The rocking curve of reflection high-energy electron diffraction (RHEED) is widely used to determine surface structures. We have built up a new apparatus of RHEED system with two pairs of magnetic coils to measure accurately the rocking curves in short time. By using this system, we succeeded in the studies of effective surface Debye temperature, dynamic structure change during epitaxial growth of Si/Si(111) and surface structure at high temperature near the melting point, and discovered an additional surface phase on the Si(111) surface. In this review, I report the new apparatus and some results of surface phase transition of the Si(111) surface at high temperatures.
Rocking-curve analysis of reflection high-energy electron diffraction (RHEED) based on dynamical diffraction theory is a powerful method to study atomic structures of solid surfaces. Also, the grazing-angle geometry of the RHEED electron gun makes itself suitable for in-situ monitoring of molecular-beam epitaxy (MBE). This is of great advantage in evaluating the real surface structure of GaAs using RHEED rocking-curve analysis, because the experimental data can be analyzed without considering the effect of the adsorption and/or desorption of As, which might occur while the sample is transferred from an MBE chamber. This paper briefly reviews recent RHEED studies on the atomic structures of GaAs(001) surfaces. Results for the As-stabilized c(4×4) and Ga-stabilized c(8×2) surfaces, and the adsorption structure of Zn on the (2×4) surface are presented.
RHEED is one of the powerful tools for surface studies and monitoring thin film growth. RHEED pattern provides information on surface structures, surface morphology and thin film crystallography. Further more information including inelastic scattering can be obtained by installation of energy-filter in RHEED apparatus. We present a newly constructed energy-loss measurement system for diffracted electrons and demonstrate the experimental data obtained from Si(111)7×7 surface. For the morphology of regular step surface, it is shown that the mean terrace length can be evaluated by the energy-filtered RHEED pattern of the vicinal Si surface. Reflected electron beams from a specimen surface into vacuum are usually concerned in conventional RHEED. On the other hand, it is shown that the incident electron density distribution (wave field) formed in a crystal surface is useful to clarify the causes of the complicated peaks in the rocking curve. This approach is also effective for surface structural study using Auger excitation at selected atomic rows.
Amethod for calculating wave functions of RHEED based on a multiple scattering theory is described. Reflection matrix is used, which makes it possible to avoid divergence problem when we take account of evanescent waves as well as absorption effect. The electron density that is the square of the absolute value of RHEED wave function is calculated from Si(100)-c(4×2) surface as an example. First the change of electron density with respect to the number of diffracted beams is studied, which shows it is necessary for quantitative analysis to take many beams including evanescent beams. Then the azimuthal dependence is shown. The maximum and minimum positions of the density in the azimuth parallel to dimers are opposite to those in the azimuth normal to dimers. The density also changes as a function of the glancing angle. The maximum density is several times larger than the minimum at the same position from the surface.
Electron energy loss spectroscopy (EELS) under reflection high-energy electron diffraction (RHEED) conditions has been measured and analyzed to investigate incident and exit angle dependence of plasmon excitation processes. As a specimen, clean Si(111)7×7 surface is used. Mean number of surface (ns) and volume (nν) plasmon excitations has been obtained by Poisson analysis of the EELS spectra. Dependency of ns on incident and exit angle of electron is slightly different from that expected from Lucas theory. It is considered that refraction effect of electrons near surfaces should be included for quantitative analysis. The nν value does not depend on incident and exit conditions within our experimental range, that indicates little dependence of effective depth for volume plasmon excitation under grazing incident conditions. From EELS spectra of Kikuchi lines and energy filtered RHEED patterns, it has been revealed that volume plasmon excitation enhancement only occurs on Kikuchi lines around a specular spot.
Energy filtering of reflection electron microscopy, diffraction and holography (REM, RHEED and REH) on clean silicon surfaces has been performed by using an omega-type energy filter built into a high-resolution UHV electron microscope with a field emission gun. Due to the glancing reflection geometry, electrons are travelling near the surface over a long distance, and multiple surface plasmon excitation occurs. By removing plasmon-loss electrons, background intensity in RHEED has been lowered and contrast of REM images and holograms has been improved. Although electrons that lost a small amount of energy due to surface plasmon excitation etc. are found to be still coherent, their coherency deteriorate due to a spread of angular distribution after the inelastic scattering. By making no-plasmon-loss hologram, improvement of spatial and phase resolution is expected.
Reflection high-energy positron diffraction (RHEPD) is a new surface analysis tool having a capability to determine structure of adsorbed layers, surface Debye temperatures and metal dipole barriers with less disturbance from bulk. We have constructed a positron beam apparatus for RHEPD experiments and succeeded in observing RHEPD patterns and totalreflectionof positrons. In this article, we introduce the principle of RHEPD and some of its applications.
Sr-Ca-Cu-O complex oxide thin films were prepared at different firing temperatures using a spin coating method. Sr0.14Ca0.86CuO2 having layer structure could be prepared at lower temperatures and the method was compared with ordinary firing methods. However, the single phase of Sr0.14Ca0.86CuO2 was not obtained under such spin coating conditions as in this study. The friction properties of four spin coating films sliding against a cemented carbide ball were investigated using a Bowden-Leben type friction tester. Comparatively low friction coefficient and high wear resistance under non-lubrication conditions were obtained with these complex oxide films.
We investigated a local Au atomic diffusion on an Au(111) surface by using a scanning tunneling microscope (STM). The diffusion, which was dependent on a tip material, was triggered by an applied voltage pulse. Using an Au tip, small one-dimensional atomic diffusion was observed during the tip scan around a position where a voltage pulse was applied. As the result, finger-like stripes grew at a step along the [1-21] direction of the Au(111) surface. In case of a Pt-Ir tip, large two-dimensional atomic diffusion was observed on the scanned area of the Au(111) surface. Movable Au atoms around the position where voltage pulse was applied were swept by the tip scan with field gradient effect.