Reflection High-energy Diffraction Research Articles

Initial etching of GaAs (001) during H2 plasma cleaning

The initial etching of GaAs (001) during remote H2 plasma etching for oxide removal was examined using atomic force microscopy, coupled with in situ reflection high energy diffraction and Auger spectroscopy. Localized etching of the GaAs surface is particularly evident for etch temperatures below ∼650 K and leads to the formation of etch pits 5–10 nm deep and 20–50 nm in diameter. Etch pit formation was found to occur well before complete oxide removal. The development of etch pits is influenced by surface mis-cut steps, possibly due to increased hydrogen incorporation into the near surface at surface defects such as steps.

Quantitative comparison of surface morphology and reflection high-energy electron diffraction intensity for epitaxial growth on GaAs

The relationship between developing surface morphology and specular spot intensity in reflection high-energy diffraction (RHEED) has been investigated during the homoepitaxial growth of GaAs on various surface orientations. The ‘morphological’ quantities of step density and effective coverage have been measured directly by means of rapid-quench scanning tunnelling microscopy (STM). For growth on the singular (001), (110) and (111)A surfaces, the step density, layer coverage and RHEED intensity oscillate with a period of one monolayer and little damping. The morphological quantities are always in phase, while the phase of the RHEED intensity depends on the diffraction conditions. On vicinal GaAs(001), the RHEED intensity oscillations are rapidly damped, as are oscillations of the morphological quantities, but again the phase of the RHEED intensity can be varied with respect to the other two. In all cases on the (001) surface, a well-ordered (2×4) surface reconstruction develops very rapidly in the growing layer. Implications for theories of RHEED intensity oscillations are discussed.

RHEED study of atomic steps on ZnSe surface growing on vicinal GaAs substrates

Atomic steps of ZnSe surface growing on vicinal GaAs(1 0 0) substrate are investigated with in-situ reflection high-energy electron beam diffraction. The diffraction patterns indicating 1 ML-height step with uniform terrace width are clearly observed as the growth proceeds. The uniformity of step array on growing surface becomes stable by ZnSe growth of 500 Å. The terrace width and substrate vicinal angle measured by the diffraction patterns agree well with the substrate nominal values. The observed diffraction patterns are compared with simulation results based on kinematical diffraction theory to discuss the fluctuation of surface step structure. The improvement of uniformity of step array is anisotropic with regard to the substrate vicinal direction. The insertion of CdSe layer does not cause broadening of full width at half maximum of reciprocal lattice rod, which indicates that the uniformity of step array is unchanged.

Dynamics of Ga Clusters on GaAs(001) Surface

AbstractDynamics of Ga clusters and GaAs epitaxial growth on a GaAs (001) surface were successfully observed by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE) were incorporated into one unit rather than in separate chambers. With the substrate temperature of 528°C, reflection high-energy diffraction (RHEED) patterns showed a (4×6) Ga-stabilized surface reconstruction and dynamics of steps and islands were clearly observed. The detaching and attaching of small Ga clusters might cause the change of steps and islands. It seems that the small Ga clusters migrated with the diameter of about 0.8 to 1.2 nm and around the steps and islands. These clusters could be observed only when they were detached from or attached to the steps and islands. Moreover, even under the substrate temperature of 440 oC and the As4 partial pressure of 2×10−6 torr, STM images were clearly observed. After 0.1 ML Ga was additionally supplied to the sample by migration enhanced epitaxy mode, step flow growth occurred, resulting in an additional GaAs layer grown on the B-step side. Moreover, the c(4×4) As-stabilized surface reconstruction was moderate. It seems that there is an equilibrium additional layer of As amorphous adatoms on the c(4x4) surface reconstructions.

Heteroepitaxial growth of Pt (0 0 1) films on SrTiO 3 by pulsed laser deposition

New functional materials are being considered having in perspective an important role in the development of microsystems. In particular, in an experiment of pulsed laser deposition (equipped with in situ reflection high-energy diffraction of the growth surface) thin Pt epitaxial layers have been deposited on SrTiO3 (0 0 1) substrates. It is shown that thin single-crystal layers of metallic Pt (0 0 1) can be heteroepitaxially grown on SrTiO3 (0 0 1). Furthermore complex SrTiO3–Pt–SrTiO3 heteroepitaxial structures have been deposited. This result is of relevant importance for the growth of new functional materials consisting of heteroepitaxial structures involving both complex oxides and standard metals. These materials may reasonably open up new prospects in the field of microsystem technologies.

Growth Control of Cubic GaN and GaAlN (GaInN) Alloys by RHEED Oscillations

We report on the optimization of growth of cubic GaN and AlN binaries together with cubic GaInN and GaAlN ternary alloys. Reflection high energy diffraction oscillations are observed for all these cubic materials which allow us to determine precisely the stoichiometric growth conditions. The alloy compositions deduced from the growth rate variations by adding indium or aluminium are compared with those obtained by Rutherford backscattering spectra.

RHEED FROM EPITAXIALLY GROWN THIN FILMS

It is shown that dynamical theory can be applied to analyze reflection high energy diffraction (RHEED) data from epitaxially grown samples. First, the theoretical description of the dynamical approach employed is presented in detail. Then examples of experimental data successfully interpreted are given. It is demonstrated that RHEED azimuthal plots may be helpful in gaining detailed information on the arrangement of atoms at the surface when the growth of samples is terminated. Furthermore, it is shown that RHEED oscillations recorded for one-beam conditions (i.e. at off-symmetry azimuths or at very low glancing angles) may be very useful in investigating growing surfaces. Finally, we discuss how the theoretical approach used in this work is related to approaches employed by other researchers, and what kind of investigations may help to develop RHEED further.

Comparison of Thin GaN and AlN Layers Deposited by Plasma Assisted Molecular Beam Epitaxy on 6H-SiC

We have studied the effect of the deposition temperature and high temperature annealing on the quality of thin GaN and AlN layers on 6H-SiC substrates. By systematic characterization using reflection high electron energy diffraction (RHEED) and atomic force microscopy (AFM), we have shown that the two nitrides act differently in the early stage of growth. GaN undergoes a Stransky-Krastanov transition enhanced by the post growth annealing. AlN is less sensitive to the effect of annealing and the layers are comparatively smoother than the GaN ones. GaN gives easily the cubic phase at low temperatures whereas it is less feasible for AlN. For each material, better morphology and crystallinity are achieved at the highest temperature studied (800°C) but AlN is preferable to GaN as no islands are formed with AlN.

Low-temperature molecular beam epitaxy of GaAs: A theoretical investigation of antisite incorporation and reflection high-energy diffraction oscillations

Surface dynamics dominate the incorporation of charged, AsGa+, and neutral, AsGa0, antisite arsenic, and the temporal variation of reflection high-energy electron diffraction (RHEED) intensity in the low-temperature molecular beam epitaxy of (100) gallium arsenide (GaAs). A rate equation model is proposed which includes the presence and dynamics of a physisorbed arsenic (PA) layer riding the growth surface. The PA layer dictates the incorporation and concentration of AsGa+ and AsGa0. Additionally, it influences the RHEED oscillations (ROs) behavior and the RO’s dependence on its coverage through its contribution to the reflected intensity. The model results for the dependence of AsGa+ and AsGa0 concentrations on beam equivalent pressure (BEP) and growth temperature are in good agreement with experimental data. The experimental observations can be explained based on the saturation of the PA coverage at one monolayer and the competing rate processes such as the AsGa incorporation into and evaporation from the crystalline surface. Using the same kinetic model for the temporal behavior of the surface, the contribution of the PA layer to the RHEED intensity is computed based on kinematical theory of electron diffraction. The experimental observation of the ROs during growth at high and low temperatures with no ROs in the intermediate temperature range of 300–450 °C is in good agreement with our model results. At low temperatures, the surface is covered by the PA layer whose step density depends on that of the subsurface crystalline GaAs. Thus, a temporal variation of the step density of subsurface crystalline GaAs results in ROs, but with a different step height, that of the PA layer, of 2.48 Å. At high temperatures, the crystalline GaAs is exposed to the RHEED beam due to the evaporation of the PA layer and the ROs appear due to periodic step-density oscillations with a step height of 1.41 Å, which is the Ga–As crystalline interplanar distance. At intermediate temperatures, the surface is partially covered by the PA layer resulting in RHEED reflection contributions from both surfaces covered by the PA layer and crystal. Due to the very different interplanar distances between the crystalline GaAs and the PA layers, complete destructive interference of the RHEED intensity results at a 0.5 surface coverage of the PA layer. The RO dependence on the As BEP is also presented and discussed.

Stabilization of high spin FCC Fe in (Fe/Pd) n multilayers

Polycrystalline (Fe/Pd) n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd) n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ B, which is about 20% larger compared to μ=2.2 μ B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.

The growth of epitaxial aluminium on As containing compound semiconductors

The growth of epitaxial aluminium on different (1 0 0) oriented compound semiconductors grown using the molecular beam epitaxy technique have been studied. After deposition of the first complete adlayer between the aluminium and the GaAs surface as evidenced by in situ reflection high electron energy diffraction (RHEED), ex situ atomic force microscopy (AFM) images agree that subsequent aluminium deposition is via a 3D nucleated growth mode. RHEED observations during continued deposition of epitaxial aluminium indicate a 2D growth mode dominated by the (1 0 0) orientation. AFM images of the surface of the aluminium reveal that the surface morphology consists of a plateau–valley structure, while transmission electron microscopy characterisation reveals that the aluminium is a (1 0 0) oriented single crystal. For growth of epitaxial aluminium on different (1 0 0) compound semiconductors the resultant hillock–valley morphology of the aluminium is remarkably similar regardless of the underlying semiconductor. There is no apparent difference between the aluminium growth on GaAs and Al 0.6Ga 0.4As indicating that the aluminium content of the semiconductor is having no effect on the growth of the aluminium, whereas there can be a difference in the hillock widths for aluminium grown on In 0.53Al 0.47As and In 0.55Ga 0.45As. The dominant orientation that the aluminium recrystallises to, appears to be determined by the strain between the aluminium 3D nucleates and the underlying semiconductor with (1 0 0) oriented aluminium for tensile strain (growth on GaAs and AlGaAs) and (1 1 0) oriented aluminium for compressive strain (growth on InAlAs and InGaAs).

Surface characterization for sputter-cone formation on InP(100)

The dynamic processes of sputter-cone formation for the InP surface, which occur during the depth profiling of InP/InGaAs multilayers, have been studied by ion scattering spectroscopy (ISS), reflection electron microscopy (REM) and reflection high-energy election diffraction (RHEED) as a function of sputtering time. Results show preferential sputtering of phosphorus atoms and the formation of an altered amorphous layer which is followed by surface roughening as a transition stage to cone formation. After reaching a steady state, the ISS spectra remain almost the same despite ion sputtering, even though the surface becomes covered with a significant number of cones. RHEED and REM observations revealed that the shank of the cone is composed of crystalline InP, which is covered by an altered layer of InP with an In metal ball situated on the top of the cone. Monte Carlo simulation has predicted that preferential sputtering of phosphorus causes the InP surface to be In rich, suggesting that the excess In atoms should agglomerate to form the metallic In ball on the surface.

RHEED study of Pd thin film growth on α-Al 2O 3 substrate

The epitaxial growth of very thin palladium layers on α-Al 2O 3 (112̄0) single-crystalline substrates was controlled in situ by Reflection High Electron Energy Diffraction. The substrates were prepared by heating in air and/or under vacuum in order to obtain well stoichiometric and partially reduced surfaces, respectively. The three-dimensional growth mode has been found, giving two populations of particles having (111) plane parallel to the (112̄0) substrate surface and different azimuthal orientations. The stoichiometry of the substrate surface did not influence the structure and epitaxial parameters of the overlayer.

Molecular beam epitaxial growth of strained layers on graded for Pt silicide Schottky diodes

Low temperature growth of a high quality strained film on graded is demonstrated using MBE. In situ monitoring of reflection high electron energy diffraction (RHEED) patterns and intensity oscillations during different stages of the growth has shown two-dimensional pseudomorphic growth. The layers have been further characterized by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM). The Schottky barrier height of platinum-silicide (PtSi) on p-type has been found to vary between 0.19-0.57 eV in the temperature range 100-300 K. At 300 K, the ideality factor has been found to be 2.0 and 1.32 for PtSi/ and PtSi/ diodes, respectively.

Epitaxial growth of n- and p-type ZnCdSe on InP

Chlorine doping and nitrogen doping of lattice-matched ZnCdSe on InP were investigated. High-quality ZnCdSe films were successfully grown on a GaInAs buffer layer using a dual-growth-chamber molecular-beam epitaxy system. The reflection high-energy diffraction oscillations on the ZnCdSe surface persisted for about 80 periods after the ZnCdSe layer growth was started. The full-width at half-maximum of the double-crystal X-ray (0 0 4) diffraction curve was as narrow as 42 arcsec. Epitaxial chlorine doping and epitaxial nitrogen doping were performed using ZnCl 2 and a radio-frequency nitrogen-plasma source, respectively. In the ZnCdSe: Cl layer, free-electron concentrations up to 7×10 18cm −3 were obtained. In the ZnCdSe: N layer, p-type conductivity was observed for the first time with a maximum net acceptor concentration of 3.5 × 10 16 cm −3.

MAGNETO-OPTICAL SPECTRA OF ULTRATHIN Co FILMS SANDWICHED BETWEEN Pt LAYERS

Magneto-optical Kerr spectra have been measured for Pt(2 nm)/Co(0.2-2 nm)/Pt(60 nm) sandwich films, prepared by a molecular beam epitaxy technique onto MgO(111) substrates. From the reflection high energy diffraction observations, the Co layers on the Pt(111) base layers were confirmed to have a twinned fcc structure. The Kerr rotation of the sandwich films increased with increasing photon energy in a similar manner to those of Pt-Co alloys and multilayers, and as the Co layers become thicker, the magnitude of Kerr rotation becomes larger throughout the range of measurement (0.8∼4.1 eV). In order to characterize the magneto-optical properties of Pt/Co interfaces, the ωσxyspectrum of a Pt monolayer was estimated from the ωσxy of the sandwich by assuming that the Pt monolayer neighboring the Co layer are spinpolarized. The ωσxy for the Pt monolayer are considerably larger in magnitude than those of thick Co and Pt-Co sputtered alloy films in the high energy region. The experimental Kerr spectra with various Co thicknesses were reproduced by the simulation using ωσxy for the spin-polarized Pt.

Simulation of GaAs growth and surface recovery with respect to galliumand arsenic surface kinetics

A Monte Carlo simulation of GaAs homoepitaxy and subsequent surface recovery is performed using a solid-on-solid configuration. It is assumed that both Ga atoms and GaAs molecules are mobile during growth and contribute to the surface-diffusion flux. When growth is terminated by stopping the Ga supply, all Ga adatoms are found to react very quickly to form GaAs molecules. Thus, during recovery only diffusion of less mobile GaAs molecules is of relevance. The simulation is parametrized by a detailed comparison with reflection high electron energy diffraction measurements. In particular, the recovery phase after growth stops, the oscillation damping, and the ${\mathrm{As}}_{4}$ sticking coefficient are analyzed. Quantitative agreement with all measurements is achieved and a simulation of the flux-ratio-dependent behavior is possible, as is demonstrated for the ${\mathrm{As}}_{4}$ sticking coefficient.

Surface structure of reduced SrTiO 3(111) observed by scanning tunneling microscopy

We have obtained clean surfaces of reduced SrTiO 3(111) crystals and observed them by means of scanning tunneling microscopy (STM) combined with reflection high energy diffraction. Two different surface structures are observed. One, obtained by annealing at the temperature ∼ 1180°C, is assigned to have a SrO 3−x outermost layer. The other, obtained by annealing at the temperature ∼ 1220°C, is assigned to have a Ti outermost layer. The STM images reveal that the reproducible preparation of clean and well-defined surfaces of SrTiO 3(111) is possible.

Structure and magnetic anisotropy of epitaxial fcc-Co(110) and hcp-Co(11̄00) films

Fcc-Co(110) and hcp-Co(11̄00) films of 200 Å thickness were grown on MgO(110) and bcc-Cr(211)/MgO(110), respectively, by the molecular beam epitaxy method. Reflection high energy diffraction was used to in situ characterize the crystal structure. Co films grown directly on MgO(110) were pseudomorphic fcc structure. Twofold symmetrical hcp-Co(11̄00) films on top of the bcc-Cr(211)/MgO(110) were grown and confirmed. The magneto-optical Kerr effect (MOKE) was used to investigate the magnetic anisotropy of these films. The magnetization of these samples was found to be in-plane. The magnetization of fcc-Co films has cubic symmetry with texture induced uniaxial anisotropy. For hcp-Co(11̄00)/bcc-Cr(211) bilayer films, the magnetization was strongly anisotropic, but independent of the thickness of the Cr layer.

Raman Spectroscopy Investigation of (SiC)1-x (Ain)x, Layers Formed by Ion Implantation in 6H-SiC

Abstract6H silicon carbide (SiC) substrates were implanted with nitrogen and aluminum at different doses and annealed in the temperature range from 1300°C-1700°C. Micro-Raman Spectroscopy (μ-RS) measurements were performed in two sample geometries (conventional plane-view and cross-sectional). Changes of the polytype from 6H- to a cubic (SiC)1-xAIN)x, and influences in the 6H-SiC wafer up to depths of 2μm were detected. The results obtained by crosssectional μ-RS are discussed in comparison to other results from Reflection High Electron Energy Diffraction (RHEED), Rutherford Backscattering (RBS), Auger Electron Spectroscopy (AES), Transmission Electron Microscopy (TEM), and Positron Annihilation Spectroscopy (PAS) measurements.