Morio Wada

Wide wavelength and low dark current lattice‐mismatched InGaAs/InAsP photodiodes grown by metalorganic vapor‐phase epitaxy

Using a very thin InP/InAsP cap layer and InAsP strained superlattices in a buffer layer, a lattice‐mismatched InGaAs/InAsP photodiode with low dark current and very wide wavelength spectral response was fabricated by metalorganic vapor‐phase epitaxy. External quantum efficiencies as high as 55% at 0.6 μm, 80%–90% at 0.8–1.9 μm, and 65% at 2.1 μm was obtained. For the 1‐mm‐diam photodiodes, typical dark current measured at 274 K and at a reverse bias voltage of 0.1 V was as low as 1×10−7 A.

Female Preponderance of Parkinson’s Disease in Japan

A male preponderance of Parkinson’s disease (PD) has been reported in European countries and the USA. To verify this issue in Japanese patients with PD, we examined the age- and gender-specific prevalence of PD in Yamagata Prefecture (population 1,244,040), Japan. The prevalence of PD was 61.3/100,000 men and 91.0/100,000 women, showing that women were significantly more affected by PD than men (p < 0.001). Contrary to the findings in Europe and the USA, the results indicate a female preponderance of PD among the Japanese population.

Zn Diffusion into InP Using Dimethylzinc as a Zn Source

A new method of Zn diffusion into InP using dimethyl-zinc(DMZ) as a Zn source is described. This diffusion method provides precise control over both diffusion depth and Zn concentration, and is easier to use and gives highly reproducible results. Diffusions were carried out from 400 to 570°C with the Zn surface concentration of 1017 to 1018 cm-3, and diffusion depth of 0.2 to 2 µm. The diffusion profiles were measured by SIMS analysis and electrolytic etching with simultaneous C-V measurement.

Characterization of Te Precipitates in CdTe Crystals

Te precipitates in CdTe crystals grown by the Bridgman method have been investigated using Auger electron spectroscopy, transmission infrared microscopy and etching techniques. The transmission infrared microscopy shows that Te precipitates with dimensions of several micrometers have well-defined crystallographic facets and are found to decorate dislocations; the density of the precipitates (<1 µm in diameter) is of the same order of the etch pit density, which suggests there exists a strong correlation between the presence of the precipitates and the dislocations.

Temperature dependence of the band gap in InAsyP1−y

The temperature dependence of the band gap in InAsyP1−y (y=0–0.67) has been determined by photoluminescence, x-ray diffraction, and absorption spectra measurements. We found that the measured data within the temperature range of 77–300 K can be expressed by the equation proposed by O’Donnell and Chen. The band gap at 77 K is given by Eg=1.407−1.073y+0.089y2, while the compositional dependence of the band gap observed at 300 K, agrees with the values previously reported. We confirmed that changes in temperature caused a slight change in the bowing parameters, and hence found that the band gap temperature dependence of InAsyP1−y (y=0–1) varies very little with changes in composition (2.5–3.5×10−4 eV/K).

Incorporation of Arsenic and Gallium in InP Layers in GaInAs/InP Heterostructures Grown by MOVPE

The effects of arsenic (As) and gallium (Ga) incorporation in InP layers in GaInAs/InP heterostructures grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) on lattice parameter and band-gap energy of InP layers were studied. It was found that As and Ga incorporation is prolonged during InP growth after arsine (AsH3) and triethylgallium (TEG) flows are turned off, resulting in a lowering of the band gap energy and change in the lattice parameter of the InP layers. This incorporation is considered to originate from both desorption and diffusion of their outgassing sources produced during the growth of a thick GaInAs layer.

Cadmium telluride β-ray detector☆

Abstract The fabrication and characteristics of a prototype CdTe (cadmium telluride) detector for a β-ray thickness gauge are described. The detector consists of a pn-junction diode, operating in photovoltaic mode with no applied voltage. It needs no cooling because its dark current is less than 7 × 10−14 A/mm2at 0.01 V and 30 °C. An output current of 5 × 10−8 A with a statistical noise of 1.5 × 10−12 A/(Hz) 1 2 has been achieved with 85Kr radiation on an active area 16 mm in diameter. The (111) CdTe wafer used in the detector is 22 mm × 22 mm × 1.5 mm. Single crystals with no twins and a low dislocation density (less than 1 × 106 cm−2) are used because it is found that the generation-recombination centres of carriers related to crystallographic defects (such as twins and dislocations) increase the dark current and decrease the output current. As the temperature coefficient of the output current for an 85Kr radiation source is approximately 0.23%/°C, a thermostatistically-controlled module is used to keep the detectors operating temperature constant to within 0.1 °C, thus solving the problem of drift of output current with temperature.

Investigation of Defects in High-Resistivity Undoped CdTe Using the EBIC Method

The Electron Beam Induced Current (EBIC) method using an aluminum Schottky barrier was applied to high-resistivity undoped CdTe crystals to obtain images of crystal defects. The defects such as cellular dislocation structures and twins were observed to produce distributions of dark spots in EBIC images. When the crystals were etched with PBr etchant, it was found that the etched patterns corresponded to high intensity regions in the EBIC images.

Diffusion of zinc acceptors in InAsP by the metal-organic vapor-phase diffusion technique

Diffusion of zinc acceptors in InAsP by a metal-organic vapor-phase diffusion technique, whereby a low-pressure metal-organic vapor-phase epitaxy with dimethylzinc and phosphine is utilized as an open tube diffusion system, is demonstrated to accurately control the diffusion depth in the submicrometer range. The annealing effect on the zinc diffusion profiles in InAsP was found to be the activation of zinc acceptors similar to that in InP, but the maximum hole concentration of 1×1019 cm−3 for the zinc diffusion in InAsP was achieved.

GaInAs/InP PIN Photodiodes Fabricated by MOVPE and a New Zn Diffusion Technique

GaInAs/InP photodiodes were fabricated by selective zinc diffusion using dimethylzinc as a zinc source to form the p+-region and selective p+-GaInAs growth to produce good ohmic contacts using low-pressure MOVPE. These photodiodes have an external quantum efficiency of 60–80% in the 0.95–1.55 µm-wave-length range, and a dark current of about 30 pA at -2 V and a -3 dB cutoff frequency of 1.4 GHz at 0 V were measured for 110 µm-diam diodes. These results show that this diffusion method is a useful process for fabricating photodiodes or other optical devices.