High-resolution Reflection Electron Microscopy Research Articles

High resolution REM studies of Si([formula omitted]) surfaces and their roughening phase transition

High resolution reflection electron microscopy has been applied to studies of a high index Si(5512) surface at a sub-unit cell resolution. Lattice fringes with different widths were found to be directly corresponding to the subunits of the (5512) surface. Defects on the surface were resolved as nano-facets with close orientation to (5512). (131331) and (7717) facets were identified based on the subunit arrangement. An incommensurate structure (q=a*bk/2.43) was revealed at a temperature just below the roughening transition. Formation of an incommensurate structure as well as defects was discussed in terms of surface steps.

Au adsorption on Si([formula omitted]) surfaces and facet formation studied by high resolution in situ REM

Adsorption processes of Au on a high index Si(5512) surface were studied in situ by high resolution reflection electron microscopy with subunit cell resolution. Growth of new phases is quite anisotropic and rapid along the [11̄0] direction reflecting quite anisotropic substrate surface structure. At around 715°C, successive changes from (5512) to two-fold (337), (5511) and (225) facet were noted as an increase of Au deposit. (7717) surface units are locally formed before formation of the two-fold (337) surfaces. At higher temperature 780°C (above roughening temperature 750°C of (5511)-Au surface), a disordered surface is formed after formation of the two-fold (337) facet. The disordered surface is a higher temperature phase of the (5511) surface. Above changes are discussed in terms of characteristics of these high index surface structures.

Simultaneous observation of scanning tunneling microscopy and reflection electron microscopy image of the Si(111)7×7 surface

Atomic structures of scanning tunneling microscopy (STM) tips were characterized by reflection electron microscopy (REM). The Si(111)7×7 surface was observed simultaneously by REM and STM in an ultrahigh vacuum electron microscope. In high-resolution REM images of the specular beam reflection, a tungsten tip apex was observed with its mirror image. The distance between the true and the mirror images was used to estimate the tip–surface distance. As the tip scanned over the Si(111) surface in STM experiments, the tip apex was scraped to give a plateau of a (110) atomic surface. An atomic-resolution STM image of the Si(111)7×7 surface was obtained when the STM tip apex had a single atom on the (110) terrace.

Dynamical calculation of high-resolution reflection electron microscopy lattice fringes from the Si(111)7 × 7 surface

High-resolution reflection electron microscopy (HR-REM) lattice fringes for the flat Si(111)7 × 7 surface are simulated using our earlier reported theory of REM image formation. This simulation procedure employs a 2D Bloch wave formulation of dynamical (multiple scattering) elastic reflection high-energy electron diffraction (RHEED) theory combined with conventional electron microscopy (Abbé) imaging theory. One of the purposes of this study is to provide the first critical comparison of HR-REM images simulated using this procedure with experimental images for the Si(111)7 × 7 surface. It is found that it is possible to reproduce in the simulations all of the fringe periodicities observed in real HR-REM images from this surface. The incorporation into the simulation procedure of the effects of primary electron beam divergence and chromatic aberration is described in detail. The primary beam divergence is essential to reproduce in simulated images the correct defocusing behaviour. When these details are incorporated, the simulation procedure gives good agreement with experiment. This work shows that quantitative HR-REM image simulation constitutes a valuable interpretative tool to be used in support of experimental HR-REM. Published by Elsevier Science B.V.

Simulation of reflection electron microscopy images: application to high-resolution imaging of the Si(001)2 × 1 surface

A theory of image-formation for the surface-imaging technique of reflection electron microscopy (REM) is presented. The theory is described in detail for the case in which scattering from the crystal is described by the 2D Bloch wave formulation of the dynamical theory of elastic reflection high-energy electron diffraction (RHEED) from periodic systems, but the procedure presented is of much more general applicability. A detailed account is given of how the theory explains the general characteristic features of REM images. Examples of calculated high-resolution REM images of the flat Si(001)2 × 1 surface and of metal (Ag) adatoms and steps on the Si(001)2 × 1 surface are then presented and the effects of varying the imaging conditions are discussed.

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.

Freezing of the 2 × 1 structure at commensurate Ag(100)Si(100) interface

The growth process of Ag on the Si(100)2 × 1 surface is observed in situ by high-resolution reflection electron microscopy (HR-REM). A silver layer covers the Si(100) surface entirely at 1.5 ML (1 ML = 6.78 × 10 14 atoms/cm 2). Above 1.5 ML deposition, two-dimensional Ag islands are grown on the Ag layer of 1.5 ML at room temperature, while no islands are nucleated and grown on the layer at 480°C. Moiré fringes appeared at the Ag islands seen in the HR-REM images and RHEED patterns show that the Ag islands after 3 ML deposition form a film with the (100) orientation and their (011) lattice planes are strained by 0.7% to be commensurate with the (011) Si lattice. The 2 × 1 structures of the Si(100) substrate are frozen at the Ag-Si interface, since the bright and dark contrast of the 2 × 1 domains of the clean surface is maintained.

Dynamic observation of oxygen-induced step movement on the Si (111)7 × 7 surface by high-resolution reflection electron microscopy

Step movement on the Si(111)7 × 7 surface at 750°C was observed in situ by high-resolution reflection electron microscopy which resolved superlattice fringes with a spacing of 2.3 nm. Oxygen gas was used to desorb the Si atoms from the surface, so that the steps moved largely even at 750°C. A step moved by units of the 7 × 7 superlattice, and the step edge always terminated at the same position of the 7 × 7 unit cell, that is, the dimer chain of the dimer adatom stacking-fault model of the Si(111)7 × 7 structure.

Surfactant epitaxy of Si on Si(111) surface mediated by a Sn layer I. Reflection electron microscope observation of the growth with and without a Sn layer mediate the step flow

Surfactant epitaxy of Si on Si(111)7 × 7 surface is studied by high-resolution reflection electron microscopy with and without a Sn layer which promotes the step flow growth. The Si adatoms deposited on the Sn layer forming the √3 × √3 structure are found to be incorporated with the bulk Si layer, so that the surface is covered always by the Sn layer and the growth proceeds by the step flow. The critical step distance above which islands nucleate on the terraces is determined for the growth of Si/Si and Si/Sn/Si. The critical step distance is found to change with the substrate temperature, T, in an approximate form, √ A exp(- E/2 kT), which increases much with the Sn layer.

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.

Dynamic observation of gold adsorption on Si(111)7×7 surface by high-resolution reflection electron microscopy

Dynamic processes of gold adsorption on Si(111)7×7 surface are observed in-situ by high-resolution reflection electron microscopy (HR-REM); the 5-fold superlattice fringes of the 5×2 structure of 1.7 nm spacing are resolved in addition to the superlattice fringes of the 7×7 structure of 2.3 nm spacing. On a substrate held at 650°C, a domain of the 5×2 structure is nucleated at the higher side of a step on the 7×7 surface and grows along the 2-fold, in preference to the 5-fold, directions to expand towards the lower and higher sides of the step. After the 5×2 structure transforms into the √3×√3 structure by further deposition, the steps change their configurations and move in an oscillatory manner. The implication for models of gold adsorption, and the influence of defects and superlattice registry on the kinetics, are discussed.

Reflection electron microscope study of the initial stages of oxidation of Si(111)-7 × 7 surfaces

A gas inlet device was constructed for an ulra-high-vacuum electron microscope and initial stages of oxidation of Si(111)-7 × 7 surfaces at various temperatures were observed in-situ by reflection electron microscopy (REM). Below 500°C the REM images did not show appreciable changes after an introduction of molecular oxygen. Above 500°C hollows with unit depth 0.31 nm were formed and grew in the central part of the terraces between the successive atomic surface steps. At the same time the steps moved so as to shrink the terraces. As oxygen pressure increased, oxide covered the surface and the surface structure transformed to the 1 × 1 structure. The oxide sublimed on heating above 750°C. During the oxygen exposure above 750°C rapid motions of steps and large hollow formations were observed throughout the oxygen exposure process. The growth kinetics of the hollows was analyzed. High-resolution REM observations showing the 7 × 7 lattices during oxidation were also carried out.