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.     

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)     

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.     

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.     

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SrTiO3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. 关键词： 反射高能电子衍射 SrTiO3 表面晶格常数及衍射强度振荡     

Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction (RHEED) patterns under different values of As 4 BEP for InGaAs films. When the As 4 BEP is set to be zero, the RHEED pattern keeps a 4×3/(n×3) structure with increasing temperature, and surface segregation takes place until 470 C. The RHEED pattern develops into a metal-rich (4×2) structure as temperature increases to 495 C. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515 C, the RHEED pattern turns into a GaAs(2×4) structure due to In desorption. While the As 4 BEP comes up to a specific value (1.33×10 4 Pa–1.33×10 3 Pa), the surface temperature can delay the segregation and desorption. We find that As 4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.     

Epitaxial film crystallography by high-energy Auger and X-ray photoelectron diffraction

We review recent work in the application of Auger and X-ray photoelectron diffraction at high electron kinetic energies to the problem of structure determination in ultrathin epitaxial overlayers. These closely-related techniques are based on the fact that outgoing Auger and photoelectrons from single-crystal specimens undergo elastic scattering and interference from near-neighbour atoms in the vicinity of the emitter. Such coherent diffraction leads to large intensity modulations as the detected emission direction is varied with respect to the crystal axes of the specimen. The measured modulations are readily interpreted by means of model quantum mechanical scattering calculation in which atomic coordinates in the epitaxial film are systematically varied. Such analyses provide several kinds of useful information, including growth modes accompanying heteroepitaxy, structural details of alloy and compound formation, and quantitative determination of tetragonal distortion at lattice-mismatched heterointerfaces. After a discussion of experimental design and theoretical modelling, we present several case studies of heteroepitaxial growth involving dissimilar materials. In addition, we review the new subfield of Auger and photoelectron holography, and discuss the current state-of-art in both data acquisition and Fourier inversion of experimental data for directly obtaining structural parameters.     

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.     

Fast measurement of rocking curve of reflection high-energy electron diffraction by using quasi-1D convergent beam

In order to investigate dynamic change in surface structure, we tried to measure the rocking curve of reflection high-energy electron diffraction (RHEED) by using a quasi-1D convergent beam method, which means a superposition of a hundred RHEED patterns at different glancing angles on one image. After we have checked the experimental accuracy, it is confirmed that the measurement time is shortened to 0.3 s by using this method. We also show an application of this method to a real time observation of surface segregation of B atoms while a highly B-doped Si is annealed.     

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.     

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [1̄1̄2] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3R30°, sharp √3 × √3 R30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.     

On the investigation of surface structure by reflection high energy electron diffraction (RHEED)

RHEED (Reflection High Energy Electron Diffraction) has been used to investigate surface structures. A formalism which allows fast, precise determination of the sizes, shapes and relative orientation of the lattices of well-ordered surface structures from the distances between the diffraction lines has been developed. This formalism is illustrated by the analysis of copper, gold and silicon surfaces and various surface structures formed on these. The relative precision of the measurements can in special cases be of the order of 10^?4 and will in general be better than 10^?3.     

Surface crystallography of thin films by electron diffraction in the stem

It is shown that by using STEM microdiffraction it is possible to obtain surface reflections from thin metallic films. The technique is demonstrated for a gold film. Reflections from the surface reproduce the (23 × 1) supernet for the (111) surface previously reported in the literature. It is also shown that the film surface consists of reconstructed (5 × 1) domains which when interlocking, produce the (23 × 1) pattern.     

Analytical treatment of surface resonance diffraction of high-energy electrons

The surface resonance RHEED problem is solved exactly using the tight-binding model for the case of the off-Bragg orientation of the incident beam. Explicit expressions are obtained for the wave function and for the resonance part of the surface reflectivity. It is shown that the dominant contribution to the reflectivity results from a process involving at least several states each localized in the potential of adjacent atomic planes parallel to the surface, and in the crystal bulk the wave function of the scattering problem belongs to the continuum of Bloch states. The analytical solution found provides an accurate approximation to the results of numerical integration of the relevant equations of the theory of RHEED, and demonstrates that no concept of a surface state is required for interpretation of the angular dependence of the reflectivity of a clean crystal surface in the vicinity of the resonance condition.     

A computerized complex for recording and processing of reflected high-energy electron diffraction patterns

An efficient and high-speed complex for visualization, reading, and processing of reflected high-energy electron diffraction patterns is described. The block diagrams and technical data of the complex, as well as its interface with devices controlling molecular-beam epitaxy of semiconductor structures, are presented. Dedicated software for processing high-energy electron diffraction patterns, including in real time, is suggested. The complex was used for studying heteroepitaxial growth and the formation of InAs nanostructures on the GaAs and Si surfaces.     

Splitting the plasmon peak in high-energy reflection electron energy loss experiments

Elastic scattering of energetic electrons over large angles (in this study 40 keV and 120°$120°$) implies momentum and hence energy transfer from an electron to a nucleus. Due to the large mass of the nucleus (relative to the mass of an electron) this energy transfer is small, but it has recently been shown that it can be resolved in a modern spectrometer. Hence the elastic peak of an overlayer/substrate system splits into different components corresponding to atoms with different mass. Here we extend this type of experiment to the plasmon part of a reflection energy loss spectroscopy (REELS) spectrum. It is shown that, for suitable systems, the plasmon peak of an overlayer/substrate system is split by the same amount as the elastic peak. This is a consequence of the fact that detection of an electron in REELS always requires a large-angle elastic scattering event. Moreover, we show that the relative intensity of the plasmon components contains information on the depth distribution of the scatterers.