Low-energy Electron Diffraction Intensity Research Articles

Investigation with low-energy electron diffraction of the adsorbate-induced metal relaxations in the Cu(100)-(2 x 2)-S surface structure.

A low-energy electron diffraction (LEED) crystallographic analysis has been undertaken to assess the lateral and vertical relaxations for the Cu(100)-(2\ifmmode\times\else\texttimes\fi{}2)-S surface structure. The motivation was provided by the recent report with angle-resolved photoemission extended fine structure (ARPEFS) by Bahr et al. [Phys. Rev. B 35, 3773 (1987)]. It was found that the surface structure given by ARPEFS does not account well for the measured LEED intensity curves, although these curves can be reasonably accommodated by a structural model with components of relaxation in some opposite senses to those previously reported.

Epitaxy of Mn On Pd {001}

A new metastable body-centered tetragonal phase of Mn has been grown epitaxially at room temperature on Pd {001} up to thicknesses of 21 layers. Low-energy electron diffraction (LEED) intensity analysis has determined two important structural parameters: the bulk interlayer spacing and a 5%-larger first interlayer spacing. Assumption that the film is a cubic phase in plane strain leads to the conclusion that the phase is probably face-centered cubic with an equilibrium lattice parameter of 3.80Å, which is much larger than a theoretical non-magnetic equilibrium value and indicates that the phase probably is magnetic.

Observation and structural determination of ( sqrt 3 x sqrt 3)R30 degrees reconstruction of the Si(111) surface.

A new reconstruction structure, having a \ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} unit cell, is reported for the Si(111) surface. Analysis of low-energy electron-diffraction intensity spectra by multiple-scattering theory revealed a vacancy model. The bonds between the first- and second-layer atoms are of the ${\mathrm{sp}}^{2}$ type, resulting in a highly compressed surface bilayer. Measured intensity spectra of the clean and Ag-deposited (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}2)R30\ifmmode^\circ\else\textdegree\fi{} surfaces are very similar. This leads us to conclude that the Ag-Si interface is not sharp and that the Ag atoms do not form a lattice structure that has long-range ordering.

A molecular representation of the potential of carbon monoxide adsorbed on a nickel surface

In a calculation of low-energy electron diffraction intensities from c(2*2) CO/Ni(100) the authors have considered a model of the potential of the adsorbate that preserves its molecular integrity. This representation of atomic muffin tins embedded in an interstitial potential bounded by an outer sphere mimics the model used in self-consistent-field X alpha calculations for molecules. This enables the authors to determine some of the parameters of such a potential by comparison with experiment. Confirmation of the model is found in X-ray absorption near-edge structure.

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001} c(2×2)- Au and Cu{001} c(2×2)- Pd, and on the bulk alloy Cu 3 Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu 3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A ̊ , c= 3.54 A ̊ and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 Å for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

Structural properties of epitaxial films of Fe on Cu and Cu-based surface and bulk alloys

Thin films of Fe were grown epitaxially at room temperature on Cu{001}, on the ordered surface alloys Cu{001}c(2×2)-Au and Cu{001}c(2×2)-Pd, and on the bulk alloy Cu3Au{001}. The maximum thicknesses attained in well-crystallized Fe films were 18 layers on the first three surfaces but only 3 layers on Cu3Au {00}. Low-energy electron diffraction (LEED) intensity data from 1-, 2- and 3-layer films on Cu{001&} could not be fitted by models with complete layers of Fe, suggesting that the growth of these ultrathin films was not layer-by-layer. The 12-layer films were found to have a tetragonally distorted face-centered-cubic (fcc) structure with a= 3.61 A, c= 3.54 A and 4%-expanded first interlayer spacing. Analysis of the elastic strain gave an equilibrium lattice constant of 3.59 A for fcc Fe at room temperature. Comparison with lattice constants from total-energy band calculations shows that the Fe cannot be in the nonmagnetic phase, but could be in the ferromagnetic phase, or possibly in an antiferromagnetic phase with the same lattice constant. It is suggested that the first interlayer spacing is enlarged owing to the larger magnetic moment of the first layer.

First-principles study of the electronic and magnetic structure of c(2 x 2) sulfur chemisorbed above Fe(001).

We have performed the first, first-principles study of the adsorption of sulfur above a magnetic, Fe surface. Our results, derived from the all-electron, film full-potential linearized augmented-plane-wave method applied to a seven-layer Fe film with and without c(2\ifmmode\times\else\texttimes\fi{}2) layers of S positioned next to the two surface Fe [Fe(S)] layers, include determinations of the equilibrium sulfur height (${H}_{\mathrm{eq}}$) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 A\r{}) of ${H}_{\mathrm{eq}}$ with the earlier result [1.09(5) A\r{}] derived by Legg, Jona, Jepsen, and Marcus from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below ${E}_{F}$ which play an important role both in reducing the magnetic moment of the Fe(S) atom (by \ensuremath{\sim}20%) and in the rather small calculated increase (0.85 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved, spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields and in the angle-resolved, spin-polarized surface state spectra, and relate our findings to questions associated with the sulfur-induced poisoning of an iron catalyst.

Electronic structure of fcc and bcc close-packed silver surfaces.

We report results of total energy and electronic structure calculations of fcc and bcc silver bulk and surfaces. We find that under normal conditions (positive pressure) bcc silver is not stable. However, if silver is grown with a volume per atom & 10% larger than that of the well-known fcc silver, the total energy of the bcc structure becomes below that of fcc silver. For bcc (110) a surface state is predicted at the Fermi level. Modern deposition techniques make it possible to grow crystals (or crystalline layers) with interatomic distances and structures different than those known so far. Nevertheless, the surface properties of differently structured materials have not been studied or compared systematically so far. Using the density-functional theory (DFT) we performed total-energy and electronic-structure calculations for fcc and bcc silver in order to investigate the electronic properties of a new (so far not known) crystalline material (bcc silver). In fact, recent experimental studies of Aristov, Bolotin, and Grazhulis' indicated that silver on InSb(110) may grow in the bcc structure up to many layers of thickness. The structure identification was drawn on the basis of Auger-electron spectroscopy and the lowenergy electron diffraction (LEED) pattern of a multilayer silver film. No LEED intensity analysis was performed, so that the interlayer distance (normal to the surface) remains uncertain. The LEED pattern indicated a somewhat perturbed two-dimensional bcc (110) layer. Figure 1 displays the results of self-consistent scalarrelativistic linear muffin-tin-orbital (LMTO) calculations for silver fcc and bcc. We use the local-density approximation (LDA) for the exchange-correlation functional3 and the atomic-sphere approximation for the effective potential. The basis set includes partial waves up

Structure determination of the (1 x 2) and (1 x 3) reconstructions of Pt(110) by low-energy electron diffraction.

The atomic geometry of the (1×2) and (1×3) structures of the Pt(100) surface has been determined from a low-energy electron-diffraction intensity analysis. Both structures are found to be of the missing-row type, consisting of (111) microfacets, and with similar relaxations in the subsurface layers. In both reconstructions the top-layer spacing is contracted by approximately 20% together with a buckling of about 0.17 A in the third layer and a small lateral shift of about 0.04 A in the second layer. Further relaxations down to the fourth layer were detectable. The surface relaxations correspond to a variation of interatomic distances, ranging from -7% to +4%, where in general a contraction of approximately 3% for the distances parallel to the surface occurs. The Pendry and Zanazzi-Jona R factors were used in the analysis, resulting in a minimum value of RP=0.36 and RZJ=0.26 for 12 beams at normal incidence for the (1×2) structure, and similar agreement for 19 beams of the (1×3) structure. The (1×3) structure has been reproducibly obtained after heating the crystal in an oxygen atmosphere of 5×10-6 mbar at 1200 K for about 30 min and could be removed by annealing at 1800 K for 45 min after which the (1×2) structure appeared again. Both reconstructed surfaces are clean within the detection limits of the Auger spectrometer. CO adsorption lifts the reconstruction in both structures. After desorption at 500 K the initial structures appear again, indicating that at least one of the reconstructions does not represent the equilibrium structure of the clean surface and may be stabilized by impurities.

Atomic geometry and electronic structure of CdTe(110)

A sp 3 tight-binding model for CdTe is constructed and validated by comparison with optical absorption and X-ray photoemission data. The energy minimization method is applied to determine the surface structure of the (110) cleavage face. The predicted surface atomic geometry compares well with that obtained from low-energy electron diffraction intensity analysis. The calculated electronic surface states give a good description of angle-resolved photoemission data.

Cu{001}c(2×2)-Pd: An ordered surface alloy

A quantitative low-energy electron diffraction (LEED) intensity analysis shows that the room-temperature reaction of Pd atoms with a Cu{001} surface produces a surface alloy, Cu{100}c(2\ifmmode\times\else\texttimes\fi{}2)-Pd, analogous to Cu{001}c(2\ifmmode\times\else\texttimes\fi{}2)-Au. The structure of Cu{001}c(2\ifmmode\times\else\texttimes\fi{}2)-Pd is almost planar, with the Pd atoms located 0.02\ifmmode\pm\else\textpm\fi{}0.03 A\r{} outwards from the Cu atoms, and an interlayer distance from the Cu atoms to second (100% Cu) layer equal to the Cu{001} bulk interlayer spacing (1.807 A\r{}). Although there are no ordered Pd atoms beyond the surface mixed layer, there is some evidence for disordered Pd either above or below the alloy layer. Angle-resolved photoemission spectra show that the valence band of the surface alloy is characterized by a well-defined Cu 3d--derived band and by the appearance of features due to Pd. The dispersion and the photon-energy dependence of these features suggest their probable origins and are consistent with the alloy character of Cu{001}c(2\ifmmode\times\else\texttimes\fi{}2)-Pd.

Improved LEED system using position-sensitive detection

An improved low-energy electron diffraction (LEED) system using a two-dimensional position-sensitive detector is described. The system is capable of storing LEED patterns at a resolution of 256×256 channels with up to 216 counts per channel. The maximum electron count rate is 100 kHz. A new position computer with edge-gating controls and a special event-counting memory interfaced to a personal computer is used to record the LEED intensities.

Atomic and electronic structure of tetrahedrally coordinated compound semiconductor interfaces

The most extensively specified atomic interface geometries for tetrahedrally coordinated compound semiconductors are those of the nonpolar (110) cleavage surfaces of zinc-blende structure semiconductors, specifically AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, and CdTe. These geometries have been determined experimentally by a variety of techniques, low-energy electron diffraction intensity analysis and ion scattering, in particular. They have been predicted by several quantum chemical methods, including extended tight-binding, pseudopotential, and ab initio models. All of the structures are known to be qualitatively identical: a bond-length-conserving rotation in the top layer followed by a relaxation of this layer toward the substrate and possibly small counter rotations in deeper layers. The atomic rotation in the top layer is driven by a rehybridization of the surface chemical bonds to redistribute the charge in the dangling bonds of the unrelaxed surface into the remaining surface and back bonds. The magnitudes of the surface atomic relaxations scale linearly with the bulk lattice constants. Atomic geometries also are available for a few overlayer structures on these surfaces, i.e., p(1×1) structures of Al on GaAs and of Sb on GaAs and InP. An area of current research is the extension of such studies to the cleavage faces, (101̄0) and (112̄0), of wurtzite-structure compound semiconductors. Theoretical predictions are available for ZnO, ZnS, ZnSe, CdS, and CdSe. Experimental structure analyses have been reported only for the (101̄0) surfaces of ZnO and CdSe. The relaxed geometries of these surfaces are analogous to those of zinc-blende (110), and are driven by the same mechanism of dangling-bond charge redistribution. Another active research area is the study of the polar surfaces of zinc-blende structure materials. These surfaces exhibit complex reconstructions which depend both on the surface stoichiometry and on the sample preparation procedure. Structure analyses have been reported for GaAs(111)-(2×2), InSb(111)-(2×2), GaP(111)-(2×2), GaAs(1̄1̄1̄)-(2×2), InP(1̄1̄1̄)-(1×1), and GaAs (100)-(2×4), as well as for GaAs(311) surfaces. These reconstructions also are driven by rehybridization of the surface chemical bonds, but the precise nature of this rehybridization differs from one structure to another. The electronic structures of all three classes of interface have been studied by a combination of photoemission spectroscopy and quantum chemical calculations. The interface states are dominated by the surface-relaxation-induced rehybridization and/or consequences of interface chemical reactions. In those cases for which the atomic geometries are known independently, the qualitative features of the observed interface-state eigenvalue spectra are well reproduced by the model calculations, although the models disagree both among themselves and with the measurements on several details.

All-electron study of c(2×2) S chemisorbed above magnetic Fe(001)

We have performed a first-principles study of the adsorption of sulfur above a magnetic Fe surface. Our results, derived from the all-electron, film full-potential augmented-plane-wave method applied to a seven-layer Fe film with and without c(2×2) overlayers of S, include determinations of the equilibrium sulfur height (heq) and vibrational frequency, as well as the associated electronic and magnetic structures. We find excellent agreement between our calculated value (1.12 Å) of heq with the earlier result (1.09±0.05 Å) derived by Legg et al. from a dynamical low-energy electron diffraction intensity analysis. The adsorption induces antibonding minority surface states immediately above and below EF which play an important role both in reducing the magnetic moment of the surface Fe atom (by ∼20%) and in the resulting calculated increase (0.86 eV) in work function. These states should be clearly resolvable in both integrated and angle-resolved spin-polarized photoemission and inverse-photoemission experiments. We present additional predictions, including the adsorption-induced changes in the hyperfine fields.

Growth of an orientationally ordered incommensurate potassium overlayer and its order-disorder transition on the Cu(111) surface.

The evaporation of potassium onto the Cu(111) surface at 80 K results in the formation of an incommensurate orientationally ordered structure as observed by low-energy electron diffraction (LEED). The incommensurate structure appears when the potassium coverage is near 0.09, and can be characterized as a non-close-packed-hexagonal phase with an interatomic spacing continuously varying with concentration from 6.3 to 4.4 A\r{}. As the temperature is increased, the incommensurate phase with larger K-K spacing (g5.2 A\r{}) undergoes three transition stages as identified by LEED intensity and beam-width (full width at half maximum) measurements: (i) a better ordered hexagonal phase, (ii) a hexatic phase, and (iii) a disordered phase. The incommensurate phase with smaller K-K spacing (l5.2 A\r{}), however, undergoes only two transition stages: (i) a better ordered hexagonal phase and (ii) a disordered phase. The incommensurate overlayer may behave as a ``two-dimensional solid.''

Epitaxial growth of CrSi and CrSi 2 on Si(1 1 1)

The possibility of epitaxial growth of chromium silicides upon thermal processing of thin Cr deposits (⪅ 30 monolayers (ML)) on Si(1 1 1) is demonstrated using low energy electron diffraction (LEED) and angle resolved X-ray (XPS) and ultra-violet (UPS) photoemission. For coverages θ ⪆ 4 ML epitaxial CrSi with a lattice misfit of ≈ 1.6% can be grown upon annealing at 350°C. Thermal treatment at 450°C for θ ⪆ 6 ML results in epitaxial CrSi 2 formation with two kinds of domains rotated by 30° with respect to each other around the surface normal. LEED intensities indicate essentially equal formation probabilities for both orientations despite the large difference in lattice misfit of ≈ 0.1 and ≈ 3.8% respectively.

Surface structure change during solid phase epitaxial growth of an amorphous Si film deposited on Si (111)

Some surface structure change of an amorphous Si film deposited on Si (111) during the solid phase epitaxy was observed by low-energy electron diffraction (LEED). The LEED intensity profile shows the formation of 7×7 structure whenever a crystallized surface is constructed. The intensity increases with an increase of the annealing temperature Ta, where the increasing rate with Ta becomes small for 590<Ta<700 °C, and for 780<Ta<850 °C. It is discussed that there exists some process inhibiting the growth of the 7×7 structure regions in these two temperature ranges.

The missing-row model for the reconstructed Pt(110)−(1 × 2) surface: A leed intensity analysis showing multilayer distortions

The (1 × 2) missing-row reconstruction of clean Pt(110) is studied with a new low-energy electron diffraction (LEED) intensity analysis. In order to obtain theoretical intensity-voltage curves in good agreement with experiment, it is necessary to consider distortions from the basic missing-row model in the theoretical treatment. In our calculations, alternating row-pairing and vertical buckling distortions down to the fourth layer are allowed for the first time. Large contractions are seen in the first three layer spacings, and significant distortions are found even in the fourth layer. The distortions are similar to those seen in Ir and Au LEED results as well as those predicted by the embedded atom method (EAM) for Pt. In the optimum geometry, the top-layer spacing contracts by 18%. The atoms in the second layer pair towards the missing rows, in agreement with Ir and Au LEED but in the direction opposite to that predicted by EAM. The R-factors are especially sensitive to the third-layer buckling. The fourth-layer pairing is also towards the missing rows, in agreement with the EAM prediction. It is shown that full dynamical LEED calculations of this kind can be realistically run on a personal computer with suitable coprocessor board.

Dynamical analysis of low-energy electron diffraction intensities from CdSe( [formula omitted

The measurement of the intensities of 14 diffracted beams of low-energy (40≤ E≤230 eV) electrons normally incident on CdSe( 10 10 ) is reported. The temperature of the CdSe surface during the measurements was T≅125 K. The surface were prepared by in situ cleavage within the ultrahigh vacuum system used to perform the intensity measurements. The measured intensities were analyzed using a relativistic, Hara-exchange electron-ion-core potential and an X-ray R-factor structure analysis methodology. This analysis leads to a best-fit structure characterized by a bond-length-conserving rotation of the dimers in the top layer by ω = 23°, Se outward and Cd inward. The X-ray R-factor for this structure is R x = 0.23 identical to the value obtained for the best-fit surface-structure of the InAs(110) which is the zincblende-structure isoelectronic counterpart of wurtzite-structure CdSe.

Relaxation and surface states on wurtzite cleavage faces: CdSe(101-bar0).

A $s{p}^{3}$ tight-binding model is utilized to predict the atomic geometry and electronic structure of ($10\overline{1}0$) surface of wurtzite-structure CdSe. The model predicts a bond-rotation relaxation in the uppermost few atomic layers resulting in a displacement of top layer Se relative to Cd of 0.78 \AA{} normal to the surface, and a surface bound state near the top of the valence band. Analyses of low-energy electron diffraction intensity data and ultraviolet photoemission spectra confirm these predictions.