Reflection High-energy Electron Diffraction Experiments Research Articles

Regular distribution of [formula omitted] Bi-reconstructed stripes on Si(553) surface

The crystallographic and electronic structures of (3×3) Bi-induced reconstructions of Si(553) surface are investigated with reflection high energy electron diffraction (RHEED), scanning tunneling microscopy and angle resolved photoelectron spectroscopy. The α and β(3×3) superstructures have been obtained at 1/3 and 1 monolayer of Bi, respectively. RHEED experiments indicate that α(3×3) is formed on wide (111) terraces separated by (331) facets while the β(3×3) phase stabilizes regular distribution of steps over entire Si(553) surface. In the latter case the double atomic height steps separate Bi-reconstructed (111) terraces of about 3nm in width. Only one orientation of (3×3) reconstruction is formed as a consequence of broken three-fold symmetry. The electronic structures of both Bi-induced superstructures are similar to the corresponding ones obtained for a flat Si(111) surface.

Pb nanoribbons on the Si(553) surface

The crystallographic structure and morphology of the Si(553) surface ordered by Pb atoms are investigated with the reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and density functional theory (DFT) techniques. Adsorption of 1.3 monolayers of Pb, and a subsequent gentle annealing, causes regular distribution of Si atomic steps over the macroscopic sample area. The step periodicity is exactly the same as in the case of the Au-induced Si(553). However, the hybridization between Pb and Si atoms is weaker, as compared to Au and Si atoms on the Si(553) surface. Instead of two Au atomic chains strongly bonded to Si atoms, Pb forms a five-atom-wide nanoribbon on each Si(111) terrace. The simulated STM topography images obtained with the DFT calculations agree very well with the results obtained in the RHEED and STM experiments.

Role of lateral interaction in the homoepitaxy of GaAs on the (001)-β(2 × 4) surface

The homoepitaxy of GaAs on the (001)-β(2 × 4) surface during molecular beam epitaxy is considered as a twodimensional first-order phase transition from the lattice gas of adsorbed growth components to the two-dimensional crystalline phase. In the context of the mean field theory of phase transitions, the parameters of lateral interaction between filled cells of the lattice gas are determined. The causes for the completion of the growth of a particular monolayer (self-ordering) before the beginning of growth of a new monolayer are clarified. The oscillations observed in the reflection high-energy electron diffraction experiments support the conclusions of the suggested theory of the phase transition for homoepitaxy.

A pulsed electron gun for ultrafast electron diffraction at surfaces

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

Kikuchi Patterns Observed by New Astrodome RHEED

A new reflection high-energy electron diffraction (RHEED) apparatus equipped with a hemispherical fluorescent screen upon a sample surface has been constructed. We call it astrodome screen, namely, the apparatus is named as astrodome RHEED. For the observation of Kikuchi pattern in RHEED experiment, the astrodome screen is effective because of the relatively short escape depth of Kikuchi electrons arising in bulk. Diffraction patterns have been observed in the astrodome screen for Si(111)7 × 7 clean surface, and the glancing angle and the acceleration voltage dependences were investigated. It has been found that some circular patterns exist in the Kikuchi patterns. The circular patterns are considered to be responsible for one-dimensional diffractions from certain atomic rows in bulk. The observed Kikuchi patterns and circular patterns are calculated using simple kinematical theory and a good agreement between the experimental and calculated results has been obtained. [DOI: 10.1380/ejssnt.2006.118]

Surfactant effect of gallium during the growth of GaN on AlN(0001¯) by plasma-assisted molecular beam epitaxy

The growth mode of N-face GaN deposited on AlN(0001¯) by plasma-assisted molecular beam epitaxy has been investigated. Based on reflection high-energy electron diffraction experiments, we demonstrate that for appropriate Ga fluxes and substrate temperature, a self-regulated 1-ML-thick Ga excess film can be formed on the growing surface. Depending on the presence of this Ga monolayer, the growth can proceed following either the Stranski–Krastanow or the Frank Van der Merwe growth modes, hence enabling the synthesis of either quantum dots or quantum wells.

Rheed in-plane rocking curve analysis of biaxially-textured polycrystalline MgO films on amorphous substrates grown by ion beam-assisted deposition

We have developed a method for biaxial texture determination in polycrystalline films using reflection high energy electron diffraction (RHEED) in-plane rocking curves. Experimental RHEED in-plane rocking curves were taken at 25keV and 2.7° incidence angle from 11nm thick, nominally [100]-textured MgO films grown on amorphous Si3N4 by ion beam-assisted deposition (IBAD). The experimental RHEED in-plane rocking curves were analyzed by comparing them with RHEED in-plane rocking curves calculated using a kinematical simulation. The model enables a quantitative correlation between biaxial texture and RHEED in-plane rocking curve measurements. RHEED results are compared to X-ray rocking curve film analysis.

Surface Morphology of Growing Al on Si(111)7×7 and Si(111)√3×√3–Al Substrates by Reflection High-Energy Electron Diffraction

Morphology of thin Al films grown on Si(111)7×7 and Si(111)√3×√3–Al substrates was investigated by glancing angle dependent reflection high-energy electron diffraction (RHEED). Comparing the experimental RHEED patterns with the calculated ones based on the refraction effect and Laue function at each glancing angle, the averaged slope angle of the deposited Al islands and the averaged domain size are estimated for both the systems. The morphological differences between the two systems are quantitatively clarified.

Quantitative Rheed Analysis of Biaxially-Textured Polycrystalline MgO Films on Amorphous Substrates Grown by Ion Beam-Assisted Deposition

AbstractWe have developed a computer simulation based on analytic calculation of reflection high energy electron diffraction (RHEED) patterns in the kinematic approximation for mosaic polycrystalline films for given values of electron beam incidence angle, polycrystalline texture, in-plane orientation distribution, and grain size. Although RHEED is most appropriately modeled using dynamical scattering theory, the computational efficiency of the kinematic approximation has enabled the development of a model suitable for real time measurement of crystallographic texture and in-plane orientation distributions for biaxially-textured films grown by ion beam-assisted deposition (IBAD). Using the simulation, we can quantitatively determine how RHEED spot shapes and relative intensities depend on the crystallographic texture and in-plane orientation distribution of polycrystalline films. RHEED patterns taken at 25 keV with incidence angle in the range 1–5 degrees from 10 nm thick, nominally [100]-textured MgO films grown on amorphous Si3N4 by IBAD were analyzed by comparing experimental RHEED phi rocking curves with those predicted by the simulation. For some films, an additional 200 nm thermally-grown MgO homoepitaxial layer was grown on top of the IBAD MgO layer. The model enables a quantitative correlation between biaxial texture and RHEED measurements. RHEED results are compared to X-ray rocking curve film analysis.

Determination of the atomic geometry of the GaAs(001)2×4 surface by dynamical RHEED intensity analysis: The β2(2×4) model

Using dynamical reflection high-energy electron diffraction (RHEED) computations, experimental RHEED rocking curves from the MBE-grown As-rich GaAs(001)2×4 surface have been analysed to provide estimates of the surface structural parameters. Ab initio total-energy computations suggest that surfaces grown in such As-rich conditions are likely to exist in the so-called β2(2×4) phase. Therefore, it is assumed that the most appropriate GaAs(001)2×4 surface model with which to analyse the experimental data is that of the β2(2×4) phase. This phase has two As dimers in the surface layer and a third As dimer in the third layer. The experimental rocking curves analysed were measured in the primary beam azimuth, permitting estimates of those β2(2×4) phase surface structural parameters in the plane perpendicular to the primary beam azimuth. The best experiment–theory fit was obtained using a standard non-linear optimisation scheme (the Marquardt algorithm). We find that by using the β2(2×4) surface model we are able to get a better fit to the experimental data than in earlier work in which the data were analysed using the β(2×4) surface model. This supports the assumption that the β2(2×4) surface model is the most appropriate to describe the surface in the As-rich MBE growth conditions used to produce the sample surface upon which the RHEED experiments were performed. Our best-fit β2(2×4) surface structure is predominantly in agreement with that obtained from a surface X-ray diffraction study and from ab initio total-energy calculations. As in these other studies, our best-fit structure shows a large in-plane relaxation of the second layer threefold coordinated Ga atoms and an outward perpendicular displacement of the first layer As dimers. The good agreement in most details between all of these independent structure determinations suggests that we now have a largely consistent picture of the structure of the GaAs(001) β2(2×4) surface.

Real time control of InxGa1−xN molecular beam epitaxy growth

The growth of GaN and InxGa1−xN on c-plane sapphire substrates was carried out by molecular beam epitaxy using NH3. In situ reflection high-energy electron diffraction (RHEED) was used to monitor the growth process. Oscillations of the specular beam intensity were observed during both GaN and InxGa1−xN deposition. This allows determining in real time the composition of InxGa1−xN alloys. The effects of the growth temperature and the Ga flux on the In incorporation rate were investigated. The critical thickness for InGaN islanding as a function of In mole fraction is also easily deduced from RHEED experiments.

Ionicity of nickel oxide: Direct determination by reflection diffraction of high-energy electrons from the (100) surface

The degree of ionicity and the inner potential of crystalline nickel oxide have been determined by using reflection high-energy electron diffraction (RHEED) and reflection electron microscopy. By matching experimental RHEED patterns obtained from an atomically flat NiO(100) surface with dynamical RHEED calculations, we found that the degree of ionicity of this oxide is close to 0.3, and that the inner potential equals 21.21 V.

STM study of the InSb(111)A-(2 × 6) surface

Scanning tunneling microscopy (STM), reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES) experiments have been performed to study details of a (2 × 6) reconstructed surface of InSb(111)A. The results obtained are as follows: The (2 × 6) surface exists purely and solely in the narrow temperature range of 200 and 215°C where Sb atoms are considerably slowly desorbed from the domain boundaries. Above 215°C, vacancies and voids are formed by the desorption of Sb which constitutes the (2 × 6) surface. However, the RHEED experiment evidences the (2 × 6) surface to leave its own structure almost unchanged up to 230°C.

Theoretical study of the Si/GaAs(001)-c(4×4) surface

The deposition of Si on the GaAs(001)-c(4×4) reconstructed surface is studied with the aid of ab initio pseudopotential calculations. With reference to the results of scanning tunneling microscopy (STM), reflectance anisotropy spectroscopy (RAS), and reflection high-energy electron diffraction experiments, models for the Si/GaAs(001)-c(4×4) surface are constructed with Si coverages corresponding to 0.25 and 1.0 ML. STM images are simulated and RAS spectra are calculated for these models and compared with experiment. By examining the local density of states and the magnitude of the optical transition matrix elements it is possible to make a link between the features in the RAS spectra and surface electronic structure. Comments are made on the validity of these surface structural models

STM and RHEED experiments on non-equilibrium surface structures of the Au(110) surface

The Au(110) surface is studied by scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED). The surface is rapidly cooled from above the order-disorder transition temperature of Tc = 650 K to room temperature. The STM reveals some details of the disordered state, i.e. a preference of 〈110〉 chains of varying lengths as the most obvious short range structural elements. The first long range order structural features are (1 × 3) rather than (1 × 2) islands. The (1 × 2) structure needs some energy input to be formed. The RHEED analysis is done while the target is heated in the range from room temperature to above Tc. The RHEED experiments give information about the disordered state, as well as the annealing and the order-disorder phase transition.

Symmetry analysis of the RHEED patterns from the Si(001)−2 × 1 surfaces

Experimental and theoretical studies of the Si(001)−2×1 reconstruction are reviewed. The review indicates that a technique, which is sensitive and fast enough to detect the basic symmetry of the Si(001)−2×1 surface, seems still unavailable and the local density approximation (LDA) adopted in calculations makes it difficult to determine the basic electronic property of the surface: metallic or semiconducting. The review of the distinct features of the reflection high-energy electron diffraction (RHEED) technique shows that it clearly has a role in resolving the controversies over the Si(001)−2×1 reconstruction. The multislice formalism of Cowley and Moodie with a recently developed edge-patching method has been applied to the analyses of several experimental RHEED patterns from the Si(001)−2×1 surface. The calculation results reject the static buckled model for the surface. The analyses also show that the information to determine which one of three possible symmetrical models for the surface (namely ‘inherent symmetry’, ‘individual-dimer-random-symmetry’ and ‘domain-random-symmetry’) is closest to the true structure can be found in the experimental RHEED patterns. The results indicate that the intensity symmetry (symmetry regarding the intensities rather than geometrical locations of spots) of a RHEED pattern from the Si(001)−2×1 surface is very sensitive to the structure variations of the surface.

90° double reflection high-energy electron diffraction experiments on vicinal surfaces of GaAs

Reflection high-energy electron diffraction (RHEED) experiments recording the intensity of the specular beam in two azimuths have been carried out simultaneously. The critical temperature for the transition from 2D nucleation to step-flow growth was simultaneously estimated in the [ 1 10] and [110] directions by observing the disappearance of RHEED intensity oscillations on GaAs(001) substrates 2° misoriented towards GaAs(111)A. It was found that the oscillations disappear at a lower substrate temperature in the [110] azimuth. The difference in the transition temperatures was 25°C.

90° double reflection high-energy electron diffraction experiments on vicinal surfaces of GaAs

Reflection high-energy electron diffraction (RHEED) experiments recording the intensity of the specular beam in two azimuths have been carried out simultaneously. The critical temperature for the transition from 2D nucleation to step-flow growth was simultaneously estimated in the [ 1 10] and [110] directions by observing the disappearance of RHEED intensity oscillations on GaAs(001) substrates 2° misoriented towards GaAs(111)A. It was found that the oscillations disappear at a lower substrate temperature in the [110] azimuth. The difference in the transition temperatures was 25°C.

Study of Epitaxy by RHEED(Reflection High Energy Electron Diffraction)-TRAXS(Total Reflection Angle X-Ray Spectroscopy).

By detecting characteristic X-rays excited during a RHEED (reflection high energy electron diffraction) experiment, a highly sensitive measurement of the adsorbate is possible if the X-ray detector is placed at the critical angle (θC) corresponding to the total reflection of the X-rays. Furthermore, by measuring the glancing angle (θg) dependence, it is possible to analyze the depth distribution of the composition at the surface. The resolution in the depth direction is about 1 ML. By applying these methods to studying the epitaxy of metals (Ag, Au, Sn, Ga and In) on Si( 111 )-n × m- M surface structures (metal-induced Si(111) reconstructed surface structures), new growth modes were observed, such as the substitution-atom growth mode, the floating-atom growth mode and some complex growth modes.

Dynamical analysis of a RHEED pattern from the Si(111)-7 × 7 surface

The multislice formalism of Cowley and Moodie [Acta Crystallogr. 10 (1957) 609] with a recently developed edge-patching method has been applied to the analysis of an experimental RHEED (reflection high energy electron diffraction) pattern from the Si(111)-7 × 7 surface. Several data sets of atomic coordinates for the Si(111)-7 × 7 reconstruction, optimized by different approaches, have been evaluated based upon comparison of visually estimated intensity ordering between the observed and calculated patterns. The results show that compared to other data sets, the coordinates optimized by ab initio-parallel computation appear closer to the true structure, so far as the RHEED in the current scheme can judge. The analysis also shows that the intensities of a RHEED pattern taken from the surface are highly surface sensitive. The surface sensitivity extends to the whole selvage region of interest with a depth of ∼10 Å. The correlation between mean inner potential V 0 and the angle of incidence θ has also been investigated. This indicates that if the angle of incidence is measured with errors less than ± 0.05° for this specific case, V 0 can be determined with the accuracy of ∼± 10%, and vice versa.