Low-energy Electron Diffraction Intensity Research Articles

Ultrathin films of body-centered-cubic Co on TiAl(010).

Slow deposition in ultrahigh vacuum of cobalt onto a clean (010) surface of the ordered alloy TiAl (the bulk structure is tetragonal of the CuAu I type) produces ultrathin films of Co that contain large numbers of steps and defects, but also large regions that are both epitaxial and pseudomorphic. In these regions, the unit mesh of the Co films in the plane of the film is a rhombus with sides equal to 2.855 \AA{} and an angle between the sides of 89.068\ifmmode^\circ\else\textdegree\fi{}. A quantitative low-energy electron-diffraction intensity analysis shows that the bulk interlayer spacing is 1.40\ifmmode\pm\else\textpm\fi{}0.03 \AA{} and that the first interlayer spacing is contracted by 8.6%. Strain analysis finds that the equilibrium (i.e., the unstrained) phase of the films is the metastable body-centered-cubic (bcc) modification of Co (lattice parameter 2.83 \AA{}). Thus, the square unit mesh of bcc Co under a tensile strain of 0.9% becomes a rhombus with a consequent contraction of the bulk interlayer spacing by about 0.86%. The close match of the unit meshes in side and angle stabilizes the metastable bcc Co structure for about 15--20 layers. This case is a rarely encountered example of pseudomorphic epitaxy in which the unit mesh of the equilibrium phase of the film differs from the unit mesh of the substrate either by the ratio of the lengths of the sides or by the magnitude of the angle between the sides or both.

EARLY STRUCTURAL STAGES OF METALLIC EPITAXY: THE MODEL CASE OF Fe/Cu (100)

By epitaxial growth of iron on Cu (100) phases of fcc-Fe can be stabilized at room temperature and below. Dependent on coverage the films show various superstructures which we investigate by quantitative analysis of low-energy electron diffraction intensities. There are three coverage regimes, whereby in the first one (Θ≤4 ML ) all iron layers are considerably reconstructed and are fcc(100) layers only on average. Upon further iron deposition (5 ML <Θ<10 ML ) subsurface iron layers flatten and only the top layer is reconstructed. Simultaneously, the epitaxial growth seems to change from pseudomorphic to nonpseudomorphic. At about 10 ML there is a 1. order phase transition fcc (100)⇒ bcc (110), the latter phase being stable at higher coverages. It turns out that the reconstruction of layers always entails an increased atomic volume available for iron atoms involved in the reconstruction. In turn this is consistent with magnetic properties known for the iron films from the literature, i.e., bulk ferromagnetism in the first coverage regime and only surface ferromagnetism in the second. So, magnetic and structural properties are shown to be closely related.

Alkali-metal adsorption on dissimilar alkali-metal monolayers preadsorbed on Cu(001): Li on Na and Na on Li.

An adsorption system of metal-on-metal growth is presented. The adsorption of alkali-metal atoms on monolayers of dissimilar alkali metals preadsorbed on the Cu(001) surface has been studied by Auger-electron spectroscopy, work-function change, and low-energy electron-diffraction intensity analysis. We have examined the following three adsorption systems on Cu(001) at 180 K; Na on a full monolayer of Li, Li on a full monolayer of Na, and Na on a submonolayer of Li, namely, the c(2\ifmmode\times\else\texttimes\fi{}2) structure of Li adatoms. It is found that a simple overlayer formation of Na takes place on the full Li monolayer, while Li atoms substitute Na adatoms for Li adsorption on the full Na monolayer. For adsorption of Na on the Li-c(2\ifmmode\times\else\texttimes\fi{}2) structure whose coverage is 5/8 of the full Li monolayer, it is found that Na atoms compress the Li adlayer to become denser monolayers. The Na atoms do not intermix with Li adatoms, but they form islands of the c(2\ifmmode\times\else\texttimes\fi{}2) structure on Cu(001).

The strain tensor in a general pseudomorphic epitaxial film

The use of strain analysis, in particular, the measurement and calculation of strain ratios in the study of ultrathin pseudomorphic epitaxial films is discussed. The general solution is given of the problem in crystal-elasticity theory of finding the three unknown out-of-plane strain components from the three known in-plane strain components of a pseudomorphic ultrathin film growing on a substrate with a unit mesh different in both size and shape from the film's unit mesh. It is shown that the experimental determination, e.g. by quantitative analysis of low-energy electron diffraction intensities, of the strains in the epitaxial ultrathin film of a cubic material grown on a general surface yields values of the ratios of two shear moduli to the bulk modulus of the film material.

Surface anharmonicities and disordering on Ni(100) and Ni(110).

Recent observations on various low-index metallic surfaces of an extraordinary thermal attenuation of diffracted peak intensities suggest that anharmonic effects are important at high temperatures, though such observations have also been interpreted in terms of disordering processes (premelting and roughening). In order to address this problem, we have carried out an extensive molecular-dynamics investigation of the evolution in temperature of the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom method potentials. We observe both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration; the latter are found in both cases to be larger in the plane of the surface than out of the plane. An analysis of our simulated low-energy electron-diffraction intensities supports the experimental interpretation of the measured ones in terms of enhanced mean-square displacements. Indeed, anharmonicities are found to appear at temperatures well below the onset of disordering, which occurs at 1200 and 1000 K for the (100) and (110) surfaces, respectively (via in-plane diffusion and adatom-vacancy formation). The onset of disordering proceeds differently on the two surfaces: while it involves essentially only the outermost layer on the (100) surface, both the first and second layer participate in the process on the (110) surface. We also calculate the phonon spectra at high-symmetry points of the surface Brillouin zone, which we find to be in good agreement with experimental values at room temperature. We predict, further, that the temperature-dependent frequency shifts are not significantly softened in comparison with bulk phonons, which suggests that surface phonons are affected by anharmonicity in a nontrivial way. Several of the observed properties can be understood in terms of the larger displacements that surface atoms can afford, because of their reduced coordination compared to bulk atoms.

Structure of Ni(100)-c(2 x 2)-Na: A LEED analysis.

The structure of Ni(100)-c(2\ifmmode\times\else\texttimes\fi{}2)-Na has been investigated by analysis of extensive new low-energy electron-diffraction (LEED) data. The structure is found to contain Na atoms adsorbed in fourfold hollow sites on an unreconstructed and essentially unrelaxed substrate. The Na-Ni layer spacing is 2.38\ifmmode\pm\else\textpm\fi{}0.04 \AA{} and the first Ni-Ni layer spacing is 1.75\ifmmode\pm\else\textpm\fi{}0.01 \AA{}. These results are in good agreement with the conclusions of an early study by Demuth et al. [J. Phys. C 8, L25 (1975)]. Good agreement between experimental and calculated LEED intensities is obtained using the dynamic theory of LEED, with a conventional, muffin-tin potential for the adsorbed Na atoms and a step potential at the surface. The good agreement pertains not only to the energy positions and relative intensities of peaks in intensity-energy spectra for the diffracted beams, but also to the relative beam intensities, which span a range of nearly two orders of magnitude.

ATOMIC STRUCTURE OF Si(113) 3×1-H BY DYNAMICAL LOW-ENERGY ELECTRON DIFFRACTION INTENSITY SPECTRA ANALYSIS

The Si ((113) 3×1-H structure has been investigated by a quantitative low-energy electron diffraction intensity analysis. A model with two Si dimers in the unit cell gives best agreement between the calculation and the experimental data. Three-dimensional atomic coordinates have been determined.

Fingerprinting technique in low-energy electron diffraction

The most important part in solving complex surface structures is a promising guess of the starting configuration if an automated structure refinement is employed. The “fingerprinting technique” is able to provide such information quasi-directly from experimental low-energy electron diffraction (LEED) data for a class of surface structures. The application of this method to LEED is based on the local scattering picture. Because of the short mean-free path of LEED electrons, the energy dependence of LEED intensities of fractional-order beams is mainly influenced by the local geometry of the adsorbate complex. By comparing experimental IV curves ( fractional-order beams) of an unknown structure with those whose structure has been successfully analyzed, it is frequently possible to obtain information on the basic structural elements of the unknown surface system. For instance, similar IV curves suggest similar structural elements. In this paper, the fingerprinting idea will be substantiated by several representative examples.

The structure of thin epitaxial Fe films on Cu(100) in the transition range fcc → bcc

The structure of epitaxial layers of Fe on Cu(100) is investigated in the coverage range near 10 monolayers where a structural transition from fcc(100) to bcc(110) proceeds in the film with increasing coverage. By measurement and full dynamical analysis of low energy electron diffraction intensities we demonstrate that the resulting bcc(110) structure of the film is fully unstrained and exhibits practically no multilayer relaxation. So it is structurally identical to the (110) surface of bulk iron samples. For the transition range we show that fcc(100) and bcc(110) domains coexist on the surface. Also, we give arguments that the surface reconstruction found earlier for the fcc(100) phase may be understood as a precursor for the transition to bcc(110).

Annealing effect on an amorphous Si film deposited on a 7×7 superlattice surface of Si(111) studied with low-energy electron diffraction

The intensity profile of low-energy electron diffraction (LEED) was observed for a Si film grown on a Si(111)–7×7 substrate under low-temperature epitaxial growth conditions. The epitaxial relation for Si(111) holds for each film with its thickness region below a certain value (dC) from the substrate, and an amorphous phase is formed above it. The values of dC were 0.5, 6.5, 17, and 100 nm at substrate temperatures (TS) of 35, 170, 200, and 250 °C, respectively. The annealing effect on the films grown at Ts=35 and 170 °C was studied. The LEED intensity (IS) is proportional to the surface area occupied by the 7×7 and 5×5 superlattices such that (1) IS depends sensitively on the film thickness (d) and TS, and (2) the dependence of IS on the annealing temperature is independent of TS and d. This is discussed from the viewpoint of the change in structural hierarchy formed in the as-grown film due to the annealing.

Atomic geometry of Ge(111) sqrt 3 x sqrt 3 R30 degrees-Ag determined by low-energy electron diffraction.

We have carried out a quantitative low-energy electron-diffraction intensity analysis of the Ge(111) \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 R30\ifmmode^\circ\else\textdegree\fi{}-Ag structure. It is found that the Ag atoms arrange according to a honeycomb chained trimer model (HCT) similar to the situation of Si(111) \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 R30\ifmmode^\circ\else\textdegree\fi{}-Ag. This result shows that the Ge-Ge bond is stronger than the Ag-Ag bond on the surface, thus causing the HCT structure to form.

Direct structural determination by inversion of experimental diffuse low-energy electron diffraction intensities

We demonstrate that two-dimensionally resolved diffuse low-energy electron diffraction intensities can be measured with sufficient accuracy and at multiple energies to allow direct inversion for a low coverage (5%) disordered K/Ni(100) surface. The data inversion reveals three-dimensional coordinates of atoms with atom images whose full width at half maximum is less that 1 A in all spatial directions. By varying the angle of incidence, first layer and second layer near-neighbor Ni atoms are separately imaged. This is the first demonstration of multiple-energy internal-source electron holography using measured elastically backscattered electrons.

Atomic and electronic structure of the corundum ( alpha -alumina) (0001) surface.

Using a tight-binding, total-energy model, we predict the atomic and electronic structure of the relaxed (1\ifmmode\times\else\texttimes\fi{}1) corundum (0001) surface. The surface shows a large, bond-length-conserving relaxation, which is allowed by the topology of the surface. The relaxation is driven by a rehybridization of the surface Al atoms to ${\mathit{sp}}^{2}$, and an accompanying drop in the energy of occupied surface states, during the relaxation. Displacements of surface atoms from bulk positions are as large as 0.7 \AA{}, and should be observable using a low-energy electron diffraction intensity analysis. Full relaxed atomic positions are reported, as well as a wavelength-resolved surface band structure, including orbital characters of surface states.

LEED structure determination of the c( 8 × 2)Mn phase on Cu(100)

The structure of the Cu(100)c(8 × 2)Mn phase was determined by a quantitative low-energy electron diffraction (LEED) intensity analysis. This structure is formed by deposition of 0.9 ML Mn on Cu(100) at 200 K. Scanning tunneling microscopy topographs and an investigation of the film morphology by high-resolution electron diffraction show that the Mn film wets the substrate and has a low step density. The LEED intensity analysis proves that Mn forms a quasi-hexagonal layer on the Cu(100) substrate. The Mn atoms in this layer have a pronounced buckling of 0.53 Å and an in-plane spacing of around 2.92 Å. This spacing is considerably larger than the nearest-neighbour spacing of 2.7 Å found in γ-Mn.

Surface relaxation of Rh{001}

The relaxation of a clean Rh[l brace]001[r brace] surface is reexamined by means of analyses of three sets of low-energy electron diffraction (LEED) intensities collected in earlier experiments. The averaged results show that the first interlayer spacing is slightly contracted by (1.2[plus minus]1.6 %) with respect to the bulk spacing (1.902 A) and that the second interlayer spacing is unrelaxed. These results are only a little different from those found by other authors in two previous LEED studies, and they lie in the range of the combined experimental errors of all the analyses. However, our results are in quite good agreement with a recent [ital ab] [ital initio] pseudopotential calculation of Morrison, Bylander, and Kleinman, who found ferromagnetism on the Rh[l brace]001[r brace] surface and a corresponding unusually small first-spacing relaxation.

Growth of ultrathin films of Fe on Au{001}

The crystallographic aspects of the growth of ultrathin films of Fe on Au[l brace]001[r brace] are studied by means of quantitative low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The first monolayer of Fe is found to be pseudomorphic to the Au[l brace]001[r brace] substrate with an interlayer spacing of 1.825 A, and to be covered by a monolayer of Au at an interlayer distance of 1.85 A, which acts as a surfactant. Thicker films of Fe [up to 45 layer equivalents (LE)] may be pseudomorphic and hence slightly strained (the lattice misfit is 0.6%) or equilibrium bcc with surface relaxation (the present analysis cannot distinguish between the two possibilities), but contain defects and disordered steps. Gold atoms are found to be segregated in the surface region of 45-LE Fe films, but mostly in a disordered arrangement. A comparison between the results obtained with two different computer programs for the calculation of LEED intensities finds that differences of 0.025 A in the values of structural parameters may occur.

LEED structure determination of two ordered surface alloys: Cu(100)-c(2 × 2)Mn and Ni(100)-c(2 × 2)Mn

The structure of the Cu(100)-c(2 × 2)Mn and Ni(100)-c(2 × 2)Mn phase was determined by a quantitative low-energy electron diffraction (LEED) intensity analysis. These structures are ordered surface alloys which form after deposition of 0.5 ML Mn on the clean Cu(100) and Ni(100) surface above 270 K. The alloy layer shows a pronounced corrugation in which Mn atoms in the first layer are displaced outwards by 0.30 ± 0.02 A ̊ on Cu(100) and 0.25 ± 0.02 A ̊ on Ni(100). The interlayer distance from the Cu (Ni) atoms to second layer Cu (Ni) atoms is close to the Cu(100)-[Ni(100)] bulk interlayer spacing of 1.807 Å (1.76 Å). Deeper interlayer distances are also very similar to the respective bulk value.

Ionicity dependence of surface bond lengths on the (110) cleavage faces of isoelectronic zincblende structure compound semiconductors: GaP, ZnS, and CuCl

Previously measured low-energy electron diffraction (LEED) intensity data for the (110) surfaces of the isoelectronic series GaP, ZnS, and CuCl, are reanalyzed using dynamical LEED calculations, a new selection of independent structural variables, and an automated optimization procedure. The bond lengths of the atoms in the top layer and perpendicular displacements of the cation-anion sublattices in the top two layers are selected as the independent structural variables. All other bond lengths are taken to be equal to their bulk values. An automated gradient method search procedure is utilized to obtain these structural variables by optimizing the fit of the measured intensities by the calculated ones as determined by the x-ray R factor. This procedure permits a precise comparison of the bond lengths of species in the top layer from one material to another. Our major finding is that the bond lengths, d, between the anion and cation in the top layer depend on the spectroscopic ionicity, f, via [(d−dB)/dB]=0.025−0.14f, in which dB is the bulk value of the bond length. The bonds between surface and subsurface species do not exhibit a systematic dependence on f.

A low-energy electron diffraction study of oxygen, water and ice adsorption on Pt(111)

Abstract We present low-energy electron diffraction (LEED) studies of a p(2 × 2) overlayer of oxygen, a monolayer of water and a multilayer ice film on Pt(111). Oxygen adsorbs in a fcc hollow site; buckling in the first two substrate layers leads to a local expansion of the substrate underneath the oxygen. Water adsorbs as a disordered monolayer and diffuse LEED intensities (DLEED) can be observed. From the structure of the diffuse background preferred next-neighbor distances can be concluded. A multilayer of water forms a thick molecular crystal of ice grown in (√3 × √3)R30° periodicity on Pt(111).

A low-energy electron diffraction study of oxygen, water and ice adsorption on Pt(111)

We present low-energy electron diffraction (LEED) studies of a p(2 × 2) overlayer of oxygen, a monolayer of water and a multilayer ice film on Pt(111). Oxygen adsorbs in a fcc hollow site; buckling in the first two substrate layers leads to a local expansion of the substrate underneath the oxygen. Water adsorbs as a disordered monolayer and diffuse LEED intensities (DLEED) can be observed. From the structure of the diffuse background preferred next-neighbor distances can be concluded. A multilayer of water forms a thick molecular crystal of ice grown in (√3 × √3)R30° periodicity on Pt(111).