Reflection High-energy Electron Diffraction Technique Research Articles

Effect of graphene domains orientation on quasi van der Waals epitaxy of GaN

We demonstrate the growth features of III-nitrides on graphene buffer layers obtained by the CVD method on a copper catalyst with different dominant grain orientations. The reflection high-energy electron diffraction technique (RHEED) is used to map the 2D reciprocal space structures of graphene buffers and growing nitride layers. The RHEED reciprocal space pattern for the graphene layer grown on a (111) textured copper foil and transferred to a SiO2/Si substrate demonstrates the sixfold symmetry characteristic of a highly oriented material. In turn, graphene grown on a Cu (100) foil consists of two types of domains that are 30° rotated relative to each other. It has also been demonstrated that III-nitride films exactly repeat the texture of the 2D graphene buffers. The GaN sample grown over the highly textured substrate demonstrates a clear sixfold symmetry of the RHEED reciprocal space map as well as {101¯3} XRD pole figure, which is close to 2D surface morphology. In turn, the GaN film grown over the graphene buffer layer transferred from the Cu (100) textured foil has 12-fold axial symmetry, which is equivalent to the essentially two-domain in-plane orientation of the initial graphene.

In-situ quantification of the surface roughness for facile fabrications of atomically smooth thin films

This work presents an in-situ technique to quantify the layer-by-layer roughness of thin films and heterostructures by measuring the spectral profile of the reflection high-energy electron diffraction (RHEED). The characteristic features of the diffraction spot, including the vertical to lateral size ratio c/b and the asymmetrical ratio c1/c2 along the vertical direction, are found to be quantitatively dependent on the surface roughness. The quantitative relationships between them are established and discussed for different incident angles of high-energy electrons. As an example, the surface roughnesses of LaCoO3 films grown at different temperatures are obtained using such an in-situ technique, which are confirmed by the ex-situ atomic force microscopy. Moreover, the in-situ measured layer-by-layer roughness oscillations of two LaCoO3 films are demonstrated, revealing drastically different information from the intensity oscillations. The experiments assisted with the in-situ technique demonstrate an outstanding high resolution down to ∼ 0.1 Å. Therefore, the new quantitative RHEED technique with real-time feedbacks significantly escalates the thin film synthesis efficiency, especially for achieving atomically smooth surfaces and interfaces. It opens up new prospects for future generations of thin film growth, such as the artificial intelligence-assisted thin film growth.

Peculiarities of the AlN crystalline phase formation in a result of the electron-stimulated reconstruction transition (√31×√31)R ± 9° − (1 × 1)

In the present work, it was found for the first time that an electron beam used in reflection high-energy electron diffraction technique stimulates a reconstruction transition from a sapphire (√31×√31)R ± 9° superstructure which is chemically inert under an ammonia flux to an unreconstructed (1 × 1) structure with subsequent surface nitridation. The electron beam initiates electron-stimulated oxygen desorption from the sapphire surface followed by formation of oxygen vacancies, which are potential energetically accessible centers for the primary nuclei formation of the AlN crystalline phase.

Role of the phase transition at GaN QDs formation on (0001)AlN surface by ammonia molecular beam epitaxy

We report an original method of GaN/AlN quantum dots (QDs) formation with low density by ammonia MBE on the (0001)AlN surface using a decomposition process of GaN thin layer. The QDs formation has been investigated in situ by reflection high-energy electron diffraction technique. Low density of quantum dots has been obtained in the range 107–109 cm−2. Single quantum dots photoluminescence lines corresponding to exciton and biexiton transitions were observed in micro-photoluminescence spectra. A lattice gas model was developed for correct description of the GaN QDs statistical ensemble on the surface. Effective interaction between QDs results in the discontinuous phase transitions (first-order phase transitions) from low density of QDs (gas branch) to condensed phase of QDs. The GaN QDs formation has been confirmed by high-resolution transmission electron microscopy and micro-photoluminescence of single quantum dots.

Behavior of Wave Field on Graphite Surface Observed Using Reflection High-Energy Electron Diffraction Technique

Behavior of Wave Field on Graphite Surface Observed Using Reflection High-Energy Electron Diffraction Technique

Epitaxial Ni nanoparticles on CaF2(001), (110) and (111) surfaces studied by three-dimensional RHEED, GIXD and GISAXS reciprocal-space mapping techniques

The development of growth techniques aimed at the fabrication of nanoscale heterostructures with layers of ferroic 3d metals on semiconductor substrates is very important for their potential usage in magnetic media recording applications. A structural study is presented of single-crystal nickel island ensembles grown epitaxially on top of CaF2/Si insulator-on-semiconductor heteroepitaxial substrates with (111), (110) and (001) fluorite surface orientations. The CaF2 buffer layer in the studied multilayer system prevents the formation of nickel silicide, guides the nucleation of nickel islands and serves as an insulating layer in a potential tunneling spin injection device. The present study, employing both direct-space and reciprocal-space techniques, is a continuation of earlier research on ferromagnetic 3d transition metals grown epitaxially on non-magnetic and magnetically ordered fluorides. It is demonstrated that arrays of stand-alone faceted nickel islands with a face-centered cubic lattice can be grown controllably on CaF2 surfaces of (111), (110) and (001) orientations. The proposed two-stage nickel growth technique employs deposition of a thin seeding layer at low temperature followed by formation of the islands at high temperature. The application of an advanced three-dimensional mapping technique exploiting reflection high-energy electron diffraction (RHEED) has proved that the nickel islands tend to inherit the lattice orientation of the underlying fluorite layer, though they exhibit a certain amount of {111} twinning. As shown by scanning electron microscopy, grazing-incidence X-ray diffraction (GIXD) and grazing-incidence small-angle X-ray scattering (GISAXS), the islands are of similar shape, being faceted with {111} and {100} planes. The results obtained are compared with those from earlier studies of Co/CaF2 epitaxial nanoparticles, with special attention paid to the peculiarities related to the differences in lattice structure of the deposited metals: the dual-phase hexagonal close-packed/face-centered cubic lattice structure of cobalt as opposed to the single-phase face-centered cubic lattice structure of nickel.

Direct growth of nano-crystalline graphite films using pulsed laser deposition with in-situ monitoring based on reflection high-energy electron diffraction technique

We report an experimental method to overcome the long processing time required for fabricating graphite films by a transfer process from a catalytic layer to a substrate, as well as our study of the growth process of graphite films using a pulsed laser deposition combined with in-situ monitoring based on reflection high-energy electron diffraction technique. We monitored the structural evolution of nano-crystalline graphite films directly grown on AlN-coated Si substrates without any catalytic layer. We found that the carbon films grown for less than 600 s cannot manifest the graphite structure due to a high defect density arising from grain boundaries; however, the carbon film can gradually become a nano-crystalline graphite film with a thickness of approximately up to 5 nm. The Raman spectra and electrical properties of carbon films indicate that the nano-crystalline graphite films can be fabricated, even at the growth temperature as low as 850 °C within 600 s.

反射高速電子回折による結晶表面研究

The reflection high-energy electron diffraction (RHEED) is a powerful tool for surface studies and widely used for surface structural analysis in monitoring epitaxial growth. In this article, it is described that RHEED is developed as a tool in surface studies such as structure determination of crystal surfaces, evolution of surface structures by epitaxial growth and monitoring thin film growth processes. Recent novel RHEED techniques for surface structural analysis are also described.

Optical Fingerprints of Si Honeycomb Chains and Atomic Gold Wires on the Si(111)-(5×2)-Au Surface

The intensively studied Si(111)-(5×2)-Au surface is reexamined using reflectance anisotropy spectroscopy and density functional theory simulations. We identify distinctive spectral features relating directly to local structural motifs such as Si honeycomb chains and atomic gold wires that are commonly found on Au-reconstructed vicinal Si(111) surfaces. Optical signatures of chain dimerization, responsible for the observed (×2) periodicity, are identified. The optical response, together with STM simulations and first-principles total-energy calculations, exclude the new structure proposed very recently based on the reflection high-energy electron diffraction technique analysis of Abukawa and Nishigaya [Phys. Rev. Lett. 110, 036102 (2013)] and provide strong support for the Si honeycomb chain with the triple Au chain model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)]. This is a promising approach for screening possible models of complex anisotropic surface structures.

Model-Driven Development for scientific computing. An upgrade of the RHEEDGr program

Model-Driven Engineering (MDE) is the software engineering discipline, which considers models as the most important element for software development, and for the maintenance and evolution of software, through model transformation. Model-Driven Architecture (MDA) is the approach for software development under the Model-Driven Engineering framework. This paper surveys the core MDA technology that was used to upgrade of the RHEEDGR program to C++0x language standards. New version program summaryProgram title: RHEEDGR-09Catalogue identifier: ADUY_v3_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADUY_v3_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 21 263No. of bytes in distributed program, including test data, etc.: 1 266 982Distribution format: tar.gzProgramming language: Code Gear C++ BuilderComputer: Intel Core Duo-based PCOperating system: Windows XP, Vista, 7RAM: more than 1 MBClassification: 4.3, 7.2, 6.2, 8, 14Does the new version supersede the previous version?: YesNature of problem: Reflection High-Energy Electron Diffraction (RHEED) is a very useful technique for studying growth and surface analysis of thin epitaxial structures prepared by the Molecular Beam Epitaxy (MBE). The RHEED technique can reveal, almost instantaneously, changes either in the coverage of the sample surface by adsorbates or in the surface structure of a thin film.Solution method: The calculations are based on the use of a dynamical diffraction theory in which the electrons are taken to be diffracted by a potential, which is periodic in the dimension perpendicular to the surface.Reasons for new version: Responding to the user feedback the graphical version of the RHEED program has been upgraded to C++0x language standards. Also, functionality and documentation of the program have been improved.Summary of revisions:1.Model-Driven Architecture (MDA) is the approach defined by the Object Management Group (OMG) for software development under the Model-Driven Engineering framework [1]. The MDA approach shifts the focus of software development from writing code to building models. By adapting a model-centric approach, the MDA approach hopes to automate the generation of system implementation artifacts directly from the model. The following three models are the core of the MDA: (i) the Computation Independent Model (CIM), which is focused on basic requirements of the system, (ii) the Platform Independent Model (PIM), which is used by software architects and designers, and is focused on the operational capabilities of a system outside the context of a specific platform, and (iii) the Platform Specific Model (PSM), which is used by software developers and programmers, and includes details relating to the system for a specific platform. Basic requirements for the calculation of the RHEED intensity rocking curves in the one-beam condition have been described in Ref. [2]. Fig. 1 shows the PIM for the present version of the program. Fig. 2 presents the PSM for the program.2.The TGraph2D.bpk package has been recompiled to Graph2D0x.bpl and upgraded according to C++0x language standards. Fig. 3 shows the PSM of the Graph2D component, which is manifested by the Graph2D0x.bpl package presently. This diagram is a graphic presentation of the static view, which shows a collection of declarative model elements and their relationships. Installation instructions of the Graph2D0x package can be found in the new distribution.3.The program requires the user to provide the appropriate parameters for the crystal structure under investigation. These parameters are loaded from the parameters.ini file at run-time. Instructions for the preparation of the .ini files can be found in the new distribution.4.The program enables carrying out one-dimensional dynamical calculations for the fcc lattice, with a two-atoms basis and fcc lattice, with one atom basis but yet the zeroth Fourier component of the scattering potential in the TRHEED1D::crystPotUg() function can be modified according to users' specific application requirements.5.A graphical user interface (GUI) for the program has been reconstructed.6.The program has been compiled with English/USA regional and language options.Unusual features: The program is distributed in the form of main projects RHEEDGr_09.cbproj and Graph2D0x.cbproj with associated files, and should be compiled using Code Gear C++ Builder 2009 compilers.Running time: The typical running time is machine and user-parameters dependent.

Movement of oxygen vacancies in oxide film during annealing observed by an optical reflectivity difference technique

The monolayer growth of Nb-doped SrTiO3 on SrTiO3 (100) substrate is prepared by a pulsed laser deposition method. The growth and annealing of the film in vacuum and in oxygen ambient are monitored in real time by an oblique-incidence optical reflectivity difference (OIRD) technique and reflection high-energy electron diffraction technique. The films annealed in different ambient result in different optical annealing signals. From the comparison of experimental OIRD signals with the simulation of OIRD signals, we prove that the optical technique can easily tell whether the oxygen vacancies are moving into or moving out of the film during the annealing. The optical signals are found to be composed of contributions from step edges and terraces.

Initial stages of MnAs/GaAs(0 0 1) epitaxy studied by RHEED azimuthal scans

We study the nucleation phase of molecular beam epitaxy of (hexagonal) MnAs on (cubic) GaAs (0 0 1) using reflection high-energy electron diffraction (RHEED) azimuthal scans. The nucleation proceeds from a non-reconstructed initial stage through randomly oriented small nuclei and two orientation stages to the final single-phase epitaxial orientation. The fascinatingly complex nucleation process contains elements of both Volmer–Weber and Stranski–Krastanov growth. The measurement demonstrates the potential of high-resolution RHEED techniques to assess details of the surface structure during epitaxy.

Nano-structure formed by nanosecond laser annealing on amorphous Si surface

The formation of nano-structure on an amorphised Si surface is studied upon annealing by a nanosecond laser pulse of an excimer laser. The resulting structures are studied by means of the reflection high-energy electron diffraction technique and atomic force microscopy. Smoothing out of the post-implanted islands and then the formation of polycrystalline Si grains are observed in surfaces annealed with the energy density up to the threshold value.

A new version of a computer program for dynamical calculations of RHEED intensity oscillations

We present a new version of the RHEED program which contains a graphical user interface enabling the use of the program in the graphical environment. The presented program also contains a graphical component which enables displaying program data at run-time through an easy-to-use graphical interface. New version program summary Title of program: RHEEDGr Catalogue identifier: ADWV Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADWV Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Catalogue identifier of previous version: ADUY Authors of the original program: A. Daniluk Does the new version supersede the original program: no Computer for which the new version is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the new version has been tested: Windows 9x, XP, NT Programming language used: Borland C++ Builder Memory required to execute with typical data: more than 1 MB Number of bits in a word: 64 bits Number of processors used: 1 Number of lines in distributed program, including test data, etc.: 5797 Number of bytes in distributed program, including test data, etc.: 588 121 Distribution format: tar.gz Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying growth and surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). The RHEED technique can reveal, almost instantaneously, changes either in the coverage of the sample surface by adsorbates or in the surface structure of a thin film. Method of solution: RHEED intensities are calculated within the framework of the general matrix formulation of Peng and Whelan [1] under the one-beam condition. Reasons for the new version: Responding to the user feedback we designed a graphical package that enables displaying program data at run-time through an easy-to-use graphical interface. Summary of revisions: (1) In the present form the code is an object-oriented extension of previous version [2]. Fig. 1 shows the static structure of classes and their possible relationships (i.e. inheritance, association, aggregation and dependency) in the code. (2) The code has been modified and optimized to compile under the C++ Builder integrated development environment (IDE). (3) A graphical user interface (GUI) for the program has been created. The application is a standard multiple document interface (MDI) project from Builder's object repository. The MDI application spawns child window that reside within the client window; the main form contains child object. (4) We have added an original graphical component [3] which has been tested successfully in the C++ Builder programming environment under Microsoft Windows platform. Fig. 2 shows internal structure of the component. This diagram is a graphic presentation of the static view which shows a collection of declarative model elements, such as classes, types, and their relationships. Each of the model elements shown in Fig. 2 is manifested by one header file Graph2D.h, and one code file Graph2D.cpp. Fig. 3 sets the stage by showing the package which supplies the C++ Builder elements used in the component. Installation instructions of the TGraph2D.bpk package can be found in the new distribution. (5) The program has been constructed according to the systems development live cycle (SDLC) methodology [4]. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is distributed in the form of a main project RHEEDGr.bpr with associated files, and should be compiled using Borland C++ Builder compilers version 5 or later.

Dynamical calculations for RHEED intensity oscillations

A practical computing algorithm working in real time has been developed for calculating the reflection high-energy electron diffraction from the molecular beam epitaxy growing surface. The calculations are based on the use of a dynamical diffraction theory in which the electrons are taken to be diffracted by a potential, which is periodic in the dimension perpendicular to the surface. The results of the calculations are presented in the form of rocking curves to illustrate how the diffracted beam intensities depend on the glancing angle of the incident beam. Program summary Title of program: RHEED Catalogue identifier:ADUY Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADUY Program obtainable from:CPC Program Library, Queen's University of Belfast, N. Ireland Computer for which the program is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the program has been tested: Windows 9x, XP, NT, Linux Programming language used: Borland C++ Memory required to execute with typical data: more than 1 MB Number of bits in a word: 64 bits Number of processors used: 1 Distribution format:tar.gz Number of lines in distributed program, including test data, etc.:982 Number of bytes in distributed program, including test data, etc.: 126 051 Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying growth and surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). Nowadays, RHEED is used in many laboratories all over the world where researchers deal with the growth of materials by MBE. The RHEED technique can reveal, almost instantaneously, changes either in the coverage of the sample surface by adsorbates or in the surface structure of a thin film. In most cases the interpretation of experimental results is based on the use of dynamical diffraction approaches. Such approaches are said to be quite useful in qualitative and quantitative analysis of RHEED experimental data. Method of solution: RHEED intensities are calculated within the framework of the general matrix formulation of Peng and Whelan [Surf. Sci. Lett. 238 (1990) L446] under the one-beam condition. The dynamical diffraction calculations presented in this paper utilize the systematic reflection case in RHEED, in which the atomic potential in the planes parallel to the surface are projected on the surface normal, so that the results are insensitive to the atomic arrangement in the layers parallel to the surface. This model shows a systematic approximation in calculating dynamical RHEED intensities, and only a layer coverage factor for the nth layer was taken into account in calculating the interaction potential between the fast electron and that layer. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is presented in the form of a basic unit RHEED.cpp and should be compiled using C++ compilers, including C++ Builder and g++.

Ge epitaxial island growth on a graphitized C-rich 4H-SiC(0 0 0 1) surface

Epitaxial Ge islands on a SiC(0 0 0 1) substrate have been examined by reflection high-energy electron diffraction (RHEED). These islands have been obtained by depositing three monolayers of Ge at 500 °C on a graphitized SiC (6√3×6√3)R30° reconstructed surface. This surface has been chosen for its ability to support epitaxial Ge island growth in a Volmer-Weber mode. The RHEED technique has allowed us to determine the epitaxy relationship and in-plane orientations between relaxed Ge and SiC(0001). Typical transmission electron diffraction patterns indicate that Ge grows according to only one epitaxy relationship Ge{1 1 1}||SiC(0 0 0 1) that corresponds to two in-plane orientations, a preferential one Ge 〈 1 ¯ 1 ¯ 2 〉 | | SiC 〈 1 1 ¯ 0 0 〉 and a minority one Ge 〈 1 ¯ 1 ¯ 2 〉 | | SiC 〈 1 0 1 ¯ 0 〉 rotated by 30° around the growth 〈1 1 1〉-Ge (or [0 0 0 1]-SiC) axis. Double spot RHEED patterns also point out a twinning for each in-plane orientation, a consequence of the three-fold order of the 〈1 1 1〉-Ge axis.

GROWTH, MORPHOLOGICAL AND STRUCTURAL PROPERTIES OF Ag THIN FILMS ON A Ru (0001) SURFACE GROWN BY MBE

Ag thin films deposited on Ru (0001) surface by molecular beam epitaxy, at temperatures of 20°C and 450°C, have been investigated using reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. For both growth temperatures, the in situ RHEED patterns of the Ag films exhibited an in-plane six-fold symmetry, indicating that the Ag deposit is in epitaxy with the Ru buffer surface. At RT, the RHEED technique indicated a three-dimensional growth (3D), while a layer-by-layer growth (2D) takes place at HT. The AFM images showed a granular structure of the surface of the deposited Ag layers with a large variation of the roughness with the growth temperature. XRD analysis gave indication of a strongly textured thin film along the growth direction. The lattice mismatch between the Ag and Ru is at the origin of a stress at the interface and defects structure in the film.

Recrystallisation and dopant diffusion in amorphised germanium layers upon pulsed laser annealing

In the present paper the results of investigations of the recrystallisation process in amorphous Ge near-surface layers induced by laser annealing are reported. Amorphised layers were created by implantation of Ge single crystal with 150 keV Sn ions at a dose 1×10 16 cm −2. Structural changes and redistribution of dopants were caused in irradiated materials by a single nanosecond laser pulse. For a study of the near-surface layer structure before and after laser annealing, the reflection high-energy electron diffraction technique was applied. Dopant diffusion was investigated by means of secondary ion mass spectrometry. Changes for polycrystalline of the near-surface layer structure amorphised by Sn implantation, occurred independently of the phase transitions and of the dopant concentration (0–8%).

Laser ablation of manganite thin films monitored by in situ RHEED

Reflection high-energy electron diffraction (RHEED) during crystal growth has become a standard analytical tool in the fabrication of semiconducting materials by molecular-beam epitaxy in an ultrahigh vacuum environment. For the fabrication of high-quality epitaxial and heteroepitaxial thin film structures of oxide materials such as perovskite manganites or high-temperature superconductors, it is also highly desirable to use RHEED technique during epitaxial growth. However, the complex oxide materials are commonly fabricated by pulsed laser deposition (PLD) at high oxygen pressure above 10 Pa preventing the use of standard RHEED systems. A two-stage differential pumping system can serve to circumvent this problem [1]. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber can be reduced considerably. Then the specular beam intensity of the RHEED system is sufficiently high to allow the observation of intensity oscillations during the growth of oxide materials. We have used high-pressure RHEED and in situ atomic force microscopy (AFM) for the investigation of the surface morphology of oxide thin films prepared by PLD.

Theoretical analysis of thin film epitaxial growth monitored by differential reflectance

The thin film growth at the level of a single molecular layer can be achieved via laser molecular beam epitaxy approach. Such a growth can be monitored not only via the reflection high-energy electron diffraction technique, but also with the use of relative reflectivity change between s- and p-polarized light. The epitaxy growth of thin film is modeled as a microscopic two-dimensional (2D) gratings on the surface of the substrate with a period over two orders of magnitude less than the wavelength of probe light. The solution of Maxwell’s equations for such a 2D gratings leads to an effective multilayer medium model, from which the reflectance-difference (RD) signal in the process of epitaxy growth can be calculated in the framework of general optics. The simulation results demonstrate that the growth of thin film at the atomic level can be detected by such an optical RD technique. The theoretical calculations of RD signal are in agreement with the experimental measurements both in the order of magnitude and variation character of RD signal curve.