Reflection Electron Microscopy Images Research Articles

Strain relaxation induced by interfacial steps of GaAs/In 0.2Ga 0.8As superlattices

A theoretical and experimental study of GaAs/InGaAs strained-layer superlattices with ideal and stepped interfaces is presented. The stable atomic positions in the interface region with a single-layer step are determined from the elastic strain energy. A local strain field is set up by the rearrangement of the atoms near the step edge. Some fringe lines running approximately perpendicular to the epilayers have been observed in the reflection electron microscopy images of GaAs/InGaAs samples grown by molecular beam epitaxy. The observations can be understood by a simple interfacial step strain-field model. Our results confirm that dislocations are not the only mechanism for surface strain relief, another case is roughness.

Incommensurate phase of Sn/Si(100) studied by HR-REM

Coverage dependent surface phases of Sn deposited on the Si(100) surface are observed by reflection electron microscopy (REM). As the coverage is decreased from monolayer coverage at 400°C, two kinds of phases are found. One is the admixture of n-fold (5 < n < 11) superstructures found in a wide coverage range: Spacing of the superlattice fringes seen locally in the high-resolution REM (HR-REM) images varies from place to place, while superlattice spots in RHEED patterns are of an average value p of the superstructure period n. The RHEED patterns show always half order spots, which indicates that the Sn layer has a twofold periodicity parallel to the dimer row of the 2 × 1 structure of the Si(100) surface. The other is an incommensurate phase of a superlattice period p = 6.7 ± δ (0 ⩽ δ ⩽ 0.2), formed at a certain coverage. The phase is well-ordered: the corresponding superlattice fringes observed in the REM image have a uniform spacing. In this phase, no half order spots appear in the RHEED pattern, indicating the disappearance of the twofold periodicity in the incommensurate Sn layer.

Incommensurate phase of Sn/Si(100) studied by HR-REM

Coverage dependent surface phases of Sn deposited on the Si(100) surface are observed by reflection electron microscopy (REM). As the coverage is decreased from monolayer coverage at 400°C, two kinds of phases are found. One is the admixture of n-fold (5 < n < 11) superstructures found in a wide coverage range: Spacing of the superlattice fringes seen locally in the high-resolution REM (HR-REM) images varies from place to place, while superlattice spots in RHEED patterns are of an average value p of the superstructure period n. The RHEED patterns show always half order spots, which indicates that the Sn layer has a twofold periodicity parallel to the dimer row of the 2 × 1 structure of the Si(100) surface. The other is an incommensurate phase of a superlattice period p = 6.7 ± δ (0 ⩽ δ ⩽ 0.2), formed at a certain coverage. The phase is well-ordered: the corresponding superlattice fringes observed in the REM image have a uniform spacing. In this phase, no half order spots appear in the RHEED pattern, indicating the disappearance of the twofold periodicity in the incommensurate Sn layer.

REM study of Au(100) reconstructed surfaces

Reconstruction on Au(100) surfaces has been directly imaged by reflection electron microscopy (REM) with the minimum-exposure technique. Domains containing different superstructures coexist on these surfaces and display certain orientation relations. A ( 27 −1 2 72 ) reconstruction with large unit cells is inferred from the observed superlattice fringes which are highly sensitive to th observation direction in REM images. These large unit cells are considered to be generated by a long-range modulation on the ( 27 −2 2 5 ) reconstruction. A model based on a compressed and twisted “Hex” overlayer on a square substrate is proposed to explain the reconstruction observed on these surfaces.

REM study of Au(100) reconstructed surfaces

Abstract Reconstruction onm Au(100) surfaces has been directly imaged by reflection electron microscopy (REM) with the minimum-exposure technique. Domains containing different superstructures coexist on these surfaces and display certain orientation relations. A ( 27 1 2 72 ) reconstruction with large unit cells is inferred from the observed superlattice fringes which are highle sensitive to the observation direction in REM images. These large unit cells are considered to be generated by a long-range modulation on the ( 27 -2 2 5 reconstruction. A model based on a compressed and twisted ‘Hex’ overlayer on a square substrate is proposed to explain the reconstruction observed on these surfaces.

Study of α-Al 2O 3(11̄02) (R) surfaces with the reflection electron microscopy (REM) technique

The (11̄02) surfaces of α-Al 2O 3 were investigated with the reflection electron microscopy (REM) technique. The morphology of these surfaces is surveyed and compared with surfaces of other orientations. The height of the atomic steps has been determined by direct measurement of the REM images. The dynamical process of surface modification by electron beams is observed.

Characterization of rutile (110) surface structure by REM

Single crystal TiO2 (rutile) (110) surface has been characterized by several experimental techniques. In this paper, we report the investigations of “optically polished” as well as high temperature oxygen annealed rutile (110) surfaces by using reflection electron microscopy (REM) and reflection high energy electron diffraction (RHEED) techniques.The crystal was purchased, “optically polished” as-received, from Commercial Crystal Laboratories, Inc.. The details in specimen cutting and surface cleaning procedures have been reported previously. The samples were annealed in pure oxygen at 1425°C for 36 h. The experimental observations were carried out in a Philips 400T microscope operated at 120 kV. The REM images were obtained by selecting the specular reflection spots satisfying surface resonance conditions.Figure 1 is a REM image of as-received rutile (110) surface. The corresponding RHEED pattern is shown in the inset. The azimuthal angle of the incident beam was at 3.9° away from [001] zone axis and the image was formed by choosing (440) specular reflection spot under surface resonance condition.

Stepped surfaces of sapphire (alpha-Al2O3) with low Miller indices.

Oxygen-annealed surfaces of sapphire with low Miller indices ((0001), [1010], [1120], [1011]) have been studied in both transmission electron microscopy (TEM) and reflection electron microscopy (REM) configurations. The significance of REM diffraction conditions for the determination of the nature of the step heights is discussed. The relationship between the TEM and REM images is explained. The structural features are those that might be expected from considerations of the atom arrangement in the low Miller index planes. The structural features on the surfaces varied with respect to annealing temperature and surface condition. Thermally stable structures that might appear from consideration of the equilibrium-annealing temperature are proposed.

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.

The Surface Structure of Oxides Studied with Reflected High Energy Electrons

AbstractReflected high energy electrons have been employed for studying surfaces of single crystal oxides in real and reciprocal space. The techniques used include reflection high energy electron diffraction (RHEED), reflection electron microscopy (REM), scanning reflection electron microscopy (SREM), and reflection electron energy loss spectroscopy (REELS). These methods have provided much information on surface morphology as well as the atomic and electronic structure of MgO, ∝-Al2O3, ∝ -Fe2O3, SiO2, TiO2(rutile), and ZnO single crystals. Analyses for chemical composition and electronic states are possible using SREM and REELS. Surfaces terminating at different atomic layers of large-unit-cell materials have been determined in REM images and also in REELS data obtained under REM and SREM. Surface modification due to bombardment with the incident electron beam is pronounced on some oxide surfaces. These processes have been investigated with the REM imaging and REELS techniques.

Atomic step structures on cleaved α-alumina (012) surfaces

Cleaved α-alumina (012) surfaces are imaged with reflection electron microscopy (REM). A RHEED-THEED diffraction method is demonstrated for on-line determination of surface orientation. The observed defect structures on the (012) surface include vacancy-type of terraces islands, kinks and ledges of a few atoms high. Steps of height possibly less than about 0.8 Å may be seen in REM images. The formation of the surface steps of different heights is described based on the atomic structure of the bulk crystal. The electron beam damage to the surface domains initially terminated with oxygen and aluminium layers, respectively, is interpreted based on the electrostatic desorption model after Auger decay. This effect is responsible for the bright-dark band-type contrast observed in REM images, and can be employed to identify the surface termination. It has been found that the REM image sensitivity is significantly improved with a field emission gun (FEG).

Alumina-induced reconstruction on annealed (001) surfaces of rutile

The rutile (TiO 2) (001) surfaces are found reconstructed due to the addition of alumina during high temperature (1500°C) annealing in an oxygen atmosphere. Reflection electron microscopy images showed that the configuration of the surface steps changed remarkably upon the structural change from unreconstructed to reconstructed surfaces. The diffusion processes of alumina on rutile (001) surfaces are also discussed.

In situ REM imaging of surface processes on ceramic bulk crystals from 300 to 1670 K in a conventional TEM

Studying the behavior of surfaces at high temperatures is of great importance for understanding the properties of ceramics and associated surface-gas reactions. Atomic processes occurring on bulk crystal surfaces at high temperatures can be recorded by reflection electron microscopy (REM) in a conventional transmission electron microscope (TEM) with relatively high resolution, because REM is especially sensitive to atomic-height steps.Improved REM image resolution with a FEG: Cleaved surfaces of a-alumina (012) exhibit atomic flatness with steps of height about 5 Å, determined by reference to a screw (or near screw) dislocation with a presumed Burgers vector of b = (1/3)&lt;012&gt; (see Fig. 1). Steps of heights less than about 0.8 Å can be clearly resolved only with a field emission gun (FEG) (Fig. 2). The small steps are formed by the surface oscillating between the closely packed O and Al stacking layers. The bands of dark contrast (Fig. 2b) are the result of beam radiation damage to surface areas initially terminated with O ions.

High-resolution REM of the reconstruction of sapphire surfaces

Sapphire (α-Al2O3) is widely used in industry and research as a high temperature insulator, a substrate for growing thin films and carrier materials in catalysis. It is important to understand the surface properties of the substrate materials for thin film growth and for metal support interactions. Recently RHEED (reflection high energy electron diffraction) and REM (reflection electron microscopy) techniques have been used to characterize surface properties of sapphire annealed in air or in oxygen. The presentpaper reports some results of high resolution REM imaging of lattice fringes of reconstructed sapphire surface.The sample preparation procedure was reported elsewhere. The results presented here were obtained from a sample annealed in pure oxygen at 1650° C for 72 h. The high resolution REM observations were done in a Philips 400T transmission electron microscope operated at 120 kV with a vacuum pressure about 10-7 Torr.

Imaging friction tracks on diamond surfaces using reflection electron microscopy (REM)

Abstract Reflection electron microscopy (REM) has been applied to image the surface structures of polished natural diamond (001) faces before and after frictional sliding. Friction tracks produced by a diamond stylus sliding at a nominal contact pressure of a few GPa can be clearly seen in REM images of the surface. The contact between the stylus and the surface is shown to be non-uniform. At low nominal contact pressures, ∼3·5 GPa, the surface shows relative dark contrast at the contact areas, which it is suggested is due to structural modification of the top atomic layers. At nominal contact pressures of ∼13 GPa, the surface is damaged on a scale less than the height of polishing lines. At high nominal contact pressures, ∼21 GPa, ‘grooves’ are produced on the surface. This result, and the absence of surface cracking, suggested that plastic deformation is involved in the process of frictional sliding on diamond at high contact pressures.

Monoatomic step observation on Si(111) surfaces by force microscopy in air

The structure of a vicinal Si(111) stepped surface is analyzed by force microscopy in air to reveal a fine structure in a step bunching area, and a monoatomic step in a terrace region. Step heights of one to three monoatomic layers were also observed on a debunched Si(111) surface. These steps have low crystallographic indices [112̄], [101̄], [01̄1], [213̄], and [ 1̄2̄3]. The force microscope images were in good agreement with scanning electron microscope and reflection electron microscope images observed in ultrahigh vacuum just after sample annealing by resistive heating.

Real-time μ-RHEED observations of GaAs surfaces during growth with alternating source supply

The microscopic surface features of GaAs and AlAs during growth with an alternating source supply were observed by scanning microprobe reflection high-energy electron diffraction in real time. Scanning reflection electron microscope images were obtained in the TV scan mode (50 frames per second). The creation of Ga droplets was observed when Ga atoms over the critical amount depending on the substrate temperature were supplied. They disappeared after a sufficient amount of As was supplied. When 5 monolayers of Ga atoms were supplied per cycle, small islands with bright contrast appeared due to three-dimensional nucleation after As was supplied.

Imaging friction tracks at diamond surfaces using reflection electron microscopy

Reflection electron microscopy (REM) is applied to image the structure of polished natural diamond (001) surfaces (of 5 x 4 mm size) after friction experiments under a pressure below the critical value. Friction tracks marked by a diamond needle after a single pass movement under a pressure of 13 GPa can be seen in REM images and show non-uniform contrast. The surface shows relatively dark image contrast at the light contacted area, which is possibly due to the structural modification at the top atomic layer. The high local contacting pressure pushes part of the needle into the surface which causes fracture, resulting in the formation of grooves at the surface. It is possible to have plastic deformation in this process, but no evidence has been found for the presence of cracking. The observations support the adhesion frictional mechanism rather than the micro-cleavage model.

Experiments on REM and SREM using a Tem/Stem Microscope with a Configurable Angle-Resolving Detector

A Philips EM 420 electron microscope equipped with a field emission gun and an external STEM unit was used to compare images of single crystal surfaces taken by conventional reflection electron microscopy (REM) and scanning reflection electron microscopy (SREM). In addition an angle-resolving detector system developed by Daberkow and Herrmann was used to record SREM images with the detector shape adjusted to different details of the convergent beam reflection high energy electron diffraction (CBRHEED) pattern.Platinum single crystal spheres with smooth facets, prepared by melting a thin Pt wire in an oxyhydrogen flame, served as objects. Fig. 1 gives a conventional REM image of a (111)Pt single crystal surface, while Fig. 2 shows a SREM record of the same area. Both images were taken with the (555) reflection near the azimuth. A comparison shows that the contrast effects of atomic steps are similar for both techniques, although the depth of focus of the SREM image is reduced as a result of the large illuminating aperture. But differences are observed at the lengthened images of small depressions and protrusions formed by atomic steps, which give a symmetrical contrast profile in the REM image, while an asymmetric black-white contrast is observed in the SREM micrograph. Furthermore the irregular structures which may be seen in the middle of Fig. 2 are not visible in the REM image, although it was taken after the SREM record.

Characterization of Surface Resonance Conditions for Surface Imaging

In order to increase intensity and contrast in the image of a surface, the surface resonance conditions have been widely used to enhance the Bragg reflection for image formation in REM (Reflection Electron Microscopy). However, detailed studies of how the resonance conditions relate to the imaging contrast have not been reported. This paper will concentrate on the general properties of the different resonance conditions, as well as the resulting image contrast.Figure 1 shows a series of RHEED (Reflection High Energy Electron Diffraction) patterns and REM images from the same region of a Pt(l11) surface with the incident electron beam in a direction close to the [112] zone axis at 200 KeV, with a glancing incident angle of about 24 mrad which corresponds to the (555) Bragg reflection condition inside the crystal. For the purpose of convenience in discussion, the four different diffraction conditions shown in figures l(al)-(dl) have been named as D1-D4. With Dl, the specular reflected spot falls in an intersection of a parallel Kikuchi line with a parabola; with D2, the specular reflected spot coincides with an intersection of the Kikuchi lines running parallel to and inclined to the crystal surface; with D3, the specular reflected spot crosses only the parallel Kikuchi line; and with D4, the specular reflected spot intersects only with a parabola. It was found that the diffraction conditions Dl and D2 can not be considered as identical, although the specular reflected spots for both cases are commonly regarded as (555) Bragg reflection in the RHEED pattern. Detailed inspection indicates that for Dl, both the Bragg reflection and the electron surface channelling wave are excited, and for D2, the excitement of simultaneous Bragg reflection occurs closely associated with the properties of three-dimensional dynamical diffraction for a bulk crystal.