Polarity determination of III-V compound semiconductors by large-angle convergent beam electron diffraction

The large-angle convergent beam electron diffraction (LACBED) technique is used for determining the crystal polarity of GaP and GaAs single crystals from < 1 1 0 > cross-sectional samples. The method which is based on an earlier approach using convergent beam electron diffraction (CBED) evaluates the polarity-sensitive contrast of high odd-index Bragg-lines in [0 0 2] dark-field patterns. The polarity is determined by application of a simple contrast rule as well as by direct comparison with dynamical simulations. For the two materials the ranges of applicability are determined by a detailed analysis of the Bragg-line contrast as a function of the sample thickness. The coexistence of the Bragg-line pattern and the of shadow image of the defect in correct rotational relationship to each other makes the analysis straightforward and free from possible sources of errors. As an example, the crystal polarity of GaP is related to the morphology of facetted voids. The LACBED method is shown to be suitable for relating the analysis of extended crystal defects. The advantages and the disadvantages of the LACBED method are discussed in comparison with the corresponding CBED method and with a recent method based on the analysis of bend contours.     

Diffracted phase and amplitude measurements by energy-filtered convergent-beam holography (CHEF)

Interference between transmitted and diffracted disks in convergent-beam electron diffraction (CBED) patterns using the CBED+EBI method proposed by Herring et al. is explored using different optical configurations on a spherical aberration corrected transmission electron microscope equipped with a biprism and imaging energy filter: the SACTEM-Toulouse. We will relate the amplitude and phase of these interference patterns, which we call convergent-beam holography (CHEF), to microscope transfer theory and the complex amplitudes of the diffracted beams. Experimental CHEF patterns recorded in the absence of aberration correction will be compared with simulations to validate the theory concerning the effect of microscope aberrations and current instabilities. Then, using aberration correction, we propose a scheme for eliminating the effect of the microscope, so that the diffracted amplitudes and phase due to dynamical scattering within the specimen can be studied. Experimental results are compared with simulations performed using the full dynamical theory. The potential for studying diffracted amplitudes and phases using CHEF analysis is discussed.     

A modified goodman's technique for obtaining rocking-beam type patterns

A new method was developed for convergent beam electron diffraction (CBED) and large angle convergent beam electron diffraction (LACBED) in the JEM-100CXII. This method is obtained in the imaging mode using the defocus objective lens and by re-setting condenser-2. A multi-dark field CBED pattern was achieved in two ways.     

Investigation of the local Ge concentration in Si/SiGe nanostructures by convergent-beam electron diffraction

SiGe multi quantum well structures were investigated by convergent-beam electron diffraction (CBED) measurements. Detailed layer characterizations were performed by acquiring series of bright field CBED patterns in the form of a line scan across the nanostructures in scanning transmission electron microscopy (STEM) mode. From the higher order Laue zone (HOLZ) lines the local lattice parameters were deduced. The Ge concentration corresponding to these lattice parameters was determined by means of the elasticity theory. In this work it is shown that the lattice constants can be determined locally with an accuracy of about ±0.001 to ±0.003 Å which leads to an accuracy of the corresponding Ge concentration of about 1–2%. The characteristics of the focused electron probe and its influence on the experimental data were used for an estimation of the spatial resolution of the CBED method. For comparison, experimental values regarding the spatial resolution were determined by investigating the abrupt interface between Si(1 1 1) and AlN(0 0 0 1).     

On the peculiarities of CBED pattern formation revealed by multislice simulation

A modified multislice method has been developed for calculations of Convergent Beam Electron Diffraction (CBED) patterns. The validity of the method for HOLZ- and Kikuchi-line calculations has been proofed by comparison to Bloch-wave calculations. The application of the method leads to the new understanding of CBED patterns formation. Dynamical scattering of weak HOLZ reflections plays the key role in the appearance of deficient lines in the central CBED disk. Different HOLZ lines do have significantly different and extended scattering areas; the central 000 CBED disk, consequently, contains structural information from an area around the primary beam which is determined by the Bragg angle of HOLZ reflections and the thickness of the sample. A variation of lattice parameters, if present within this area, results in artificial symmetry violations of the pattern and in changes of line profiles.     

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.     

A comparison of techniques for obtaining convergent beam electron diffraction patterns with a JEOL 200CX

Three different techniques for obtaining convergent beam electron diffraction (CBED) patterns using a JEOL 200CX transmission electron microscope will be described and compared. The first technique, described by Goodman, will be shown to yield clear, undistorted patterns, but only with relatively large camera length, and a limited field of view. A second technique, which is a modification of Goodman's technique, will be shown to yield CBED patterns of both large camera length and small camera length with a much larger angular coverage, but the magnitude of the beam convergence is limited by distortion in the pattern. A third technique will then be presented which permits the formation of small camera length, relatively undistorted CBED patterns with large angular coverage and greatly increased beam convergence; high quality large camera length CBED patterns can also be obtained by simply increasing the strengths of the diffraction lenses.     

Optimizing use of the structural chemical analyser (variable pressure FESEM‐EDX raman spectroscopy) on micro‐size complex historical paintings characterization

The novel Structural Chemical Analyser (hyphenated Raman spectroscopy and scanning electron microscopy equipped with an X‐ray detector) is gaining popularity since it allows 3‐D morphological studies and elemental, molecular, structural and electronic analyses of a single complex micro‐sized sample without transfer between instruments. However, its full potential remains unexploited in painting heritage where simultaneous identification of inorganic and organic materials in paintings is critically yet unresolved. Despite benefits and drawbacks shown in literature, new challenges have to be faced analysing multifaceted paint specimens. SEM?Structural Chemical Analyser systems differ since they are fabricated ad hoc by request. As configuration influences the procedure to optimize analyses, likewise analytical protocols have to be designed ad hoc. This paper deals with the optimization of the analytical procedure of a Variable Pressure Field Emission scanning electron microscopy equipped with an X‐ray detector Raman spectroscopy system to analyse historical paint samples. We address essential parameters, technical challenges and limitations raised from analysing paint stratigraphies, archaeological samples and loose pigments. We show that accurate data interpretation requires comprehensive knowledge of factors affecting Raman spectra. We tackled: (i) the in‐FESEM?Raman spectroscopy analytical sequence, (ii) correlations between FESEM and Structural Chemical Analyser/laser analytical position, (iii) Raman signal intensity under different VP‐FESEM vacuum modes, (iv) carbon deposition on samples under FESEM low‐vacuum mode, (v) crystal nature and morphology, (vi) depth of focus and (vii) surface‐enhanced Raman scattering effect. We recommend careful planning of analysis strategies prior to research which, although time consuming, guarantees reliable results. The ultimate goal of this paper is to help to guide future users of a FESEM‐Structural Chemical Analyser system in order to increase applications.     

Effect of electron beam parameters on simulated CBED patterns from edge-on grain boundaries

Convergent beam electron diffraction (CBED) at vertical grain boundaries (parallel to the electron beam) can be applied to determine the symmetry of bicrystals. It can also be used to investigate the structure of the boundary region itself when subnanometre probe sizes are employed. In this paper it is shown that (sub)nanometre-probe CBED patterns are largely influenced by the electron-beam geometry. In particular, simulations of coherent CBED patterns based on the multislice algorithm show that the CBED pattern of an edge-on interface depends on the defocus distance between the probe position and the specimen midplane, the probe size and the beam-convergence angle. The pattern symmetry may be lower than the theoretically predicted symmetry in case of large spherical aberration. This effect increases with smaller accelerating voltages. An increase in the beam-convergence angle also increases the possibility of a non-optimum symmetry due to spherical aberration of a coherent probe. Thus, for the determination of an interface structure using subnanometre (coherent) probes, the imaging conditions play an important role.     

Convergent beam electron diffraction extinction distance measurements for quantitative analysis of si(1-x),Ge(x)

A new method to determine the concentration of germanium in Si(1-x) Ge(x) single crystals is presented. It is based on extinction distance measurements by means of convergent beam electron diffraction (CBED). The two-beam condition CBED intensity oscillation (the so-called rocking curve) is measured for the 004 diffracted beam and compared with a numerical simulation. Using the two-beam dynamical diffraction approximation theory, this approach yields very precise values for both specimen thickness and effective extinction distance (Ultramicroscopy 87 (2001) 5). First a theoretical extinction distance zetag(x) for strain relaxed Si(1-x)Ge(x) is calculated assuming a solid solution and using tabulated atomic scattering factors of silicon and germanium atoms. It is found that for single crystals zetag(x) decreases from 156 nm in pure silicon to 90 nm in pure germanium. Measurements on calibrated strain relaxed SiGe layers with variable germanium concentrations show an excellent agreement between experimental and calculated extinction distances zetag(x). As a consequence the experimental extinction distance zetag(x) becomes an indirect measure of the germanium concentration with a 1-2 at % sensitivity. The method turns out to be insensitive to strain as experimental zetag(x) values obtained on strained SiGe layers fit the theoretical extinction distance curve calculated for strain relaxed SiGe.     

Morphology study of progesterone polymorphs prepared by polymer‐induced heteronucleation (PIHn)

In this article, morphology of progesterone polymorphs prepared by polymer‐induced heteronucleation (PIHn) technique was studied. Hydroxypropyl methylcellulose(HPMC), such as dextran T‐500 and gelatin G‐9382, polyisoprene (PI), and acrylonitrile/butadiene copolymer (NBR) were used as substrates. The crystallizations were performed by solvent evaporation at room temperature from 0.5, 10, and 40 mg/ml solutions in chloroform and acetone. Progesterone polymorphs were identified by X‐ray diffraction. Differential scanning calorimetry and total attenuated reflectance infrared spectroscopy were used as complementary techniques in the identification. Depending on the polymeric matrix and the concentration used, form 1, form 2, or mixture of both polymorphs were obtained. Scanning electron microscopy pictures evidenced difference in morphology and in homogeneity of the two progesterone polymorphs. These polymorphs prepared by PIHn, did not present a distinctive morphology that allows identifying polymorph by its crystal habit. Hence, polymeric matrix induced the crystallization, affecting polymorphism and morphology. SCANNING 35:213‐221, 2013. © 2012 Wiley Periodicals, Inc.     

In situ observation of oxygen deficiency occurring during electron irradiation in high Tc superconductor YBa2Cu3Oy

The decreasing process of oxygen in YBa₂Cu₃O _y is investigated through high resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED). Measurements of the axial length in HREM images show that oxygen content y decreases faster near a twin boundary than at the inner part of a twin lamella. The transformation from an orthorhombic to a tetragonal phase starts at a twin boundary and the transformed region propagates to an inner region of lamella. Lattice strains are observed near boundaries between transformed and non-transformed regions. The transformation is almost complete within 30 s during observation of HREM images at 400 kV and at room temperature. A value of y was quantitatively measured by analysing observed intensities of energy-filtered CBED patterns with the dynamical theory. The value of y decreases from 6.9 to 6.5 when 200 kV electrons are irradiated for 160 s in a microscope at 108 K. More precise analysis of the intensities provides information on charge distribution along the c -axis as well as local oxygen content at a spatial resolution of several nanometres.     

Aberration-compensated large-angle rocking-beam electron diffraction

The application of convergent beam electron diffraction (CBED) to determine symmetry, refine structure factors, and measure specimen thickness requires rather thick specimen and is very difficult or even impossible in the case of large unit cell materials. The large-angle rocking-beam electron diffraction (LARBED) technique introduced in this paper gives access to the kind of experimental data contained in CBED patterns but over a much larger angular range. In addition to symmetry determination and thickness measurement even for thin samples this technique also allows, in principle, very accurate measurements of structure factors. Similar to precession electron diffraction (PED), LARBED uses the illumination tilt coils to sequentially change the angle of incidence of the electron beam over a very large range. I will present results obtained by a recently developed self-calibrating acquisition software which compensates for aberration-induced probe shifts during the acquisition of LARBED patterns and keeps the probe within a few nm, while covering a tilt range from 0 to 100 mrad. This paper is dedicated to Prof. John C. H. Spence on the occasion of his 65th birthday.     

HANSIS software tool for the automated analysis of HOLZ lines

A software tool, named as HANSIS (HOLZ analysis), has been developed for the automated analysis of higher-order Laue zone (HOLZ) lines in convergent beam electron diffraction (CBED) patterns. With this tool, the angles and distances between the HOLZ intersections can be measured and the data can be presented graphically with a user-friendly interface. It is capable of simultaneous analysis of several HOLZ patterns and thus provides a tool for systematic studies of CBED patterns.     

Cryogenic‐temperature electron microscopy direct imaging of carbon nanotubes and graphene solutions in superacids

Cryogenic electron microscopy (cryo‐EM) is a powerful tool for imaging liquid and semiliquid systems. While cryogenic transmission electron microscopy (cryo‐TEM) is a standard technique in many fields, cryogenic scanning electron microscopy (cryo‐SEM) is still not that widely used and is far less developed. The vast majority of systems under investigation by cryo‐EM involve either water or organic components. In this paper, we introduce the use of novel cryo‐TEM and cryo‐SEM specimen preparation and imaging methodologies, suitable for highly acidic and very reactive systems. Both preserve the native nanostructure in the system, while not harming the expensive equipment or the user. We present examples of direct imaging of single‐walled, multiwalled carbon nanotubes and graphene, dissolved in chlorosulfonic acid and oleum. Moreover, we demonstrate the ability of these new cryo‐TEM and cryo‐SEM methodologies to follow phase transitions in carbon nanotube (CNT)/superacid systems, starting from dilute solutions up to the concentrated nematic liquid‐crystalline CNT phases, used as the ‘dope’ for all‐carbon‐fibre spinning. Originally developed for direct imaging of CNTs and graphene dissolution and self‐assembly in superacids, these methodologies can be implemented for a variety of highly acidic systems, paving a way for a new field of nonaqueous cryogenic electron microscopy.     

New approach for the dynamical simulation of CBED patterns in heavily strained specimens

A new method for the dynamical simulation of convergent beam electron diffraction (CBED) patterns is proposed. In this method, the three-dimensional stationary Schrödinger equation is replaced by a two-dimensional time-dependent equation, in which the direction of propagation of the electron beam, variable z, stands as a time. We demonstrate that this approach is particularly well-suited for the calculation of the diffracted intensities in the case of a z-dependent crystal potential. The corresponding software has been developed and implemented for simulating CBED patterns of various specimens, from perfect crystals to heavily strained cross-sectional specimens. Evidence is given for the remarkable agreement between simulated and experimental patterns.     

Spine micromorphology of normal and hyperhydric Mammillaria gracilis Pfeiff. (Cactaceae) shoots

Artificial conditions of tissue culture affect growth and physiology of crassulacean acid metabolism plants which often results in formation of hyperhydric shoots. In in vitro conditions Mammillaria gracilis Pfeiff. (Cactaceae) growth switches from organized to unorganized way, producing a habituated organogenic callus which simultaneously regenerates morphologically normal as well as altered hyperhydric shoots. In this study, influence of tissue culture conditions on morphology of cactus spines of normal and hyperhydric shoots was investigated. Spines of pot‐grown Mammillaria plants and of in vitro regenerated shoots were examined with stereo microscope and scanning electron microscope. The pot‐grown plants had 16–17 spines per areole. In vitro grown normal shoots, even though they kept typical shoot morphology, had lower number of spines (11–12) and altered spine morphology. This difference was even more pronounced in spine number (six to seven) and morphology of the hyperhydric shoots. Scanning electron microscopy analysis revealed remarkable differences in micromorphology of spine surface between pot‐grown and in vitro grown shoots. Spines of in vitro grown normal shoots showed numerous long trichomes, which were more elongated on spines of the hyperhydric shoots; the corresponding structures on spine surface of pot‐grown plants were noticed only as small protrusions. Scanning electron microscopy morphometric studies showed that the spines of pot‐grown plants were significantly longer compared to the spines of shoots grown in tissue culture. Moreover, transverse section shape varies from elliptical in pot‐grown plants to circular in normal and hyperhydric shoots grown in vitro. Cluster and correspondence analyses performed on the scanning electron microscope obtained results suggest great variability among spines of pot‐grown plants. Spines of in vitro grown normal and hyperhydric shoots showed low level of morphological variation among themselves despite the significant difference in shoot morphology.     

Measurement of lattice parameter and strain using convergent beam electron diffraction

A brief review is presented of the methods of measuring lattice parameters and strain using diffraction techniques. The presence of strain leads to broadening of diffraction maxima, which is normally separable from any broadening caused by size. The special advantages of the convergent beam electron diffraction (CBED) techniques in measuring lattice parameters and strain are given from studies of precipitation (including misfit measurements) and from investigations of partially recrystallised microstructures. These examples are used to illustrate the advantages and limitations of the CBED technique.     

CBED and LACBED: characterization of antiphase boundaries

Convergent-beam electron diffraction (CBED) and large-angle convergent-beam electron diffraction (LACBED) techniques are well adapted to the characterization of several types of crystal defects. In fact, dislocations, grain boundaries and stacking faults have already been successfully characterized with these methods. In the present paper, we describe the CBED and LACBED characterization of another type of crystal defect showing a special interest in materials science: antiphase boundaries (APBs). The first part of the paper is devoted to the determination of the effects of antiphase boundaries on CBED and LACBED patterns that could be expected from a theoretical point of view. It indicates that the superlattice excess lines present on these patterns are split into two lines with equal intensity when the incident beam is located on an APB. In the second part, we experimentally test these theoretical predictions on a specimen showing two different known types of antiphase boundaries. In a third part we indicate how these methods could be used to identify unknown APBs in a specimen. Finally, the advantages and disadvantages of both methods for the characterization of antiphase boundaries are discussed.