Toyoshi Yamaoka

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 µm

Dark current, quantum efficiency, multiplication noise, and pulse response of germanium avalanche photodiodes with n+-p junction were studied to find an optimum structure. The dark current can be separated by graphical means into a leakage current component and a multiplied component which flows through the junction. The dark current components are also evaluated by using diodes with various diameters. The quantum efficiency and the multiplication noise are shown to be strongly affected by the n+ layer thickness. An n+ layer thickness optimized for signal-to-noise ratio is estimated from experimental and calculated results, using a figure of merit for avalanche photodiodes. The response waveform for mode-locked Nd:YAG laser shows a rise time of 100 ps and a half pulsewidth of less than 200 ps.

Degradation of high‐radiance Ga1−xAlxAs LED’s

The slow degradation of single‐heterostructure Ga1−xAlxAs LED’s has been investigated. The samples which show the fast degradation are completely rejected by the selection of DLD‐free LED’s. Selection procedures and the results of accelerated aging tests for over 10 000 h are presented. The activation energy of the slow degradation is found to be 0.57 eV. Extrapolated room‐temperature half‐life in excess of 5×106 h is estimated. The degradation coefficients are not affected significantly by the operating current density (3.5–10 kA/cm2).

Determination of In‐Ga‐As phase diagram at 650 °C and LPE growth of lattice‐matched In0.53Ga0.47As on InP

The liquidus isotherm in the In‐rich corner of the In‐Ga‐As system at 650 °C was experimentally determined by an improved seed dissolution technique using InP seeds. The solidus isotherms at this temperature were also determined in the composition range close to lattice‐matched In0.53Ga0.47As on InP. The solidus data are strongly affected by the crystallographic orientation of the substrate, but are not significantly affected by the degree of lattice matching. The calculated phase diagrams have been compared with the experimental results. The conditions for equilibrium LPE growth of exactly lattice‐matched ternary layers on InP (100) and (111) B substrates were obtained from the results of the phase diagram and lattice‐constant measurements. It was found that the distribution coefficient, growth rate, and surface morphology are strongly dependent on the substrate orientation. The distribution coefficient for Ga and the growth rate at 650 °C are both larger on the (100) face than on the (111) B face. Hillo...

Ionization rates for electrons and holes in GaAs

Ionization rates in GaAs are determined from the measurement of photocarrier multiplication. Pure electron and hole initiations are achieved by using the novel crater mesa structure and appropriate optical‐injection wavelengths. The ionization rates for holes are greater than that for electrons except at highest fields. This agrees with the studies of Stillman et al., except for the individual values. The ionization rates for electrons and holes are expressed as α=5.6×106 exp(−2.41×106/E) and β=1.5×106 exp(−1.57×106/E), respectively.

High-efficiency long-lived GaAlAs LED's for fiber-optical communications

A new fiber-mounted GaAlAs LED has been proposed and developed to satisfy efficient performance and high-reliability requirements in Practical fiber-optical communication systems. By utilizing the lens effect of a spherical-ended fiber, the LED-to-fiber coupling is greatly improved and a coupling efficiency of 8.5 percent is obtained. At a forward current of 100 mA, the output power of 305 µW is obtained from 0.14-numerical-aperture 85- µm-core step-index fiber of 3-cm length. The power in the cladding part of the fiber is almost completely removed by the epoxy. Long-term operating-life tests of LEDs described here are in progress. After about 8000 h of operation, no power reduction has occurred. The accelerated aging at elevated temperatures has also been performed and an activation energy of 0.5 eV has been determined for DLD-free LEDs. A half-life in excess of 106h is estimated for room-temperature operation at a constant-current density of 10 kA/cm2.

Self‐aligned contact process for Nb/Al‐AlOx/Nb Josephson junctions

High quality Nb/Al‐AlOx/Nb Josephson junctions have been fabricated by a novel process named the self‐aligned contact process. After the definition of the junction area by a reactive ion etching (RIE) technique, the exposed Nb layer and the junction edge are anodized to protect against electrical shorts and an Al film is deposited as the etching stopper layer. After base electrode patterning by RIE, an insulation layer is deposited. The contact hole, with the diameter of the junction, for connecting a counter electrode and a wiring layer, can be made without registration because of the deposited Al film. The junctions fabricated by this process have exhibited excellent current‐voltage characteristics (Vm =70 mV at the critical current density  jJ =1.8 kA/cm2, Vm =11 mV at  jJ =20 kA/cm2).

9 ps Gate Delay Josephson OR Gate with Modified Variable Threshold Logic

A Josephson logic OR gate suitable for high speed logic circuits is proposed and tested. This gate has a structure modified from Variable Threshold Logic (VTL) to obtain large operating margin and small occupation area. The operating margin is calculated as ±19% for fan-out of 2, even with the critical current variation of ±20%. The circuit area is 40×60 µm2. A chain of 5-stage OR gates was fabricated. The gates had Josephson junctions of 4 µm and 7 µm diameter made with Pb-alloy technology. The minimum gate delay of 9 ps was measured using a Josephson sampler.

A model for reach‐through avalanche photodiodes (RAPD’s)

A model for silicon RAPD’s, which have an n+–p–π‐p+ structure and are fabricated by using the ion‐implantation techniques for forming the p layer, is studied. Useful relations for both an excess noise factor and a temperature dependence of an avalanche breakdown voltage are derived and are found to be in good agreement with experiments. The diodes discussed are useful in optical communication systems because of a relatively low operating voltage for the higher quantum efficiency.

Transmission electron microscope observation of dark‐spot defects in InGaAsP/InP double‐heterostructure light‐emitting diodes aged at high temperature

Dark‐spot defects revealed in the electroluminescence patterns of degraded InGaAsP/InP double‐heterostructure light‐emitting diodes that are operated at the current density of 8.0 kA/cm2 at 200 °C were investigated using transmission electron microscopy. Bar‐shaped defects lying in the direction of 〈100〉 or 〈110〉 were observed corresponding to the dark defects. These defects were precipitates of a certain kind of metal or compound and not ’’dislocationlike’’ ones.

Avalanche buildup time of silicon reach‐through photodiodes

The avalanche buildup time t, which is related to the multiplication factor (M) by t=τ1 M, where τ1 is the intrinsic response time, is studied for silicon reach‐through avalanche photodiodes (RAPD’s) by a shot‐noise measurement in the GHz region. The dependences of t on both the length of the avalanche region la and the wavelength exciting the avalanche process (λ) are investigated. The τ1 values obtained increase with la in the region la≳1.0 μm and also take larger values for λ∼6300 A than for λ∼8300 A. These values are in good agreement with a calculation using the modified Emmons equation τ1=Nkeffla/vs except for the very narrow avalanche region (la≲0.4 μm), where N is a constant dependent on la, keff is the effective ratio of hole to electron ionization rates, and vs is the carrier saturation velocity. These results are very useful to investigate the frequency response of RAPD’s. The diodes discussed are used extensively in fiber transmission systems.