Hiroshi Saijo

Structure and formation mechanism of V defects in multiple InGaN/GaN quantum well layers

A variety of different transmission electron microscopy techniques, and particularly high-angle annular dark-field scanning transmission electron microscopy, has been used to reveal that V defects or inverted hexagonal pyramid defects in multiple InGaN∕GaN quantum well (QW) layers nucleate on threading dislocations that cross the InGaN QW. The defects have thin walls lying parallel to {101¯1} with the InGaN∕GaN QW structure. A formation mechanism for the V defects is proposed taking into account the growth kinetics of GaN and the segregation of In atoms in the strain field around the cores of the threading dislocations.

Mapping of multiple-quantum-well layers and structure of V defects in InGaN/GaN diodes

Cathodoluminescence mapping reveals threading defects, frequently formed by the lattice misfit between GaN and sapphire substrate, as a dark contrast connected with changes in the energy state. Multiple quantum wells, 2.5 nm In0.25Ga0.75N and 13.9 nm GaN layers, are resolved in the secondary electron image as well as in the backscattered electron image. The backscattered electron image, providing compositional mapping without surface effects such as cleaved steps, reveals the presence of V defects and confirms the thin six-walled structure of the V defect with InGaN/GaN {1011} layers. These scanning electron microscopy observations can be performed after very simple specimen preparation, namely just cleaving the sapphire substrate with the epilayers.

Structural and compositional analyses of a strained AlGaN∕GaN superlattice

We investigated the nanostructure of AlGaN∕GaN strained-layer superlattice (SLS) cladding in a GaN-based violet laser diode (LD) using a scanning-transmission electron microscope (STEM). Metal-organic vapor-phase epitaxy was used to grow 200 pairs of n-Al0.14Ga0.86N∕n-GaN layers directly on the n-GaN:Si contact layer that was deposited on a (0001) sapphire substrate. The Al0.14Ga0.86N and GaN layers were distinguished as dark and bright bands in the high-angle annular dark-field (HAADF) images taken in the [1¯21¯0] zone axis. The widths of the Al0.14Ga0.86N and GaN layers were determined to be 2.24±0.09 and 2.34±0.15nm, respectively. The lattice parameters of the Al0.14Ga0.86N were measured to be a=0.32±0.01nm and c=0.50±0.02nm, and those of the GaN, a=0.32±0.02nm and c=0.52±0.03nm. This is a direct illustration of the SLSs, where a good lattice matching in the basal plane caused by shrinkage of the Al0.14Ga0.86N lattice normal to the basal plane suppresses the generation of misfit dislocations. Dislocati...

Determination of thickness and lattice distortion for the individual layer of strained Al0.14Ga0.86N∕GaN superlattice by high-angle annular dark-field scanning transmission electron microscopy

Al0.14Ga0.86N∕GaN and GaN layers in the strained-layer superlattice (SLS) in GaN-based laser diodes were distinguished as dark and bright bands, respectively, in a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. From the HAADF-STEM images the thickness of the AlGaN layers was determined to be 2.24±0.09nm and that of GaN layer 2.34±0.15nm, which corresponds to nine atom planes in the [0001] direction. The parameters of the distorted AlGaN and GaN lattices were evaluated to be a=0.32, c=0.50nm and a=0.32, c=0.52nm, respectively. This shows that the resultant good lattice matching on the (0001) AlGaN∕GaN interfaces suppressed the generation of misfit dislocation in the SLS cladding.

High-resolution scanning electron microscopy observation of GaN/AlGaN strained-layer superstructures in GaN-based violet laser diodes

Two hundred coupled layers of n-Al0.14Ga0.86N (3 nm)/n-GaN (3 nm) strained-layer superstructures (SLSs) with a n-GaN:Si layer were grown directly on a (0001) sapphire substrate by metalorganic vapor-phase epitaxy. With the aid of image processing, each SLS was definitely resolved as a bright or dark fringe 3 nm wide in the mapping of secondary electrons in a high-resolution scanning electron microscope.

MORPHOLOGY AND STRUCTURE OF PHOTOSENSITIVE DYE J-AGGREGATES ADSORBED ON AGBR MICROCRYSTALS GROWN IN GELATIN

Though the cyanine dye J‐aggregates carry the role to sense the exposing light in the silver halide photographic system, little research on the morphology of the aggregates in adsorption has been made with modern surface analytical methods. In this paper, we describe the size, epitaxy, multi‐layered array formation, nucleation and preferential adsorption, and irregular distribution of population between particles and the segregation on a particle, of J‐aggregates adsorbed on AgBr grown in gelatin. We employed cathodoluminescence microscopy, low energy high resolution scanning electron microscopy, and atomic force microscopy. Dye molecules aggregate together near the surface of AgBr and adsorb on the surface. The growth of adsorbed aggregates is controlled by the diffusion of dye molecules from the surrounding solution. The population of J‐aggregates adsorbed on an AgBr particle varies from almost none to full coverage. Each aggregate is about (20–30) × (30–50) nm in size and is 2.1 nm thick for thiacarbocyanine with sodium ion, 1.04 nm for thiacarbocyanine with tosyl ion, and 0.5 nm for an oxacarbocyanine. The aggregates connect their longer edges to each other to form arrays, and the arrays build up multi‐layered stacks. The arrays align parallel and segregate to form terraces. The longer edges of J‐aggregates align along [210] on AgBr (100) or [632] on AgBr (111). Microsc. Res. Tech. 42:123–138, 1998.

High resolution transmission electron microscopy of layer structure and stacking faults in tungsten disulphide lubricants

Abstract Tungsten disulphide crystals used as lubricants have been studied by high resolution transmission electron microscopy (HRTEM). Theoretical calculation of the HRTEM image using the multislice method shows that the WS2 [2 1 1 0] images can distinguish between the W and S columns along the incident electron beam and enable one to determine not only the crystal structure but also the fault structure. Observation of a WS2 [2 1 1 0] crystal, prepared by grinding, discloses stacking faults and an array of edge dislocations whose extra planes are single S-W-S layers, revealing the stacking sequence of W and S layers, and also the stepwise structure of its (0001) surfaces as a result of the shear strain. The lubrication mechanism for solid lubricants such as WS2 and MoS2 is interpreted microscopically in terms of the crystallography on the basis of the observation.

J-aggregates of cyanine dye molecules adsorbed on AgBr emulsion crystal surfaces

Abstract The photosensitive dyes thiacarbocyanine and oxacarbocyanine on 0.7 μm AgBr cubic crystals are observed by analytical-color-fluorescence electron microscopy (ACFEM), ultra-low-energy scanning electron microscopy (SEM), and atomic force microscopy (AFM). ACFEM proves that the dye makes J-aggregates, exhibiting cathodoluminescence spots about 30 × 70 nm. SEM reveals dye stripes epitaxially grown on the (001) AgBr surface. AFM gives the thickness of the stripes, varying from 1 to 20 nm or more, and resolves single J-aggregate particles arranged in the stripe. It is concluded that the dye forms in the stripes by island growth rather than in monolayer adsorption.

Multi-slice calculation for InP crystals using different slices

Abstract Electron diffraction intensity and high-resolution transmission electron microscopy (HRTEM) images are calculated for [001] InP crystals using the multi-slice method. The calculation is performed for two prevalent models: (I) atoms within each slice are projected on their representative plane, and (II) atoms are equally shared out among slices and projected on representative planes. Intensities of redlections such as 200, 220 and 400 for model II are appreciably different from those for model I, which have very few errors. Forbidden reflections such as 110 and 310 appear only in the calculation for model I, changing abruptly their intensity every slice in the unit cell, and deviate from Friedels law. The [001] HRTEM images, visualizing these results, also indicate that model II is inferior to model I in utility for quantitative analysis.

Growth and crystallographic, surface and defect structures of antimony particles deposited in a high-resolution transmission electron microscope

Sb fine particles vacuum-deposited by in situ electron beam heating in an electron microscope were investigated by high-resolution transmission electron microscopy. Round amorphous particles less than 40 nm in size were formed far from the evaporation source, and crystalline particles having definite crystal habits near the source. Observed wedge-shaped crystallites have smooth (110), (111) and/or (112) surfaces, where lattice relaxation and superlattice structure cannot be seen. Stacking faults represented by ab ca/ca bc/bc ab, which are caused by slippings by a glide vector of 13[112]a0, are found in particles having the normal sequence ab ca bc ab ca bc ab ca bc, where a, b and c denote atomic layers in the rhombohedral structure.