Reflection High-energy Electron Diffraction Study Research Articles

RHEED Study of the Epitaxial Growth of Silicon and Germanium on Highly Oriented Pyrolytic Graphite

Two-dimensional silicon (silicene) and germanium (germanene) have attracted special attention from researchers in recent years. At the same time, highly oriented pyrolytic graphite (HOPG) and graphene are some of the promising substrates for growing silicene and germanene. However, to date, the processes occurring during the epitaxial growth of silicon and germanium on the surface of such substrates have been poorly studied. In this work, the epitaxial growth of silicon and germanium is studied directly during the process of the molecular beam epitaxy deposition of material onto the HOPG surface by reflection high-energy electron diffraction (RHEED). In addition, the obtained samples are studied by Raman spectroscopy and scanning electron microscopy. A wide range of deposition temperatures from 100 to 800 °C is considered and temperature intervals are determined for various growth modes of silicon and germanium on HOPG. Conditions for amorphous and polycrystalline growth are distinguished. Diffraction spots corresponding to the lattice constants of silicene and germanene are identified that may indicate the presence of areas of graphene-like 2D phases during epitaxial deposition of silicon and germanium onto the surface of highly oriented pyrolytic graphite.

Solid-phase epitaxial growth of the correlated-electron transparent conducting oxide SrVO3

$\mathrm{SrV}{\mathrm{O}}_{3}$ thin films with a high figure of merit for applications as transparent conductors were crystallized from amorphous layers using solid phase epitaxy (SPE). Epitaxial $\mathrm{SrV}{\mathrm{O}}_{3}$ films crystallized on $\mathrm{SrTi}{\mathrm{O}}_{3}$ using SPE exhibit room-temperature resistivities as low as $5.2\ifmmode\times\else\texttimes\fi{}{10}^{--5}$ and $2.5\ifmmode\times\else\texttimes\fi{}{10}^{--5}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, residual resistivity ratios of 2.0 and 3.8, and visible light transmission maxima of 0.89 and 0.52 for film thicknesses of 16 and 60 nm, respectively. $\mathrm{SrV}{\mathrm{O}}_{3}$ layers were deposited at room temperature using radiofrequency sputtering in an amorphous form and subsequently crystallized by heating in a controlled gas environment. The lattice parameters and mosaic angular width of x-ray reflections from the crystallized films are consistent with partial relaxation of the strain resulting from the epitaxial mismatch between $\mathrm{SrV}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$. A reflection high-energy electron diffraction study of the kinetics of SPE indicates that crystallization occurs via the thermally activated propagation of the crystalline/amorphous interface, like SPE phenomena in other perovskite oxides. Thermodynamic calculations based on density functional theory predict the temperature and oxygen partial pressure conditions required to produce the $\mathrm{SrV}{\mathrm{O}}_{3}$ phase and are consistent with the experiments. The separate control of deposition and crystallization conditions in SPE presents possibilities for the crystallization of transparent conductors in complex geometries and over large areas.

Submonolayer epitaxy growth of fractional monolayer CdSe/ZnSe quantum dots

A heterostructure containing several sheets of CdSe fractional monolayer quantum dots with nominal coverages of 0.5 and close to 0.25 of a monolayer (ML) embedded in a ZnSe matrix was successfully grown by a combination of submonolayer epitaxy and molecular beam epitaxy. Due to surface reconstruction properties of cation- and anion-terminated surfaces, one complete Cd–Se alternate source exposure cycle produces a submonolayer saturation coverage of around 0.5 ML of CdSe. To obtain specific coverages lower than 0.5 ML, it is necessary to work with an unsaturated Cd surface, then the control of Cd exposure time becomes crucial. To determine the appropriate Cd exposure times, a careful study of the adsorption process and knowledge of the time to obtain the saturation coverage are required. Four separated sheets of CdSe fractional monolayer quantum dots with coverage Θ ≃ 0.5 ML were deposited in the first section of the heterostructure to analyze the surface reconstruction streaks in the reflection high energy electron diffraction (RHEED) patterns. The analysis performed during the adsorption process to reach a Cd saturated surface (nominal coverage Θ = 0.5 ML) suggests that the large Cd atoms produce strain-induced disorder in the underlying layers during the initial stages of the Cd chemisorption process. The results of the RHEED study were employed to estimate the Cd exposure time for the growth of the last sheet of CdSe fractional monolayer quantum dots with Θ ≃ 0.25 ML. The low temperature photoluminescence spectrum presented two intense and narrow excitonic peaks at energies of 2.750 eV and 2.778 eV, corresponding to the CdSe fractional monolayer quantum dots with nominal coverages of 0.5 ML and ∼0.25 ML, respectively, with narrower linewidth for lower CdSe coverage.

Large-area epitaxial growth of MoSe2 via an incandescent molybdenum source

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe2 thin films. The as-grown MoSe2 thin films were characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence (PL), reflection high energy electron diffraction (RHEED) and angular resolved photoemission spectroscopy (ARPES). A new Raman characteristic peak at 1591 cm−1 was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe2 could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

Growth of Ag nanostructures on high-index Si (5 5 12) surfaces under UHV conditions: effect of prior surface treatment before deposition

High-index substrates, such as Si (5 5 12), provide self-assembled one-dimensional periodic array of structures upon reconstruction and can be used as templates for growth of aligned nanoscale structures. Crystalline quality of Ag nanostructures grown under UHV conditions using a molecular beam epitaxy system on reconstructed Si (5 5 12) surface is compared with those structures grown on HF-treated and native-oxide-covered Si (5 5 12) surfaces. These have been characterized by using reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM) and in situ scanning tunneling microscopy methods. The RHEED and TEM studies confirm the polycrystalline, textured and crystalline nature of the Ag nanostructures grown on Si (5 5 12) with native oxide, HF-treated, and reconstructed surfaces, respectively. The crystalline quality of Ag nanostructures on HF-treated substrate was found to improve at higher substrate temperatures during growth.

RHEED Study of the Texture in Polycrystalline Films of Magnetite Grown on Oxidized Silicon Surface

Polycrystalline films of magnetite (Fe3O4) with thickness of 75 nm were grown on SiO2/Si (001) surface by reactive deposition of Fe in O2 atmosphere. The growth of Fe3O4 films was conducted with the different oxygen pressures. The structure of films was monitored by reflection high-energy electron diffraction (RHEED) during the film growth. It was found that there is a range of oxygen pressure in which the growth of only textured Fe3O4 film takes place. Reactive deposition of Fe at lower oxygen pressure results in a growth of Fe3O4 film without a texture. In contrast at the deposition with higher oxygen pressure the texture in the Fe3O4 film remains but the appearance of hematite (α-Fe2O3) is observed.

Strain in epitaxial Bi2Se3 grown on GaN and graphene substrates: A reflection high-energy electron diffraction study

Topological insulator (TI) has been one of the focus research themes in condensed matter physics in recent years. Due to the relatively large energy bandgap, Bi2Se3 has been identified as one of the most promising three-dimensional TIs with application potentials. Epitaxial Bi2Se3 by molecular-beam epitaxy has been reported by many groups using different substrates. A common feature is that Bi2Se3 grows readily along the c-axis direction irrespective of the type and condition of the substrate. Because of the weak van deer Waals interaction between Bi2Se3 quintuple layers, the grown films are reported to be strain-free, taking the lattice constant of the bulk crystal. At the very initial stage of Bi2Se3 deposition, however, strain may still exist depending on the substrate. Strain may bring some drastic effects to the properties of the TIs and so achieving strained TIs can be of great fundamental interests as well as practical relevance. In this work, we employ reflection high-energy electron diffraction to follow the lattice constant evolution of Bi2Se3 during initial stage depositions on GaN and graphene, two very different substrates. We reveal that epitaxial Bi2Se3 is tensile strained on GaN but strain-free on graphene. Strain relaxation on GaN is gradual.

RHEED Study of Epitaxial Graphene on Conductive and Semi-Insulating 6H-SiC (0001) Substrates

The technique of reflection high-energy electron diffraction (RHEED) has been applied to study the graphene growth on conductive and semi-insulating 6H-SiC (0001) substrates using two RHEED devices. It was found the oriented growth of graphene on the conductive wafer and both oriented as disordered graphene growth on semi-insulating wafer due to the partial formation of polycrystalline component in nanocarbon film. It was shown that the appearance of the graphene polycrystalline phase was caused by the lower perfection structure of the surface of the semi-insulating substrate as compared to the conductive substrate.

RHEED study on continuously repeated step flow and layer-by-layer growth modes in SrRuO3/SrMnO3 superlattice

Nanolayered superlattices composed of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 layers were grown on SrTiO3 (100) substrates by pulsed laser deposition. Multilayers were grown under predetermined synthesis conditions resulting in growth of SrRuO3 and SrMnO3 by step flow and layer-by-layer modes, respectively. The growth of SrMnO3 was observed to occur through the layer-by-layer during the entire deposition process despite the expected increase in surface roughness because of the incorporation of SrRuO3 upper layers. Monitoring by reflection high-energy electron diffraction (RHEED) revealed that the growth of every SrMnO3 layer consisted of a pre-stage during which the gaps on the relatively rough SrRuO3 surface were filled before the actual growth of the SrMnO3 layer, which resulted in incomplete half oscillation and change from spot patterns to streaky patterns. The in-plane lattice constant did not show any considerable change in the case of SrRuO3 and SrMnO3 layers, despite the considerable lattice mismatch between the two materials (SrRuO3, SrMnO3) and SrTiO3. On the other hand, the RHEED patterns showed the existence of lattice mismatch effects in the out-of-plane lattice constant, which showed significant strains of opposite signs in the different layers, indicating a strong dependence on the composition of the layers and superlattice periodicity. In this paper, the growth characteristics of a SrRuO3/SrMnO3 multilayer along with its magnetic properties will be discussed.

Modified energetics and growth kinetics on H-terminated GaAs (110)

Atomic hydrogen modification of the surface energy of GaAs (110) epilayers, grown at high temperatures from molecular beams of Ga and As4, has been investigated by friction force microscopy (FFM). The reduction of the friction force observed with longer exposures to the H beam has been correlated with the lowering of the surface energy originated by the progressive de-relaxation of the GaAs (110) surface occurring upon H chemisorption. Our results indicate that the H-terminated GaAs (110) epilayers are more stable than the As-stabilized ones, with the minimum surface energy value of 31 meV∕Å(2) measured for the fully hydrogenated surface. A significant reduction of the Ga diffusion length on the H-terminated surface irrespective of H coverage has been calculated from the FFM data, consistent with the layer-by-layer growth mode and the greater As incorporation coefficient determined from real-time reflection high-energy electron diffraction studies. Arsenic incorporation through direct dissociative chemisorption of single As4 molecules mediated by H on the GaAs (110) surface has been proposed as the most likely explanation for the changes in surface kinetics observed.

RHEED and XPS study of cerium interaction with SnO2 (110) surface

Abstract Interaction of cerium with thin epitaxial film of tin dioxide with (110) surface orientation was investigated by methods of X-ray Photoelectron Spectroscopy (XPS) and Reflection High Energy Electron Diffraction (RHEED). Strong interaction of cerium and tin leads to cerium diffusion into the tin dioxide bulk and formation of tin–cerium mixed oxide indicated by highly non-stoichiometric cerium oxide Ce 3d core level spectrum. This process is more pronounced with higher temperature during the deposition of cerium. With higher amount of deposited cerium, the tin dioxide substrate is gradually saturated with cerium atoms and the cerium atoms start to create 3D epitaxial clusters of cerium oxide with the (100) and (111) crystallographic planes parallel to the surface. The clusters show a higher oxidation state than the cerium atoms located in the tin oxide bulk. Oxygen required for the creation of these clusters comes from the tin dioxide substrate, which is partially reduced to metallic tin in this process. At the end of the experiment, we obtained a multilayer system consisting of layers of partially reduced tin dioxide, tin–cerium mixed oxide and top layer of partially reduced epitaxial grains of cerium oxide.

A look inside epitaxial cobalt-on-fluorite nanoparticles with three-dimensional reciprocal space mapping using GIXD, RHEED and GISAXS

In this work epitaxial growth of cobalt on CaF2(111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise non-achievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF2 layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright 〈111〉 streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles.

Publisher’s Note: “Azimuthal reflection high-energy electron diffraction study of MnAs growth on GaAs(001) by molecular beam epitaxy” [J. Appl. Phys. 110, 023505 (2011)

Publisher’s Note: “Azimuthal reflection high-energy electron diffraction study of MnAs growth on GaAs(001) by molecular beam epitaxy” [J. Appl. Phys. 110, 023505 (2011)

Azimuthal reflection high-energy electron diffraction study of MnAs growth on GaAs(001) by molecular beam epitaxy

Azimuthal reflection high-energy electron diffraction (ARHEED) and in situ grazing incidence synchrotron x-ray diffraction techniques are employed to investigate the growth, epitaxial orientation, and interfacial structure of MnAs layers grown on GaAs(001) by molecular beam epitaxy (MBE). We demonstrate the power and reliability of ARHEED scans as a routine tool in characterizing the formation of epitaxial films. The ARHEED scans clearly reveal the formation of the rectangular MnAs unit cell during growth on GaAs(001) for a MnAs layer thickness of 2.1 ± 0.2 monolayers with a tensile strain along the MnAs[112−0] direction. A periodic coincidence site lattice, which is known to form along the MnAs [0001] direction to release the strain due to the huge lattice mismatch (∼30%) also produces periodic satellites of the diffraction spots in the ARHEED scan. The formation of different epitaxial orientations of MnAs during MBE growth can be directly observed using ARHEED scans. ARHEED is demonstrated to have a resolution similar to synchrotron x-ray diffraction with a double crystal monochromator, yielding full width at half maximum values of reflections as small as 0.005 reciprocal lattice units.

Formation of the GaAs-Ge heterointerface in the presence of oxide

The data of the X-ray photoelectron spectroscopy and reflection high-energy electron diffraction studies of the formation of the GaAs-Ge heterointerface under incomplete removal of all oxide phases from the GaAs substrate surface are presented. It is shown that the combination of the processes of final Ga2O desorption and Ge deposition allows one to avoid the evaporation of As and the stoichiometry distortion near the interface.

In situ measurements of the critical thickness for strain relaxation in β-GaN/MgO structures

It is demonstrated by in situ reflection high-energy electron diffraction (RHEED) studies that the growth of cubic GaN on MgO occurs in a layer-by-layer mode under Ga-rich conditions, growth rates 0.25–0.30 ML/s and 700 °C substrate temperature. From streak-spacing analysis of the observed RHEED patterns, it is possible to infer that the GaN layer grows pseudomorphically on MgO up to 3 monolayers (ML). After that, a relaxation process begins and the layer-by-layer or 2-dimensional growth changes to a columnar or 3-dimensional growth, as evidenced by the transformation of the RHEED patterns, which change from streaky to spotty. The experimental critical thickness ( h c ) value of 3 ML is close to the prediction of the Frank–van der Merwe classical model of 2.5 ML. Other models predict h c values 3–6 times the experimental value experimentally observed.

RHEED study of titanium dioxide with pulsed laser deposition

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the growth of thin films of titanium dioxide (TiO 2) on (1 0 0) magnesium oxide (MgO) substrates by pulsed laser deposition (PLD). The deposition is performed with a synthetic rutile TiO 2 target at low fluence. The topography and structure of the deposited layers are characterized using in situ high pressure RHEED and atomic force microscope (AFM). Based on these observations the growth mode of the films is discussed. The results will be compared to earlier results obtained for the growth of TiN films on (1 0 0) MgO.

An effect of As flux on GaAs/AlAs quantum wells: A combined photoluminescence and reflection high-energy electron diffraction study

Photoluminescence (PL) measurements and in situ reflection high-energy electron diffraction (RHEED) observations were used to optimize GaAs/AlAs multi-quantum-well (MQW) structures grown by molecular beam epitaxy (MBE) under different As fluxes. PL peaks were identified with computer simulations, intensity behavior as a function of temperature, and with previous results in literature. The room temperature PL intensity from our MQW samples first increases with the As flux and then decreases as the flux is increased. Combining our RHEED and PL observations, we propose that an optimum As flux for growth of GaAs/AlAs structures is close to the special As flux at which the GaAs(1 0 0) surface reconstruction changes from ( 2 × 4 ) to ( 4 × 2 ). The reasons for the findings are discussed.

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.

RHEED and XPS study of Pd–Sn bimetallic system growth

An early stage of Pd–Sn bimetallic system growth on highly oriented pyrolytic graphite HOPG(0 0 0 1) was observed by means of reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). Both metals were deposited using a molecular beam epitaxy under the UHV conditions. RHEED measurements were performed in real-time during all stages of the experiment. Metal clusters’ growth exhibited strong dependence on metals’ amount, succession of deposited materials and substrate conditions during the deposition. At very low coverages, the epitaxial as well as the polycrystalline phases of tin were observed. With increasing amount of deposited tin, the epitaxial phase became dominant. Palladium depositions with the substrate held on elevated temperature exhibited epitaxial growth. Metallic clusters of both metals grew in Volmer–Weber mode. The formation of Pd–Sn alloy was observed under the specific preparation procedure and was confirmed by both structural and chemical analysis.