Toshihiro Ichikawa

Structural study of Sn-induced superstructures on Ge(111) surfaces by RHEED

Abstract When less than 0.3 monolayer of tin was deposited onto a clean Ge(111) surface at room temperature and then heated to high temperatures, the 2 × 8 superstructure characteristic of the clean Ge(111) surface changed to a 1 × 1 structure with twofold-splitting diffuse scattering. As the mean thickness of Sn exceeded 0.3 monolayer, (√3 × √3)R30° and 7 × 7 superstructures were formed on the surface. The periodicity of the former structure is familiar to many metal-induced superstructures on Ge(111) and Si(111) surfaces, while the latter is very interesting as having the same periodicity as a well known 7 × 7 superstructure of clean Si(111) surfaces. By increasing the amount of the deposit beyond 1 2 monolayer in mean thickness, a 5 × 5 superstructure appeared on the surface. This structure also is to be noted, because a surface structure with the same periodicity is observed on a homoepitaxial and a heteroepitaxial Si(111) surface. Structural models for the Ge(111)(7 × 7)-Sn and Ge(111)(5 × 5)-Sn structures are proposed.

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.

Structural study of ultrathin Sn layers deposited onto Ge(111) and Si(111) surfaces by RHEED

Abstract In order to investigate the possibility of the appearance of substrate-stabilized gray Sn, a small amount of Sn was deposited onto cleaned and annealed Ge(111) (2 × 8) and Si(111) (7 × 7) surfaces in an ultrahigh vacuum and then the structure of the Sn films formed was in-situ investigated by RHEED. The RHEED observations showed that ultrathin overlayers of gray Sn were formed, though not well grown, at the initial stage of the deposition onto Ge(111) substrates at room temperature, but on further deposition they became a disordered structure and did not grow into thick films of gray Sn. Subsequent high-temperature annealing of the Sn-covered Ge(111) and Si(111) surfaces led to the appearance of five new surface structures such as (3 × 2 3 ) and ( 91 × 3 ) structures at the Ge(111)Sn surfaces and ( 133 × 4 3 ) , (3 7 × 3 7 ) R (30 ± 10.9°) and (2 91 × 2 91 ( R (30 ± 3.0°) structures at the Si(111)Sn surfaces.

Double diffraction spots in RHEED patterns from clean Ge(111) and Si(001) surfaces

Abstract In RHEED patterns from clean Ge(111) and Si(001) surfaces, extra diffraction spots have been observed with superlattice reflection spots due to Ge(111) 2 × 8 and Si(001) 2 × 1 surface structures. The extra spots have not been found out in many previous LEED and RHEED patterns of clean Ge(111) and Si(001) surfaces. When the Ge(111) and Si(001) samples were rotated about an axis normal to the surfaces so as to vary the incident direction of the primary electron beam, the intensity of the extra spots showed a remarkable dependence upon the incident direction and they became invisible in some incident directions, in spite of the experimental condition that an Ewald sphere intersected reciprocal lattice rods of the extra spots. In this study, the extra spots are understood as forbidden reflection spots resulting from double diffraction of superlattice reflections of the surface structures, and the remarkable dependence of their intensity upon the incident direction is explained in terms of excitation of the surface wave of the superlattice reflections. These results suggest that the intensity of diffraction spots in RHEED patterns may be greatly influenced by the surface wave excitation of fundamental and superlattice reflections.

Chemical Analysis of Surface by Fluorescent X-Ray Spectroscopy Using RHEED-SSD Method

A new RHEED apparatus combined with a Si(Li) solid-state detector has been constructed, and spectroscopy of fluorescent X-rays emitted during RHEED observation has been carried out using the detector. Preliminary measurements have revealed the usefulness of this apparatus both for chemical analysis of surfaces and for investigating the behaviour of incident electrons in surface layers. When Ag thin films evaporated on a clean Si(111) surface were studied, the smallest detectable amount was about 0.02 A in mean thickness. The penetration depth of the primary electron beam in the Ag film at an accelerating voltage of 20 kV was estimated to be 33.0 A from analysis of the intensity of AgLα and AgLβ lines with increasing thickness of the Ag films.

Missing spots in rheed patterns from the Si(111)(6 X 1)-Ag surface structure

Abstract Some of the 1 6 -order superlattice spots in RHEED patterns from the Si(111)(6 × 1)-Ag surface structure exhibit a peculiar feature that they are visible when the primary electron beam is incident in the 〈112〉 azimuth, but are missing in the 〈110〉 incidence azimuth. This feature can be interpreted by double diffraction enhanced through excitation of surface waves. The interpretation reveals that the 6 × 1 structure has twofold glide planes. Two structural models for the Si(111)(6 × 1)-Ag surface structure are proposed, and a general analysis for the intensity of diffraction spots in RHEED is given.

RHEED Study of Surface Reconstruction at Clean Ge(110) Surface

Surface reconstruction at a clean Ge(110) surface is reported to show uncommon features such as the reappearance of the low-temperature phase at higher temperature and temperature-dependent faceting. In order to investigate the features in detail, in-situ RHEED observations of cleaned and annealed Ge(110) surfaces were undertaken. The reap-pearance phenomenon was confirmed in the present RHEED observation, but no faceting occurred.

ESCA study on the mechanism of adherence of metal to silica glass

Indium oxide films formed on the surface of silica glass samples by selective oxidation have been shown to greatly improve the adherence of gold-indium alloy to the silica glass. In order to clarify the role that the oxide films play in the reaction, thin indium films have been evaporated onto silica glass and heated at temperatures of between 973 and 1473 K, both in air and in hydrogen gas. Electron spectroscopy for chemical analysis (ESCA) measurements have then been made to investigate the chemical environments around oxygen and indium atoms at the reacting interface between the oxide and glass. Measured 01s and In3d5/2 spectra reveal the formation of non-bridging oxygen atoms at the interface, in addition to the original bridging oxygen atoms in silica glass. Introduction of the non-bridging oxygen atoms and indium ions into the silica glass is concluded to be an essential factor in promoting good adherence between the gold alloy and silica glass.

Study of Metastable Structure of Pb-Sb Films Condensed at Low Temperature

The structure of Pb–Sb films condensed onto a substrate kept at the liquid helium or nitrogen temperature by vacuum evaporation has been investigated by means of electron diffraction and electric resistance measurements. Diffraction patterns of the films with concentrations between 17 and 100 at. % Sb show diffuse halos. The radial distribution analysis has been carried out for these films in order to decide whether they are in a true amorphous state or consist of very fine crystallites. The results lead to the conclusion that the state of atomic aggregate of these films is certainly amorphous. The crystallization temperature of the films decreases as Pb concentration increases. The behavior of electric resistance in an amorphous state differs remarkably from that in a crystalline state after the crystallization.

RHEED study of In-induced superstructures on Ge(111) surfaces

Abstract When submonolayer and monolayer amounts of indium were deposited onto clean Ge(111) surfaces at room temperature and then heated, (13 × 2√3), (12 × 2√3), (11 × 2√3), (10 × 2√3), (4√3 × 4√3) R30°-related, (√31 × √31) R (±9°), (√61 × √61) R (30 ± 4°) and (4.3 × 4.3) structures appeared on the surfaces at fixed In coverages and at fixed surface temperatures. General intensity features of superlattice reflections are derived from intensity estimations by eye of superlattice spots in their RHEED patterns, and some structural characteristics of the superstructures are clarified from the analysis of the general intensity features. The former four superstructures are long-period (2 × 2)-related antiphase structures whose period changes, depending on the coverage. The wavevector characterizing the (13 × 2√3) structure, which appears at the smallest coverage, almost coincides with those of structural fluctuation emerging at the clean Ge(111) (1 × 1) surface around 350°C. The coincidence suggests that the longperiod (2 × 2)-related antiphase structures have a close relationship to the structural fluctuation and, besides, to the (2 × 8) structure in their origin.