Katsumichi Yagi

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.

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.

Step bunching, step wandering and faceting: self-organization at Si surfaces

Step bunching, in-phase step wandering and faceting are new morphological evolutions from smooth vicinal surfaces. They are formed by giving changes of externally controlled parameters on surfaces. Two cases on Si surfaces are presented in detail. One is caused by unidirectional drift forces on adatoms on the surfaces, i.e., surface electromigration due to direct current specimen heating. Newly formed step instabilities are due to a change of a kinetic parameter on the surface. The other case is caused by adsorption of foreign metal atoms on the Si surfaces. In this case step instabilities are due to changes of thermodynamic parameters on the surface. Step bunching, in-phase step wandering and faceting processes in two cases are interesting dynamics of self-organization at surfaces and can be useful for microfabrication of surfaces. Recent results of these dynamic processes on vicinal surfaces of Si(1 1 1) and (0 0 1) surfaces and high index Si surfaces studied by real space and reciprocal space observation methods mainly done by our group are reviewed.

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.

REM Observation on Conversion between Single-Domain Surfaces of Si(001) 2×1 and 1×2 Induced by Specimen Heating Current

A current-induced conversion between Si(001)2×1 and 1×2 surfaces is observed in situ by ultrahigh-vacuum reflection electron microscopy. When the specimen is annealed with a certain current above 700°C, the surface is composed of two domains divided by single-height steps, but one type of domain is much wider than the other. When the current direction is reversed, narrow terraces widen their areas as a result of movements of higher side steps to the higher side and slight movements of lower side steps to the lower side. The wider terraces reduce their width. The process is quicker at higher temperature. It was found that the type of major domain is determined on the basis of whether the current is in the step-up or the step-down direction.

Roles of lattice fitting in epitaxy

Abstract The epitaxial orientations of three series of materials-alkali halides, chalcogenide compounds and face-centered cubic metals-and of indium (face-centred tetragonal structure) and iron (body-centred cubic structure) on a (001) MgO substrate were examined by in situ electron microscopy and diffraction. The lattice parameter ratio ϱ = a d / a s (d, deposit;s, substrate) ranges from 0.95 for LiF to 1.68 for KI, from 1.41 for PbS to 1.53 for PbTe and from 0.84 for nickel to 1.18 for lead. A systematic dependence of the epitaxial orientation on ϱ is found: alkali halides and chalcogenide compounds with ϱ around 1 or 1.5 prefer the (001) parallel orientation with (001) d (001) s , [100] d [100] s , but those with a value of ϱ around √2 prefer the (001) orientation with (001) d (001) s , [110] d [100] s . Metals with ϱ near 1 also prefer the (001) parallel orientation, but copper and nickel with ϱ around 0.85 prefer the (1 1 0) orientation with (1 1 0) d (001) s , [11 2 ] d [100] s , and gold, indium and lead with values of ϱ of 0.97, 1.17 (1.09) and 1.18, respectively, favour the (111) orientation with (111) d (001) s , [1 1 0] d [1 1 0] s . The epitaxial relation is discussed in terms of the fit of the lattice spacings of the deposit and substrate crystals.

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.