theoretical-calculation-experimental-verification-for-dislocation

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the ...

The University of Tokyo

the university of tokyo

Quasicrystals represent aperiodically ordered form of solids with symmetries long thought forbidden in nature. Since their discovery, the fundamental key question has been "where are the atoms?" in these novel aperiodic solids, and electron microscopy has indeed provided images of real atomic arrangements in quasicrystals. In this tutorial review, we describe the microscopic view of quasicrystals using state-of-the-art scanning transmission electron microscopy, providing intriguing details that had never been unveiled by the early diffraction-based structural analyses.

Electron microscopy of quasicrystals -where are the atoms?

The structure of a highly ordered Al58Cu26Ir16 decagonal quasicrystal (d-QC), which is constructed by a periodic stack of quasiperiodic atomic planes, was investigated using aberration-corrected scanning transmission electron microscopy. The entire quasiperiodic structure can be well described based on the pentagonal Penrose lattice decorated with a decagonal columnar cluster 2 nm across, and the individual clusters apparently showed up with localized atomic disorder to various degree that blurs the underlying cluster symmetry. Multivariate analysis of the cluster images with principal component analysis has successfully extracted two fundamental clusters, which are with definite mirror and 10-fold-rotation symmetries; this is the first solid example of the d-QC composed of multiple fundamental clusters with different symmetry. Consequently, it is found that all the observed variations can be reasonably interpreted by a linear combination between these two fundamental clusters of mirror and 10-fold symmetries, indicating that the possible atomic disorder is strongly restricted by these particular symmetries. Characteristic distributions of the mirror/10-fold clusters on the pentagonal Penrose lattice are also described. 

Local cluster symmetry of a highly ordered quasicrystalline Al58Cu26Ir16 extracted through multivariate analysis of STEM images.

The atomic structure of a 0.2 atom % Pt-doped complex metallic alloy, monoclinic AlFe, was investigated using a single crystal prepared by the Czochralski method. High-angle annular dark-field scanning transmission electron microscopy showed that the Pt atoms were dispersed as single atoms and substituted at Fe sites in AlFe. Single-crystal X-ray structural analysis revealed that the Pt atoms preferentially substitute at Fe(1). Unlike those that have been reported, Pt single atoms in the surface layers showed lower activity and selectivity than those of AlPt and bulk Pt for propyne hydrogenation, indicating that the active state of a given single-atom Pt site is strongly dominated by the bonding to surrounding Al atoms.

Probing Single Pt Atoms in Complex Intermetallic AlFe.

We describe the optical conditions that are essentially necessary for phase-contrast imaging with aberration-corrected scanning transmission electron microscopy (STEM), whose depth of field has reached almost comparable to the specimen thickness. For such state-of-the-art STEM, contrast-transfer-function (CTF) should be defined not solely for the projected potential but multiply for each wavefront during the beam propagation across the specimen thickness; an integration of multiple CTFs (iCTF). We show that the iCTF concept explains fairly well characteristic annular-bright-field (ABF) imaging behaviors of heavy/light atom sites against the defocus changes, and also provide notable concerns on possible artifacts that arise from different imaging-depth dependences between the heavy/light atom sites.

Eiji Abe

Department of Materials Engineering

The University of Tokyo

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Eiji Abe

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Materials Engineering

The University of Tokyo

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Department of Materials Engineering