Yasumasa Tanishiro

Structural analysis of Si(111)‐7×7 by UHV‐transmission electron diffraction and microscopy

Structural analysis of the surface reconstructions investigated by ultrahigh vacuum (UHV) transmission electron microscopy(TEM) and diffraction (TED) is shown. By TED intensity analysis a new structural model of Si(111)‐7×7 is derived. The model basically consists of 12 adatoms arranged locally in the 2×2 structure, nine dimers on the sides of the triangular subunits of the 7×7 unit cell and a stacking fault layer. UHV–HREM of Si (111)‐7×7 surface is commented.

Structure analysis of Si(111)-7×7 reconstructed surface by transmission electron diffraction

Abstract The atomic structure of the 7 × 7 reconstructed Si(111) surface has been analysed by ultra-high vacuum (UHV) transmission electron diffraction (TED). A possible projected structure of the surface is deduced from the intensity distribution in TED patterns of normal electron incidence and from Patterson and Fourier syntheses of the intensities. A new three-dimensional structure model, the DAS model, is proposed: The model consists of 12 adatoms arranged locally in the 2 × 2 structure, a stacking fault layer and a layer with a vacancy at the corner and 9 dimers on the sides of each of the two triangular subcells of the 7 × 7 unit cell. The silicon layers in one subcell are stacked with the normal sequence, CcAaB + adatoms, while those in the other subcell are stacked with a faulted sequence, CcAa/C + adatoms. The model has only 19 dangling bonds, the smallest number among models so far proposed. Previously proposed models are tested quantitatively by the TED intensity. Advantages and limits of the TED analysis are discussed.

Direct observation of the phase transition between the (7 × 7) and (1 × 1) structures of clean (111) silicon surfaces

Abstract The phase transition process of clean (111) silicon surfaces between the (7 × 7) and (1 × 1) structures at about 830°C was directly observed by reflection electron microscopy, which had been briefly reported in a previous short communication (Osakabe et al., Japan. J. Appl. Phys, 19 (1980) L309). Smooth atomic steps, whose shapes change spontaneously and continually in a microscopic scale at high temperature of the (1 × 1) structure, transform into zig-zag steps at low temperature of the (7 × 7) structure, where the changes of the step shape stop. On cooling, domains of the (7 × 7) structure nucleate preferentially on upper terraces along the steps and expand on the terraces to the neighbouring steps. Out of phase boundaries with phase differences of 2π n /7 are seen to be formed. On heating the reversed process takes place. The out of phase boundaries are easy places to transform to the (1 × 1) structure. The observations clearly suggest the phase transition of the first order and the models of the (7 × 7) structure of ordered vacancies or adatoms rather than of static displacements of surface atoms.

Reflection electron microscopy of clean and gold deposited (111) silicon surfaces

Abstract Reflection electron microscopy in ultra high vacuum has been performed during the process of thermal cleaning which converts the dirty (111) silicon surface of the 1 × 1 structure to the clean surface of the 7 × 7 structure, and during the transformations of the 7 × 7 structure to the 5 × 1, √3 × √3 and 6 × 6 structures produced by deposition of gold up to a few monatomic layers. The reflection images showed the microtopographical aspects of the process and of the transformations to be in close correlation with the structural information given by the reflection diffraction patterns observed simultaneously.

STEM imaging of 47-pm-separated atomic columns by a spherical aberration-corrected electron microscope with a 300-kV cold field emission gun

A spherical aberration-corrected electron microscope has been developed recently, which is equipped with a 300-kV cold field emission gun and an objective lens of a small chromatic aberration coefficient. A dumbbell image of 47 pm spacing, corresponding to a pair of atomic columns of germanium aligned along the [114] direction, is resolved in high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) with a 0.4-eV energy spread of the electron beam. The observed image was compared with a simulated image obtained by dynamical calculation.

UHV transmission electron microscopy on the reconstructed surface of (111) gold: I. General features

The reconstructed surface structure of (111) gold was studied by ultra-high vacuum transmission electron microscopy and diffraction and the results are described in this series of papers. In part I the observed fringes spaced about 6.3 nm in the images of (111) gold platelets grown on molybdenite, magnesium oxide and graphite are shown to be due to a reconstructed surface structure of unidirectional shrinkage of the surface layer by about 4% along one of the 〈110〉 directions on the (111) surface. The shrinkage of the surface layer of the same amount from the bulk lattice was found to take place on gold crystals of different lattice parameters, which were formed by pseudomorphic overgrowth of gold on gold containing various amounts of palladium and indium. Evidence is given for the fact that the shrinkage is not uniform and the observed fringes are not simple interference fringes between the shrunk surface layer and the underlying bulk lattice. At high temperatures the structure first transforms gradually and reversibly to an isotropically shrunk one, and finally the reconstruction disappears above about 900°C.

Direct imaging of lithium atoms in LiV2O4 by spherical aberration-corrected electron microscopy

We visualized lithium atom columns in LiV₂O₄ crystals by combining scanning transmission electron microscopy with annular bright field (ABF) imaging using a spherical aberration-corrected electron microscope (R005) viewed from the [110] direction. The incident electron beam was coherent with a convergent angle of 30 mrad (semi-angle), and the detector collected scattered electrons over 20-30 mrad (semi-angle). The ABF image showed dark dots corresponding to lithium, vanadium and oxygen columns.

Surface study by an UHV electron microscope

Abstract An anomalous surface superstructure along 〈110〉 of a clean (111)Au surface and two-dimensional nucleus formation of Pd on an atomically flat (111)Ag surface have been observed by an UHV electron microscope.

On the phase transition between the (7 × 7) and (1 × 1) structures of silicon (111) surface studied by reflection electron microscopy

Abstract The phase transition process between the (7 × 7) and (1 × 1) structures of the clean silicon (111) surface has been studied by means of reflection electron microscopy (REM). In an earlier report, the first-order phase transition was suggested from the observation of preferential nucleation of the new phases along the steps for the 〈1ovbar|10〉 electron incidence. In the present paper movements of the phase boundaries on the terraces during the transition and their terrace-width-dependence were studied. Changes of the step shape and a sudden stop of the shape fluctuation during the transition on cooling are found to be due to the formation of out-of-phase boundaries in the (7 × 7) structure. Image contrast in REM observations of the 〈11ovbar|2〉 incidence suggests the second-order type of phase transition. On the basis of these observations the phase transition process is discussed. Electron energy loss spectroscopic study of the phase transition is also reported.

Surface Structures Observed by High-Resolution UHV Electron Microscopy at Atomic Level

The dynamic processes and structure of an Au cluster growing from a few to several tens of atoms have been revealed using a new high-resolution electron microscope designed specially for in situ surface study at atomic level. The in situ microscopy has also clarified details of the reconstructed structure of Au(100)5?1 and (110)2?1 surfaces formed on particles using profile images. In the reflection microscope mode, the 7?7 reconstructed structure of the Si(111) surface has been seen with 2.3 nm superlattice fringes. In situ high-resolution electron microscope observations in transmission and reflection modes are thus demonstrated to be useful for studies of the dynamic behavior of surfaces.