Structure of AuSi nanoparticles on Si(111) from reflection high-energy electron diffraction and scanning tunneling microscopy

Gold-rich Au _x Si_1−x particles grown on Si(111)7 × 7 are studied by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The diffraction patterns reveal that (1) at least two different crystal structures coexist on the substrate; (2) the most prominent data correspond to a rhombohedral or quasi closed-packed structure; and (3) the particles show formation of an unusual contact facet to the substrate. Complete crystal alignment of the particles to the substrate lattice is found with no hints of random orientation. The findings are compared to STM images in terms of their structure, orientation, and morphology.     

Initial stages of Si-molecular beam epitaxy on Si(111) studied with reflection high-energy electron diffraction intensity measurements and Monte Carlo simulations

This is a report on the initial behaviour of homoepitaxial growth of Si on Si(111) based on reflection high-energy electron diffraction (RHEED) intensity measurements and kinetic solid-on-solid Monte Carlo simulations. A quenching of the first RHEED-oscillation corresponding to the first double layer is observed over a wide temperature range. This suppression could be simulated by introducing a defect related, extra activation energy for growth on top of the first and subsequently formed bilayers (BL). As a result of the increased activation energy, the interface width was shown to increase with the substrate temperature at the beginning of the deposition. The gradual return to regular BL-by-BL growth was accomplished by assigning an increased hopping probability to adatoms situated on top of step edges.     

An accurate dynamical electron diffraction algorithm for reflection high-energy electron diffraction

The conventional multislice method (CMS) method, one of the most popular dynamical electron diffraction calculation procedures in transmission electron microscopy, was introduced to calculate reflection high-energy electron diffraction (RHEED) as it is well adapted to deal with the deviations from the periodicity in the direction parallel to the surface. However, in the present work, we show that the CMS method is no longer sufficiently accurate for simulating RHEED with the accelerating voltage 3–100 kV because of the high-energy approximation. An accurate multislice (AMS) method can be an alternative for more accurate RHEED calculations with reasonable computing time. A detailed comparison of the numerical calculation of the AMS method and the CMS method is carried out with respect to different accelerating voltages, surface structure models, Debye–Waller factors and glancing angles.     

High energy electron diffraction at Si(001) surfaces

Elastic scattering of 10 keV electrons at Si(001) surfaces at grazing incidence was investigated. The intensity of the specularly reflected elastically scattered electrons as a function of the angle of incidence I_el(γ) was measured for different azimuthal angles and was compared with calculations using the dynamical diffraction theory. It turned out that the contribution of the elastically scattered electrons to the total intensity of the reflections strongly decreases with decreasing angle of incidence. Exciting the reflection (008) the elastic contribution is around 30%, decreasing to about 12% in the case of the reflection (004). In the calculations multiple beam effects, absorption, a smooth variation of the potential at the surface and a reduction of the topmost interlayer spacing were taken into account. There is satisfactory agreement between the structures of experimental and calculated intensity curves, I_el(γ) indicating a slight compression of the surface lattice to be probable. Quantitative agreement, however, for absolute intensities was not obtained.     

Weissenberg reflection high-energy electron diffraction for surface crystallography

The principle of a Weissenberg camera is applied to surface crystallographic analysis by reflection high-energy electron diffraction. By removing inelastic electrons and measuring hundreds of patterns as a function of sample rotation angle phi, kinematical analysis can be performed over a large volume of reciprocal space. The data set is equivalent to a three-dimensional stack of Weissenberg photographs. The method is applied to analysis of an Si(111)-square root of 3 x square root of 3-Ag surface, and the structural data obtained are in excellent agreement with the known atomic structure.     

Multiple scattering analysis of reflection high-energy electron diffraction intensities from GaAs(110)

An accurate and fast dynamical theory for calculating reflection high-energy electron diffraction (RHEED) rocking-curve intensities at 10–40 keV is presented. The application of RHEED to surface structural analysis is demonstrated for the first time on a reconstructed surface by comparing data to calculated spectra for GaAs(110). The technique is highly sensitive to structural changes along specific directions due to its forward-scattering geometry.     

System for recording and analysis of reflection high-energy electron diffraction patterns

An efficient and fast system for recording and analysis of reflection high-energy electron diffraction (RHEED) patterns is described. The software developed for this system includes three program packages: one for operating in the single-window mode, one for operating in the four-window mode, and one for the linear regime. Examples are given of the use of the system for monitoring and control of growth of III–V semiconductor compounds by molecular-beam epitaxy. Using this system, we discovered an effect wherein a periodic splitting of the RHEED peaks occurs during the growth of GaAs (100). Zh. Tekh. Fiz. 67, 111–116 (August 1997)     

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

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.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

SrTiO3同质外延过程中的反射高能电子衍射图案分析

在激光分子束外延实验中,用RHEED原位监测了SrTiO3基片初始、退火以及同质外延过程中的表面形态.通过对RHEED图案分析,获取了表面面内的晶格常数振荡与衍射条纹的半高宽振荡现象,前者是由退火重构表面与薄膜之间的界面造成的,后者与二维岛边界的弛豫相关.另外还观察到了等离子体对入射电子束的影响而导致的RHEED强度振荡行为的相位移现象.     

In-situ monitoring of interface formation using the phase shift of reflection high-energy electron diffraction intensity oscillations

A novel phase shift of Reflection High-Energy Electron Diffraction (RHEED) intensity oscillations during heterointerface formation allows direct monitoring of segragation at GaAs/Al _x Ga_1–x As interfaces. The effect should be applicable to other materials systems as it is due to the reconfiguration of the surface structure at the heterointerface.     

Low energy electron diffraction from Na(110) and Na2O(111) surfaces

Surfaces ofNa(110) are grown, investigated and oxidised to give Na₂O(111) surfaces. LEED spectra are taken for these surfaces and compared with theory to determine the surface composition of sodium oxide: the surface terminates the crystal in an integral number of electrically neutral NaONa sandwiches, with a bulk-like inter layer spacing. The effective Debye temperature for the Na(110) surface was found to be 107 K.     

Anisotropic photoemission and chemisorption-bond geometry on Ge(111) and Si(111) surfaces

In order to study the effects of photon-polarization selection rules on chemisorptionbond geometry, we have measured photoelectron spectra as a function of angle of incidence, θ_i, in the range 28° ? θ ? 80°. A noble-gas UV resonance lamp and cylindrical mirror analyzer were used to measure both bonding and non-bonding surface Orbitals. A large enhancement (200–400%) of the photocmission is found when photon electric field intensity is near the maximum normal to the surface. This indicates a spatial variation of microscopic fields which is approximately independent of adsorbate bonding since it is determined by the optical properties of the substrate. In addition, we observe some effects on adsorbate photoelectron peaks due to different orbital symmetry. The case of atomic hydrogen chemisorption is discussed as an example of this latter effect.     

Lattice vibrations at ideal Si(111) surfaces

Surface vibrations at an ideal (111) silicon surface with vanishing wavevector components parallel to the surface are calculated within the shell-model for a simple model of surface force constants.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.