S. Matsumura

Strain contrast of coherent precipitates in bright-field images under zone axis incidence

Abstract It has been proposed that bright-field imaging of coherent spherical precipitates under zone axis incidence would give improved resolution in the strain contrast compared with the weak-beam dark-field imaging. We observed the strain contrast from coherent Co-precipitates in Cu-Co alloys, and simulated the images on the basis of many-beam dynamical electron diffraction, taking into account the elastic anisotropy. Zone axis incidence such as [001] or [001] considerably reduces the effective extinction distance of the transmitted beam, and suppresses the intensity oscillation with foil thickness, in comparison with a conventional two-beam condition. The contrast in the bright-field images is straightforwardly related to the precipitate geometry. Individual precipitates are well resolved in the image when the number density of precipitates is fairly high. It has been demonstrated that the imaging under zone axis incidence is quite useful to understanding of microstructures in phase transformation in ...

Dynamical diffraction effect on HOLZ-pattern geometry in Si-Ge alloys and determination of local lattice parameter

Abstract Higher-order Laue zone (HOLZ) patterns are observed in convergent-beam electron diffraction (CBED) patterns of Si-Ge alloys and the dynamical effect on HOLZ lines is discussed in relation to the kinematical approximation to simulate the HOLZ lines. Double lines often appear from one HOLZ reflection due to the excitation of the first and second branches of the zeroth-layer dispersion surface. The magnitude of the systematic line shift from the line positions predicted by the kinematical simulation changes sensitively with specimen thickness and alloy composition depending on which branch is dominantly excited. The kinematical simulation is applicable to accurate determination of lattice parameters with a proper correction δ E : a difference between the effective electron energy and the actual microscope operating voltage. Usefulness of the kinematical simulation is demonstrated in the investigation of the segregation in a Si-Ge alloy.

Phase separation and domain structure of iron-based ordering alloys

Abstract The process of ordering with phase separation and domain structure of Fe-Al and Fe-Ga alloys in the regions of mixed phases A2 + B2 and A2 + DO3 have been investigated by transmission electron microscopy. Observation by dark field micrographs with superlattice reflections revealed a complicated process of phase separation strongly depending on alloy composition and temperature, which was in good agreement with thermodynamics based on Landaus phenomenological theory.

HRTEM image contrast of short range order in Ni4Mo

The high resolution transmission electron microscope (HRTEM) imaging of short range order (SRO) in Ni4Mo was investigated by means of multi-slice image simulations. The HRTEM images of Ni4Mo exhibit locally bright dot patterns corresponding to the [001] projections of the N2M2-type (chalcopyrite-like) structure. The multi-slice simulations revealed that the N2M2 patterns are rationalized as the projection patterns of the SRO structure which consists of subunit cell clusters of D1a, D022 and Pt2Mo structures. The N2M2-type image contrast appears when both the fundamental fcc lattice reflections and the 1 1/2 0 diffuse scattering of SRO contribute enough to imaging. This suggests that a good coincidence in intensity distribution between the Fourier power spectra of HRTEM images and the electron diffraction patterns is one of the conditions for the image contrast of SRO to be interpreted in terms of the projection contrast.

Dynamical Monte Carlo simulation of L11(CuPt)-type ordering during (001) epitaxial growth of III-V semiconductor alloys

Abstract In this study, we have carried out Monte Carlo simulation of L11(CuPt)-type ordering in II–V semiconductor alloys of A0.5B0.5C during the (001) epitaxial growth, on the basis of an Ising-like model for the layer-by-layer stacking including surface migration of atoms. Fourier power spectra of the atom configurations thus obtained exhibit diffuse streaks running along the growth direction through peaks at 1 2 1 2 1 2 positions indicative of the L11-type ordering. An additional tendency for first nearest neighboring atoms toward clustering gives rise to wavy modulation of the streaks such as observed in electron diffraction patterns of epitaxial layers of Ga0.5In0.5P or GaAs0.5Sb0.5. The result suggests that the epitaxial growth develops L11 ordered domains with numerous stacking faults and small like-atom clusters simultaneously within the epilayers.

Dynamical diffraction effects on higher-order Laue zone lines in CBED patterns of semiconductors

Abstract We studied the dynamical diffraction effects on the defect lines of higher-order Laue zone reflections (HOLZ lines) in bright field disks of convergent-beam electron diffraction (CBED). The positions of the HOLZ lines were confirmed experimentally to deviate by a small amount from the positions expected from the kinematical approximation. The amount of line shift is represented by the corresponding deviation of the accelerating voltage Δ E . In the case of [111] incidence, one HOLZ reflection can generate two lines with negative and positive values of Δ E . Calculations based on the dynamical diffraction theory clarified that the HOLZ lines with negative and positive Δ E arise from the first and second branches of the zeroth-layer dispersion surface, respectively. Either, or occasionally both, of the lines is observed in the HOLZ pattern, depending upon which branch is dominantly excited. The dependency of Δ E on the material species and thickness of specimen and on the zone axis is discussed in detail for Si, Ge and some III–V compound semiconductors.

Phase equilibria of L10-type order in Cu-Au-Pd and Cu-Au-Ni ternary systems

Abstract The IKL-ALCHEMI (intersecting Kikuchi-line-atom-location by electron channeling-enhanced X-ray microanalysis) method of electron diffraction was employed to study the atomic configuration in L10-type ordered phases in Cu-Au-Pd and Cu-Au-Ni ternary alloys annealed at 573 K. In the L10 phase of Cu-Au-Pd, Au and Pd atoms tend to share the same sublattice, while Cu atoms prefer the other sublattice. If the sum of Au and Pd contents exceeds 50 at.%, most Pd atoms still remain on the favourite sites, while excess Au atoms are driven to the opposite sublattice. In the ordered Cu-Au-Ni alloys, Ni atoms weakly prefer the Cu sublattice rather than the other for Au, but the distribution of Ni atoms depends on the alloy composition. Most Ni atoms reside on the Cu sublattice when the Au content is 50 at.%, while about two thirds of Ni atoms are found on the Au sublattice in the alloy with 50 at.% Cu. The cluster variation method (CVM) in the tetrahedral approximation explains well not only the experimental re...

Monte Carlo simulation of L11-type ordering due to surface step migration in the epitaxial growth of III-V semiconductor alloys

We investigated L11-type ordering due to surface step migration in III-V semiconductor alloys of (A0.5B0.5)IIICV, using an Ising-like crystal growth model. A surface step along the [110] direction (the [110] step), which has a preference for unlike-atom pairs along the step, bears (111) and (111) variants (CuPtB-type) of the four possible L11 variants. On the other hand, a surface step along the [110] direction (the [110] step), which has a preference for like-atom pairs, produces either of the two CuPtB-type variants depending on the direction of the step motion. Antiphase boundaries are inserted parallel to (001) planes in the former case, and (111) or (111) in the latter case. The ordering rate in the case of the [110] step is larger than that for [110] step migration. These results explain well the previous experimental results obtained by transmission electron microscopy.

High-voltage electron diffraction study of structure factors of Pd and Pt

Abstract In order to determine the atomic scattering factors of Pd and Pt for g = 200 and 220 reflections as well as their temperature factors, we measured critical voltages under the systematic excitation and the splitting of Kikuchi lines of high-order reflections. A simulation of the many-beam dynamical diffraction showed that the absorption of electrons in Pd and Pt is so significant as to narrow the splitting of kikuchi lines by 1.1% and 5.5%, respectively. The analysis of the experimental results, taking into account the absorption effect, determined the atomic scattering factors for both elements and the B -factor of Pd with quite good accuracies. In contrast, the B -factor of Pt involves relatively large uncertainty, owing to diffuse Kikuchi lines.