M. Horn-von Hoegen

A new two-dimensional particle detector for a toroidal electrostatic analyzer

We describe a new two‐dimensional detector for the detection of ions scattered from a solid target, analyzed in energy and scattering angle by a toroidal electrostatic analyzer. The detector resolves the scattering angle with a resolution of 0.4° over a range of 25°, and the ion energy with a resolution of 120 eV over a range of 2000 eV, at 100 keV ion energy. The energy resolution of the spectrometer was improved with a factor 4 relative to its previous performance with a one‐dimensional scattering angle detector, while−at the same time−the dose efficiency (count/μC) was improved by a factor 5–10.

The interplay of surface morphology and strain relief in surfactant mediated growth of Ge on Si(111)

Abstract The growth of Ge on Si is strongly modified by surface active species called surfactants. While the effectiveness of Sb as a surfactant in forcing layer-by-layer growth of Ge on Si(111) and in creating a misfit adjusting dislocation network confined to the Si Ge-interface has been demonstrated in previous studies, the dynamic growth process on an atomic scale leading to this result is still unknown. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by spot profile analysing low energy electron diffraction during the deposition. The change from islanding to layer-by-layer growth is seen in the oscillatory intensity variation of the (00)-spot. To relieve the strain the Ge-film forms a microscopically rough surface of small triangular and defect free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. After the overgrowth of the dislocations the surface smoothes again showing a much larger terrace length. The periodic dislocation network at the interface gives rise to an elastic deformation of the surface, which results in a spot splitting in LEED. Thus, for the first time the dynamics of the formation of a dislocation network has been observed in situ during the growth process. Surprisingly, the dislocation network is already completed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro-rough surface morphology, providing innumerous nucleation sites for dislocation.

Selecting a single orientation for millimeter sized graphene sheets

We have used low energy electron microscopy and photo emission electron microscopy to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99% composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based, electronic devices.

Layer-by-layer growth of germanium on Si(100)" strain-induced morphology and the influence of surfactants

Abstract Germanium grows on pure Si(100)-(2×1) in the Stranski-Krastanov mode. Layer-by-layer growth is found for coverages below 3 ML before the onset of 3D islanding. In this regime the morphology of the Ge layer is strongly influenced by the misfit of 4.2% between layer and substrate. Around 1 ML aligned missing dimer defects are created which form a semiperiodic (2×12) arrangement. With increasing coverage this periodicity is gradually compressed and reaches a (2×8) reconstruction around 2.3 ML. This behaviour is discussed in terms of partial relaxation of the local strain. When further Ge layers grow on this (2xN) arrangement, only part of the missing-dimer defects of the lower layer are buried and a network of trenches partly reaching down to the substrate remains. Layer-by-layer growth up to higher coverage can be obtained using As as a “surfactant” during growth. Under these conditions no (2×8)-like arrangement is found. Up to 12 ML Ge coverage the layer grows free of defects forming extremely anisotropic Ge islands. At higher coverage a network of trenches arises which decorate an array of V-shaped defects previously found with TEM. The arrangement and the start of the overgrowth of these defects is studied.

Formation of interfacial dislocation network in surfactant mediated growth of Ge on Si(111) investigated by Spa-Leed: Part I

The relief of strain during hetero-epitaxial growth of non-lattice matching materials is an important and unavoidable process, which includes the formation of strain relieving defects such as dislocations. Spot profile analysing low energy electron diffraction (SPA-LEED) has been used to observe for the first time the dynamics of the formation of those dislocations in situ during the growth process. Using Sb as surfactant in the growth of Ge on Si(111) confines all strain relieving defects into a periodic network of dislocations at the interface. The dislocations at the interface give rise to an elastic deformation of the film up to the surface, which, due to its regularity, is seen as a spot splitting in LEED. The exact form of the deformation and thus the arrangement of the dislocations is deduced from the intensity variation of the satellite spots with energy. The data are in excellent agreement with elasticity theory. The dislocations of the three sets do not intersect in one point but form an extended node with a size of 18 A. The first dislocations are generated at a Ge coverage of 8 monolayers, the final dislocation network is completed just after 10 additional monolayers of coverage. The network is detectable at the surface up to 60 monolayer thickness of the film.

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.

Enhanced Sb segregation in surfactant-mediated-heteroepitaxy: High-mobility, low-doped Ge on Si

Surfactant-mediated epitaxy (SME) allows the growth of smooth, continuous, relaxed, and principally defect free Ge films directly on Si(111); however, the very high surfactant doping level in the range of the solid solubility limit made them unacceptable for most device applications. By using high temperature SME we have reduced the Sb surfactant background doping level by more than three orders of magnitude. This is attributed to an enhanced surfactant segregation without kinetic limitations. The low Sb incorporation has been determined by an electrical characterization: An electron concentration of 1.1×1016 cm−3 and a very high electron Hall mobility of 3100 cm2/V s at 300 K (12 300 cm2/V s at 77 K) suggest an interesting potential of SME grown Ge films for future device applications.

A pulsed electron gun for ultrafast electron diffraction at surfaces

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

Electromigration in self-organized single-crystalline silver nanowires

We present electromigration experiments on single-crystalline silver nanowires. The wires were grown on 4° vicinal silicon (100) substrates by self-organization and were contacted by electron beam lithography. The electromigration experiments were performed in situ in a scanning electron microscope at room temperature with constant dc conditions. In contrast to other experiments we observe void formation at the anode side of the wires. If the current is reversed, the electromigration behavior is also reversed.

Growth temperature dependent graphene alignment on Ir(111)

The morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moire spots reflect graphene in a single orientation epitaxial incommensurate phase.