Itsuo Umebu

Atomic Structure of Ordered InGaP Crystals Grown on (001)GaAs Substrates by Metalorganic Chemical Vapor Deposition

InGaP crystals grown on (001)GaAs substrates by metalorganic chemical vapor deposition are structurally evaluated by transmission electron microscopy. High-resolution electron microscopy observation and electron diffraction analysis of both the (110) and the (10) cross-section specimens strongly suggest that ordering of column III atoms on only two sets of the (111) planes with doubling in periodicity of (111) layers, i.e., In/Ga/In/Ga/In/Ga. . ., is occurring in the crystal. The ordering of the crystal is not perfect, and the ordered regions are assumed to be plate-like microdomains.

Direct observation of electron leakage in InGaAsP/InP double heterostructure

The electron leakage through the heterobarrier consisting of a thin InGaAsP active layer and a p‐InP confining layer was directly observed by using a novel InGaAsP/InP light‐emitting diode (LED) structure. In the present structure, electrons leaking from the active layer were confined in a subsidiary quaternary layer having a crystal composition different from that of the active layer, and the recombination emission caused by these electrons was optically detected. Experimental results showed that significant electron leakage can occur in the present InGaAsP/InP double heterostructure (DH) system, suggesting a possibility of the electron leakage being one of the dominant mechanisms of sublinearity in the light intensity‐current characteristics in InGaAsP/InP DH LED’s operating at wavelengths shorter than 1.3 μm and also of the temperature dependence of threshold current in laser diodes.

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.

TEM investigation of modulated structures and ordered structures in InAlAs crystals grown on (001) InP substrates by molecular beam epitaxy

Abstract InAlAs crystals grown on (001) InP substrates by molecular beam epitaxy have been structurally investigated for the first time by transmission electron microscopy. Modulated structures lying in two equivalent directions of 〈100〉 and 〈010〉 are found in crystals grown at 500–560°C. In the crystals grown at temperature below 500°C, anomalous diffraction contrast elongating in the 〈110〉 direction is present. Ordered strutures of CuPt-type are also found in these crystals. Only two of the four equivalent variants are observed in the (110) cross-section. The super-spots are observed only in the crystals grown at 435–505°C and their intensity is weaker than in MOCVD-grown InGaP. These results suggest that the ordering is not perfect and that ordered regions are likely to be microdomains.

Transmission electron microscopic observation of InGaP crystals grown on (001)GaAs substrates by metalorganic chemical vapor deposition

Abstract The detailed nature of ordered structures and the effect of rotation of the substrates on their formation are studied by transmission electron microscopy, for the first time, in InGaP crystals grown on (001)GaAs subtrates by atmospheric metalorganic chemical vapor deposition. In InGaP crystals grown with substrate rotation, ordering occurs in column III atoms on only one of the four equivalent (111) planes with doubling in periodicity of (111) layers, i.e. … In/Ga/In/Ga/In/Ga/…. The ordering of the crystals is not perfect, and the ordered regions are found to be plate-like micro-domains lying on nearly the (001) plane. On the other hand, without rotation of the substrate, ordering occurs on two equivalent (111) planes and the size of the ordered domains becomes larger with a drastic increase of relative volume of the ordered regions. Independent of substrate rotation, modulated structures are observed in two equivalent 〈100〉 directions.

Catastrophic degradation of InGaAsP/InGaP double‐heterostructure lasers grown on (001) GaAs substrates by liquid‐phase epitaxy

Catastrophically degraded InGaAsP/InGaP double‐heterostructure lasers grown on (001) GaAs substrates by liquid‐phase epitaxy, emitting at 727 and 810 nm are investigated by photoluminescence topography, scanning electron microscopy, transmission electron microscopy, and energy dispersive x‐ray spectroscopy. The degradation is mainly due to catastrophic optical damage at the facet, i.e., development of 〈110〉 dark‐line defects from the facet, and rarely due to catastrophic optical damage at some defects, i.e., development of 〈110〉 dark‐line defects from the defects inside the stripe region. These 〈110〉 dark‐line defects correspond to complicated dislocation networks connected with dark knots, and are quite similar to those observed in catastrophically degraded GaAlAs/GaAs double‐heterostructure lasers. The degradation characteristics of the InGaAsP/InGaP double‐heterostructure lasers are rather similar to those in GaAlAs/GaAs double‐heterostructure lasers concerning the catastrophic degradation.

Nature of dark defects revealed in InGaAsP/InP double heterostructure light emitting diodes aged at room temperature

The nature of the dark defects which have appeared in the InGaAsP/InP double heterostructure light emitting diodes aged at room temperature were studied by transmission electron microscopy and other related methods. According to the observation of many diodes by electroluminescence topography, the dark defects were classified into five types: cross‐hatched 〈110〉 dark line defects, regular tetragonal 〈110〉 dark line defects, regularly distributed dark spot defects, dark band defects, and dark spot defects whose circumference was bright. These dark defects corresponded to misfit dislocations, stacking faults which originated from the interface between the epitaxial layer and the substrate, precipitate‐like spherical defects, mechanical damages induced during epitaxial growth, and alloyed regions produced by the penetration of the electrode metals, respectively. It was found that both the dislocation glide process and the dislocation climb motion due to the nonradiative recombination of the minority carriers...

V-grooved substrate buried heterostructure InGaAsP/InP laser by one-step epitaxy

A V‐grooved substrate buried heterostructure (VSB) InGaAsP/InP laser by one‐step epitaxy emitting at 1.3 μm wavelength is developed. The one‐step epitaxy VSB laser is realized by using Cd diffusion for the formation of the p‐InP internal current restriction layer. Efficient current restriction is confirmed in the Cd‐diffused current restriction structure to be comparable to that by two‐step epitaxy. The VSB laser by one‐step epitaxy showed a cw threshold current of about 15 mA at 25 °C and a far field pattern as smooth as that of the two‐step epitaxy VSB laser. The T0 value was 50–60 K below 55 °C and was 30 K above 55 °C. The threshold current obtained here is the lowest among various InGaAsP/InP lasers by one‐step epitaxy.

Mechanism of catastrophic degradation in 1.3‐μm V‐grooved substrate buried‐heterostructure lasers with the application of large pulsed currents

Catastrophic degradation of V‐grooved substrate buried‐heterostructure InGaAsP/InP lasers (λ=1.3 μm), by large pulsed currents, has been investigated using scanning electron microscopy, etching technique, energy dispersive x‐ray spectroscopy, and spatially resolved photoluminescence topography. After the degradation, the diode stops lasing and becomes ohmic. These are associated with the following phenomena: (i) facet erosion inside or outside the stripe region; (ii) penetration of the electrode‐metals into the epitaxial layer. These phenomena are presumably caused by the abrupt passage of large current along the facet or by local heating at the contact region outside the stripe region. Catastrophic optical damage, which frequently occurs in GaAlAs/GaAs double‐heterostructure lasers, is not observed in any part of the degraded diodes.

Characterization of interfacial atomic steps in GaAs/AlAs superlattices by transmission electron microscopy

We have studied interfacial atomic steps in GaAs/AlAs superlattices using high‐resolution transmission electron microscopy (HRTEM) and lattice image simulation. We find arrays of bright spots at the interface in the TEM image to be good indicators of the interface configuration. Doubling of the bright spot arrays and step‐shaped arrays in TEM lattice images indicate a ‘‘type 1’’ monolayer step whose front is perpendicular to the direction of the electron beam and a ‘‘type 2’’ monolayer step whose front is parallel to the direction of the electron beam. Our HRTEM observations indicate that the atomic steps at GaAs and AlAs interfaces grown at 700 °C are denser than at interfaces grown at 500 °C.