J. Falta

Electron diffraction at stepped homogeneous and inhomogeneous surfaces

Defect analysis at surfaces with LEED or RHEED have so far been essentially restricted to steps or superstructure domains. Even if there are no superstructures, variations of the scattering factor influence the spot profile. With the now available high-resolution instruments more details of the spot profile may be observed. Therefore it will be discussed how a careful study of the spot profile at different energies may be used to analyze a surface with simultaneous steps and inhomogeneities. It will be shown how the asperity height is obtained without fitting parameters and how inhomogeneities are analyzed with respect to coverage and size distribution.

Spatial pattern formation in a catalytic surface reaction: The facetting of Pt(110) in CO+O2.

The term “dissipative structures” which was introduced by Prigogihe [17] describes a broad class of non-equilibrium systems where a constant flow of energy and/or matter leads to structures ordered in space or time, e.g. causes kinetic oscillations or spatial pattern formation. Temporal oscillations and spatial pattern formation are closely related since the same kinetic instabilities which lead to a periodic variation of the system variables in time may also induce a periodic variation in space. Consequently one often observes spatio-temporal structures, but one may also consider the case of a non-equilibrium structure which is only periodic in space. Such structures which rarely have been observed in chemical reaction systems were first discussed by Turing [20] and have been termed accordingly as “Turing structures”.

THE INITIAL STAGES OF GROWTH OF SILICON ON Si(111) BY SLOW POSITRON ANNIHILATION LOW-ENERGY ELECTRON DIFFRACTION

Abstract Low-energy electron diffraction (LEED) is used to investigate the initial stages of growth of silicon on Si(111) between 556 and 900 K. The partial coverages θ h of the growing film are derived from the intensity variation during evaporation of the 00 and 7 × 7 spot. A preferred growth is observed in the second layer long before the first layer is completed. Surface defects caused by superstructure disorder on the grown islands act as nucleation centers for the diffusing adatoms, so at 633 K substrate temperature the grown islands show no ordered superstructure, although the film is epitaxial. The 7 × 7 spots show all the features of diffraction of a substrate with a non-scattering overlayer for coverages below two monolayers. The grown films show below 650 K an unordered superstructure, in an intermediate temperature range a mixture of 5 × 5 and 7 × 7 domains, and finally, above 870 K, perfect 7 × 7 superstructure. The average size of islands increases from 30 to 40 000 atoms.

Strain status of self-assembled InAs quantum dots

Grazing incidence x-ray diffraction experiments employing the asymmetric (202) Bragg diffraction have been performed to characterize self-assembled InAs quantum dots grown by molecular-beam epitaxy. We find that the strain is elastically relaxed with different components. The volume distribution of partially strained InAs inside islands is peaked at intermediate strain values. The fraction of both almost fully strained and totally relaxed InAs is found to be small. In addition, a small volume fraction of relaxed InxGa1−xAs is found.

Surfactant coverage and epitaxy of Ge on Ga‐terminated Si(111)

We have investigated the role of surfactant coverage and bonding for growth of Ge on Si(111). At 470 °C Ge grows on Si(111)‐(7×7) in a Stranski–Krastanov fashion. Preadsorption of 1‐ML Ga at 500 °C forms a Ga:Si(111)‐(6.3×6.3) structure and alters the Ge growth mode from three‐dimensional (3D) islanding to continuous film formation. However, the epitaxial layer contains defects, caused by the presence of domain boundaries of both A‐ and B‐type material. Growth properties depend strongly on the initial Ga coverage: if a (√3×√3) surface with 1/3‐mL Ga is used, a modified Stranski–Krastanov growth mode is observed, with 3D islands of a uniform predominant thickness.

Ordering and shape of self-assembled InAs quantum dots on GaAs(001)

Quantitative grazing-incidence small-angle x-ray scattering experiments have been performed on self-assembled InAs quantum dots (QDs) grown by molecular-beam epitaxy. We find pronounced nonspecular diffuse scattering satellite peaks with high diffraction orders, indicating a lateral ordering in the spatial positions of the InAs QDs. The mean-dot–dot distance and correlation lengths of the dot lateral distribution are found to be anisotropic. We observe the sharpest dot distribution in the [110] direction. Additional broad diffraction peaks are observed and associated with dot facet crystal truncation rods of the {111} and {101} facet families. This suggests an octagonal-based dot shape.

Self-limited oxide formation in Ni(111) oxidation

The oxidation of the Ni(111) surface is studied experimentally with low energy electron microscopy and theoretically by calculating the electron reflectivity for realistic models of the NiO/Ni(111) surface with an ab-initio scattering theory. Oxygen exposure at 300 K under ultrahigh-vacuum conditions leads to the formation of a continuous NiO(111)-like film consisting of nanosized domains. At 750 K, we observe the formation of a nano-heterogeneous film composed primarily of NiO(111)-like surface oxide nuclei, which exhibit virtually the same energy-dependent reflectivity as in the case of 300 K and which are separated by oxygen-free Ni(111) terraces. The scattering theory explains the observed normal incidence reflectivity R(E) of both the clean and the oxidized Ni(111) surface. At low energies R(E) of the oxidized surface is determined by a forbidden gap in the k_parallel=0 projected energy spectrum of the bulk NiO crystal. However, for both low and high temperature oxidation a rapid decrease of the reflectivity in approaching zero kinetic energy is experimentally observed. This feature is shown to characterize the thickness of the oxide layer, suggesting an average oxide thickness of two NiO layers.

Bi: Perfect surfactant for Ge growth on Si(111)?

We have investigated the growth of Ge on Bi-terminated Bi:Si(111)-∛×∛. In-situ measurements of x-ray standing waves, crystal truncation rods and scanning tunneling microscopy clearly show that, at substrate temperatures around 485 °C, smooth and homogeneous Ge films of thicknesses exceeding 30 bilayers Ge can be grown. For Ge coverages larger than 10 bilayers, the Ge film is completely relaxed. Bi is found to segregate to the surface during Ge deposition, and can be removed from the surface after growth by mild annealing at 520 °C as proven by Auger electron spectroscopy.

Surfactants in Si(111) homoepitaxy

Si epitaxy is strongly affected by the presence of an adsorbate (surfactant). We have examined both film quality and dopant incorporation in homoepitaxy for Sb, As, and Ga terminated Si(111). The efficency of site exchange between Si and adsorbate depends sensitively on binding energy and binding geometry of the adsorbate. For a weakly bound adsorbate (Ga or Sb), there is no inhibition of epitaxy, but a strongly bound adsorbate (As) kinetically inhibits growth

Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface

For a characterization of interface and “bulk” properties of Ge films grown on Si(111) by Sb surfactant-mediated epitaxy, grazing incidence x-ray diffraction and transmission electron microscopy have been used. The interface roughness, defect structure, and strain state have been investigated in dependence of film thickness and growth temperature. For all growth parameters, atomically smooth interfaces are observed. For thin Ge layers, about 75% of the strain induced by the lattice mismatch is relaxed by misfit dislocations at the Ge∕Si interface. Only a slight increase of the degree of relaxation is found for thicker films. At growth temperatures below about 600°C, the formation of twins is observed, which can be avoided at higher temperatures.