Papers by Author: Shinobu Onoda

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: Micrometer-scale patterning was performed using the particle beam writing technique with a focused heavy-ion microbeam, allowing the creation of a unique two-dimensional distribution of fluorescent centers in single-crystalline diamond. The focused nitrogen microbeam was scanned over the target of single-crystalline diamond prepared by chemical vapor deposition to create nitrogen-vacancy (NV) centers at defined positions. Imaging using a custom-built confocal fluorescence microscopy system revealed that the desired NV distribution was generated in the target crystal with a spatial resolution similar to the beam resolution. A two-dimensional matrix barcode test pattern was successfully generated in a diamond substrate to demonstrate the encryption of information inside a solid-state target.

Abstract: Luminescence centers formed in the vicinity of SiC surface are expected to be utilized as Single Photon Sources (SPSs) because of the high-brightness and the potential of electric control at room temperature. In order to gain more insight into the surface SPSs, 4H-SiC pin-diodes are fabricated and the surface SPSs formed in the pin diodes are investigated using a confocal laser scanning fluorescence microscope (CFM). Locations where the surface SPSs appear as well as photoluminescence spectra of the observed surface SPSs are presented. Antibunching characteristics of the surface SPSs are also investigated by the second order autocorrelation function measurement. We conclude that two different types of surface SPSs appear in the surface of 4H-SiC. The location dependence of the observed surface SPSs indicates that the oxide layer on 4H-SiC plays an important role in the formation of surface SPSs, whereas neither ion implantation nor donor ions had an effect. The peak wavelength of luminescence spectra widely varies depending on their locations, indicating lattice strain introduced by the oxide layer has the potential to affect the luminescence spectra.

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: Proton beam writing was carried out into high purity semi-insulating 4H-SiC bulk substrates. Luminescent defects created in the SiC by proton beam writing using 1.7 MeV-proton micro beams were investigated at room temperature using confocal laser scanning microscope. As a result, photoluminescence peak around 900 nm associated with silicon vacancy was observed for the irradiated SiC without post implantation process such as annealing. The overall depth profile of photon counts detected from irradiated areas is in good agreement with simulated vacancy depth profile. This suggests that silicon vacancy can be applied to ion tracking detector. In addition, since silicon vacancy is known as single photon source of which spins can be controlled at RT, PBW is expected to be a useful tool to fabricate spin qubits.

Abstract: Gamma-ray irradiation effects of motor-driver circuit composed of SiC MOSFETs under motor driving with different PWM frequencies were investigated. The driving current and voltage waveforms were normal when the irradiation exceeded 1.1 MGy at PWM frequency of 10 kHz. In addition, the motor was still rotating in this total dose. We compared the irradiation responses of SiC MOSFETs between the cases of driving states and no bias. The drain current – gate voltage characteristics with no bias shifted to the negative voltage side wider than the driving states. Also the leakage current in the case of driving state is fewer than that of no bias.

Abstract: The charge enhancement in SiC-Schottky Barrier Didoes (SBDs) with different epi-layer thicknesses under the condition of the single-species ion irradiation was simulated to find out the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. The value of ion induced charge depended on the thickness of epitaxial-layer in the SBDs. The simulation result suggests that the impact ionization is one of the key effects to lead ion induced charge enhancement.

Abstract: Radiation response of 4H-SiC vertical power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) was investigated at 150°C up to 10.4 MGy. Until irradiation at 1.2 MGy, the drain current – gate voltage curves of the SiC MOSFETs shifted to the negative voltage side, and the leakage of drain current at gate voltages below threshold voltage increased with increasing absorbed dose. However, no significant change in the electrical characteristics of SiC MOSFETs was observed at doses above 1.2 MGy. For blocking characteristics, there were no degradations of the SiC MOSFETs irradiated at 150°C even after irradiated at 10.4 MGy.

Abstract: The charge induced in SiC-SBDs with different epi-layer thicknesses by ion incidence was measured to understand the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. SiC SBD of which epitaxial-layer thicknesses is close to ion range show larger anomalous charge collection than SBD with thicker epi-layer although the former one has lower electric field than the later one. The gains of collected charge from the SBDs suggest that the impact ionization under 0.16 - 0.18 MV/cm of the static electric field in depletion layer is not dominant mechanisms for the anomalous charge collection. It is suggested that the epitaxial-layer thickness and ion-induced transient high electric field are key to understand the anomalous charge collection mechanisms in SBDs.

Abstract: Effects of gamma-ray irradiation and subsequent thermal annealing on the characteristics of vertical structure power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) fabricated on 4H-SiC were studied. After irradiation at 1.2 MGy, the drain current – gate voltage curves of the MOSFETs shifted to the negative voltage side and the leakage drain current at inverse voltage increased. No significant change in the degraded electrical characteristics of SiC MOSFETs was observed by room temperature annealing. The degraded characteristics of SiC MOSFETs began to recover by annealing above 120 °C, and their characteristics reached almost the initial ones by annealing at 360 °C.