J.M. Cowley

Atomic and other structures of cleaved GaAs(110) surfaces

Abstract Reflection electron microscopy (REM) has been applied to studying the (110) cleavage planes of GaAs crystals. Experiments were performed in commercial electron microscopes with minimal specimen preparation. Surface steps on an atomic scale and dislocations were imaged, as well as crystal fibers and other features which have not been reported before. Effects of contamination are discussed.

REM and REELS identifications of atomic terminations at α-alumina (011) surface

Abstract Reflection electron microscopy (REM), reflection high energy electron diffraction (RHEED) and reflection electron energy-loss spectroscopy (REELS) have been combined to investigate the atomic structures at a α-alumina (011) surface. The α-alumina (011) shows light and dark regions which are interpreted as arising from termination of the structure by O-rich and Al-rich layers. It is the reactions of these different top layers with the atmosphere that are considered to give quite different contrast domains in the REM images. The surface domains with O-rich termination are found to be flat and create strong reflection intensity. The surface regions terminated by Al-rich layers are found to be rough and easily form surface oxides with the adsorbed oxygen from air. The structure modulation in a surface monolayer can significantly affect the excitation property of the surface and reduce the main travelling distance of the electrons along the surface. It is demonstrated that the combined techniques of REM, RHEED and REELS are useful tools for studying the chemical reactions between surface monolayers with gases.

The alignment of gold particles on MgO crystal faces

Abstract Gold, evaporated indirectly on the faces of magnesium oxide smoke crystals, is shown by scanning transmission electron microscopy to form well dispersed, well epitaxed crystals of diameter about 2 nm, apparently located randomly on the surface. Scanning reflection electron microscopy reveals that these crystallites tend to be aligned, presumably along straight-line steps on the MgO surfaces. While many of these linear arrays are parallel to the [100] edges of the MgO cubes, many are not but make small angles (1–5°) with such edges and with each other. There is some evidence of preferred angles between these lines and the cube edges, suggesting that jogs in the MgO steps tend to occur regularly with periodicities of 2 to 7 nm.

Electron Microscopy in the Study of Materials

Electron microscopy in the study of materials , Electron microscopy in the study of materials , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Electron microscopy of atoms in crystals

From its beginnings the field of electron microscopy has sought, as its ultimate aim, the ability to study the structure of matter by imaging the individual atoms that compose it. The resolution necessary to attain this goal is obtainable in principle, because the electron wavelengths of the beams normally used are less than 0.1 A, What has kept this goal from realization are practical difficulties in the design and construction of electron microscopes. These difficulties have now been overcome to the extent that resolutions of 2–3 A, close to the interatomic distances in some solids, have been achieved.

A multislice approach to the RHEED and REM calculation

Abstract A computing method for the forward dynamical scattering calculations which has been successfully applied to the Bragg case of high energy electron diffraction is described. This method is based on the physical optics theory of dynamical diffraction of Cowley and Moodie and is used for calculations of reflection high energy electron diffraction (RHEED) diffraction amplitudes and the reflection electron microscopy (REM) image intensities for the extended surface of a perfect crystal and also for a crystal surface with a defect. The physical basis of the method and some practical computing considerations are described.

A (√3 × √3)R30° reconstruction on annealed (111) surfaces of MgO

Abstract We have performed non-UHV studies on the (111) surface of MgO subjected to high temperature annealing (1550–1700°C) in an oxygen rich atmosphere. Reflection electron microscopy shows that the surface is atomically smooth over extended regions and exhibits a (√3 × √3)R30° reconstruction. Superlattice spots were observed in the RHEED patterns after two months exposure to air indicating that meaningful results can be obtained on this system under non-UHV conditions. Surface sensitive REELS indicates that the structure is mainly composed of oxygen.

Contrast and resolution in REM, SEM and SAM

Abstract Reflection electron microscopy (REM) in fixed beam and scanning modes, secondary electron microscopy (SEM) and scanning Auger electron microscopy (SAM) are related in that they provide nanometer-scale resolution of surfaces in moderate or ultra-high vacuum using incident electron beams having energies of the order of 100 keV. They may, in many cases, be used in conjunction or in parallel to provide complementary information on crystal structure, morphology and compositional variations with sensitivity to single surface-layers of atoms. Experiments have shown in each case that the achievable resolution is considerably better than had previously been thought possible. Recent theoretical developments and exploratory investigations have provided at least a partial explanation of why sub-nanometer resolutions can be achieved. In the case of REM, the main limitations to resolution and contrast of the images arise from the inelastic scattering processes. Experiments with a new energy-filtering electron microscope have confirmed that images are considerably improved if formed by using only elastically scattered electrons.

Electron holography and holographic diffraction for surface studies

Abstract The use of a reference wave coming from a particular type of atom has been applied in diffraction experiments from the early days of X-ray diffraction as a means for defining the relative phases of diffracted beams. Modern diffraction techniques for surface study, including diffuse LEED and photoelectron diffraction, apply the same basic concept of what has been called holographic diffraction in a variety of ways. The concept of holography was introduced by Gabor as a basis for the improvement of resolution in electron microscope imaging. An essential aspect was the use of a reference wave to allow relative phases of the image wave function to be defined. The use of holographic imaging principles for the imaging of surfaces with high-energy electrons in the reflection mode has been developed by analogy with off-axis transmission holography. Other forms of reflection-mode electron holography appear feasible by analogy with the Gabor scheme and other in-line holographic transmission modes. The development of very sharp field emission tips has introduced the possibility of low-energy electron holography in transmission and reflection modes. A plea is made for the use of appropriate terminology to distinguish clearly between these various uses of the holographic principle.

Experimental studies of surface resonance scattering processes in RHEED

Abstract A high energy electron diffraction experimental has been carried out in which both the reflection high energy electron diffraction pattern (RHEED) and transmission high energy electron diffraction pattern (THEED) are recorded together from the edge of a GaAs bulk crystal specimen. The combined diffraction pattern shows clearly the existence of the surface resonance waves when the surface resonance diffraction conditions are satisfied, and the close correlation between the intensity enhancement and the surface resonance waves. Strong intensity dependence of the specular reflection and surface resonance spot on the positions of the specular reflection spot along the Kikuchi envelopes, and experimental identification of the surface resonace region, demonstrates the relationship of the resonance diffraction processes to the three-dimensional diffraction conditions. However, the fact that the wavefield for the surface-channelled electrons is confined to a few atomic layers on the surface is indicated by the elongatition of the transmitted diffraction spots. Generation of diffusely scattered electrons by the surface-channelled electrons produces diffuse bands in the diffraction pattern, parallel to the surface.