Computer experiments for coherent convergent-beam electron diffraction

1983 ◽  
Vol 41 ◽  
pp. 304-305
Author(s):  
William Krakow
Keyword(s):  
Electron Diffraction ◽  
Amorphous Materials ◽  
Three Dimensional ◽  
Computer Experiments ◽  
Convergent Beam Electron Diffraction ◽  
Specimen Thickness ◽  
Beam Electron ◽  
Wide Range ◽  
Stem Type ◽  
Convergent Beam

Considerable effort has been expended to use convergent beam electron diffraction (CBED) from small specimen areas with an incoherent thermionic source. Here space group classification and even three dimensional analysis have proven to be possible by observing the diffraction disks and the fine detail seen within these disks. The use of a coherent convergent beam has been attempted for a field emission STEM type instrument and a number of novel interference effects have recently been observed in both crystalline and amorphous materials. Preliminary CBED computer calculations were performed for a dislocation in Si3 however no structural detail was observed in the diffraction disks because the computation only considered a 30Å thick crystal. Computations covering a wide range of materials, specimen thickness values and STEM type probe conditions has been obtained by the present author. In these papers only results for zone axis patterns and 100kV electrons were given. It is now the intent to present some new results at high voltages (200kV) and for non-symmetric crystal orientations and with larger reciprocal space sampling distributions

Burgers Vector Analysis of Vertical Dislocations in Ge Crystals by Large-Angle Convergent Beam Electron Diffraction

2015 ◽  
Vol 21 (3) ◽  
pp. 637-645 ◽  
Author(s):  
Heiko Groiss ◽  
Martin Glaser ◽  
Anna Marzegalli ◽  
Fabio Isa ◽  
Giovanni Isella ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Large Angle ◽  
Three Dimensional ◽  
Chemical Vapor ◽  
Dark Field ◽  
Convergent Beam Electron Diffraction ◽  
Vector Analysis ◽  
Burgers Vector ◽  
Beam Electron ◽  
Convergent Beam

AbstractBy transmission electron microscopy with extended Burgers vector analyses, we demonstrate the edge and screw character of vertical dislocations (VDs) in novel SiGe heterostructures. The investigated pillar-shaped Ge epilayers on prepatterned Si(001) substrates are an attempt to avoid the high defect densities of lattice mismatched heteroepitaxy. The Ge pillars are almost completely strain-relaxed and essentially defect-free, except for the rather unexpected VDs. We investigated both pillar-shaped and unstructured Ge epilayers grown either by molecular beam epitaxy or by chemical vapor deposition to derive a general picture of the underlying dislocation mechanisms. For the Burgers vector analysis we used a combination of dark field imaging and large-angle convergent beam electron diffraction (LACBED). With LACBED simulations we identify ideally suited zeroth and second order Laue zone Bragg lines for an unambiguous determination of the three-dimensional Burgers vectors. By analyzing dislocation reactions we confirm the origin of the observed types of VDs, which can be efficiently distinguished by LACBED. The screw type VDs are formed by a reaction of perfect 60° dislocations, whereas the edge types are sessile dislocations that can be formed by cross-slips and climbing processes. The understanding of these origins allows us to suggest strategies to avoid VDs.

scholarly journals Extracting Local Crystallographic Structure Using 4D-STEM Datasets

2014 ◽  
Vol 70 (a1) ◽  
pp. C1455-C1455 ◽  
Author(s):  
Colin Ophus ◽  
Peter Ercius ◽  
Michael Sarahan ◽  
Cory Czarnik ◽  
Jim Ciston
Keyword(s):  
Electron Diffraction ◽  
Three Dimensional ◽  
Convergent Beam Electron Diffraction ◽  
Separate Experiment ◽  
Crystallographic Structure ◽  
Beam Electron ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Diffraction Patterns ◽  
Convergent Beam

Traditional scanning transmission electron microscopy (STEM) detectors are monolithic and integrate a subset of the transmitted electron beam signal scattered from each electron probe position. These convergent beam electron diffraction patterns (CBED) are extremely rich in information, containing localized information on sample structure, composition, phonon spectra, three-dimensional defect crystallography and more. Many new imaging modes become possible if the full CBED pattern is recorded at many probe positions with millisecond dwell times. In this study, we have used a Gatan K2-IS direct electron detection camera installed on an uncorrected FEI Titan-class transmission electron microscope to record 4D-STEM probe diffraction patterns on a variety of samples at up to 1600 frames per second. As an example, a 4D-STEM dataset for a multilayer stack of epitaxial SrTiO3 and mixed LaMnO3-SrTiO3 is plotted in Figure 1. Figure 1A shows a HAADF micrograph of the multilayer along a (001) zone axis. Only the A sites (Sr and La) are visible in this micrograph and the composition can be roughly determined from the relative brightness. One possible 4D-STEM technique is position-averaged convergent beam electron diffraction (PACBED) described by LeBeau et al. [1]. We can easily construct ideal PACBED patterns by averaging the probe images over each unit cell fitted from Figure 1A, which is shown in Figure 1B. By matching these patterns to PACBED images simulated with the multislice method we can precisely determine parameters such as sample thickness and composition, the latter of which is plotted in Figure 1C. For comparison, the composition has also been determined with electron energy loss spectroscopy (EELS) in a separate experiment, shown in Figure 1D. The composition range of 0-85% LaMnO3 measured by PACBED is in good agreement with the EELS measurements. In this talk we will demonstrate several other possible uses for 4D-STEM datasets.

Introduction: A Special Issue on Electron Diffraction

2003 ◽  
Vol 9 (5) ◽  
pp. 377-378 ◽  
Author(s):  
John C.H. Spence
Keyword(s):  
Electron Diffraction ◽  
Strain Measurement ◽  
Direct Methods ◽  
Convergent Beam Electron Diffraction ◽  
Special Issue ◽  
Beam Electron ◽  
Wide Range ◽  
Hours Of Work ◽  
Precession Electron Diffraction ◽  
Convergent Beam

This special issue of Microscopy and Microanalysis explores quantitative electron diffraction from nonbiological materials. Jim Turner and I have put many hours of work into bringing it together, and we thank the authors for their fine contributions. The articles cover a wide range of materials and techniques, from convergent-beam electron diffraction (CBED) to the new Kohler SAD mode, as well as the use of direct methods, the study of diffuse elastic scattering from defects, strain measurement, and multiwavelength methods. We were sorry that we could not obtain recent work using the precession electron diffraction camera by our deadline, but readers should be aware of that promising method also.

On-line measurement of specimen thickness by Convergent Beam Electron Diffraction

1990 ◽  
Vol 48 (2) ◽  
pp. 480-481
Author(s):  
Max T. Otten
Keyword(s):  
Electron Diffraction ◽  
Thickness Measurement ◽  
Dark Field ◽  
Convergent Beam Electron Diffraction ◽  
Specimen Thickness ◽  
Bright Field ◽  
Beam Electron ◽  
Kikuchi Line ◽  
On Line ◽  
Convergent Beam

Convergent Beam Electron Diffraction (CBED) thickness measurement is the easiest and most accurate way of determining the thickness of crystalline materials. The method was described by Kelly et al. The specimen thickness can be calculated from a few measurements on a recorded diffraction pattern in a matter of minutes (by hand) or seconds (by a computer program).For thickness measurement a CBED pattern is needed that contains a two-beam diffracting condition, with a dark Kikuchi line going through the centre of the Bright-Field disc and the corresponding bright Kikuchi line through the centre of a Dark-Field disc. Parallel to the bright Kikuchi line, the Dark-Field disc contains a number of fringes (Fig. 1) whose distance from the Kikuchi line varies with specimen thickness. The data needed for a measurement are the electron wavelength, the d-spacing dhkl of the diffraction used, the distance 2θB between the Bright-Field disc and Dark-Field disc in the CBED pattern, and the distances Δθi between the dark thickness fringes and the bright Kikuchi line in the Dark-Field disc (Fig. 2).

Convergent beam electron diffraction

1987 ◽  
Vol 51 (359) ◽  
pp. 33-48 ◽  
Author(s):  
P. E. Champness
Keyword(s):  
Electron Beam ◽  
Electron Diffraction ◽  
Point Group ◽  
Convergent Beam Electron Diffraction ◽  
Objective Lens ◽  
Specimen Thickness ◽  
High Symmetry ◽  
Beam Electron ◽  
Cell Parameters ◽  
Convergent Beam

AbstractIn convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (⩽50 nm) area of the sample. Instead of the diffraction spots that are obtained in the back focal plane of the objective lens with parallel illumination in conventional selected-area electron diffraction, CBED produces discs of intensity. The point group can be determined uniquely from the symmetry within the individual discs and the overall pattern. In order to determine the point group, it is usually necessary to record a number of CBED patterns with the electron beam aligned along different zone axes, but sometimes only one, high-symmetry pattern is required. The positions of reflections in higher-order Laue zones can be used to identify the crystal system and lattice type and to detect the presence of certain glide planes. The repeat along the zone axis that is parallel to the beam can be calculated from the diameters of the Laue zones. Hence the presence ofpolymorphs can be detected. Doubly-diffracted discs in CBED often contain a ‘line of dynamic absence’, the orientation of this line with respect to the symmetry seen in the bright field disc allows the symmetry element responsible for it (glide plane or screw diad) to be identified. This allows 191 of the 230 space groups to be uniquely identified. The measurement of specimen thickness, extinction distance and cell parameters are also briefly discussed.

Symmetries and Extinction Rules in CBED

1982 ◽  
Vol 40 ◽  
pp. 684-685
Author(s):  
K. Ishizuka
Keyword(s):  
Electron Diffraction ◽  
Incident Beam ◽  
Convergent Beam Electron Diffraction ◽  
Beam Direction ◽  
Beam Electron ◽  
Convergent Beam

The technique of convergent-beam electron diffraction (CBED) has been established. However there is a distinct discrepancy concerning the CBED pattern symmetries associated with translation symmetries parallel to the incident beam direction: Buxton et al. assumed no detectable effects of translation components, while Goodman predicted no associated symmetries. In this report a procedure used by Gjønnes & Moodie1 to obtain dynamical extinction rules will be extended in order to derive the CBED pattern symmetries as well as the dynamical extinction rules.

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