Kunio Takayanagi

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.

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.

In-situ observations of growth processes of multiply twinned particles

Abstract Growth kinetics of multiply twinned particles (MTPs) of Ag and Au formed by vacuum deposition on MgO and MoS 2 were studied by in-situ electron microscopy. Three types of growth process were observed: (1) a MTP nucleated ab initio and grown to a certain size while maintaining its equilateral shape; (2) a small MTP re-formed after coalescence of a MTP with another MTP or an epitaxial particle; and (3) a MTP formed by a successive twin formation starting from an epitaxial particle. Based on these observations, the growth mechanism is discussed with recognition that a MTP is a stable or metastable entity of a small cluster of atoms. Stress is laid on the importance of stability considerations rather than on the detailed growth model obtained by adding atom-by-atom.

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.

In situ observation of formation of misfit dislocations in pseudomorphic monolayer overgrowth of metals and non-metals

Abstract Epitaxial overgrowth in cases of deposit and substrate combinations of PbSe/PbS, Au/Pd and Pd/Au were observed in situ in an electron microscope. It was found that the growth proceeds with the very characteristic features of monolayer overgrowth as implied by the theory of Frank and Van der Merwe; no indication of nucleation was observed and the formation of misfit dislocations was noticed over the whole area of films at certain critical thicknesses of the overgrowths. Points were noted which distinguish the monolayer overgrowth from the case where misfit dislocations are introduced in nucleated islands, to which Matthews has counted the growth processes in these combinations. The importance of climb motion as well as slip motion was stressed as the mechanism of introduction of misfit dislocations. Some results of electron diffraction and electron microscopy of films prepared in separate evaporation units were also quoted. These were to examine the relation between misfit dislocation density and mean lattice parameter and the effect of degree of vacuum on the growth mode and to indicate that the same growth mode takes place for some other combinations of IV–VI compounds, SnTe/PbSe, PbTe/PbS and PbTe/PbSe.

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.

High resolution surface study by In-situ UHV transmission electron microscopy

Abstract Results of in-situ UHV transmission electron microscopy on surfaces were summarized. The observations of a reconstructed structure on a clean (111)Au surface and of the condensation processes in Ag(111)Ag, Pb/ (111)Ag, Pb/ (111)Au, Au/ (111)Pb and Ag/ (111)Pb systems gave structural information at the same monatomic layer levels as LEED together with additional microtopographic details which can hardly be obtained by other conventional techniques for surface analysis. The possibility of further development of the observation to reveal directly the atomic arrangement in condensed monolayers was investigated by a computer simulation and a model experiment.

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.