Tomoyuki Ueki

Application of femtosecond laser irradiation to low-temperature diffusion at the Ni/SiC interface

We discovered low-temperature diffusion caused by femtosecond (fs) laser irradiation at the Ni/SiC interface. Before annealing, strained layers were introduced into the near-interface region in the SiC substrate by fs laser irradiation. Transmission electron microscopy unveiled Ni diffusion into SiC, occurring preferentially along strained layers, associated with the diffusion of Si and C into the Ni film, resulting in the formation of an interdiffusion layer. The strained layers acted as a sink for absorbing the Kirkendall voids close to the interface. These results indicate the potential application of fs laser irradiation for low-temperature fabrication of contacts at the Ni/SiC interface.

High temperature degradation mechanism of a red phosphor, CaAlSiN3:Eu for solid-state lighting

Thermal properties of a red phosphor CaAlSiN3:Eu (CASN) at elevated temperatures were evaluated. A heat treatment at 800 °C degraded the photoluminescence property of CASN and caused irreversible changes in both the excitation and emission intensities. The heat treatment in air simultaneously decreased the N elements and increased the O elements. Consequently, the Eu2+ luminescence center was oxidized and CASN lost its photoluminescence property. Although the crystal structure of CASN host was stable even after the heat treatments, the local structure change around the Eu2+ ions is the origin of the thermal degradation of CASN. We found that the heat treatment in N2 atmosphere suppresses the thermal degradation. This is due to the suppression of N evolutions and the incorporation of O elements, which sustains the optically active Eu2+ state.

Femtosecond laser-induced modification at aluminum/diamond interface

We investigated femtosecond-laser-induced modification at an Al/diamond interface. The interface was irradiated from the backside through the diamond substrate, which is transparent to the laser beam. Extremely high pulse energies, i.e., 200 and 100 µJ/pulse, were used to irradiate the interface. The cross-section of the laser-irradiated line was observed with conventional and high-voltage transmission electron microscopy. The modification of the laser-irradiated interface was characterized by the formation of an amorphous phase sandwiched between the deformed Al film and the diamond substrate. The major chemical component of the amorphous phase was identified as carbon, blown from the diamond substrate. The newly formed interface between the amorphous phase and the diamond substrate was concave. In addition, a fine ripple structure with an average spacing one-quarter the wavelength of the laser light was formed only in the sample irradiated by the higher-energy pulses.

Low-temperature diffusion assisted by femtosecond laser-induced modifications at Ni/SiC interface

We investigated low-temperature diffusion at the Ni/SiC interface with the assistance of femtosecond laser-induced modifications. Cross sections of the laser-irradiated lines of two different pulse energies — 0.84 and 0.60 J/cm2 in laser fluence — were compared before and after annealing at 673 K. At the laser fluence of 0.60 J/cm2, a single flat Ni-based particle was formed at the interface after annealing. The SiC crystal under the particle was defect-free. The present results suggest the potential application of femtosecond laser-induced modifications to the low-temperature fabrication of contacts at the interface without introducing crystal defects, e.g., dislocations and stacking faults, in SiC.

Femtosecond-laser-induced modifications on the surface of a single-crystalline diamond

We investigated femtosecond-laser-induced modifications on the surface of a single-crystalline diamond and the changes that occur in them with annealing by observing the cross sections of laser-irradiated lines by transmission electron microscopy (TEM). Femtosecond laser pulses were irradiated on the (001) surface of a diamond by scanning along straight lines. An aluminum film was deposited on the (001) surface after the irradiation, and a TEM sample was prepared. Element mapping and electron diffraction analyses showed that the laser-modified regions were composed of single-crystalline diamond, graphite, and amorphous carbon. The existence of aluminum in the laser-modified regions suggested that the regions were porous when laser-irradiated. After the bulk sample was annealed, another cross-sectional TEM sample was prepared. The annealing induced the diffusion of aluminum from the film into the laser-modified regions where the extension of amorphous areas was observed.

Preparation of micro-biosensor for continuous glucose monitoring

Amperometric glucose sensors with a diameter of less than 10 μm were fabricated by the immobilization of glucose oxidase (GOx) on carbon fiber electrodes. Carbon fiber electrodes with platinum thin film were also prepared using electroplating and sputtering. The combination of electrodeposition and electropolymerization was employed for the immobilization of GOx. Although the introduction of Pt film satisfactorily improved the oxidation current of the electrode to hydrogen peroxide, sensitivities of obtained GOx-immobilized electrodes to glucose were not significant. Formation of electroplated-platinum thin film on carbon fiber was effective to reduce the influence of electroactive compounds existing in biological fluid such as ascorbic and uric acids.