Analysis of grain boundary dislocations by large angle convergent beam electron diffraction

Large angle convergent beam electron diffraction (LACBED) is used to analyse secondary dislocations in sigma3 and sigma9 grain boundaries in silicon. By selecting reflections from crystal planes common to the adjoining grains, LACBED images are insensitive to the boundaries except where dislocations are present. The dislocation images are closely similar to those for dislocations in single crystals and can be analysed by standard Cherns-Preston rules. It is shown that, for both boundaries, sufficient common reflections can be selected for a complete analysis, and that dislocations can be analysed assuming integer values of g x b, implying that the Burgers vectors are Displacement Shift Complete (DSC) lattice vectors. For both sigma3 and sigma9 boundaries, DSC dislocations are identified which are specific to these boundaries. The experimental conditions for the analysis of grain boundaries are explained, and the extension of the method to other coincidence boundaries is discussed.     

Convergent-beam electron diffraction study of transverse stacking faults and dislocations

Transverse stacking faults and dislocations have been studied by convergent-beam electron diffraction (CBED). Stacking faults and dislocations induce splitting in some reflections in the CBED patterns. The splitting and unsplitting of the reflections correspond to the visibility and invisibility of the defect in the kinematic theory of diffraction contrast of imperfect crystals of Hirsch, Howie and Whelan. This method provides a powerful means for the study of crystal defects.     

Investigating the optical properties of dislocations by scanning transmission electron microscopy

The scanning transmission electron microscope (STEM) allows collection of a number of simultaneous signals, such as cathodoluminescence (CL), transmitted electron intensity and spectroscopic information from individual localized defects. This review traces the development of CL and atomic resolution imaging from their early inception through to the possibilities that exist today for achieving a true atomic-scale understanding of the optical properties of individual dislocations cores. This review is dedicated to Professor David Holt, a pioneer in this field.     

Towards automated diffraction tomography: part I--data acquisition

The ultimate aim of electron diffraction data collection for structure analysis is to sample the reciprocal space as accurately as possible to obtain a high-quality data set for crystal structure determination. Besides a more precise lattice parameter determination, fine sampling is expected to deliver superior data on reflection intensities, which is crucial for subsequent structure analysis. Traditionally, three-dimensional (3D) diffraction data are collected by manually tilting a crystal around a selected crystallographic axis and recording a set of diffraction patterns (a tilt series) at various crystallographic zones. In a second step, diffraction data from these zones are combined into a 3D data set and analyzed to yield the desired structure information. Data collection can also be performed automatically, with the recent advances in tomography acquisition providing a suitable basis. An experimental software module has been developed for the Tecnai microscope for such an automated diffraction pattern collection while tilting around the goniometer axis. The module combines STEM imaging with diffraction pattern acquisition in nanodiffraction mode. It allows automated recording of diffraction tilt series from nanoparticles with a size down to 5nm.     

Multiple-Shockley unfaulting of single-layer faulted defects

The general principles laid down by Swart & Kritzinger (1974a) in a paper dealing with the unfaulting of Frank dislocation loops by the simultaneous action of two shearing Shockley dislocations are applied to explain the structure and electron diffraction contrast behaviour of certain complex single-layer dislocation configurations observed in quenched aluminium and dilute Al-Mg alloys. In these cases the simultaneous operation of three Shockley dislocations is employed. It is also demonstrated that adequate stresses, such as those encountered during annealing at temperatures ～150°C, serve to transform butterfly hexagons to normal hexagonal prismatic dislocation loops, confirming the expectation that the butterfly defects are in a metastable state.     

Towards automated diffraction tomography. Part II--Cell parameter determination

Automated diffraction tomography (ADT) allows the collection of three-dimensional (3d) diffraction data sets from crystals down to a size of only few nanometres. Imaging is done in STEM mode, and diffraction data are collected with quasi-parallel beam nanoelectron diffraction (NED). Here, we present a set of developed processing steps necessary for automatic unit-cell parameter determination from the collected 3d diffraction data. Cell parameter determination is done via extraction of peak positions from a recorded data set (called the data reduction path) followed by subsequent cluster analysis of difference vectors. The procedure of lattice parameter determination is presented in detail for a beam-sensitive organic material. Independently, we demonstrate a potential (called the full integration path) based on 3d reconstruction of the reciprocal space visualising special structural features of materials such as partial disorder. Furthermore, we describe new features implemented into the acquisition part.     

Spatially resolved diffractometry with atomic-column resolution

We demonstrate spatially resolved diffractometry in which diffraction patterns are acquired at two-dimensional positions on a specimen using scanning transmission electron microscopy (STEM), resulting in four-dimensional data acquisition. A high spatial resolution of about 0.1 nm is achieved using a stabilized STEM instrument, a spherical aberration corrector and various post-acquisition data processings. We have found a few novel results in the radial and the azimuthal scattering angle dependences of atomic-column contrast in STEM images. Atomic columns are clearly observed in dark field images obtained using the excess Kikuchi band intensity even in small solid-angle detection. We also find that atomic-column contrasts in dark field images are shifted in the order of a few tens of picometers on changing the azimuthal scattering angle. This experimental result is approximately interpretable on the basis of the impact parameter in Rutherford scattering. Spatially resolved diffractometry provides fundamental knowledge related to various STEM techniques, such as annular dark field (ADF) and annular bright field (ABF) imaging, and it is expected to become an analytical platform for advanced STEM imaging.     

Extended defect related energy loss in CVD diamond revealed by spectrum imaging in a dedicated STEM

This article aims at investigations of the low EEL region in the wide band gap system diamond. The advent of the UHV Enfina electron energy loss spectrometer combined with Digital Micrograph acquisition and processing software has made reliable detection of absorption losses below 10 eV possible. Incorporated into a dedicated STEM this instrumentation allows the acquisition of spectral information via spectrum maps (spectrum imaging) of sample areas hundreds of nanometers across, with nanometers pixel sizes, adequate spectrum statistics and 0.3 eV energy resolution, in direct correlation with microstructural features in the mapping area. We aim at discerning defect related losses at band gap energies, and discuss different routes to simultaneously process and analyse the spectra in a map. This involves extracting the zero loss peak from each spectrum and constructing ratio maps from the intensities in two energy windows, one defect related and one at a higher, crystal bandstructure dominated energy. This was applied to the residual spectrum maps and their first derivatives. Secondly, guided by theoretical EEL spectra calculations, the low loss spectra were fitted by a series of gaussian distributions. Pixel maps were constructed from amplitude ratios of gaussians, situated in the defect and the unaffected energy regime. The results demonstrate the existence of sp2-bonded carbon in the vicinity of stacking faults and partial dislocations in CVD diamond as well as additional states below conduction band, tailing deep into the band gap, at a node in a perfect dislocation. Calculated EEL spectra of shuffle dislocations give similar absorption features at 5-8 eV, and it is thought that this common feature is due to sp2-type bonding.     

STEM nanodiffraction technique for structural analysis of CoPt nanoparticles

Studying the structure of nanoparticles as a function of their size requires a correlation between the image and the diffraction pattern of single nanoparticles. Nanobeam diffraction technique is generally used but requires long and tedious TEM investigations, particularly when nanoparticles are randomly oriented on an amorphous substrate. We bring a new development to this structural study by controlling the nanoprobe of the Bright and Dark Field STEM (BF/DF STEM) modes of the TEM. The particularity of our experiment is to make the STEM nanoprobe parallel (probe size 1 nm and convergence angle <1 mrad) using a fine tuning of the focal lengths of the microscope illumination lenses. The accurate control of the beam position offered by this technique allowed us to obtain diffraction patterns of many single nanoparticles selected in the digital STEM image. By means of this technique, we demonstrate size effects on the order-disorder transition temperature in CoPt nanoparticles when their size is smaller than 3 nm.     

Scanning electron microscopy imaging of dislocations in bulk materials, using electron channeling contrast

The imaging and characterization of dislocations is commonly carried out by thin foil transmission electron microscopy (TEM) using diffraction contrast imaging. However, the thin foil approach is limited by difficult sample preparation, thin foil artifacts, relatively small viewable areas, and constraints on carrying out in situ studies. Electron channeling imaging of electron channeling contrast imaging (ECCI) offers an alternative approach for imaging crystalline defects, including dislocations. Because ECCI is carried out with field emission gun scanning electron microscope (FEG-SEM) using bulk specimens, many of the limitations of TEM thin foil analysis are overcome. This paper outlines the development of electron channeling patterns and channeling imaging to the current state of the art. The experimental parameters and set up necessary to carry out routine channeling imaging are reviewed. A number of examples that illustrate some of the advantages of ECCI over thin foil TEM are presented along with a discussion of some of the limitations on carrying out channeling contrast analysis of defect structures.     

Subject index to volume 10

The scanning transmission electron microscope with a field emission electron source operated at 100 kV allows X-ray microanalysis using electron probes as small as 1 to 2 nm. Measurements of the probe in a Vacuum Generators HB-501 STEM show that spherical aberration in the objective lens controls the probe size and shape at beam convergence half-angles of 10 mrad and greater typically used for X-ray microanalysis. A virtual objective aperture eliminates X-ray contributions from the probe-forming system, but must be aligned exactly to avoid asymmetrical broadening of the probe by spherical aberration. It is estimated that 5 nm X-ray spatial resolution can be achieved in low to medium atomic number materials. Even at this resolution however, probe broadening in the specimen controls the resolution; the main limitation is one of specimen preparation and a knowledge of the final specimen thickness. Determination of composition profiles near voids, dislocations and other individual defects in thin foils also requires a knowledge of the defect depth position and deconvolution of the probe and composition profiles.     

Observation of dislocations and microplasma sites in semiconductors by direct correlations of STEBIC,STEM and ELS

A high voltage electron microscope, equipped with scanning transmission (STEM) attachment, electron beam induced conductivity (EBIC) facilities, and electron energy loss spectrometer (ELS), has been used to investigate semiconductor devices. The capability of STEM to produce, simultaneously or sequentially, conductive and transmission images of the same specimen region, which can also be ELS analysed, is exploited in order to establish direct and unambiguous correlations between EBIC and STEM images of defective regions (dislocations and microplasma sites) in silicon devices. The results obtained are discussed in terms of correlations, resolution, contrast, and radiation damage; in addition, a comparison is made between this method and the other correlation methods based on EBIC/SEM (scanning electron microscope) and TEM (transmission electron microscope).     

Weak beam under convergent beam illumination

The weak beam technique is now used widely for the determination of stacking fault energies, in particular for intermetallic alloys, and the accuracy of the approach is critically dependent upon the reliability of the relationship between the image and the actual position of the dissociated dislocations. Examining as a model case a dislocation dissociated into two Shockley partial dislocations in Cu at 100 kV for orientations ranging through the g(3g) weak beam condition, image simulations are used to explore the accuracy to which the true spacing between the partial dislocations can be determined from the spacing measured on the image as a function of the dislocation character, the foil thickness, the dislocation depth in the foil, the diffraction condition and the beam convergence. It appears that for image simulations and for the given conditions a beam convergence of about 5 mrad allows to greatly improve the accuracy, and that beam convergence must be taken into account quantitatively when deducing the true partial dislocation spacing as it is the principal parameter controlling the precision in this type of measurement.     

A high-speed area detector for novel imaging techniques in a scanning transmission electron microscope

A scanning transmission electron microscope (STEM) produces a convergent beam electron diffraction pattern at each position of a raster scan with a focused electron beam, but recording this information poses major challenges for gathering and storing such large data sets in a timely manner and with sufficient dynamic range. To investigate the crystalline structure of materials, a 16×16 analog pixel array detector (PAD) is used to replace the traditional detectors and retain the diffraction information at every STEM raster position. The PAD, unlike a charge-coupled device (CCD) or photomultiplier tube (PMT), directly images 120–200 keV electrons with relatively little radiation damage, exhibits no afterglow and limits crosstalk between adjacent pixels. Traditional STEM imaging modes can still be performed by the PAD with a 1.1 kHz frame rate, which allows post-acquisition control over imaging conditions and enables novel imaging techniques based on the retained crystalline information. Techniques for rapid, semi-automatic crystal grain segmentation with sub-nanometer resolution are described using cross-correlation, sub-region integration, and other post-processing methods.     

Identifying threading dislocations in GaN films and substrates by electron channelling

Electron channelling contrast imaging of threading dislocations in GaN (0002) substrates and epitaxial films has been demonstrated using a conventional polepiece-mounted backscatter detector in a commercial scanning electron microscope. The influence of accelerating voltage and diffraction vector on contrast features denoting specific threading dislocation types has been studied. As confirmed by coordinated transmission electron microscopy analysis, electron channelling contrast imaging contrast features for edge-type threading dislocations are spatially smaller than mixed-type threading dislocations in GaN. This ability to delineate GaN edge threading dislocations from mixed type was also confirmed by defect-selective etch processing using molten MgO/KOH. This study validates electron channelling contrast imaging as a nondestructive and widely accessible method for spatially mapping and identifying dislocations in GaN with wider applicability for other single-crystal materials.     

Diffraction effects and inelastic electron transport in angle‐resolved microscopic imaging applications

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle‐resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in‐depth theoretical modelling of the energy‐ and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.     

CBED and LACBED characterization of crystal defects

Convergent‐beam electron diffraction (CBED) obtained with a focused incident beam is well known for the identification of the point and space groups but it can also be used for the analysis of stacking faults and antiphase boundaries. Large‐angle convergent‐beam electron diffraction (LACBED) is performed with a large defocused incident beam and is well adapted to the characterization of most types of crystal defects: point defects, perfect and partial dislocations, stacking faults, antiphase boundaries and grain boundaries. Among the advantages of these methods with respect to the conventional transmission electron microscopy methods, are that one or few patterns are required for a full analysis and the interpretations are easy and unambiguous. The LACBED technique is particularly useful for the analysis of dislocations present in anisotropic and beam‐sensitive materials.     

Subtleties in ADF imaging and spatially resolved EELS: A case study of low-angle twist boundaries in SrTiO3

A screw dislocation network at the low-angle SrTiO3/Nb:SrTiO3 twist grain boundary has been analyzed by annular dark field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). The cores of one set of dislocations running parallel to the beam direction appear dark in the ADF STEM images. EELS on the dislocation core reveals a reduced Sr/Ti ratio compared to the bulk suggesting Sr-deficient cores. The second set of dislocations, orthogonal to the latter, is imaged by its strain field using low-angle annular dark field (LAADF) imaging. Multislice image simulations suggest channeling of the electron probe on the atomic columns for small tilts, theta < 1 degree, where the Sr columns act as beam guides. Only for larger tilts is the channeling effect strongly reduced and the fringe contrast approaches the value predicted by a purely incoherent imaging model. Ti-L(2,3) EELS across the dislocation core shows an asymmetry between the EELS and the ADF signal which cannot be explained by the geometry or beam broadening. This asymmetry might be explained by an effective nonlocal potential representing inelastic scattering in EELS.     

Nanometric crystal defects in transmission electron microscopy

Transmission electron microscopy (TEM) is revisited in order to define methods for the identification of nanometric defects. Nanometric crystal defects play an important role as they influence, generally in a detrimental way, physical properties. For instance, radiation-induced damage in metals strongly degrades mechanical properties, rendering the material stronger but brittle. The difficulty in using TEM to identify the nature and size of such defects resides in their small size. TEM image simulations are deployed to explore limits and possible ways to improve on spatial resolution and contrast. The contrast of dislocation loops, cavities, and a stacking fault tetrahedra (SFT) are simulated in weak beam, interfering reflections (HRTEM), and scanned condensed electron probe (STEM) mode. Results indicate that STEM is a possible way to image small defects. In addition, a new objective aperture is proposed to improve resolution in diffraction contrast. It is investigated by simulations of the weak beam imaging of SFT and successfully applied in experimental observations.     

Combined HREM and STEM microanalysis on decorated dislocation cores

Impurity precipitation at dislocation cores in silicon and germanium have been identified by lattice imaging and electron energy loss spectroscopy on a STEM. The results lead to the conclusion that oxygen segregates at dislocations with large Burgers vectors forming a crystalline phase in the form of a continuous cylinder all along the dislocation line. The ceosite phase is tentatively proposed as being this crystalline phase. Severe radiation-induced damage has been observed under the high flux employed in a STEM. This results in complete disappearance of the precipitates by desorption of oxygen at the surfaces of the thin foil after an integrated dose of 10¹⁰ C/m².