Kenichi Takarabe

Study of Amorphous Carbon Nitride Films Aiming at White Light Emitting Devices

The possibility for white light emitting devices using carbon nitride (CNx) thin films has been studied. Microwave electron cyclotron resonance (ECR)–plasma chemical vapor deposition (CVD) and RF-sputtering apparatuses have been used for the formation of CNx thin films. In both cases, CH4 was used as the source or sub-source of carbon in order to investigate the effect of hydrogenated carbon nitride for luminescence. The cathodoluminescence (CL) measurement of the film grown by ECR–plasma CVD method showed three peaks of red, green, and blue (R/G/B). The photoluminescence (PL) measurement of the film grown by RF-sputtering also showed the red peak, which could not be observed in the film without hydrogen. Together with the X-ray photoelectron spectroscopy (XPS) analysis data, we concluded that the red peak originates from the level relating to H atom and blue peak from C–N bonds.

Site-selective x-ray absorption fine structure: Selective observation of Ga local structure in DX center of Al0.33Ga0.67As:Se

In order to discuss the local structure of deep level carrier traps, the site-selective x-ray absorption fine structure (XAFS) by measuring the x-ray photon-energy dependence of the capacitance of a Schottky barrier diode is proposed. Dropping of the localized electron into a core hole arising from x-ray absorption of the atom in the carrier trap, not in bulk, increases the capacitance. The site-selective XAFS is adopted for the local structure analysis of DX center in Al0.33Ga0.67As:Se. The Ga K-edge site-selective XAFS is different from the conventional XAFS, suggesting that Ga in the DX center with a large lattice relaxation is selectively observed.

Thermoelectric and electrical transport properties of Mg2Si multi-doped with Sb, Al and Zn

Enhanced thermoelectric and electrical transport properties of Mg2Si-based thermoelectric materials have been achieved by multi-doping with Sb, Al and Zn. Results on the investigation of the electrical transport and thermoelectric properties of multi-doped samples prepared using the spark plasma sintering technique are reported. Synchrotron radiation powder X-ray diffraction was used to characterize the structures of the doped samples. The electrical transport properties were determined from mid-infrared reflectivities, Hall effect and conventional quasi-four probe conductivity measurements. Using the electron concentrations (N) determined from the Hall coefficients, the effective masses (m*) were calculated from the frequency of the plasma edge (ωP) of the infrared reflectivities. The thermoelectric performance and thermoelectric figure of merits (ZT) in the temperature range of 300 K to 900 K of the doped Mg2Si compounds were calculated from the measured temperature dependent electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ). A maximum ZT of 0.964 was found for Sb0.5%Zn0.5% doped Mg2Si at 880 K. This value is comparable to those of PbTe based thermoelectric materials.

Optical properties of nanocrystalline FeSi2 and the effects of hydrogenation

Various optical measurements confirm that optical absorption in uniform thin films made from nanocrystalline iron disilicide (nc-FeSi2) with a 3–5nm radius is larger by about 10% than that of single crystalline β-FeSi2. It is also found that the hydrogenation of nc-FeSi2 changes strongly its optical characteristic energies. The nanocrystalline state appears characteristically in the imaginary part of dielectric constants of β-FeSi2 around 2–3eV.

Electronic structure of some I–III–VI2 chalcopyrite semiconductors studied by synchrotron radiation

Photoemission spectra were measured for the I–III–IV2 chalcopyrite semiconductors CuInSe2, CuInS2, CuInTe2, and CuGaS2 with various photon energies (32 to 140, and 1253 eV). The partial density‐of‐states (DOS) of Cu 3d orbital is selectively observed at the photon energies beyond 100 eV. The DOS of Cu 3d spreads from the top of valence band (VB) to 5 or 6 eV below. Three dominant peaks are labeled as A, B, and C, the structures of which are interpreted as antibonding, nonbonding, and bonding states. Reflection spectra were also measured for CuInSe2 from 2 to 100 eV at room temperature and 98.5 K. The In 4d core reflection is observed at 17.5, 18.9, 19.8, and 21.2 eV at 98.5 K; very similar characteristics are also observed at 5.28, 6.21, 7.35, and 8.67 eV. These contributions are due to the Cu 3d nonbonding electrons. Photoemission and reflection experiments thus confirm conclusively the existence of nonbonding Cu 3d states in I–III–VI2 chalcopyrites. This result proves the theoretical model that attribut...

Significant enhancement of thermoelectric properties and metallization of Al-doped Mg2Si under pressure

We report results of investigations of electronic transport properties and lattice dynamics of Al-doped magnesium silicide (Mg2Si) thermoelectrics at ambient and high pressures to and beyond 15 GPa. High-quality samples of Mg2Si doped with 1 at. % of Al were prepared by spark plasma sintering technique. The samples were extensively examined at ambient pressure conditions by X-ray diffraction studies, Raman spectroscopy, electrical resistivity, magnetoresistance, Hall effect, thermoelectric power (Seebeck effect), and thermal conductivity. A Kondo-like feature in the electrical resistivity curves at low temperatures indicates a possible magnetism in the samples. The absolute values of the thermopower and electrical resistivity, and Raman spectra intensity of Mg2Si:Al dramatically diminished upon room-temperature compression. The calculated thermoelectric power factor of Mg2Si:Al raised with pressure to 2–3 GPa peaking in the maximum the values as high as about 8 × 10−3 W/(K2m) and then gradually decreased ...

Hole mobility of p-type beta-FeSi2 thin films grown from Si/Fe multilayers

The hole mobility of intentionally undoped p-type β‐FeSi2 thin films grown by a multilayer method was investigated. With increasing annealing temperature and time, the hole mobility increased to approximately 450cm2∕Vs at room temperature (RT). The observed hole mobility was analyzed by considering various carrier scatterings such as acoustic-phonon and polar-optical-phonon scatterings, intervalley scattering, ionized impurity scattering, and grain-boundary scattering. The nice fit of the mobility to the experimental results reveals that the polar-optical-phonon scattering determines the hole mobility at RT.

Safety, efficacy, and quality control of a photoelectric dye-based retinal prosthesis (Okayama University-type retinal prosthesis) as a medical device

Patients with retinitis pigmentosa lose photoreceptor cells as a result of genetic abnormalities and hence become blind. Neurons such as bipolar cells and ganglion cells remain alive even in the retina of these patients, and ganglion cells send axons to the brain as the optic nerve. The basic concept of retinal prostheses is to replace dead photoreceptor cells with artificial devices to stimulate the remaining neurons with electric currents or potentials. Photodiode arrays and digital camera-type electrode arrays are the two main approaches for retinal prostheses to stimulate retinal neurons, but these arrays have the problems of poor biocompatibility, low sensitivity, and low output of electric currents, and hence have a requirement for external electric sources (batteries). To overcome these problems, we are developing photoelectric dye-based retinal prostheses that absorb light and convert photon energy to generate electric potentials. The prototype, using a photoelectric dye-coupled polyethylene film, could induce intracellular calcium elevation in photoreceptor-lacking embryonic retinal tissues and cultured retinal neurons. The subretinal implantation of the prototype in the eyes of Royal College of Surgeons (RCS) rats led to vision recovery as proved by a behavior test. The photoelectric dye that was chosen for the prototype did not exhibit any cytotoxicity. The surface potentials of the photoelectric dye-coupled film showed a rapid on-and-off response to illumination with a threshold for light intensity as measured by a Kelvin probe system. Photoelectric dye-based retinal prostheses are thin and soft, and therefore, a sheet of the film of large size, corresponding to a large visual field, could be inserted into the vitreous and then to the subretinal space through a small opening by rolling up the film. Clinical studies of photoelectric dye-based retinal prostheses in patients with retinitis pigmentosa who lose sight will be planned after the manufacturing control and the quality control had been established for the medical device.

Infrared Reflectivity and Crystal Strusture of InS

Infrared reflection spectra of InS single crystals are observed in the wavenumber range between 50 and 900 cm -1 for each polarization in c -plane. The vibrational frequencies of longitudinal (ω L ) and transverse (ω T ) modes are determined by the Kramers-Kronig analyses as follows: ω L =309 and ω T =217 cm -1 for E // a , and ω L =110, 236, 322, and ω T =104, 196, 296 cm -1 for E // b polarizations. Almost the same frequencies are obtained from the fitting procedure by using the classical oscillator model. It is confirmed that the obtained dispersion parameters are more explicit values than those of previous works for twin crystals. The difference in crystal structures between InS and other layer semiconductors, e.g . GaS, GaSe, InSe, is explainable qualitatively in terms of effective charges or ionicities.

Use of nitrogen atmospheric pressure plasma for synthesizing carbon nitride

Carbon nitride (CNx) with a high nitrogen content was synthesized using a nitrogen atmospheric pressure plasma. In this method, a reaction space with a high temperature and a high nitrogen content was generated. Under this condition, it was expected that a dense nitrogen radical reacting with a carbon radical would saturate carbon bonds, result in a simultaneous increase in the composition ratio (N/C), and produce a C–N single bond. The N/C ratio of the synthesized CNx reached 1.0, as determined by X-ray photoelectron spectroscopy (XPS) analysis. XPS and FT-IR analyses showed that the synthesized CNx consists of a dominant C–N bond with a C=N bond and a C≡N bond. The synthesized CNx appeared like a sphere with a diameter of about 100 nm. The optical emission of a CN radical was observed under the CNx growth conditions, and similarities and differences between a nitrogen atmospheric pressure plasma and plasma-enhanced chemical-vapor deposition were determined.