Tetsuichiro Ohno

InP-Based Planar-Antenna-Integrated Schottky-Barrier Diode for Millimeter- and Sub-Millimeter-Wave Detection

An InP-based Schottky-barrier diode (SBD) is monolithically integrated with a wideband log-periodic toothed antenna for detecting millimeter- and sub-millimeter-waves at frequencies of up to the terahertz (THz) range. A module with a quasi-optical collimation lens fabricated for practical use exhibits sensitivities of 1000 V/W at 300 GHz and 125 V/W at 1.2 THz with good linearity. Near-distance wireless transmission of 5 Gbit/s data with a 240 GHz carrier is also examined using the fabricated SBD module.

Comparison of GaN- and ZnSe-based materials for light emitters

Abstract Recent progress in research on GaN- and ZnSe-related materials is remarkable. Based on these results, this paper compares the characteristics of GaN- and ZnSe-based materials from the viewpoint of achieving reliable light emitting devices, in particular laser diodes (LDs) based on experience gained in the development of LDs such as the InGaAsP/InP and GaAs/AlGaAs LDs widely used at present. The relationship between lattice constant and bandgap energy, the dispersion of refractive indices, the equilibrium vapor pressure of elements over materials, and self-diffusion coefficients can be compared. The current status of both materials is reviewed with respect to light emitters. The transport devices are also described, briefly. Finally, future prospects are discussed.

Growth of high-quality cubic ZnS crystals and their application to MIS blue light-emitting diodes

Cubic ZnS crystals, some 1.5 cm 3 in size, have been successfully grown at about 850 ° C using a conventional iodine-transport method. The quality of the crystal obtained can be significantly improved by prebaking the ZnS powder in H 2 S gas prior to growth. Low-resistive n-type crystals, with a room temperature (RT) resistivity as low as 3 Ω cm, have been characterized by their excitonic and impurity-related photoluminescence emission spectra at 4.2 K. A correlation between the emission properties and surface morphology was found with the various oriented homoepitaxial ZnS layers grown by low-pressure metalorganic chemical vapour deposition (MOCVD). Using the high-quality n-ZnS crystals doped with I donors, an MIS-structured blue LED has been fabricated and yielded an external quantum efficiency as high as 0.05% at RT when a ZnS insulating (I) layer with a thickness of 300 A was deposited on the (110)-oriented ZnS substrate.

Identification of Na acceptor in MOCVD-grown ZnS films and the effect of UV light illumination

Abstract Low-temperature photoluminescence (PL) studies have been carried out to identify a Na acceptor level in ZnS films grown by low-pressure metalorganic chemical-vapour deposition. A neutral Na acceptor bound-exciton line at 3.781 eV and related edge-emission bands at about 3.67 eV were observed. In particular, the edge-emission band is attributed to a free-to-Na acceptor transition by a time-resolved spectra experiment. These PL properties have also been investigated by changing growth and Na cell temperatures. The ionization energy of the Na acceptor is estimated to be about 170 meV which is in good agreement with the theoretical value calculated by Baldereschi and Lipari. The effect of UV light illumination was preliminary done to incorporate Na impurities into ZnS films during epitaxial growth. It was found that the Na acceptor bound-exciton line can be significantly enhanced in intensity.

ZNCDSE/ZNSE QUANTUM-WELL LASER DIODE ON A (711)A GAAS SUBSTRATE

For the purpose of increasing the acceptor concentration in p-type ZnSe, nitrogen-doped ZnSe was grown on ( n11) GaAs substrates. An acceptor concentration twice as large as that on a (100) substrate was obtained and found to be reproducible, except for (111) substrates, on which ZnSe films showed high resistivity. The PL characteristics of ZnCdSe/ZnSe single quantum wells (SQWs) on a ( n11) substrate were also examined. The PL spectrum showed a blue shift mainly caused by the smaller Cd composition on a ( n11) substrate than that on a (100) substrate. Finally, a ZnCdSe/ZnSe MQW SCH laser with HR-coated facets, a 900-µ m-long cavity, and a 20-µ m stripe contact was fabricated on a (711)A substrate. The built-in voltage of a LD on the (711)A substrate is 5 V lower than a LD on the (100) substrate. The former oscillated under pulsed operation at 25°C with a threshold current of 3.1 A and an oscillation wavelength of 501 nm.

The effects of photo-assisted MOCVD on the doping of Na acceptors into ZnS epitaxial layers

Abstract Photo-assisted (PA) MOCVD has been used under low-pressure conditions of 3 Torr to grow ZnS films on (100) GaAs substrates. This method has been shown to enhance the Na acceptor-bound exciton line at 3.783 eV at 4.2 K in intentionally Na-doped ZnS films. The growth rate was not enhanced by the PAMOCVD, but the crystal quality was found to be significantly improved as evidenced by the appearance of free-exciton emission around 3.80 eV. It is tentatively proposed that the observed effects are due to Zn atom desorption which allows Na ions to be incorporated as acceptors.

Surface cleaning with hydrogen plasma for low-defect-density ZnSe homoepitaxial growth

This article investigates in depth the conditions for cleaning of the (100) ZnSe substrate surface for ZnSe homoepitaxial growth. Wet etching with a K2Cr2O7-based etchant as a pre-growth treatment is found to result in a Se-rich ZnSe surface that retains its original flatness. Reflection high energy electron diffraction patterns show that plasma irradiation during thermal cleaning is indispensable for removing the oxide layer on ZnSe substrate and keeping the flatness. In hydrogen plasma cleaning, the etch pit density (EPD) of the homoepitaxial film increases at rf power higher than 265 W, indicating the optimum cleaning temperature is between 260 and 280 °C. As for the dependence of the EPD on the cleaning time, a minimum EPD of 2.7×104/cm2 can be obtained for a plasma cleaning time of 20 min. A correlation between the interfacial residual oxygen concentration and the defect density in an epitaxially grown ZnSe film is also found.

Investigation of degradation in homoepitaxially grown ZnCdSe/ZnSe light emitting diode

From the transmission electron microscope (TEM) observation of ZnSe homoepitaxial films, it is clarified that the major pre-existing defects in the film are Shockley extended dislocations. Correlation between the etch pits and the Shockley extended dislocations are also confirmed. The degradation mode of a ZnCdSe/ZnSe homoepitaxial light emitting diode (LED) is discussed on the basis of the microscopic observation. Many dark spot defects (DSDs) are observed in the emission pattern just after turn-on, and they enlarge and become pronounced keeping their round shape. The growth velocity of the DSD is less than 0.056 µm/min for the current density of 408 A/cm2. The DSD density is almost the same as the etch pit density (EPD) of the as-grown LED wafer.

30-km Error-Free Transmission of Directly Modulated DFB Laser Array Transmitter Optical Sub-Assembly for 100-Gb Application

We fabricated the first compact 100-gigabit Ethernet (100GbE) transmitter optical sub-assembly (TOSA) using a directly modulated DFB laser (DML) array monolithically integrated with an optical multiplexer. Specially designed InGaAlAs/InGaAlAs multiple quantum wells make it possible to obtain a large gain in the 1295-1310-nm wavelength range and to operate at a high temperature of 55 °C. The four-channel DML array chip is only 2.0 mm × 2.4 mm in size, and the fabricated TOSA is only 7.2 mm(W ) × 14.3 mm(L) × 6.5 mm(H), which fits a CFP2 or smaller 100-GbE transceiver. For all the lanes, the 3-dB bandwidths of the TOSA exceeded 17 GHz, and it could operate at 25.8 Gb/s. With 25.8-Gb/s × four-channel simultaneous operation, error-free transmissions over a 30-km single-mode fiber were demonstrated for all the lanes.

Compact High-Responsivity Receiver Optical Subassembly With a Multimode-Output-Arrayed Waveguide Grating for 100-Gb/s Ethernet

We developed a compact receiver optical subassembly (ROSA) for 100-Gigabit Ethernet (100 GbE) using multimode-output-arrayed waveguide grating (MM-AWG) and arrayed photodiodes (PDs). The four-channel silica-based MM-AWG has low insertion loss of less than 1 dB. The back-side-illumined PD with maximized-induced current structure provides high responsivity of 0.95 A/W. To avoid the distortion of the responsivity spectrum due to poor coupling between the multimode beam and PD, we employ a racetrack shape for the active region of the PD. The optical coupling between the MM-AWG and lens-coupled PD is optimized in terms of the flattening ratio of the racetrack shape. The fabricated ROSA has high responsivity of over 0.7 A/W with a flat-top spectrum and obtains high environmental stability thank to its large assembly tolerance of over ± 4 μm. For 4 × 25-Gb/s signals after 10-km transmission, the minimum receiver sensitivity of the ROSA has a margin of 4.8 dB for the Ethernet standard. The transmission performance with a 100-GbE transmitter optical subassembly was also confirmed.