M.B. Webb

Scanning tunneling microscopy studies of structural disorder and steps on Si surfaces

Scanning tunneling microscopy observations of several forms of disorder on Si surfaces are presented. These include dimer vacancies on Si(001), step bunches associated with a morphological phase transition on vicinal Si(111), and step structure on vicinal Si(001). A recipe for cleaning of Si surfaces to produce a minimum amount of disorder is presented.

Surface self-diffusion of Si on Si(001)

The surface diffusion coefficient of Si atoms on a Si(001) surface is quantitatively determined using scanning tunneling microscopy. The method rests on counting the number of islands that form at various substrate temperatures for a given deposited dose at a given deposition rate. In the simplest situation, the diffusion coefficient is related to the island density by N α D−13 and to the width of denuded zones at steps by WDZαD16. The activation energy for diffusion is Ea = 0.67±0.08 eV an D0≅10−3±1 cm2. The diffusion is highly anisotropic, with the fast direction along the dimer rows.

Low‐energy electron diffraction analysis of the Si(111)7×7 structure

We have used the dynamical theory of low‐energy electron diffraction to analyze data of the Si(111)7×7 surface and determined the atomic structure. The method includes the use of symmetrized wave functions in real and reciprocal spaces. Individual atomic coordinates for the first five atomic planes (containing 200 atoms) are determined. The low‐energy electron diffraction optimized structure shows an oscillatory relaxation: atomic planes with stretched bonds followed by planes with compressed bonds. Geometric displacements from the bulk dimer–adatom–stacking fault model are presented.

An electron diffraction study of the structure of silicon (100)

The (100) surface of silicon has been studied by low-energy electron diffraction and a preferred model of its atomic arrangement is proposed. A clean, well ordered, and reproducible surface was prepared by repeated heating to 1250°C, annealing at 900°C, and cooling at less than l°Csec. This surface is reconstructed from the bulk structure and has a c(4 × 2) unit mesh. The diffracted intensity consists of the Bragg maxima expected from the bulk and secondary peaks whose properties suggest they are due to the diffraction from the reconstructed selvedge. The data are averaged at constant momentum transfer to extract the quasikinematic intensity. The magnitude and width of the secondary structure suggest the selvedge extends three to five atomic layers into the material. No previously proposed model of the surface is consistent with the data. A model which gives a reasonable fit to the data has alternate rows of atoms in each of the outermost two planes removed, which exposes small facets of (111) surface. Then, atoms in the third layer are displaced — half moving to the plane of their back-bonding neighbors and the other half making orthogonal back bonds. Additionally, there are outward displacements of atoms in the first two layers. The proposed model can be rationalized by arguments due to W.A. Harrison in which rehybridization is the major contribution to the energy of the reconstruction.

Scanning tunneling microscopy study of diffusion, growth, and coarsening of Si on Si(001)

The growth, diffusion, and coarsening of Si on Si(001) have been investigated with scanning tunneling microscopy (STM). A diffusion coefficient for Si has been determined. Anisotropy in the island shapes during epitaxy is shown to be principally a growth structure due to an anisotropic accommodation coefficient. Diffusional anisotropy is small. An ordered ‘‘diluted‐dimer’’ structure is observed at low coverages and temperatures.

Xe adsorption on Ag(111): Experiment

Abstract Precise structural and thermodynamic studies of Xe adsorbed on Ag(111) are made using low-energy diffraction. Spacings are measured relative to those of bulk adsorbed film. Effective equilibrium experiments are done using a directed gas beam and cryopumping. The density of only that gas in the coherently diffracting area is measured by the attenuation of the Ag(01) beam. With decreasing temperature at fixed gas flux one observes the sequential of 2D gas, monolayer, bilayer and bulk films. The effective pressure of the impinging gas is determined from the known bulk vapor pressure so bypassing absolute pressure measurements. The phase diagram, lattice spacings in the film at equilibrium and at zero pressure, latent heats of adsorption and isosteric heats are determined.

Ar and Kr adsorption on Ag(111)

Abstract Precise structural and thermodynamic studies of Kr and of Ar adsorbed on Ag(111) are made using low energy electron diffraction. The phase diagram, lattice constants of the unconstrained monolayer and of the monolayer in equilibrium with the bilayer, latent heats of adsorption and isosteric heats are measured. The results are similar to those of an earlier study of Xe adsorbed on Ag(111). The results are compared to model calculations using effective lateral interactions which are similar to those for Xe/Ag(111). Comparison of the results for Xe, Kr, and Ar on Ag(111) is made using corresponding states scalings. A comparison is also made with properties of the non-registry phases of Xe, Kr, and Ar on basal plane graphite.

Elastic Scattering of Low-Energy Electrons from Surfaces*

Publisher Summary The extensive activity and progress of the last decade have clarified the important ingredients of a satisfactory understanding of the elastic scattering of low-energy electrons by surfaces. The electrons are scattered within a few atomic planes of the surface and their diffraction is intermediate between the kinematic and dynamic limits. The large cross sections for elastic scattering lead to significant multiple scattering, but for energies reasonably above the plasmon threshold the inelastic processes rapidly attenuate higher-order scattering processes. For the usual experiments, the momentum transfer is large, so the scattering is mainly sensitive to the potentials rather deep in the ion core, and the scattering factors are similar to those for free atoms. Consequently, the core potentials must be treated correctly in any realistic calculations. However, the scattering is quite insensitive to the valence electrons and the surface potential. Thus elastic scattering is primarily a tool for studying the atomic rather than the electronic structure of surfaces. Both because the momentum transfer is large and because the electrons are scattered from the surface region where thermal vibrational amplitudes are large, temperature effects are very important and must be accounted for in interpreting diffracted intensities. Low-energy electron scattering also provides the most direct technique for investigating surface lattice dynamics.

Xe monolayer adsorption on Ag(111) I. Structural properties

The monolayer of adsorbed xenon on the (1) surface of silver was studied using low- energy electron diffraction. The structure is an hexagonal lattice aligned with but out of registry with the substrate. The Xe-Xe spacing at 25 K is 4.44 ± 0.01 A in the full monolayer and 4.47 ± at less than full coverage. The Xe-Ag spacing is 3.5 ± 0.1 A. Here 〈u⊥〉2T, the meansquare vibrational amplitude normal to the surface divided by the temperature is (1.83 ± 0.4 × 10−4 A2/K. The heat of adsorption is 0.28 ± 0.03 eV/atom.

Surface step configurations under strain: kinetics and step-step interactions

Abstract Strain is an important ingredient in the physics of surfaces as has been shown in experiments with thin films and alloys. It is desirable to have strain as an externally applied and continuous variable. We have studied the effect of strain by loading a cantilevered bar and observing the effects on the surface with both LEED and the STM. On the Si(100) surface, strain produces a reversible asymmetry in the relative population of the 2 × 1 and 1 × 2 domains. This requires the motion of monatomic steps which are the domain boundaries. The effect is driven by the relaxation of the energy associated with a long-range strain field extending into the bulk due to the anisotropy of the intrinsic stress tensor of the two reconstructed domains. It is similar to magnetic domain structures reducing the magnetic field energy. These long-range strain fields have important consequences for a number of surface phenomena. Here we first report experiments on the terrace width distributions which gives information about the effective step-step interactions. Then we report experiments on the kinetics of the step migration which gives information about surface diffusion.