changed compared with the pristine sample. After drain voltage (V_D) was abruptly applied to 6 V, I_D at V_G= 0 V increased slowly as a function of time. This indicates that variation of transfer characteristics is attributed to capture and emission process at defects generated in channel region.

Abstract: We demonstrated optically detected magnetic resonance (ODMR) measurements using three-dimensional (3D) arrayed silicon vacancies (V_Sis) in in-plane SiC pn diodes. Proton beam writing successfully created 3D arrayed V_Sis using different ion (proton) energies. The results of PL mapping analysis indicate that the features of luminescent spot such as size and depth can be estimated by a Monte Carlo simulation (SRIM). This suggests that diagnosis at any locations in SiC devices can be realized using V_Si quantum sensors. Luminescent spots with different depth ranging 4-60 μm showed similar ODMR spectra including its contrast, which means that a similar sensor sensitivity is expected. The results suggest that 3D arrayed V_Si can act as quantum sensor elements with uniform sensitivity in SiC devices.

Abstract: Spin defects of which states can be manipulated in Silicon Carbide (SiC) have drawn considerable attention because of their applications to quantum technologies. The single negatively-charged pairs of V_Si and nitrogen atom (N) on an adjacent C site (N_CV_Si^- center) in SiC is suitable for them. This paper reports the formation of N_CV_Si^- centers on 4H-SiC epilayers with different nitrogen concentrations using light/heavy ion irradiation and subsequent thermal annealing. The formation of N_CV_Si^- centers is characterized by the near infrared photoluminescence (PL) spectroscopy. It is shown that the PL intensity from N_CV_Si^- centers depends on the N concentration and the ion irradiation conditions. The PL intensity increases monotonically with increasing the N concentration when the N concentration is above 2.6×10¹⁶ cm^-3, whereas no linear correlation between them does not appear below that N concentration. Although the PL intensity increases with increasing defects induced by ion irradiation, the PL quenching due to neighboring residual defects appear at above the areal vacancy concentration of 10¹⁷ vac/cm² and the broad Raman scattering spectra originated from vibration modes of amorphized regions hinder the PL from N_CV_Si^- centers at above 10¹⁸ vac/cm². The formation mechanism and the charge state stability of N_CV_Si^- centers are discussed based on the obtained results.

Abstract: This paper reports optical propertites of negatively charged N_CV_Si^- centers in silicon carbide (a nitrogen substituting for a carbon atom adjacent to a silicon vacancy) whose emission wavlength is 1100-1500 nm at room temperature. High-purity semi-insulating (HPSI) 4H-SiCs are implanted with high energy N ion beams and subsequently thermally annealed to form N_CV_Si centers. We investigated a wide range of N ion implantation dose using a micro ion beam implantation technique and observed the photoluminescence intensity from the SiC-NV centers. We show that under conditions of heavy implantation, the excitation laser power excites residual defects and their fluorescences intereferes with the emission from the N_CV_Si^- centers. These results allow us to clarify the requirements to optically detect isolated single N_CV_Si^- centers at lightly implanted conditions.

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: In this work, quenching effect in the photoluminescence (PL) spectrum of divacancy defects in 4H SiC is investigated. Quenching in PL occurs when photoexcitation with an energy below a certain threshold is applied. In order to understand this phenomenon, we carried out Kohn-Sham density functional theory (DFT) calculations. In accordance with recent experimental results, we propose a physical model which explains the quenching phenomenon.

Abstract: 4H-SiC and SOI substrates were bonded by SiO₂-SiO₂ direct bonding method with diluted HF solution (0.5 wt.%). After the bonding process, the handle layer and the BOX layer of the SOI substrate were etched by TMAH solution, and finally the silicon active layer with a thickness of 1.5 μm was remained on the 4H-SiC substrate. Using this silicon layer, Si photodiodes on 4H-SiC for the radiation hardened image sensors were fabricated and demonstrated.