Michio Ohkubo

980-nm aluminum-free InGaAs/InGaAsP/InGaP GRIN-SCH SL-QW lasers

The design of 980-nm InGaAs/InGaAsP/InGaP GRIN-SCH lasers with aluminium-free GaAs-based materials is discussed. The first approach was successful MOCVD growth of InGaAsP alloy lattice matched to GaAs. It was found that an immiscible region existed, as determined by photoluminescence measurements. The main approach was to introduce the graded bandgap structure consisting of InGaAsP layers lattice matched to GaAs into GaAs/InGaP interfaces. The graded structure suppresses the heterojunction spikes, especially of the valence band at these interfaces. As a result, series resistance of GRIN-SCH lasers with the graded bandgap structure was reduced compared with simple SCH lasers with abrupt bandgap interfaces due to improved hole injection. Furthermore, the optimum graded structures for optical confinement region were investigated to improve the carrier injection efficiency, especially electron injection efficiency into a single quantum well active layer. Also, this graded bandgap structure formed the graded refractive index profile in an active region, which is the so-called GRIN-SCH waveguide. The GRIN-SCH profile could be controlled to narrow the transverse beam divergence for high coupling efficiency into a single-mode fiber and to reduce the optical power density at facets for high reliability. Finally the results of a life-test of GRIN-SCH lasers was shown, and the lifetime of GRIN-SCH lasers with immiscible InGaAsP layers was discussed. >

InGaAs/InP double‐heterojunction bipolar transistors with step graded InGaAsP between InGaAs base and InP collector grown by metalorganic chemical vapor deposition

We have fabricated InGaAs/InP double‐heterojunction bipolar transistors (DHBTs) grown by metalorganic chemical vapor deposition (MOCVD). By inserting step graded InGaAsP layers between p+‐InGaAs base and n−‐InP collector, the current gain of DHBTs with the graded layers was about twice as large as that without the graded layers, and the dependence of collector current on collector/emitter voltage was smaller than that without graded layers. The current gain was measured up to 2300 with 25×25 μm2 emitter area at a collector current density of 1×104 A/cm2.

0.98 mu m InGaAs-InGaAsP-InGaP GRIN-SCH SL-SQW lasers for coupling high optical power into single-mode fiber

A CW optical power of 75 mW coupled into a single-mode fiber (SMF) at a driving current of 200 mA was achieved by InGaAs-InGaAsP-InGaP graded-index separate-confinement-heterostructure strained-layer single-quantum-well (GRIN-SCH Sl-SQW) ridge waveguide lasers emitting at 0.98 mu m. The GRIN-SCH profile was optimized to minimize the series resistance due to spikes at GaAs-InGaP heterointerfaces. Highly efficient coupling into a SMF with cutoff wavelength of 0.88 mu m by precisely controlling the ridge mesa width (around 2 mu m) and reducing the aspect ratio of the far-field pattern to 1.9. As a result, 0.98- mu m InGaP cladding lasers show performance comparable to the conventional AlGaAs cladding lasers. >

Preparation of Fluorescent Poly(methyl methacrylate) Beads Hybridized with Y3Al5O12:Ce3+ Nanophosphor for Biological Application

Poly(methyl methacrylate) beads were hybridized with Y3Al5O12:Ce3+ nanoparticles through electrostatic interactions using the layer-by-layer adsorption technique to prepare fluorescent beads for biological application. The subsequent sequential adsorption of poly(sodium 4-styrenesulfonate) and poly(allylamine hydrochloride) onto the composite beads prevented the detachment of the Y3Al5O12:Ce3+ nanoparticles. The fluorescent beads were observed as tight dot plots by flowcytometry. Bovine serum albumin can be immobilized on the surface of the composite beads and tagged with a red dye through an antigen–antibody reaction. This result indicates that the composite beads are useful for bead assays such as flowcytometry.

Development of an Immunochromatographic Assay Kit Using Fluorescent Silica Nanoparticles for Rapid Diagnosis of Acanthamoeba Keratitis

ABSTRACT We developed an immunochromatographic assay kit that uses fluorescent silica nanoparticles bound to anti-Acanthamoeba antibodies (fluorescent immunochromatographic assay [FICGA]) and evaluated its efficacy for the detection of Acanthamoeba and diagnosis of Acanthamoeba keratitis (AK). The sensitivity of the FICGA kit was evaluated using samples of Acanthamoeba trophozoites and cysts diluted to various concentrations. A conventional immunochromatographic assay kit with latex labels (LICGA) was also evaluated to determine its sensitivity in detecting Acanthamoeba trophozoites. To check for cross-reactivity, the FICGA was performed by using samples of other common causative pathogens of infectious keratitis, such as Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermidis, and Candida albicans. Corneal scrapings from patients with suspected AK were tested with the FICGA kit to detect the presence of Acanthamoeba, and the results were compared with those of real-time PCR. The FICGA kit detected organisms at concentrations as low as 5 trophozoites or 40 cysts per sample. There were no cross-reactivities with other pathogens. The FICGA was approximately 20 times more sensitive than the LICGA for the detection of Acanthamoeba trophozoites. The FICGA kit yielded positive results for all 10 patients, which corresponded well with the real-time PCR results. The FICGA kit demonstrated high sensitivity for the detection of Acanthamoeba and may be useful for the diagnosis of AK.

Experimental Study of Beam Steering in 980-nm InGaP Cladding Lasers

The beam steering phenomenon was directly observed by means of backplane electroluminescence in 980 nm InGaP cladding power lasers. The electroluminescence of the lasers which showed lateral beam steering had a periodic pattern along the active region, which was assumed to be due to spatial hole burning. The period of this pattern was roughly estimated using the effective refractive index method. A laser satisfying the cut-off condition for higher lateral modes did not exhibit a periodic pattern of electroluminescence.

Magnesium-doped InGaAs using (C2H5C5H4)2Mg: application to InP-based HBTs

Heavily magnesium-doped p-type-InGaAs layers on InP(100) substrates were successfully grown, for the first time, by low-pressure metalorganic chemical vapor deposition (MOCVD) using bis-ethylcyclopentadienyl-magnesium, (C 2 H 5 C s H 4 ) 2 Mg (EtCp 2 Mg), as organometallic precursor for the Mg. It was experimentally verified that the room-temperature hole concentration of Mg into InGaAs increased with increase of the V/III ratio and decrease of the growth temperature. A maximum hole concentration of over 4x10 19 cm -3 was obtained. The diffusion coefficient of Mg in InGaAs was experimentally derived to be 10 -12 cm 2 /s at 800°C, which was comparable to that of Be. Finally, InP/InGaAs heterojunction bipolar transistors (HBTs) with Mg-doped bases were fabricated successfully. Measured maximum current gain was about 320 with a 90 nm thick base and a sheet resistance of the base layer of 1.28 kΩ/sq.

Emitter Material Comparison between InGaP and InGaAsP in GaAs-Based Heterojunction Bipolar Transistors

In0.49Ga0.51P ( Eg=1.87 eV)/GaAs and In0.38Ga0.62As0.22P0.78 ( Eg=1.77 eV)/GaAs Npn heterojunction bipolar transistors (HBTs) were fabricated. In devices with 110 µm×110 µm emitter areas, the curtent gains of both devices were the same value of 14 at a collector current density of 700 A/cm2. By contrast, the turn-on voltage of InGaAsP/GaAs HBTs was 0.01 V smaller than that of InGaP/GaAs HBTs, which indicated that the conduction band discontinuity of the InGaP/InGaAsP heterojunction was much smaller than the valence band discontinuity.

Heavily Zn-doped graded-base AlGaAs/GaAs HBTs grown by MOCVD

Heavily Zn-doped graded-base AlGaAs/GaAs HBTs with base doping concentrations of 3.5*10/sup 19/, 5.5*10/sup 19/, and 8.1*10/sup 19/ cm/sup -3/ were fabricated using metalorganic chemical vapor deposition (MOCVD)-grown epitaxial wafers. The maximum measured current gain was 42 with a base doping concentration of as high as 8.1*10/sup 19/ cm/sup -3/. The current gain decreased drastically from 96 to 42 when doping concentration increased from 3.5*10/sup 19/ to 8.1*10/sup 19 /cm/sup -3/. Assuming that the emitter efficiency is unity, the minority electron lifetime in the base is estimated by the current gain by consideration of drift in the graded base. The dependence of the electron lifetime in the graded base on Zn doping concentration was similar to that in a uniform base. >

Compositionally graded C-doped In/sub 1-x/Ga/sub x/As base in InP/InGaAs D-HBTs grown by MOCVD with low base sheet resistance and high current gain

MOCVD-grown carbon (C)-doped InGaAs layers using CBr/sub 4/ as a C source were investigated with the van der Pauw method and PL measurement. A hole concentration of as high as 7/spl times/10/sup 19/ cm/sup -3/ was obtained at a growth temperature of 385/spl deg/C. However, PL intensity of the C-InGaAs depends on the growth temperature, and was weaker than that of Mg- or Zn-doped InGaAs at a range of over 1/spl times/10/sup 19/ cm/sup -3/. Furthermore, DC measurement of D-HBTs revealed that there existed a strict tradeoff between the current gain and base sheet resistance of C-InGaAs uniform-base D-HBTs. To break through the tradeoff, we have fabricated D-HBTs with 150-nm-thick strain-compensated graded-In/sub 1-x/Ga/sub x/As-base (X=0.42/spl rArr/0.53). As a result, a current gain of 55 with a base sheet resistance of 480 /spl Omega///spl square/ was achieved.