Low Energy Electron Reflection Research Articles

Dynamical low-energy electron diffraction analysis of the structure of nitric oxide on Pt( [formula omitted

The adsorption structure of NO on the Pt(1 1 1) surface at low temperature (90–130 K) was studied by dynamical analysis of low-energy electron diffraction and reflection absorption infrared spectroscopy. Three different 2×2 patterns were identified and their experimental intensity vs. voltage curves ( I– V curves) were compared with dynamically calculated I– V curves for the models including the tilted (bent) NO geometry. The differences of I– V curves can be recognized by virtue of the wider incident electron energy range than the previous dynamical analysis. The most promising interpretation is that NO occupies three sites sequentially depending on the NO coverage. In this interpretation, NO first adsorbs at the threefold fcc hollow site, next at the atop site and last at the threefold hcp hollow site. The NO saturation coverage is 0.75 ML ( 1 ML=1.5×10 −15 molecules/cm 2). The NO species at the atop site is tilted and those at both hollow sites are upright. Our new model agrees well with the recent density functional theory calculation and explains the experimental results better than the previous model, in which NO transits from the bridge site to the atop site with increasing coverage.

NO2-assisted molecular-beam epitaxy of wustitelike and magnetitelike Fe oxynitride films on MgO(100)

In an attempt to obtain wustite ${\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ as epitaxial films on MgO(100), ${\mathrm{NO}}_{2}$-assisted molecular-beam epitaxy was applied. At low ${\mathrm{NO}}_{2}$ fluxes, the low-energy electron diffraction and reflection high-energy electron diffraction images indeed indicate the formation of a rocksaltlike structure. In addition, M\"ossbauer spectroscopy provides evidence for the formation of a phase that is paramagnetic at room temperature. However, the layers are not pure oxides but are well-ordered oxynitrides with composition ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{O}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}.$ The nitrogen atoms occupy substitutional sites on the oxygen-anion sublattice. Similarly, at slightly higher ${\mathrm{NO}}_{2}$ fluxes, magnetitelike oxynitride films with composition ${\mathrm{Fe}}_{3+\ensuremath{\delta}}{\mathrm{O}}_{4\ensuremath{-}y}{\mathrm{N}}_{y}$ are obtained. By correlating x-ray photoelectron spectroscopy spectra with the intensity oscillation periods observed during reflection high-energy electron diffraction, it is possible to derive the complete stoichiometry of the films. We propose that the abrupt incorporation of nitrogen atoms only occurs if the atomic oxygen provided by the ${\mathrm{NO}}_{2}$ flux is insufficient to form a stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$

A compact ultrahigh-vacuum system for the in situ investigation of III/V semiconductor surfaces

A compact ultrahigh vacuum (UHV) system has been built to study growth and properties of III/V semiconductor surfaces and nanostructures. The system allows one to grow III/V semiconductor surfaces by molecular beam epitaxy (MBE) and analyze their surface by a variety of surface analysis techniques. The geometric structure is examined by scanning tunneling microscopy (STM), low-energy electron diffraction and reflection high-energy electron diffraction. The electronic properties of the surfaces are studied by angular resolved photoemission either in the laboratory using a helium discharge lamp or at the Berlin Synchrotron Radiation Facility BESSY. In order to meet the space restriction at BESSY the system dimensions are kept very small. A detailed description of the apparatus and the sample handling system is given. For the UHV-STM (Park Scientific Instruments, VP2) a new, versatile tip handling mechanism has been developed. It allows the transfer of tips out of the chamber and furthermore, the in situ tip cleaning by electron annealing. In addition, another more reliable in situ tip-preparation technique operating the STM in the field emission regime is described. The ability of the system is shown by an atomically resolved STM image of the c(4×4) reconstructed GaAs(001) surface.

Surface morphology of homoepitaxially grown (111), (001), and (110) diamond studied by low energy electron diffraction and reflection high-energy electron diffraction

The surface morphology and crystallinity of homoepitaxially grown (111), (001), and (110) diamonds were comparatively investigated by using low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED). We found that the crystal quality and the surface smoothness of the homoepitaxial diamond on (111) strongly depends on the substrate temperature; the highest quality homoepitaxial diamond on (111) can be successfully grown at 690 °C. The highest quality homoepitaxial diamond had the smoothest C(111)-1×1 surface, which was evidenced by the first observation of sharp and clear Kikuchi patterns, comparable to those of a C(001) surface. Caution has to be exercised when judging the structure of a C(111) surface from a LEED(1×1) pattern, because its observation can encompass surface morphologies ranging from single-crystal to polycrystalline diamonds. In contrast, surface roughness has a dramatic effect on the RHEED diffraction pattern. In the smoothest C(001)-2×1/1×2 surface, the RHEED pattern shows spots rather than streaks. Growth on a (110) diamond surface results in {111}-oriented microfacets.

A novel electron spin-polarization detector with very large analyzing power

The low energy electron reflectivity from a magnetic surface depends on the relative orientation between the electron spin and the sample magnetization. This effect has been exploited for realizing a new electron spin polarization detector. As a scattering surface we have used a well ordered Fe(001)-p(1×1)O surface, which is stable against surface contamination and gives rise to very large spin dependent effects. We describe in detail the preparation in vacuum of such a target surface and show that it can be transferred in a separate system, without performance losses. The analyzing power S of the detector is given by the relative variation of its response when the target magnetization is reversed. We find extremely large S values for electrons reflected at 3–6 eV kinetic energy, with maximum above 45%, i.e., roughly 3 times better than other polarimeters presently used.

Adsorbate-induced faceting of high-index semiconductor surfaces: antimony adsorbed on Ge(103)

Scanning tunneling microscopy, low-energy electron diffraction and reflection high-energy electron diffraction were used to monitor changes in the morphology of Ge(103) surfaces after adsorption of antimony and annealing at different temperatures for various durations. After deposition of Sb at room temperature, the Sb-covered Ge(103) surface breaks up on annealing into small irregular pyramids, which coalesce and form more regular quasi-one-dimensional `hill-and-valley' structures bounded by reconstructed {113} facets. The average periodicity of the structures approaches a constant maximum value in the nanometer range. The symmetry of the substrate with respect to the facets constrains the formation of single domain nanostructures.

Oxygen-induced enhancement of the spin-dependent effects in electron spectroscopies of Fe(001)

Oxygen adsorption in an ordered $p(1\ifmmode\times\else\texttimes\fi{}1)$ fashion on the Fe(001) surface has been studied by empty states spectroscopies using a spin-polarized electron beam. The low-energy electron absorption and reflection display a much larger spin dependence in Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ than in Fe(001). Peak values of the spin asymmetries of the absorbed (reflected) current larger than 13% (45%) at around 9--11 eV above the Fermi level are found. Spin-resolved inverse photoemission reveals that in the same energy range the exchange splitting between majority- and minority-spin states in Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ is $\ensuremath{\sim}50%$ larger than in the clean surface.

Large spin asymmetry in electron absorption and reflection from oxidized single crystal Fe/MgO(001) films

High quality single crystal Fe films to be used in spin polarized electron spectroscopy have been prepared in ultra-high vacuum by epitaxial growth on MgO(001) substrates. Oxygen absorption on the clean surface followed by annealing at 600°C produces a well-ordered p(1×1) overlayer. The low energy electron absorption and reflection from the Fe(001)- p(1×1)O surface display a huge dependence on the polarization of the primary beam. Peak values of the spin asymmetries of the absorbed (reflected) current larger than 13% (45%) around 4–8 eV electron kinetic energy are obtained. The presence of the surface oxide protects the system against contamination, so that an almost negligible asymmetry deterioration is observed after prolonged operation in vacuum. The results are discussed in view of applications in electron spin polarimetry.

Au-induced superstructure formation on vicinal Si(001) studied by low-energy electron diffraction and reflectance anisotropy spectroscopy

Au-induced superstructure formation on vicinal Si(001) studied by low-energy electron diffraction and reflectance anisotropy spectroscopy

Elastic reflection of low-energy electrons from polycrystalline gold targets

Monte Carlo calculations to study the influence of solid-state, polarization, exchange and relativistic effects on the angular distribution and the reflection coefficient for electrons elastically reflected from gold surfaces have been performed. Elastic differential cross sections were determined using relativistic Dirac equations with a solid-atom potential including the polarization and exchange effects. Inelastic differential inverse mean free paths were computed using the extended Drude dielectric function for volume and surface excitations. These showed that the polarization effect made a contribution to elastic differential cross sections at small scattering angles, whereas, the exchange and relativistic effects contributed to these cross sections at large scattering angles. These effects are important to the angular distribution and the elastic reflection coefficient for electron energies below 400 eV. Monte Carlo results calculated in this work are in very good agreement with experimental data.

An ultrahigh vacuum (UHV) apparatus to study the interaction between adsorbates and photons

An UHV apparatus has been designed and constructed for studying the interaction between adsorbates and photons. Since a variety of light sources and experimental geometries are used in these studies, the system was designed to be as flexible as possible. The system is put on a single frame so that it can be transported from one photon source to another. A compact, exchangeable, high-pressure cell is designed to provide easy access for photons to the crystal. To allow the study of small poly-atomics the following combination of detection techniques are installed: thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and reflection - adsorption infrared spectroscopy (RAIRS). Furthermore, a sample exchange system in combination with liquid nitrogen sample cooling is incorporated. The exchange system allows three samples to be stored in the main chamber. The sample exchange capability does not restrict the cooling capabilities - 85 K within an hour after turning on the liquid nitrogen flow and 85 K within minutes after flashing to 800 K.

Design considerations and performance of a combined scanning tunneling and scanning electron microscope

We designed and built a combination of a scanning tunneling microscope (STM) and a scanning electron microscope (SEM) which is working under ultrahigh vacuum conditions (base pressure typically 7⋅10−11 mbar). The SEM is ideally used for surveying the sample and to control the STM tip positioning, while the STM extends the resolution range into the atomic scale. The design concept allows moving the STM tip freely over the sample under SEM control and using both imaging techniques simultaneously. The system is equipped with an electron energy analyzer (cylindrical sector analyzer) providing Auger electron spectroscopy, scanning Auger microscopy (SAM) and x-ray photoelectron spectroscopy capabilities. In addition, low energy electron diffraction and reflection high energy electron diffraction facilities are installed. In order to use these very different imaging techniques in situ, several special solutions had to be incorporated in the design of the system; they are described in detail. Some results are presented which demonstrate the performance of the STM/SEM system. Atomic resolution of the STM, a SEM resolution of up to 20 nm, and a SAM resolution of better than 100 nm were achieved.

Translationally activated dissociation of CO2 on Si(100)2×1

The reaction of hyperthermal (0.1 eV→2.6 eV) CO2 with Si(100)2×1 at ≤300 K has been studied with high-resolution electron energy loss spectroscopy (HREELS), low energy electron reflectivity (LEER), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). The initial sticking coefficient depends on the total kinetic energy of the incident molecule, but is insensitive to the vibrational state. Adsorption results in a dramatic reduction in specular electron reflectivity which is reversed upon desorption of one of the dissociatively adsorbed products, CO, at 400 K. The adsorbed oxygen remains on the surface after annealing to as high as 900 K. Continued exposure results in the population of a CO state which desorbs near 330 K as well as a molecular CO2 state desorbing at 110 K.

Surface Adsorption Kinetics of Ga Wire Arrays on Si(112)

AbstractThe growth and energetics of Ga chains on a Si(112) facet surface have been investigated. Low energy electron diffraction (LEED) and Reflectance Difference Anisotropy (RDA) experiments have been performed in UHV to examine the kinetics of the Ga atom chain formation process along the Si steps. This is a self-limiting process for substrate temperatures in the 400°C to 500°C range, since the binding energy of the metal atoms at the step edges is greater than that on the (11l)-type terraces. At saturation coverage, the Ga chains exhibit sixfold periodicity along the step edges, but below saturation, these chains exhibit fivefold periodicity. A model has been suggested from these results in which the periodicities of the chains are determined by the metal atom vacancies, which balance the tensile stress created by the larger metal atoms. We have used RDA to study the time evolution of the surface kinetics and changes in the surface reconstruction of Ga deposited on these facet (112) Si surfaces, as a function of substrate temperature and Ga deposition rate. The RDA measurement is shown to be sensitive to the formation of both the 5×1 as well as 6 × 1 reconstructions and to island formation at low substrate temperatures. The diffusion-controlled Ga line formation process at low temperatures has also been characterized as well as the evaporation controlled regime, which is evident at substrate temperatures over 400°C.

The growth and removal of molybdenum films at platinum (111) surfaces

LEED, AES, work function measurements and low-energy (<30 eV) electron reflectivity spectra were used to characterise films of 1 to 5 monolayers thickness of Mo on a Pt(111) surface. With the Pt surface at 300 K, vapour deposition of Mo produced poorly ordered multilayer films, which gave no LEED pattern but showed the electron reflectivity properties of a Mo(110) surface after completion of the first monolayer. The evolution of surface microstructure during annealing at T > 750 K was dominated by diffusion of Mo into Pt. A new surface phase was seen only during removal of Mo films thicker than three monolayers. The distinctive electron reflectivity features and strong (√3 × √3)R30° LEED pattern characteristic of this phase are attributed to separation of the ordered alloy Pt 2 Mo when the concentration of Mo diffusing into the Pt surface layers reaches the solubility limit. The electron reflectivity spectral features of the alloy were also seen during deposition of Mo on Pt(111) at 600 K.

Production and characterization of smooth, hydrogen-terminated diamond C(100)

We report the production of smooth and well-ordered C(100) surfaces by exposure to a pure hydrogen plasma. A two domain 2×1 surface reconstruction is observed by low energy electron diffraction with half-order spots visible using incident electrons with energies as low as 13 eV. High-resolution electron energy loss spectroscopy reveals a large enhancement in specular reflectivity of low energy electrons following plasma treatment. The hydrogenated surface is stable in air and free of adsorbed hydrocarbons upon insertion into ultrahigh vacuum.

Structure induced magnetic anisotropy behavior in Co/GaAs(001) films

Epitaxial Co has been grown on GaAs(001) and studied by both low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED), and by the magneto-optic Kerr effect (MOKE) and polarized neutron reflection (PNR). Three samples were fabricated using different growth procedures: (1) ‘‘interrupted’’ growth (including an anneal); (2) and (3) continuous growth of similar thicknesses. For sample 1, RHEED patterns indicate an initial growth in the bcc phase followed by a relaxation into a distorted single phase at completion of growth, whereas samples 2 and 3 showed a multicrystalline structure after growth. LEED patterns were used to check the existence of the 2×4 reconstruction patterns before growth, but no LEED patterns could be obtained after more than 2 Å Co was deposited, in contrast to the RHEED patterns which remained visible throughout the growth. Structural analysis of the completed films indicates the formation of a ∼10 Å CoO layer on the Co/air interface, and gives thicknesses for magnetic material of (1) 30 Å and (2) 80 Å. Sample 1 showed a dominant fourfold magnetic anisotropy with the easy axis parallel to the (100) direction and with a strength 2K4/M of ∼0.5 kOe, smaller in magnitude than that reported for bcc films on GaAs(110) but along the same axis [G. A. Prinz et al., J. Appl. Phys. 57, 3672 (1985)]. However, samples 2 and 3 showed only a large uniaxial anisotropy along the (110) direction of strength 2K1/M of ∼1.5 kOe and ∼2.5 kOe, respectively, similar in magnitude to those previously observed [G. A. Prinz et al., J. Appl. Phys. 57, 3676 (1985)]. We attribute the origin of the contrasting magnetic anisotropy behavior observed to the differences in final structure.

A molecular beam study of ethane on Si(111)7×7: Energy accommodation and trapping

The interaction of a molecular beam of ethane with the Si(111)7×7 surface has been studied using electron energy loss spectroscopy (EELS), low energy electron reflectivity (LEER), and temperature programmed desorption (TPD). Dissociative adsorption of ethane is not observed on this surface at &amp;lt;90 K for incident energies up to 1.5 eV and nozzle temperature of 1000 K. Ethane adsorbs molecularly on Si(111)7×7 at &amp;lt;80 K in submonolayer coverages and forms multilayers at temperatures below 66 K. The monolayer is found to desorb with first order kinetics with an activation energy of 0.25 eV and pre-exponential factor of 1014±1 s−1. At normal incidence, the probability of trapping is found to decrease with increasing translational energy, falling by a factor of 10 as the incident energy is increased from 0.1 to 1.5 eV. The data are fit by a hard cube model with an effective surface mass of 100 amu, which is between three and four times the mass of a silicon atom. For impact energies ≳0.8 eV, trapping is observed to be higher than would be expected from the hard cube model. Possible explanations for this deviation are discussed. The trapping probability is found to obey total energy scaling. Changes in the internal energy of the ethane due to variations in the nozzle temperature from 300 to 1000 K are observed to produce negligible effects on the trapping probability.

Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy

We describe the formation of ordered ultrathin films of chloroindium phthalocyanine (InPcCl) on thin films of the layered semiconductor SnS 2, both of which are deposited by a molecular beam epitaxy process to form InPcCl/ SnS 2/mica assemblies. The resultant thin films show markedly narrowed linear absorbance spectra versus those seen previously for Pc thin films. Low energy electron diffraction and reflection high energy electron diffraction data collected for the first few monolayers of InPcCl deposited on the SnS 2 substrate suggest that three (equally populated) ordered Pc domains are formed, with the Pc rings parallel to the SnS 2 surface, aligned along the principle axes of this hcp SnS 2 substrate. The red-shift and peak shape of the absorbance spectra of these thin films (about 758 nm, full width at half maximum of about 60 nm) are consistent with the type of splitting of the exciton energies expected of Pc aggregates in which the adjacent Pc rings are cofacial, but staggered in both x and y directions by about half a molecular diameter. This is the structure determined from related trivalent metal Pc's by single crystal X-ray diffraction. Loss of the flat-lying configuration of Pc in these ultrathin films is immediately apparent from the surface electron diffraction data.

Absorption and elastic and inelastic reflection of spin-polarized low-energy electrons from Fe(110).

The spin dependence of low-energy electron absorption and reflection from ferromagnetic Fe(110) is investigated using the normalized difference between the absorption (or reflection) of electrons polarized parallel and antiparallel to sample magnetization. The resulting absorption and reflection spin asymmetries are found to be reciprocal to each other, with their magnitudes related in a simple manner. Structure in the spin asymmetries is identified with features in the spin-split bulk band structure and is thus found to be related to the spin-polarization fine structure of secondary electrons emitted from the same surface. The general structure of the spin asymmetry as a function of incident energy is dominated by elastic-scattering events, although inelastic scattering is found to play a major role in determining the sign and magnitude of the absorbed and total (energy-integrated) reflected asymmetries. Furthermore, the existence of nonzero elastic-reflection asymmetries indicates the existence of spin-split unoccupied energy bands up to 50 eV above the vacuum level. Inelastic-scattering spin asymmetries show a 2-eV energy-loss feature that is identified as due to the creation of Stoner excitations in the ferromagnet. This feature is determined to be surprisingly insensitive to the incident electron energy (from 0 to 50 eV) and angle (from 0\ifmmode^\circ\else\textdegree\fi{} to 65\ifmmode^\circ\else\textdegree\fi{} from the normal) in contrast to predictions of a recent theory of spin-polarized electron scattering in ferromagnets.