Shingo Fuchi

Wideband Infrared Emission from Yb3+- and Nd3+-Doped Bi2O3–B2O3 Glass Phosphor for an Optical Coherence Tomography Light Source

We have investigated the spectroscopic properties of Bi2O3–B2O3 glasses doped with Nd3+ and Yb3+ separately, and doubly doped with the same ions, in order to search for appropriate phosphors for an optical coherence tomography (OCT) light source. The effects of Yb3+ and Nd3+ co-doping and varying Yb3+ concentration on the spectrum shape were studied to obtain an optimum line shape for OCT. We successfully obtained a Gaussian-like shape spectrum from 5.0Yb2O3–1.0Nd2O3–46.0Bi2O3–46.0B2O3 (in nominal molar composition) with a full-width-at-half-maximum (FWHM) of 101 nm in the wavelength region around 1000 nm. This FWHM corresponds to a depth resolution of 4.5 µm, which is about twice that of conventional light emitting diodes (LEDs) and super luminescent diodes (SLDs). The Gaussian-like line shape is very important to suppress ghost images in OCT. These results indicate that a Yb3+, Nd3+ co-doped Bi2O3–B2O3 glass phosphor can indeed be applied as a new type of OCT light source.

First observation of In(x)Ga(1-x)As quantum dots in GaP by spherical-aberration-corrected HRTEM in comparison with ADF-STEM and conventional HRTEM.

In(x)Ga(1-x)As quantum dots in GaP(100) crystals prepared by the OMVPE technique are observed along the [011] direction with a newly developed 200-kV spherical aberration(Cs)-corrected HRTEM, a 200-kV annular dark-field (ADF)-STEM, and a 200-kV conventional HRTEM equipped with a thermal field-emission gun. The dots are 6-10 nm in size and strongly strained due to the misfit of about 9% with the GaP substrate and GaP cap layer. All of the cross-sectional high-resolution electron micrographs show dumbbell images of Ga and P atomic columns separated by 0.136 nm in well-oriented and perfect GaP areas, but the interpretable images are limited to those taken with the Cs-corrected HRTEM and ADF-STEM with Fourier filtering of the images. The Cs-corrected HRTEM and ADF-STEM are comparable from the viewpoint of interpretable resolution. A detailed comparison between the Cs-corrected HRTEM images and the simulated ones with electron incidence tilted by 1 degree to 5 degrees from the [011] zone axis gives information on local lattice bending in the dots from the images around 0.1 nm resolution. This becomes one of the useful techniques newly available from electron microscopy with sub-angstrom resolution.

High Power and High Resolution Near-Infrared Light Source for Optical Coherence Tomography Using Glass Phosphor and Light Emitting Diode

We have fabricated a novel near-infrared light source for optical coherence tomography (OCT) using a glass phosphor and a light emitting diode (LED) as the excitation source. We used 1.0Yb2O3–4.0Nd2O3–47.0Bi2O3–47.0B2O3–1.0Sb2O3 glass as the phosphor to obtain wideband light emission around 1000 nm. We observed a full-width at half-maximum (FWHM) as wide as 98 nm with an output power as high as 1 mW. The interference signal showed a coherence length of 4.6 µm. This value indicates that the depth resolution of the OCT is twice as high as that of conventional LEDs and super luminescent diodes (SLDs).

Effects of defects and local thickness modulation on spin-polarization in photocathodes based on GaAs/GaAsP strained superlattices

The spin-polarization of electrons from the GaAs/GaAsP superlattice on a GaAs substrate (∼90%) is higher than that from the same superlattice on a GaP substrate (∼60%). Transmission electron microscopy and atomic force microscopy observations revealed that stacking faults were the main defects in the superlattice on the GaAs substrate, while local thickness modulation of the superlattice layers was prominent in the superlattice on the GaP substrate. According to the density of stacking faults and the areal ratio of the thickness modulation, it was concluded that the thickness modulation in the superlattice was the main reason for the spin-polarization reduction in the photocathode on the GaP substrate. Growth of a thin GaAs layer on a GaP substrate prior to superlattice growth eliminated the thickness modulation and the spin-polarization was recovered to 90%.

InAs dots grown on InP (001) by droplet hetero-epitaxy using OMVPE

Abstract We have successfully obtained InAs dots on InP (001) by droplet hetero-epitaxy and observed room-temperature photoluminescence (PL) spectrum with a peak at around 1.6 μm. Dependences of the surface morphology on the substrate temperature and on the trimethylindium (TMIn)-supply time were investigated. From atomic force microscopy (AFM), images of the samples prepared by varying the TMIn-supply time, a critical coverage for supplied In to form droplets was found to exist between 1.5 monolayers (MLs) and 3 MLs. This result was confirmed by low-temperature PL measurements. At the TMIn-supply of 3 MLs, which was above the critical coverage, small dots of height ≈5 nm were formed with the density of 9.8x10 9 cm −2 . A PL spectrum of the sample grown using the same sequence followed by the growth of a 10 nm InP cap-layer, exhibited a 1.6 μm emission, with a full width at half maximum (FWHM) of 150 meV at room temperature.

X-ray characterization of GaN and related materials at growth temperatures–system design and measurements

To investigate the phenomena that occur at the growth temperatures, an MOVPE (metalorganic vapor phase epitaxy) growth system was installed in the X-ray diffractometer of the laboratory level. The present MOVPE system is capable of growing GaN and related materials that are advanced in the device applications but very little is known, especially experimentally, what is going on at the growth front and in the environment. Since MOVPE growth is conducted at an atmospheric pressure or at a low pressure, very limited tools can be used to probe the growing surface. It is demonstrated that the X-ray diffraction, X-ray CTR (crystal truncation rod) scattering, and X-ray reflectivity can be used even at 1000 o C that is a normal growth temperature for GaN and related materials.

Growth mode transition of InGaAs in OMVPE growth on GaP (001)

Abstract We investigated growth of InGaAs with different In compositions on GaP (001) substrates by low-pressure organometallic vapor phase epitaxy (LP-OMVPE). From atomic force microscope (AFM), plan-view transmission electron microscope (TEM), cross-sectional TEM and high-resolution TEM observations, it was found that the growth depended clearly on the ratio of TMIn flow rate to the total flow rate of group-III sources ( x TMIn ). In the low x TMIn region, InGaAs grew as a layer. In the high x TMIn region, on the other hand, island formation took place. At x TMIn =0.5, InGaAs islands were formed by the Stranski–Krastanow (S–K) growth mode.

Effects of GaP Cap Layer Growth on Self-Assembled InAs Islands Grown on GaP (001) by Organometallic Vapor Phase Epitaxy.

We have fabricated GaP/InAs islands/GaP structures using various growth sequences for the GaP cap layer, by low-pressure organometallic vapor phase epitaxy (LP-OMVPE). The effects of the growth sequences on the buried InAs islands and the GaP cap layers have been investigated by cross-sectional transmission electron microscopy (TEM). The two-step growth sequence is effective for the growth of GaP cap layers, whereby the first GaP layer is grown at the same temperature as InAs islands and the second GaP layer is successively grown at the optimized growth temperature of GaP. The growth sequence yields good-quality GaP/InAs islands/GaP structures with small InAs islands (15 nm in width and 5 nm in height) and a high-quality GaP cap layer. Droplet heteroepitaxy of GaP cap layers is feasible for the control of the size of buried InAs islands.

Ultrawide-band near-infrared light source over 1 mW by Sm3+,Pr3+-codoped glass phosphor combined with LED

We have synthesized 0.3Sm2O3–0.12Pr2O3–10Bi2O3–45Sb2O3–45B2O3 glass as an ultrawide-band near-infrared phosphor for LED-based light sources. We have achieved an ultrawide-band luminescence from 760 to 1100 nm with an output power of over 1 mW by combining a blue LED with Sm3+,Pr3+-codoped borate glass in one package. Absorption bands corresponding to C–H and O–H stretchings in aqueous solutions of glucose have been observed using this light source and a multichannel spectrometer. These results indicate that the Sm3+,Pr3+-codoped borate glass combined with the blue LED is a useful novel light source for absorption spectrometry.

In situ X-ray reflectivity of indium supplied on GaN templates by metal organic vapor phase epitaxy

The indium supplied on c-plane GaN templates using Metal organic vapor phase epitaxy was studied by in situ X-ray reflectivity (XRR) at 800 °C. The presence of liquid indium layers on the GaN (0001) surface was demonstrated using data-fitting of XRR measurements, ex situ atomic force microscope, auger electron spectroscopy, and cross-sectional scanning electron microscope. These measurements demonstrated that a liquid indium layer coexisted with indium droplets on top of the GaN (0001) surface at 800 °C. The liquid indium film thicknesses increased with increasing TMIn supply time and did not change during cooling from 800 °C to room temperature.