Kikuchi Electron Research Articles

Influence of microscope settings on dislocation imaging in transmission forescattered electron imaging (t-FSEI)

This work analyzes the influence of several microscope settings, namely, sample-forescattered electron detector (FSD) distance, and tilting conditions on the characteristics of the dislocation contrast imaged in transmission forescattered electron imaging (t-FSEI). The dislocation contrast behaviors of characteristic dislocation configurations of two Fe-based alloys, namely an α’-martensitic (body-centered cubic, bcc) Fe33Ni alloy (wt%), and an austenitic (face-centered cubic, fcc) Fe-30Mn-6.5Al-0.3C alloy (wt%) were investigated on thin foil samples by using different on-axis transmission Kikuchi diffraction (TKD) configurations, namely t-FSEI, bright-field (BF) t-FSEI and electron channeling contrast imaging (ECCI). The set-ups use transmission Kikuchi electron patterns to orient the crystal into controlled diffraction conditions. Imaging parameters such as dislocation contrast intensity and information depth are analyzed and compared to those obtained in the ECCI mode under the same microscope conditions. These effects are associated with the attenuation of Bragg scattering by high-angle scattering processes and the electron channeling mechanism, respectively. The experimental analysis sets the microscope settings for optimum dislocation imaging in t-FSEI.

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.

Simultaneous presence of unindexed lines with different signs of contrast in Kikuchi patterns

We have obtained Kikuchi patterns of silicon. The patterns contain simultaneously bright and dark unindexed lines. Formation and the contrast of these Kikuchi lines are explained by the phenomenon of double diffraction of Kikuchi electrons. Conclusion is drawn that the sign of contrast of unindexed Kikuchi lines does not depend on the thickness of a crystal.

Nonidentified Kikuchi lines with reverse contrast

Electron diffraction patterns of silicon in transmission with contrast reversal from bright to dark for an unidentified Kikuchi line along its length have been obtained. The contrast reversal of an unidentified line is explained within the elementary mechanism of Kikuchi pattern formation 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.

Kikuchi Patterns Observed by New Astrodome RHEED

A new reflection high-energy electron diffraction (RHEED) apparatus equipped with a hemispherical fluorescent screen upon a sample surface has been constructed. We call it astrodome screen, namely, the apparatus is named as astrodome RHEED. For the observation of Kikuchi pattern in RHEED experiment, the astrodome screen is effective because of the relatively short escape depth of Kikuchi electrons arising in bulk. Diffraction patterns have been observed in the astrodome screen for Si(111)7 × 7 clean surface, and the glancing angle and the acceleration voltage dependences were investigated. It has been found that some circular patterns exist in the Kikuchi patterns. The circular patterns are considered to be responsible for one-dimensional diffractions from certain atomic rows in bulk. The observed Kikuchi patterns and circular patterns are calculated using simple kinematical theory and a good agreement between the experimental and calculated results has been obtained. [DOI: 10.1380/ejssnt.2006.118]

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.

Surface structures by direct transform of electron diffraction patterns

We find that electron diffraction patterns can be directly inverted to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either holography or a Patterson inversion scheme is used. For diffraction patterns which were generated from a localized emitter source or predominantly by an inelastic-scattering feature like low-energy Kikuchi electrons, holography inversion is needed. The information obtained from Kikuchi electron holography includes the building blocks on the surface and their relative position to the atoms below the surface layer. On the other hand, for diffraction patterns generated predominantly by an elastic-scattering feature like low-energy electron diffraction (LEED), a Patterson inversion is needed. The information obtained from the Patterson transform of the LEED I(E) curves is the relative positions of surface atoms to the atoms in underlying layers; no intra-layer information can be extracted with this method. High-fidelity and artifact-free three-dimensional atomic structures obtained by inversion of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The results from the two inversion methods are complementary and can be used to construct or to discriminate the surface atomic structural models. The future of these direct methods by inverting diffraction patterns is discussed.

Atomic structures by direct transform of diffraction patterns

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed.

Structure determination of the Si3N4/Si(111)- (8 x 8) surface: a combined study of Kikuchi electron holography, scanning tunneling microscopy, and ab initio calculations.

A comprehensive atomic model for the reconstructed surface of Si3N4 thin layer grown on Si(111) is presented. Kikuchi electron holography images clearly show the existence of adatoms on the Si3N4(0001)/Si(111)-(8x8) surface. Compared with the ab initio calculations, more than 30 symmetry-inequivalent atomic pairs in the outmost layers are successfully identified. Scanning tunneling microscopy (STM) images show diamond-shaped unit cells and nine adatoms in each cell. High-resolution STM images reveal extra features and are in good agreement with the partial charge density distribution obtained from total-energy calculations.

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.

Atomic structures and phase transitions of Si(113) reconstructed surfaces: Kikuchi electron holography studies

Atomic structures of the reconstructed Si(113) surfaces were studied by using Kikuchi electron holography (KEH). Three-dimensional images show clearly the characteristics of the puckering model for both Si(113)-(3\ifmmode\times\else\texttimes\fi{}2) and (3\ifmmode\times\else\texttimes\fi{}1) surfaces. The KEH results support the puckering model. Based on our studies, the tetramers are puckering alternatively in the (3\ifmmode\times\else\texttimes\fi{}2) surface. Whereas in (3\ifmmode\times\else\texttimes\fi{}1) structures, there are two domains, within each of them, tetramers buckled uniformly, but the overall directions are opposite. When doped with H atoms on a (3\ifmmode\times\else\texttimes\fi{}2) surface, the asymmetric tetramers change into symmetric form.

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.

HOLOGRAPHIC IMAGES OF ADATOMS, DIMERS AND REST ATOMS ON THE Si(111)-(7 × 7) SURFACE

The adatoms, dimers and rest atoms in the three outermost atomic layers of the Si(111)-(7 × 7) surface are directly imaged with glancing Kikuchi electron holography. The applicability of Kikuchi electron holography to complicated multiple-emitter surfaces is evident. The three-dimensional relative positions of atoms on the Si(111)-(7 × 7) surface are in good accordance with the LEED-optimized DAS model.

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.