Kikuchi Electron Diffraction Research Articles

Enhancement of several Kikuchi lines in the vicinity of spot reflections

Kikuchi electron diffraction patterns of silicon have been obtained which demonstrate the simultaneous enhancement of several excess Kikuchi lines near different point reflections. This enhancement is explained within the elementary mechanism of the formation of Kikuchi patterns while taking into account the Kikuchi electron double diffraction.

Mechanism of formation of curved Kikuchi lines

The mechanism of formation of curved Kikuchi lines, observed at displacement of point reflections from their normal positions, is proposed. Curving of Kikuchi lines is explained taking into account the participation of diffracted electron beams in the formation of Kikuchi electron diffraction patterns.

HOLOGRAPHIC ANALYSIS OF KIKUCHI ELECTRON DIFFRACTION DATA FROM ${\rm Si}(111)(\sqrt{3}\times\sqrt{3}){\rm R}30^\circ{\rm -Al}$

Holographic analysis was applied to Kikuchi electron diffraction data collected from the [Formula: see text] structure to produce real-space images of the surface structure. Methods for calibrating the initial diffraction data are fully described. The images clearly show features caused by the Al atoms in the T4 absorption site. Measurements of the Al–Si bond length and the height of the Al above the top layer of Si agree with the values known from dynamical LEED studies.

Holographic images of Pt(111) using Kikuchi electron diffraction

Three-dimensional atomic images of a Pt(111) surface are obtained by direct inversion of multiple low-energy Kikuchi electron-diffraction patterns. The images are in the backscattering direction, and the positions of the images are consistent with those expected from the atomic structure near the Pt(111) surface. The strong electron scattering of the Pt atoms causes no observable problems in the Kikuchi electron holography.

Perpendicular magnetization in epitaxial Cu/Fe/Cu/Si(111) ultrathin films

Epitaxial Fe films, with thickness in the range between 2.5 and 12 Å, were grown in UHV conditions on the 7×7 reconstructed (111)-Si surface, with a Cu 35 Å thick buffer layer, using the so-called metal–metal epitaxy on silicon (MMES). Kikuchi electron diffraction showed that the growth of Fe on Cu/Si(111) occurs first by formation of a pseudomorphic film of γ-Fe(111), about two to three atomic layers thick, and by the successive growth of bcc Fe(110) domains in the Kurdjumov–Sachs orientation, in agreement with our previous low-energy electron diffraction observations. Kerr effect measurements carried out at low temperatures (20–150 K) revealed that Fe films thinner than 5–6 Å are ferromagnetic with an easy axis magnetization orthogonal to the film plane. With increasing Fe thickness, in coincidence with the fcc-to-bcc structural transformation, the easy axis switches to the in-plane orientation over a finite range of thickness.

Metal–metal epitaxy on silicon: Cu/Ni/Cu ultrathin films on 7×7-Si(111)

Cu/Ni/Cu heterostructures have been in situ deposited on the 7×7 reconstructed Si(111) surface. A number of complementary techniques, such as in situ low-energy, medium-energy and Kikuchi electron diffraction and ex situ X-ray diffraction, were used in order to characterise the growth process and the structural properties of the films. It is found that the growth mode of metallic films is characterised by the presence of twinned islands induced by the 7×7 reconstruction of the Si(111) substrate. This work can stimulate further application of the metal–metal epitaxy on silicon to grow high quality ultrathin magnetic films to be integrated in microelectronic devices.

Fcc–bcc phase transition of epitaxial Fe/Cu(111) films: a structural and magnetic study

Epitaxial Cu/Fe/Cu trilayers with Fe film thickness between 4 and 110 Å have been deposited on the 7×7 reconstructed surface of Si(111) single crystal. The structural and chemical characterization of Fe films was accomplished by in situ low energy electron diffraction, Kikuchi electron diffraction and Auger electron spectroscopy. As the Fe thickness exceeds about 6–8 Å, a fcc–bcc phase transition occurs by the appearance of bcc(110) domains in the Kurdjumov–Sachs orientation. The structural transformation is also confirmed by ex situ X-ray diffraction experiments in the θ–2 θ configuration. The evolution of the magnetic properties with the Fe film thickness has been studied by means of surface magneto-optic Kerr effect. Hysteresis loops measurements in the longitudinal configuration show a marked increase of the saturation signal which can be associated with the structural fcc–bcc phase transition of Fe.

Holographic inversion of Kikuchi electron diffraction patterns for thin epitaxial NaCl films grown on GaAs(001)

Normal-incidence, multiple-energy Kikuchi electron diffraction patterns (holograms) have been measured for clean GaAs(001) surface and for thin NaCl films epitaxially grown on a GaAs(001) substrate. Three-dimensional atomic images of both structures have been obtained directly by the integral-energy phase-summing method. For the clean semiconductor surface, the superimposed images of Ga and As bulk atoms were well resolved within an accuracy of 0.5 Å. A distinctly different local atomic configuration was found after the deposition of the epitaxial NaCl film. Possible models of the NaCl/GaAs(001) epitaxy are discussed. A detailed analysis of the reconstructed diffraction patterns revealed that an initial formation of the Cl–Ga bond occurred in the system, and after deposition of 5 ML NaCl, the substrate was uniformly covered by a layer at least 3 ML thick.

Inversion of low-energy electron diffraction data with no pre-knowledge factor beyond optical holography

In this review, I describe how low-energy (typically below 500eV) electron diffraction spectra can be inverted to produce three-dimensional coordinates of atoms neighbouring a reference atom with no prior knowledge of what type or types of atom are present. The reference atom may be one of many equivalent near-surface atoms from which a core-level photoelectron is excited or, in the case of diffuse low-energy electron diffraction, one of many equivalent adsorbate atoms (lacking in long-range order) on the surface of a crystalline substrate. Other variants apply to low-energy electron diffraction, Kikuchi electron diffraction and time-reversed versions in which the wavenumber (energy) and direction of the incident beam are varied. I show that, for such low-energy electron diffraction spectra, the relative phases between the reference wave and scattered waves have a known geometric form if the spectra are taken from within a small angular cone in the near-back-scattering directions. By using the back-scattering small cone at each direction of interest, a simple algorithm is developed to invert the spectra and extract object atomic positions with no input of calculated dynamical factors. The article also reviews key ideas and works which led to the current understanding of this field.

The potential engineering of grain boundaries through thermomechanical processing

Grain boundary character distribution is a relatively new microstructural feature that describes the proportions of random and special grain boundaries as defined by the coincident site lattice model. The combination of the availability of a new experimental technique based on the automatic indexing of backscatter Kikuchi electron diffraction patterns in the scanning electron microscope (orientation imaging microscopy) and reports in the literature describing the optimization of the grain boundary character distribution through thermomechanical processing are making the potential for enhanced materials properties in commercial metals and alloys a reality. Although the effects of optimizing the grain boundary character distribution in the cost-effective improvement of properties have been documented, the potential for commercialization has limited the disclosure of processing details. In this article, two separate approaches to the optimization of the grain boundary character distribution in oxygen-free electronic copper at Lawrence Livermore National Laboratory are discussed.

Perpendicular and in-plane magnetic anisotropy in epitaxial Cu/Ni/Cu/Si(111) ultrathin films

Epitaxial ultrathin Cu/Ni/Cu films with Ni thickness in the range 1.5--6 nm have been grown by UHV evaporation on the $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surface. In situ characterization made by low-energy electron diffraction and Kikuchi electron diffraction revealed that both Ni and Cu films grow epitaxially on Si, with a (111) orientation and with their (1\ifmmode\bar\else\textasciimacron\fi{}10) axis parallel to the (12\ifmmode\bar\else\textasciimacron\fi{}1) axis of the Si substrate. Magneto-optical Kerr effect measurements performed at room temperature have shown that the preferential direction of magnetization lies in the film plane for Ni thickness above 3 nm, while it is perpendicular to the film plane for lower thickness. Brillouin light scattering was then exploited to study the spin-wave dispersion as a function of both the applied magnetic field and the wave vector direction on the surface plane. In order to interpret the Brillouin data, we have used a macroscopic model which takes into account both dipolar and exchange interactions, as well as bulk and interface anisotropy for the (111) plane of a cubic crystal. This enabled us to determine, in addition to the other magnetic parameters, both the in-plane and the out-of-plane anisotropy constants. The observed dependence of these constants on the film thickness indicates that the magnetic anisotropy is mainly of magnetoelastic origin.

New method for rapid determination of crystal orientation via Kikuchi patterns

Kikuchi electron diffraction patterns are used extensively to determine crystal orientations via transmission electron microscopy (TEM) or in the electron backscattering pattern (EBSP) mode of scanning electron microscopy (SEM). A new method is presented that is capable of finding crystal orientations, the camera length, and the projection center from only one pattern per grain using a least-squares technique. This method eliminates the need to perform an alignment with a reference crystal in the backscattering mode. Application to a Σ = 13a silicon bicrystal is presented for TEM patterns and EBSP's. A complete analysis of the propagation of random measurement errors into the disorientation axis/angle pair is carried out. The root mean square deviation from the nominal disorientation angle is found to be 0.3°in the case of TEM and 0.5°in the case of EBSP. The root mean square inclination between the nominal and measured disorientation axis is found to be 1°in the case of TEM and 0.5°in the case of EBSP.

Surface sensitivity of Kikuchi-electron diffraction patterns.

Since the realization by several workers that the electron diffraction from localized sources can be interpreted in a holographic sense, there has been increasing interest in electron holography. Photoelectron and Auger electrons are considered good localized electron sources, but their use experimentally in holography usually requires access to a synchrotron radiation source. We report here the use of our technique to obtain Kikuchi-electron diffraction (KED) patterns using normal low-energy electron-diffraction facilities. This technique improves the KED signal-to-noise ratio and allows us to take KED patterns over a wide range of energies within a short time. Using this method we have shown that Kikuchi electrons can be treated as having come from localized emitters. The results presented here clearly show the surface sensitivity of KED images in the energy range 450--800 eV. This sensitivity is revealed by the subtraction of the bulk contribution to the KED images by acquiring two image sets under exactly the same conditions, one from Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-Pb and one from Si(111)7\ifmmode\times\else\texttimes\fi{}7, and subtracting them. Furthermore, these difference images show intensities which are very strongly dependent on the primary energy, unlike the raw KED images. This energy dependence suggests that a backscattering geometry is dominating the contributions to the intensities in these difference images.

Improved X-ray and electron diffraction methods for twin determination in hexagonal crystals

Two improved methods of characterizing twin orientations in deformed crystals have been developed and compared; the back-scattered Kikuchi electron diffraction method in a standard scanning electron microscope and a new high-resolution X-ray pole-figure technique. Comparative tests on titanium and magnesium crystals deformed in plane strain compression up to high strains demonstrate their complementary features. The back-scattered Kikuchi diffraction technique, which combines imaging and microdiffraction, is well adapted to localized twin-orientation studies in both single and polycrystals after careful surface preparation. The high-resolution X-ray pole figures can be used for more quantitative studies of twinned volume fractions, including very fine twins, in crystallographically oriented samples such as deformed single crystals or large-grained polycrystals.

A new approach to obtaining Kikuchi electron diffraction patterns for holographic imaging

A newly developed method is reported here for taking Kikuchi electron diffraction patterns using conventional low-energy electron diffraction facilities. Kikuchi electron forward-scattering diffraction patterns from a single crystal Cu(111) surface have been obtained using a hemispherical rear-view LEED screen and camera interfaced to a computer. The new technique allows us to take a set of Kikuchi electron diffraction images over a wide primary energy range (50–950 eV in steps of 5 eV) within a couple of hours. With this technique we show that the Kikuchi electron, like the photoelectron and Auger electron, can be considered as being emitted from inside the crystal.

Concentric-shell algorithm for Auger and core-level photoelectron diffraction: Theory and applications.

A full multiple-scattering method is described for the calculation of angle-resolved photoelectron, Auger-electron, and Kikuchi-electron diffraction patterns. The method is particularly useful for the range of electron kinetic energies from about 50 eV to a few thousand eV. Approximations that take advantage of the finite experimental angular resolution, and, for the higher electron kinetic energies, the dominant electron forward scattering, are implemented, and may be used to improve computational efficiency. Comparisons of calculations with an experimental LVV Auger dif- fraction pattern from a Cu(001) surface show excellent agreement. The simulated diffraction patterns are also used to investigate the effects of holographic reconstruction in the case of the adsorbate system O/Ni(001), where we examine the influence of multiple scattering. In the case of the Cu(001) diffraction pattern, we examine the effects of the thickness of a film on holographic reconstruction.

Computer-aided prediction and analysis of electron and x-ray diffraction patterns

Using a personal computer, electron and x-ray diffraction patterns are analytically generated and printed to scale. The structure factor equation is used to determine which crystalline planes diffract according to the supplied crystal lattice type, atoms present and their positions in the unit cell, lattice dimension(s), crystal orientation, and radiation wavelength. Single crystal spot and Kikuchi electron diffraction patterns as well as polycrystalline ring patterns can be generated. Powder x-ray diffraction patterns are also generated using this code. Complex composite patterns are generated by overlaying multiple patterns resulting from crystallographic orientation effects. A user friendly program with a direct approach to generate diffraction patterns for any defined crystal aids in the analysis and indexing of these patterns.

Dissociated Σ=27 boundaries in silicon

In the last twenty years, a significant amount of work has been done in the theoretical understanding of grain boundaries. The various proposed grain boundary models suggest the existence of coincidence site lattice (CSL) boundaries at specific misorientations where a periodic structure representing a local minimum of energy exists between the two crystals. In general, the boundary energy depends not only upon the density of CSL sites but also upon the boundary plane, so that different facets of the same boundary have different energy. Here we describe TEM observations of the dissociation of a Σ=27 boundary in silicon in order to reduce its surface energy and attain a low energy configuration.The boundary was identified as near CSL Σ=27 {255} having a misorientation of (38.7±0.2)°/[011] by standard Kikuchi pattern, electron diffraction and trace analysis techniques. Although the boundary appeared planar, in the TEM it was found to be dissociated in some regions into a Σ=3 {111} and a Σ=9 {122} boundary, as shown in Fig. 1.

On the determination of intergranular orientation relationships by kikuchi electron diffraction

A generally applicable method of determining orientational relationships between grains by Kikuchi electron diffraction is described, and illustrated by means of observations of magnesium. Es wird eine allgemein anwendbare Methode zur Bestimmung der Orientierungsbeziehungen zwischen Kornern durch Kikuchi-Elektronenbeugung beschrieben und durch Beobachtungen an Magnesium illustriert.

Nucleation of recrystallization in compressed aluminium: studies by electron microscopy and Kikuchi diffraction

Following the earlier investigation of recrystallization of aluminium by Bellier and Doherty [1] by transmission Kossel diffraction, the details of the nucleation process were studied by transmission electron microscopy and Kikuchi electron diffraction. This showed that nucleation appeared to occur via a sub-grain coalescence process that occurred selectively at deformation bands and at deformation band, grain boundary junctions. Nucleation occurred only at grain boundaries and at deformation bands. The condition for continued growth, of enlarged sub-grains of length 2L, along the grain boundary L>2r(γs/γg) where γs is the sub-boundary energy and γg the grain-boundary energy, was found to be obeyed. The values of the stored energy calculated from the measured sub-grain sizes and misorientations were less than the reported experimental value, indicating that in as-deformed aluminium the dislocation arrays in the sub-boundaries may not have the lowest energy structure assumed in the calculation.