Reflection High-energy Electron Diffraction Results Research Articles

Cubic GaN films grown below the congruent sublimation temperature of (0 0 1) GaAs substrates by plasma-assisted molecular beam epitaxy

Gallium nitride films were synthesized on GaAs (0 0 1) substrates at temperatures lower than the congruent sublimation temperature of GaAs. By controlling isothermal desorption of the substrate and setting experimental parameters in the early growth stage, the authors obtained cubic GaN films. No nitridation process or growth of a buffer layer was necessary prior to GaN growth of GaN. In situ reflection high-energy electron diffraction (RHEED) and ex situ high-resolution x-ray diffraction were used to study the crystalline qualities of the films. The measured pole diagram of cubic GaN at 2θ = 34.5° was consistent with RHEED results and confirmed the crystalline structure. Photoluminescence measurements showed a strong emission only at 3.21 eV.

Cleaning of SiC surfaces by low temperature ECR microwave hydrogen plasma

N-type 4H–SiC (0001) surfaces were cleaned by low temperature hydrogen plasma in electronic cyclotron resonance (ECR) microware plasma system. The effects of the hydrogen plasma treatment (HPT) on the structure, chemical and electronic properties of surfaces were characterized by in situ reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). The RHEED results indicate that the structures of the films are strongly dependent on the treatment temperature and time. Significant improvements in quality of 4H–SiC films can be obtained with the temperature ranging from 200°C to 700°C for an appropriate treatment period. The XPS results show that the surface oxygen is greatly reduced and the carbon contamination is completely removed from the 4H–SiC surfaces. The hydrogenated SiC surfaces exhibit an unprecedented stability against oxidation in the air. The surface Fermi level moves toward the conduction band in 4H–SiC after the treatment indicating an unpinning Fermi level with the density of surfaces states as low as 8.09×1010cm−2eV−1.

Investigation of the surface properties of CaF2 layers on (1 1 1) Si as a function of growth temperature

This work reports on the study of surface properties of CaF2 films (30 and 10 nm thick) grown on (1 1 1) Si by molecular beam epitaxy at substrate temperatures from 400 to 700 °C. Reflection high-energy electron diffraction (RHEED) analysis indicated that CaF2 films with smooth surfaces were obtained in temperature ranges 500–550 °C and 620–700 °C, while at temperatures from 400 to 500 °C and in the vicinity of 600 °C the films showed grains randomly oriented on top of the surface. Atomic force microscopy (AFM) investigation corroborated with the RHEED results and confirmed the presence of grains on the film surface, with an evident transition near 600 °C. The dependence of grain density on the growth temperature followed the expectation from the RHEED analysis. The arithmetical average roughness of the CaF2 surface obtained from the AFM images remained below 1 nm for the best quality films. The x-ray reflectivity curves of all samples exhibited well-defined interference fringes, whose oscillation damping behaviour agreed with the RHEED and AFM results. The CaF2 layer thickness and roughness were accurately determined by a best-fit procedure applied to the x-ray reflectivity data. By combining all results, the temperature range between 525 and 550 °C was found to be the most suitable to grow CaF2 layers on (1 1 1) Si. For growth temperatures above 650 °C, pinholes and cracks started to reduce the CaF2 surface quality.

Two-Dimensional Phase Diagram of Lead on Molybdenum (110) Substrate

Surface structures constructed by metallic ultrathin layers on molybdenum (110) or tungsten (110) substrates have been investigated for many years with using reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and along with other effective techniques, for examples, see refs. 1–7. Historically, Tikhov et al. investigated the lead surface structures on molybdenum (110) substrate by using LEED, AES and TDS, however primarily restricted to room temperature (RT). After that, Jo et al. investigated the lead surface structures on molybdenum (110) substrate as the functions of both film thicknesses and substrate temperatures. At present, it is well-known that four kinds of surface structures appear in case of the lead-on-molybdenum (110) system. A two-dimensional phase diagram of this system has been developed based on the RHEED results. The change process of the surface structures from the (10 4) to (2 10) via (2 3) structures in the temperature range from RT to 873K is also discussed mainly based on the lattice coincidency. And more, the growth modes of the lead-onmolybdenum (110) system have been firstly confirmed by using RHEED specular beam intensity oscillations. All experiments were performed in an ultrahigh-vacuum (UHV) chamber with a base pressure of 1 10 10 Torr. A molybdenum single crystal with a (110) plane within a 0.05 deviation was prepared as a substrate. Details of the experimental procedure have been described elsewhere. To investigate both surface structures and growth modes in detail, the lead deposition thicknesses were changed from 0.1 to 60 monolayers (ML) in 0.1ML steps and the substrate temperatures were changed from RT to 1173K in 50K steps. Resultantly, four kinds of surface structures, as shown in Fig. 1, such as (a) the (10 4), (b) (2 3), (c) (2 10), and (d) (3 2) structures could be confirmed to appear in the leadon-molybdenum (110) system. The detailed discussion for RHEED analyses has already been performed in the previous paper. Each regions appearing each given structures are indicated in Fig. 2 as a two-dimensional phase diagram. The (10 4) structure appeared in the thickness range from 0.4 to 1.1ML and in the temperature range from RT to 1023K, and in the thickness range above 1.1ML and in the temperature range from 973 to 1023K, as indicated by [ Pb]. Unit vectors of the (10 4) structure are ja1j 1⁄4 3:50 A and jb1j 1⁄4 3:44 A, and the angle between them is 121 . The line coincidency can be recognized as follows; in every ninth lead-row and tenth molybdenum-row along the 1⁄2 110 Mo direction, and in every third lead-row and every fourth leadrow along the 1⁄2001 Mo direction. The atomic density is calculated as 9:6 10 atoms cm , corresponding to the 1.02ML. As with the (10 4) structure, the basic informations on the other surface structures are summarized as follows; (2 3) structure [ Pb]: the thickness range of 1.1– 1.3ML and the temperature range of RT to 923K, and the thickness range above 1.3ML and temperature range of 923 to 973K, unit lengths ja2j 1⁄4 3:15 A and jb2j 1⁄4 3:43 A, unit angle of 103 , and atomic density of 9:52 10/cm; the (2 10) structure [ Pb]: 1.3–1.5ML and RT to 923K, above 1.3ML and 873–923K, ja3j 1⁄4 3:15 A, jb3j 1⁄4 3:30 A, 105.8 , b1 a1

Reflection High-Energy Electron Diffraction Characterization of Cobalt Clusters Electrodeposited on a Au(100) Electrode

Cobalt was electrodeposited onto a reconstructed Au(100) electrode in 0.25 M Na2SO4 + 0.1 mM CoSO4 under potentiostatic conditions. The Au(100) electrode loaded with the Co deposit was transferred via an argon environment to ultrahigh vacuum and subsequently examined with reflection high-energy electron diffraction (RHEED). The resultant RHEED results were analyzed in detail to unveil the structures of the as-prepared Co deposit, which assumed nanometer-sized clusters on the Au(100) substrate. Aided by coulometric results, profile analysis of the RHEED results indicated that the Co clusters on average were 3 nm in width and 15 atomic layers in height. The packing habits of the Co clusters, determined by the obtained RHEED results, were the face-centered cubic (fcc) phase, rather than the hexagonal closed-packed (hcp) phase. Cobalt clusters grew with their fcc (1̅10) plane aligned parallel to the Au(100) substrate and with their [110] axis aligned in the [011] direction of the Au(100) substrate. The preferential growth of the fcc Co phase is driven by the optimal lattice match between the (1̅10) plane of Co clusters and the Au(100) substrate network.

Structure of the SiC(0001)- [formula omitted] × [formula omitted]-R30° surface after initial oxidation

The structure of the 3 × 3 -R30° reconstruction on the (0001) surface of silicon carbide before and after initial oxidation at room temperature was determined with reflection high-energy electron diffraction (RHEED) rocking curves in the one-beam condition and density-functional theory calculations. The structural parameters of the clean surface are in good agreement with previous reports. For the oxidized surface, good agreement between experimental and calculated RHEED rocking curves was obtained for a model with oxygen atoms in the backbonds of the Si adatom and on its top (ad-ins × 3). The atomic positions of the oxidized surface determined from the density-functional theory calculations agree reasonably well with the RHEED results.

Growth of EuTe islands on SnTe by molecular beam epitaxy

Semiconductor magnetic quantum dots are very promising structures, with novel properties that find multiple applications in spintronic devices. EuTe is a wide gap semiconductor with NaCl structure, and strong magnetic moments S=7/2 at the half filled 4f 7 electronic levels. On the other hand, SnTe is a narrow gap semiconductor with the same crystal structure and 4% lattice mismatch with EuTe. In this work, we investigate the molecular beam epitaxial growth of EuTe on SnTe after the critical thickness for island formation is surpassed, as a previous step to the growth of organized magnetic quantum dots. The topology and strain state of EuTe islands were studied as a function of growth temperature and EuTe nominal layer thickness. Reflection high energy electron diffraction (RHEED) was used in-situ to monitor surface morphology and strain state. RHEED results were complemented and enriched with atomic force microscopy and grazing incidence X-ray diffraction measurements made at the XRD2 beamline of the Brazilian Synchrotron. EuTe islands of increasing height and diameter are obtained when the EuTe nominal thickness increases, with higher aspect ratio for the islands grown at lower temperatures. As the islands grow, a relaxation toward the EuTe bulk lattice parameter was observed. The relaxation process was partially reverted by the growth of the SnTe cap layer, vital to protect the EuTe islands from oxidation. A simple model is outlined to describe the distortions caused by the EuTe islands on the SnTe buffer and cap layers. The SnTe cap layers formed interesting plateau structures with easily controlled wall height, that could find applications as a template for future nanostructures growth.

Fabrication and Luminescent Properties of 6H-SiC/3C-SiC/6H-SiC Quantum Well Structure

Quantum well structure film of 6H-SiC/3C-SiC/6H-SiC was fabricated on 6H-SiC (0001) with the substrate temperature of 1350 K by solid source molecular beam epitaxy (SSMBE) through the variation of Si flux rate. The crystal polytypes and luminescent properties of the film were characterized by reflection high energy electron diffraction (RHEED) and photoluminescence (PL), respectively. The results of RHEED indicated that the film was 6H-SiC/3C-SiC/6H-SiC with the quantum well structure. The results of PL excited by He-Gd laser at room temperature showed that there were intense emissions in the range of 480–600 nm, which could not be observed from the substrate. The fitting peaks were consistent with the results calculated from the model of quantum well structure, which showed that such intense emissions were probably from the quantum wells with different widths.

Room-temperature growth of AlN/TiN epitaxial multi-layer by laser molecular beam epitaxy

We have fabricated epitaxial AlN thin films at room temperature on sapphire (0001) substrates with a TiN (111) epitaxial buffer layer by pulsed laser deposition in ultra-high vacuum (laser molecular beam epitaxy method). The TiN buffer layers were also fabricated at room temperature. Four-circle X-ray diffraction analysis and reflection high-energy electron diffraction results indicate the heteroepitaxial structure of AlN (0001)/TiN (111)/sapphire (0001) with the epitaxial relationship of AlN [10–10]||TiN [11–2]||sapphire [11–20]. The surface of the room-temperature grown AlN film was found to be atomically flat, reflecting the nano-stepped surface of ultrasmooth sapphire substrates. Then, we could achieve the room-temperature epitaxial growth of [AlN/TiN] multi-layer. The temperature dependence of resistivity of the AlN/TiN multi-layer film was also measured.

Temperature dependence of Bi behavior in MBE growth of InGaAs/InP

The behavior of bismuth (Bi) in the molecular beam epitaxy (MBE) growth of InGaAs/InP was investigated in situ by reflection high energy electron diffraction (RHEED) and ex situ by X-ray diffraction (XRD) and atomic force microscopy (AFM). It was found that the behavior of Bi in InGaAs growth can be divided into two categories: incorporation and surfactant, depending on the growth temperature. A low growth temperature was favored for Bi incorporation in InGaAs to form the quaternary InGaAsBi alloy. The amount of incorporated Bi rapidly decreased with increasing growth temperature. At the normal MBE growth temperature for InGaAs/InP, 450 °C, no Bi is incorporated in InGaAs films. However, Bi can act as a surfactant during the MBE growth of InGaAs/InP at that temperature because it changes the growth mode from 3D to 2D as growth begins, and it improves surface smoothness, as confirmed by RHEED and AFM results.

Growth and surface characterization of sputter-deposited molybdenum oxide thin films

Molybdenum oxide thin films were produced by magnetron sputtering using a molybdenum (Mo) target. The sputtering was performed in a reactive atmosphere of an argon–oxygen gas mixture under varying conditions of substrate temperature ( T s) and oxygen partial pressure ( pO 2). The effect of T s and pO 2 on the growth and microstructure of molybdenum oxide films was examined in detail using reflection high-energy electron diffraction (RHEED), Rutherford backscattering spectrometry (RBS), energy-dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) measurements. The analyses indicate that the effect of T s and pO 2 on the microstructure and phase of the grown molybdenum oxide thin films is remarkable. RHEED and RBS results indicate that the films grown at 445 °C under 62.3% O 2 pressure were stoichiometric and polycrystalline MoO 3. Films grown at lower pO 2 were non-stoichiometric MoO x films with the presence of secondary phase. The microstructure of the grown Mo oxide films is discussed and conditions were optimized to produce phase pure, stoichiometric, and highly textured polycrystalline MoO 3 films.

Surface morphology of the Si(1 1 1) surface induced by Co-deposition of Si and CH 4

Co-deposition of Si and CH 4 gas on a Si(1 1 1) surface at different temperatures is studied by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). For comparison, Si deposition on Si(1 1 1) and exposure of a Si surface to CH 4 are also investigated. From RHEED patterns, it is found that a SiC layer was only obtained by exposing Si to 1.8×10 4 L of CH 4 at 800 °C. The AFM images corresponding to RHEED results show that the step width of the clean 7× 7 surface is similar to that of SiC grown at 800 °C, while the step width of 3D-Si grown by co-deposition of Si and CH 4 at 600 °C is about five times wider. It is considered that the presence of CH 4 in the co-deposition with Si enhances the step width on the surface. Triangular Si islands with average size about 30 nm are observed at the terrace region in the co-deposition case, while in the case of CH 4 deposition, circular shaped SiC islands with an average size of 10 nm are observed.

Epitaxial growth of Au(1 1 1) on α-Al 2O 3(0 0 0 1) by using a Co seed layer

The influence of a Co seed layer on the structure of Au films grown onto α-Al 2O 3(0 0 0 1) single crystal substrates by molecular beam epitaxy has been studied by reflection high energy electron diffraction (RHEED). The RHEED results proved that the initial deposition of a few monolayers of Co onto Al 2O 3(0 0 0 1) prior to the Au deposition yielded high-quality fcc Au(1 1 1)-oriented epitaxial films and a well-known (23×1) surface reconstruction structure. Based on the RHEED results, it was concluded that the adhesion energy between the Co seed layer and Al 2O 3 substrate was rather important than the lattice mismatch between them.

Interpretation of initial stage of 3C-SiC growth on Si(100) using dimethylsilane

Abstract The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).

Atomically engineered interfaces for spin injection: ultrathin epitaxial Fe films grown on As- and Ga-terminated GaAs(0 0 1) substrates

We compare ultrathin Fe films grown epitaxially at 50°C on GaAs(001) substrates having different surface reconstructions in a film thickness range of 2.5–140 monolayers (ML). The surface reconstruction-dependent nucleation of the Fe films was characterized by azimuthal reflection high-energy electron diffraction (RHEED) scans. The Fe/GaAs interface is more abrupt for Fe growth on As-terminated GaAs(001) templates than on Ga-terminated ones. Our studies of the magnetic properties of the Fe films show that the strong uniaxial anisotropy with the easy axis of magnetization along [110] is not directly related to the specific GaAs(001) surface reconstruction. The onset of ferromagnetism in our Fe films starts at 3.5ML at room temperature. This is only slightly larger than that for Fe films grown on metallic single crystal substrates. The saturation magnetization for Fe films above 4ML thickness is very close to the bulk value of Fe indicating the absence of interfacial Fe–Ga–As compounds. Fe films with smaller thickness show a reduced magnetic moment which we ascribe to the unfinished coalescence process of Fe islands at the initial stage of growth in agreement with RHEED results.

Fabrication and magnetoresistance of tunnel junctions using half-metallic Fe3O4

Magnetite (Fe3O4) is believed to be half metal, providing 100% spin-polarized conduction electrons. The half-metallic nature of magnetic electrodes for tunneling junction devices is expected to induce a large magnetoresistance. We investigated the structural and chemical properties of interfaces in ferromagnet–insulator–ferromagnet (Fe3O4/MgO/Fe) tunnel junctions. Al/Ag/Fe3O4/MgO multilayers for magnetic tunnel junction have been fabricated on α-Al2O3 (001) and MgO (100) substrates by a molecular beam epitaxy system. The Fe3O4 quality was examined by reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), superconducting quantum interference device magnetometry, atomic force microscopy (AFM), and in situ x-ray photoelectron spectroscopy. RHEED and XRD results showed that the epitaxial Fe3O4 layer with a smooth surface was successfully grown on substrates. The stoichiometric Fe3O4 was confirmed by Verway transition in temperature dependence of magnetization. AFM data showed relatively smooth surface for Fe3O4 prepared at Ts=250 °C and P(O2)=3×10−3 Pa. The Fe 2p3/2 and Fe 2p1/2 peak profiles for Fe3O4 layer are little changed by overlaying MgO in the XPS measurements. These results suggest the Al/Ag/Fe3O4/MgO multilayers available for spin-dependent tunnel junction.

Isothermal adsorption and desorption studies of In/Si( [formula omitted]) by reflection high energy electron diffraction and fluorescent X-ray spectroscopy

Isothermal adsorption and desorption processes of In/Si(1 1 1) were studied by reflection high energy electron diffraction (RHEED) and fluorescent X-ray spectroscopy. In the isothermal adsorption process at room temperature, a sticking probability of In on Si(1 1 1) is unity even in a multilayer region, and the 7×7 structure is replaced by 1×1 during adsorption of In up to 2 monolayers (ML). In the isothermal desorption process at temperatures between 360 and 460 °C, superstructure of In/Si(1 1 1) changes to 1×1, 4×1, 31 × 31 , 3 × 3 , 7×7, in turn. Structure change and In desorption from each phase go together and all desorption rates are zeroth order. Activation energies of In desorption from 1×1, 4×1, 31 × 31 and 3 × 3 phases are estimated to 0.48, 1.5, 1.6 and 1.9 eV, respectively. From RHEED and isothermal desorption results, saturation coverage of 4×1 and 31 × 31 phases are estimated to be 0.96±0.15 and 0.56±0.06 ML, respectively.

Investigation of YBCO film growth by post-annealing of precursor films including BaF2 at low-pressure oxygen atmosphere

Precursor films were deposited by the co-evaporation technique using Y, BaF2 and Cu as evaporation sources. Then, the films were annealed at low-pressure oxygen atmosphere without the introduction of water vapour, which was different from the so-called BaF2 ex situ process. Previous studies have indicated that the c-axis-oriented YBCO films can be prepared by annealing at low-pressure oxygen atmosphere, and that the reaction of the precursor films during sample temperature elevation is very important in the present process according to reflection high-energy electron diffraction (RHEED) observation. Therefore, two kinds of samples, rapidly cooled down from 400 °C and 630 °C, were prepared for microstructural study. YBCO film growth in this process is discussed on the basis of the results of x-ray diffraction (XRD), RHEED and transmission electron microscopy (TEM).

Occurrence of CuPt-A and CuPt-B type ordering in GaInP layers grown by solid source molecular beam epitaxy

Transmission electron diffraction (TED) and reflection high-energy electron diffraction (RHEED) have been used to investigate GaInP molecular beam epitaxial layers grown at temperatures in the range 445–510°C. TED results reveal the presence of diffracted intensity at 1 2 (−1 1 1) and 1 2 (1 −1 1) positions, indicating the occurrence of CuPt-B ordering. As the growth temperature decreases, the superlattice spots move toward 1 2 {−1+δ,1−δ,0} positions, where the value of δ is 0.15. It is further shown that for the layers grown at temperatures in the range 445–490°C, weak diffracted intensity is observed at 1 2 (1 1 −1) and 1 2 (1 1 1) positions, indicating the formation of CuPt-A ordering. Based on the TED and RHEED results, explanations are given to describe the formation of CuPt-A and CuPt-B type ordering. Photoluminescence results show that the samples grown at temperatures in the range 470–510°C undergo a bandgap reduction. This is attributed to the presence of ordered structures and composition modulation.

Surface structure of anataseTiO2(001):Reconstruction, atomic steps, and domains

The surface structure of anatase ${\mathrm{TiO}}_{2}(001)$ was investigated using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED), and low-energy electron diffraction (LEED). A two-domain $(1\ifmmode\times\else\texttimes\fi{}4)/(4\ifmmode\times\else\texttimes\fi{}1)$ reconstruction, similar to those previously reported in LEED and ion scattering studies, was observed by STM and RHEED. This reconstruction was found to be stable not only from room temperature to 850 \ifmmode^\circ\else\textdegree\fi{}C in ultrahigh vacuum and oxygen-rich environments, but also during the anatase film growth. High-resolution STM images obtained at positive sample biases revealed two types of atomic row within each surface unit cell, indicating different Ti-derived states at the surface. At the same time, XPS of the reconstructed surfaces showed no evidence of ${\mathrm{Ti}}^{3+}.$ Based on the STM, XPS, RHEED, and LEED results, an ``added''-and-``missing''-row model is proposed to account for the $(1\ifmmode\times\else\texttimes\fi{}4)$ reconstruction. Atomic steps and their relationship to the population of $(1\ifmmode\times\else\texttimes\fi{}4)$ and $(4\ifmmode\times\else\texttimes\fi{}1)$ domains were also investigated. The results showed that for vicinal surfaces the domain population depended strongly on the overall surface step orientation. While populations of the $(1\ifmmode\times\else\texttimes\fi{}4)$ and the $(4\ifmmode\times\else\texttimes\fi{}1)$ domains were nearly equal on flat (001) surfaces, they became significantly lopsided on a surface with its normal 2\ifmmode^\circ\else\textdegree\fi{} away from the (001) direction, demonstrating a strong correlation between surface steps and domain population on vicinal surfaces.