Fresnel Fringes Research Articles

Lunar Occultations of the SiO Masers in RLeo

VLBI observations show that the SiO maser emitting regions in oxygen-rich stars are very clumpy and that these clumps extend over a few stellar radii (Mclntosh et al. 1987; Colomer et al. 1992). These observations indicate that the ideal instrument for the study of the SiO masers is an interferometer with baselines covering between a few and several hundreds/thousands km. Such an instrument is so far unavailable.A classical way to get high angular resolution and a full beam synthesis with a single telescope of moderate size is through lunar occultations. This observing technique provides the angular resolution of a single linear antenna several kilometers long. However, at millimeter wavelengths the Fresnel fringes produced by the Moon limb as the source under study is occulted have never been observed. We present here the observation with the 30-m IRAM radio telescope of the v=1 J=2-1 line of SiO during an occultation (and reappearance) of R Leo by the Moon.

Atomic structure of grain boundaries in YBa2Cu3O7-? as observed by Z-contrast imaging

Determination of the atomic structure of grain boundaries is the key to fundamental understanding of the critical current density in polycrystalline superconductors. High-resolution images with incoherent characteristics, obtained using a high-angle annular detector on an atomic resolution scanning transmission electron microscope, are used to study the atomic arrangements of these technologically important boundaries. The incoherent Z-contrast images do not experience contrast reversals with defocus or sample thickness and display no Fresnel Fringe effects at boundaries. Observed rigid shifts of atomic columns at grain boundaries are independent of sample thickness and objective lens defocus. These characteristics allow unambiguous and intuitive interpretations of the generated images. We find the atomic structures at grain boundaries in YBa2Cu3O7-δ are strongly influenced by the strong tendency of this compound to exist only as complete unit cells terminated at {001} and {100} planes. The weak-link behavior associated with high-angle grain boundaries may follow from this structure in which there is no clear connection between the {100} facets of adjacent grains. Symmetric grain boundaries where adjacent grains share a common boundary plane have also been observed in YBa2Cu3O7-δ. In these boundaries partial structural coupling of the grains is maintained. There is evidence that these two boundary forms produce junctions with very different superconducting properties.

The defocus contrast of a θ′ precipitate in Al-4wt%Cu: Fresnel fringe analysis applied to an atomically abrupt interface

The through-focal contrast of a θ′ precipitate in Al-4wt%Cu is examined. The aim of the work presented here is to establish whether Fresnel contrast analysis can be used to characterise an atomically abrupt interface. Simulations are presented which illustrate the sensitivity of variations in image contrast, at relatively low resolution and as a function of the defocus, to atomically localised changes in compositional spread. The effects of dynamical contributions to the contrast variations are also considered in this context. Preliminary experimental data are analysed and the problems associated with matching experiment and theory are discussed.

Measuring the height of steps on MgO cubes using Fresnel contrast in a scanning transmission electron microscope

The possibility of using Fresnel (or phase) contrast in a scanning transmission electron microscope as a means of measuring the heights of steps on the surface of MgO smoke cubes is examined. It is unknown that once the microscope parameters are known it is possible to match the intensities of the Fresnel fringes quantitatively with multislice simulations for step heights of the order of a few monolayers on a cube of thickness 44 nm.

Quantitative Comparison of TEM Techniques for Determining Amorphous Intergranular Film Thickness

The question of which transmission electron microscopy technique can best be used to quantitatively determine the thickness of thin intergranular films is addressed. Measurement of the film thickness by diffuse dark‐field imaging resulted in values 50% to 100% larger than those determined by high‐resolution lattice imaging. Defocus Fresnel fringe imaging is an indirect method of obtaining intergranular film thickness; with this method thicknesses were overestimated by 20% to 35%, with the largest error caused by uncertainty in the exact location of fringe maxima. Although technically difficult to use, high‐resolution lattice imaging is a method capable of the resolution necessary to obtain detailed information of the boundary and the intergranular phase itself; it was found to be the most applicable to quantitative measurements of film thicknesses in Si3N4 materials, with an accuracy of ± 1 Å (±0.1 nm).

High resolution study of the structure of the Σ=5 [001] twist grain boundary in NiO

The existence of grain boundaries (GBs) in many advanced ceramics can significantly alter their performance, rendering the study of GB structure an essential part of understanding ceramic properties on a fundamental level. Furthermore, the debate regarding the stability of [001] twist GBs in rocksalt-structured oxides provides the impetus for comparing experimental observations of the Σ=5 [001] twist GB structure in NiO with the stable GB structure calculated by Tasker and Duffy (Fig. 1). The Tasker and Duffy structure was obtained by introducing a Schottky defect in each of the GB unit cells, resulting in a 20% decrease in atomic density at the GB. High resolution TEM (HRTEM) employing Fresnel fringe and lattice imaging techniques (Fig. 2) supplemented with image simulations of six possible configurations for the NiO Σ=5 [001] twist GB structure using the TEMPAS program (Fig. 3) were the primary experimental methods used in an attempt to detect this reduced atomic density in NiO Σ=5 [001] twist GBs manufactured by a modified Schober-Balluffi method. Since the Fresnel fringe technique consists of a change in Fresnel fringe contrast with defocus due to a change in mean inner potential at the GB, it becomes necessary to consider the contribution of the various causes of a change in mean inner potential to the resulting contrast. These causes include: (i) a change in atomic density or a change in atomic structure at the GB, (ii) the existence of a space charge at the GB, (iii) the existence of a thin amorphous film at the GB, (iv) the presence of impurities at the GB or a change in composition across the GB, and (v) GB grooving. The contribution of a GB space charge to the change in mean inner potential at the GB is assumed to be small, given that the space charge associated with with a GB in an ionic crystal is on the order of a few tenths of a volt, whereas the mean inner potential at the GB is on the order of 15 - 20 V. An attempt to address the impurity and grooving issues was made by performing EDS and EELS across the GB in an ultra-high vacuum, high resolution STEM.

Interpretation of low-voltage field-emission projection interferograms

Remarkable transmission electron interference patterns have been reported from very thin crystals at energies Eo< 400 volts using a field emission tip, distance z1 ≈ 200 nm from a semi-transparent crystal sample, which acts as the grounded anode. As predicted, “atomic resolution” Fourier images ar observed on a screen distance z2 from the sample, with magnification z2/z1 ≈ 106at interior sample regions, confused by Fresnel fringes at edges. The same geometry is used to observe lattice images without scanning in coherent CBED patterns with overlapping orders. The interpretation of these patterns must be based on the theory of transmission LEED (TLEED), including multiple scattering. Figure 1 shows the Ewald sphere construction for 250 volt electrons along [110] gold. Image resolutio is limited to the inner reflections by the small sphere (large wavelength). TLEED computations using the Bloch-wave method of Collela are compared with the single scattering approximation in figure 2 Convergence tests show that 58 forward and backscattered beams are sufficient (Backscattered beams hop along the surface under the repulsive influence of the tip field).

Reflection electron energy-loss spectroscopy and imaging for surface studies in transmission electron microscopes.

A review is given on the techniques and applications of high-energy reflection electron energy-loss spectroscopy (REELS) and reflection electron microscopy (REM) for surface studies in scanning transmission electron microscopes (STEM) and conventional transmission electron microscopes (TEM). A diffraction method is introduced to identify a surface orientation in the geometry of REM. The surface dielectric response theory is presented and applied for studying alpha-alumina surfaces. Domains of the alpha-alumina (012) surface initially terminated with oxygen can be reduced by an intense electron beam to produce Al metal; the resistance to beam damage of surface domains initially terminated with Al+3 ions is attributed to the screening effect of adsorbed oxygen. Surface energy-loss near-edge structure (ELNES), extended energy-loss fine structure (EXELFS), and microanalysis using REELS are illustrated based on the studies of TiO2 and MgO. Effects of surface resonances (or channeling) on the REELS signal-to-background ratio are described. The REELS detection of a monolayer of oxygen adsorption on diamond (111) surfaces is reported. It is shown that phase contrast REM image content can be significantly increased with the use of a field emission gun (FEG). Phase contrast effects close to the core of a screw dislocation are discussed and the associated Fresnel fringes around a surface step are observed. Finally, an in situ REM experiment is described for studying atomic desorption and diffusion processes on alpha-alumina surfaces at temperatures of 1,300-1,400 degrees C.

Cross‐sectional transmission electron microscopy of x‐ray multilayer thin film structures

A simple method for preparing cross-sectional transmission electron microscopy specimens and discussions of possible artifacts from specimen preparation and observation of x-ray multilayer thin film structures are presented. The specimen preparation method employs mechanical grinding and polishing to approximately 20 microns, followed by ion milling, without dimpling. Artifacts such as preferential ion milling and crystallization under the electron beam, as well as effects of Fresnel fringes at interfaces, are important factors in interpretation of the images. Care in identifying them is required to avoid erroneous results in studies of morphology and microstructures within the layers and at their interfaces. Example high-resolution TEM results of cross-sectional W/C, Ru/C, and Mo/Si multilayers are presented.

The characterisation of GaAs/(Al,Ga)As heterostructure interface roughness using Fresnel analysis

The composition profiles of two GaAs/AlAs heterostructures are determined by the electron microscopical method of analysing the Fresnel fringe contrast observed along the GaAs/AlAs interfaces. Accuracies of ±0.25 nm in the determination of interface diffuseness and ±0.2 nm in the determination of layer widths are demonstrated using this Fresnel Method and the implications for the modelling of the electronic properties of such heterostructures are discussed.

Analysis of pure tilt grain boundary in silicon by Fresnel fringe technique

One of the main emphases of research on interfaces is to characterize them both structurally and chemically. The atomic structure of interfaces and the existence of thin interfacial glassy phases are being investigated mainly by HREM imaging methods. In addition, a few other methods including dark field diffuse imaging and Fresnel Fringe techniques have been developed to identify the interfacial films and quantify their width. Fresnel fringe profiles from the interface images have been used to deduce the mean inner potential of thin interfacial films. In our research the Fresnel fringe contrast behavior of a tilt grain boundary in silicon whose structure and chemistry were previously characterized by HREM and high spatial resolution AEM was analyzed by comparing the experimental Fresnel fringe profiles with computer simulations of these images.

Fresnel fringe contrast in the TEM: Application to study the microstructure of amorphous silicon

Amorphous silicon-hydrogen (α - Si:H) alloy films deposited by plasma enhanced chemical vapor deposition (PECVD) of silane exhibit a remarkable property; namely, the films are practically insulators in the as deposited state. When sandwiched between two metals or highly conductive refractory metal silicides, the films can be converted to near conductivity state by “programming” the sandwich structure to complete dielectric breakdown of the insulator. Such α - Si:H films are used in permanently programmable integrated circuits such as PGA (Programmable Gate Arrays) as “antifuse” materials.TEM technique has, usually, limited application to study the microstructure of amorphous materials since they do not exhibit the well understood diffraction contrast as seen in crystalline materials. However, Fresnel fringe contrast due to electron interference effects at either sides of interfaces or at the edges can be used to provide the sort of information needed to characterize amorphous films. The profiles of Fresnel fringes are often complex for interface thickness of <100 A and their shape and contrast are very sensitive to compositional changes on either sides of the interfaces.

High-resolution Z-contrast imaging of crystals

The use of a high-angle annular detector in a scanning transmission electron microscope is shown to provide incoherent images of crystalline materials with strong compositional sensitivity. How this occurs, even in the presence of strong dynamical diffraction of the low-angle beams, becomes very clear in a Bloch wave description of the imaging, which shows that only tightly bound s-type Bloch states contribute significantly to the image. Interference effects are therefore precluded and the image can be described as a convolution. There are no contrast reversals with thickness or defocus and no Fresnel fringe effects at interfaces. Each atomic column contributes to the image independently of its neighbors until the s-states themselves overlap. With an optimum imaging probe the nature of the convolution can be visualized intuitively to a scale well below the resolution limit. To first order, therefore, each object has only one possible image, and since the same probe is used for all objects, an unknown structure can be interpreted directly. These ideas will be illustrated with images from semiconductors, superconductors, and alloys.

Theoretical image processing of simulated supported particles (1-3 nm)

The images of supported nanoparticules obtained by High Resolution Transmission Electron Microscopy are often difficult to interpret. The small size of the particles, the existence of Fresnel fringes and the presence of a stabilizing carbon film all contribute to blur the images. Quantitative results on the size and the morphology of the particles or information about the interface substrate-particle have to be extracted from the images as a direct interpretation is difficult (and hazardous). Simulation of images, starting from a model and computing all the changes in the electron

A study of the initial stages of the oxidation of silicon using the Fresnel method

Abstract The initial stages of the oxidation of silicon are studied using a new analytical technique which we have developed for use in the transmission electron microscope. This technique, the Fresnel method, is based on the use of a through-focal series of images of an interface viewed edge-on to characterise any compositional discontinuity across it. The appearance of the Fresnel fringes seen along the interface as a function of defocus, as matched with computer models, can be used to determine the change in elastic scattering behaviour of the material due to the change in projected potential at such a compositional discontinuity. A set of relatively low-resolution images is used to provide high-resolution data on both the form and the magnitude of the composition change present. The method is applied in conjunction with conventional high-resolution imaging to study the relationship between the structural and stoichiometric changes occurring at the interface between silicon and its oxide. The compositi...

Fresnel fringe effects at interfaces of thin multilayer structures

Recent developments in the TEM Fresnel-fringe technique have provided an alternative method to the determination of structures and morphology of interfaces in multilayer thin film structures. This method has been employed in the investigation of structures and defects in grain boundaries, dislocations, precipitate platelets, twin boundaries, metal interfaces, and multilayer structures. It has been demonstrated that the fringe spacing primarily relates to the layer thickness, while the fringe contrast as a function of defocus relates to the magnitude of the localized change in the scattering potential and thus to the interfacial composition. The profile of the fringes is more closely related to the abruptness of the composition change at the interface. Other factors affecting the Fresnel fringe intensity are the specimen thickness, aperture size, beam convergence, and degree of tilt of the interface from the incident electron beam.Fresnel fringes result from the electrons experiencing an abrupt change in the inner scattering potential parallel to the electron beam path. Most previous calculations of the Fresnel contrast with defocus have simulated models based on one square or symmetrical trapezoid-shaped potential well, which are applicable only to a single interface or grain boundary, or to a few layer pairs of the multilayer.

Recent applications of TEM to the study of interfaces

This paper will review the current state of understanding of interface structure and highlight some of the future needs and problems which must be overcome. The study of this subject can be separated into three different topics: 1) the fundamental electron microscopy aspects, 2) material-specific features of the study and 3) the characteristics of the particular interfaces. The two topics which are relevant to most studies are the choice of imaging techniques and sample preparation. The techniques used to study interfaces in the TEM include high-resolution imaging, conventional diffraction-contrast imaging, and phase-contrast imaging (Fresnel fringe images, diffuse scattering). The material studied affects not only the characteristics of the interfaces (through changes in bonding, etc.) but also the method used for sample preparation which may in turn have a significant affect on the resulting image. Finally, the actual nature and geometry of the interface must be considered. For example, it has become increasingly clear that the plane of the interface is particularly important whenever at least one of the adjoining grains is crystalline.A particularly productive approach to the study of interfaces is to combine different imaging techniques as illustrated in the study of grain boundaries in alumina. In this case, the conventional imaging approach showed that most grain boundaries in ion-thinned samples are grooved at the grain boundary although the extent of this grooving clearly depends on the crystallography of the surface. The use of diffuse scattering (from amorphous regions) gives invaluable information here since it can be used to confirm directly that surface grooving does occur and that the grooves can fill with amorphous material during sample preparation (see Fig. 1). Extensive use of image simulation has shown that, although information concerning the interface can be obtained from Fresnel-fringe images, the introduction of artifacts through sample preparation cannot be lightly ignored. The Fresnel-fringe simulation has been carried out using a commercial multislice program (TEMPAS) which was intended for simulation of high-resolution images.

Calculations and experimental observations of shadow images from multilayers

The properties of multilayer thin film materials are strongly influenced by the structure of their interfaces. The numerous applications for these materials motivate electron microscopy studies of cross sectional samples in which the interfaces are observed edge on. High resolution imaging is the most established technique, but other techniques, such as Fresnel fringe method and refraction at interfaces, have also been employed to characterize the structure and abruptness of amorphous/polycrystalline multilayers. The aim of this work is to explore the applicability of shadow images from coherent sources to the studies of multilayers.Coherent interference effects are readily observable in the diffraction plane of a VG HB-5 STEM which is equipped with a cold field emission gun. The shadow image of the studied Si/Mo multilayer is obtained on the detection plane when the specimen is illuminated by a stationary convergent beam. This beam is formed with a very large or no objective aperture. Low magnification images are produced at large values of defocus (Fig. 1a), while for lower defocus the magnification increases and the shadow images become more and more distorted because of the objective lens aberration (Fig. 1b and 1c). In addition to the high sensitivity to the defocus. spherical aberration and probe position, the shadow images also appear to be dependent on the interface abruptness.

High-resolution Z-contrast imaging in the STEM

With a high-angle annular detector in a STEM, incoherent imaging can be realized at atomic resolution. This provides improved image resolution, from 0.66 CS¼λ¾ to 0.43 CS¼λ¾, based on the Scherzer definition of point-to-point resolution, although in both cases finer scale detail can be transmitted to the image at higher defocus values. Equally important, the incoherent mode provides a simply interpretable image; no contrast reversals occur with defocus or sample thickness, no Fresnel fringe effects are seen at interfaces and no speckle pattern is seen within an amorphous phase. In addition, the high-angle (Rutherford) scattering imparts a strong compositional sensitivity to the image through the practically Z2 dependence of the cross section.How these effects occur have been discussed in detail elsewhere. The three components of high-angle scattering for a zone axis orientation are the HOLZ reflections, the zero layer reflections, and the thermal diffuse (Rutherford scattering) component. The contribution of the HOLZ is simple to estimate experimentally in selected area diffraction, either from bright field line traces or from the high-angle channeling pattern, which is obtained from the annular detector (Fig. 1). A HOLZ reflection would be visible in the center of the channeling pattern only if its intensity was greater than the total diffuse scattering reaching the annular detector.

Influence of the mean crystal potential on the Fresnel fringes in cross-sectional transmission electron microscopy bright-field images of a flat projecting Si/SiOzinterface

Abstract An analysis of the Fresnel fringe contrast in defocused bright-field images of a Si-SiO2 interface imaged parallel to the beam is carried out with contrast simulations. The drop in the mean potential across the interface is assumed to be continuous and of error function type. The width of the potential drop is shown to have a strong influence on the visibility of the Fresnel fringes for different values of the defocus.