Shozo Ino

Stability of Multiply-Twinned Particles

A theory is developed which allows one to discuss the stability of multiply-twinned particles and to calculate critical sizes for stable and quasi-stable states. An icosahedral particle is essentially stable for r ≦ r i w * , quasi-stable for r i w * r i t 0 where r is an edge length of the particle, while a decahedral particle is not essentially stable but quasi-stable for r ≦ r d t 0 and unstable for r > r d t 0 . The critical sizes r i w * , r i t 0 and r d t 0 are formulated as functions of the specific surface energy γ h k l , the twin boundary energy γ t , the elastic strain energy W and the adhesive energy γ a to the substrate. Calculated critical diameters 2 r i w * ,2 r i t 0 and 2 r d t 0 for the particles grown in free space for several elements range between 15.5A and 106.8A, between 109.8A and 486.1A and between 725A and 3961A, respectively. These values are in good agreement with experimental results.

Some New Techniques in Reflection High Energy Electron Diffraction (RHEED) Application to Surface Structure Studies

A new apparatus for reflection high energy electron diffraction (RHEED) provided with an electron gun with a sharply focused electron beam and operated under ultrahigh vacuum condition has been constructed. It has a specially devised fluorescent screen which enables us to take new type RHEED patterns corresponding to sections of the reciprocal lattice rods and similar to LEED patterns. The oscillating- and the rotating-crystal methods have been applied to observe the intensity distributions along the reciprocal lattice rods. On the basis of experimental results obtained in the studies of the Si(111)7×7, Si(111)√19×√19 Ni and Si(111)5×1 Au structures, it is concluded that the RHEED method including the new technique is superior in principle for the surface structure research to LEED almost in all respects. It is expected that RHEED will find wide applications in the field of surface research taking the place of LEED.

Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihalide Crystals

Succeeding to previous studies (Ino, 1966; Ogawa et al. , 1966), the orientations and the structures of thin gold films formed in ultrahigh vacuum by evaporation on to NaCl and KCl have been studied at earlier stages of film formation by electron diffraction and electron microscopy, especially by the dark field technique. The present paper describes some results obtained after the previous reports. Some particles which show rhombic shape in the bright field image and various contrasts in the dark field images have newly been observed. In order to explain such contrasts, the “multiply twinned particle model with a nucleus of (001) orientation” is proposed. Abnormal diffraction spots and image contrasts observed from thin gold films have satisfactorily been explained by using this model in some cases and by using the multiply twinned particle models with a nucleus of (111) orientation in other cases.

Surface Structures of Ag on Si (111) Surface Investigated by RHEED

The Si (111) surface structures induced by Ag evaporation have been investigated by RHEED. A phase diagram of the surface structures in relation of the deposition thickness and the annealing temperature has been established. When the specimen temperature was elevated after Ag evaporation of 1 A in mean thickness, the √3×√3 (R 30°) structure appears in the range of temperature between 290 and 660°C, and the 3×1 structure appears between 600 and 700°C. After the specimen showing the 3×1 structure was cooled below 220°C, the 6×1 structure appears. The transformation of the 6×1 structure is perfectly reversible at 220°C. The critical temperatures at which these surface structures appear or disappear depend slightly on the deposition thickness (0–3.0 A). If the deposition thickness is less then 0.8 A, the 7×7 structure also appears in addition to these surface structure.

Study of the Si(111) 7 × 7 surface structure by alkali- metal adsorption

Abstract When alkali-metals such as Li, Na, K, Rb and Cs were adsorbed on clean Si(111) 7 × 7 surface at room temperature, the intensity distribution of the original 7 × 7 RHEED pattern changes gradually with the increase of alkali-metal adsorption, and at last a new superstructure with 7 × 7 periodicity, here named δ-7 × 7 structure, has been observed. This change at room temperature can be explained if the 7 × 7 structure is mostly made of displacement. The structure model estimated from the intensity distribution of the δ-7 × 7 pattern is the one that has one vacancy at the corners of the 7 × 7 unit mesh and relaxed surrounding atoms. The change of the 7 × 7 structure by alkali-metal adsorption to this model is naturally understood with our new model (1984). For all alkali metals, by adsorption at high temperature 3 × 1 superstructure has also been observed for the first time.

Chemical Analysis of Surfaces by Total-Reflection-Angle X-Ray Spectroscopy in RHEED Experiments (RHEED-TRAXS)

A new method for chemical analysis of surfaces by total-reflection-angle X-ray spectroscopy in RHEED experiments (RHEED-TRAXS) has been developed. When the X-ray take-off angle is set to be the critical angle for total reflection of the characteristic X-ray emitted from the deposited atoms on surfaces, the detection efficiency for the deposit becomes drastically higher owing to the refraction effect of the X-ray. This enhancement of surface sensitivity is demonstrated with Ag on Si(111). The smallest detectable amount of Ag is about 0.01 monolayer or less. This sensitivity is comparable to or higher than that of AES.

A study of adsorption and desorption processes of Ag on Si(111) surface by means of RHEED-TRAXS

The Si(111)-Ag system was investigated by a new experimental technique for chemical analysis of solid surfaces, called total reflection angle X-ray spectroscopy in RHEED experiments (RHEED-TRAXS). By applying this method to the isothermal condensation and desorption experiments of Ag atoms, saturation coverages of surface structures and desorption energies of Ag atoms could be measured. The sticking probability and the desorption rate of Ag atoms can be directly measured in principle by the RHEED-TRAXS experiment, which is generally difficult by AES. By comparing the experimental results with those of AES, the features of the X-ray spectroscopy are discussed

Epitaxial Growth of Face-Centred Cubic Metals on Alkalihalide Crystals Cleaved in Ultrahigh Vacuum

Succeeding to a previous study (Ino, 1966), the orientations of nuclei of face-centred cubic metal crystals evaporated onto alkalihalide crystals cleaved in ultrahigh vacuum have been studied mainly by electron diffraction using the following combinations of deposit and substrate: Au on KCl, Ag on NaCl and KCl, Ni on NaCl and KCl, Pd on NaCl and KCl and Al on NaCl and KCl. According to the previous and present studies, only two orientations of nuclei, (111) and (001), exist in the epitaxial growth of these metals, the preference of one orientation to the other depending upon the substrate temperature. the pressure in the evaporation chamber, the kind of substrate crystal and the nature of evaporated metal atom. The mechanism of the orientation of nuclei on alkalihalide crystals is investigated, taking into consideration the interaction between a metal atom and substrate ions which controls the staying period of the metal atom, and hence, the size of a stable nucleus.

Improvement of the spherical mirror analyzer

With the use of some obstacle rings and a set of retarding grids, the resolution of the prototype spherical mirror analyzer, which has been developed in a previous paper [H. Daimon, Rev. Sci. Instrum. 59, 545 (1988)], was improved. The uniformity of the resolution with respect to the emission angle was also achieved. Some examples of electron energy‐loss spectra and XPS spectrum were presented.

Rheed study on the Ge/Si(111) and Si/Ge(111) systems: Reaction of Ge with the Si(111)(7 × 7) surface

Abstract According to the structural viewpoint that surface strain plays a substantial role in (7 × 7) reconstruction at the clean Si(111) surface, it is conjectured that a superstructure with a periodicity shorter than (7 × 7) may be induced at Ge-dissolving Si(111) surfaces, because a large strain is accommodated in the surface region by the dissolution of Ge atoms. The present RHEED observations have revealed that a (5 × 5) structure appears at the Ge-dissolving Si(111) surfaces, which are prepared by the deposition of more than 1.5 monolayers of Ge onto the surfaces and by subsequent annealing at high temperatures.