Bert Voigtländer

Simultaneous molecular beam epitaxy growth and scanning tunneling microscopy imaging during Ge/Si epitaxy

A high temperature scanning tunneling microscope (STM) capable of simultaneous imaging and molecular beam epitaxy (MBE) growth at 600–900 K sample temperature is described. The formation of the two‐dimensional Stranski–Krastanov layer and the evolution of three‐dimensional islands during further growth of Ge on Si(111) was observed. An inversion of the aspect ratio of the islands with increasing coverage indicates a transition from coherent to dislocated 3D islands. This method (MBSTM) opens the possibility to follow MBE growth processes with STM in a real in situ way and gives access to the evolution of specific features during growth.

Nucleation and growth of CoSi2 on Si(100) studied by scanning tunneling microscopy

Abstract The initial stages of CoSi 2 formation on the Si(100) surface are investigated by scanning tunneling microscopy (STM). We find a quasi-periodical reconstruction of the Si surface for very low Co coverages of 0.01 ML which is similar to the Ni induced (2 × 8) structure. At higher Co coverage, in reactive deposition epitaxy, the formation of qualitatively different two and three-dimensional islands is observed. We have evidence that the former are Si terminated, with Co probably being positioned in substitutional sites beneath the island. The growth of the 3D CoSi 2 islands is connected with substantial mass transport from the substrate into the islands to enable the silicide formation. Their elongated shape is attributed to strain and they occur in different epitaxial relations to the substrate. CoSi 2 islands in (100) orientation are identified by the c(2 × 2) surface lattice with mixed Co and Si occupation. Simultaneous deposition of Co and Si up to 30 ML results in the formation of CoSi 2 island clusters and a rough surface. The roughness exponent β = 0.66 is in agreement with an existing Monte-Carlo simulation.

Temperature-dependent morphologies of gold surfaces

Abstract Periodic surface profiles on Au{111} and Au{100} single crystals annealed at 1023 K under ultra-high vacuum conditions were studied ex situ by scanning tunneling microscopy (STM). The shapes exhibited several facets in each case separated from each other by sharp edges. Aside from the main {111} and {100} facets, extra facets at 3.7° and 11.7° for Au{111}〈112〉 and at 8.0° for Au{100}〈110〉 were observed. These orientations seem to represent stable surface phases, in the sense of “magic” orientations due to the influence of surface reconstruction.

SCANNING TUNNELING MICROSCOPY OF EQUILIBRIUM CRYSTAL SHAPES

Scanning tunneling microscopy (STM) has been used to image the equilibrium shape of small 2d and 3d crystallites supported on single crystal substrates. Due to the high resolution of STM, highly accurate shape information is obtained. Concerning 3d particles, special attention was given to the step structure of facets and the curved portions near facets. The shape of the (111) facet of Pb particles, prepared and equilibrated on Cu(111) under ultrahigh vacuum conditions, exhibits a threefold symmetry. The facet edge can be determined with atomic resolution. Hence an accurate analysis of the Pb equilibrium crystal shape (ECS) near facets is possible yielding shape exponents. Expected universal exponents of 3/2 or 2 are not found. Instead, shape exponents exhibit an azimuthal dependence, with values ranging between 1.4 and 1.7. This variation, consistent with the threefold symmetry of the (111) facet, suggests a structure dependent step interaction behavior and, furthermore, that the facet to vicinal surface...

Structure of the Stranski-Krastanov layer in surfactant-mediated Sb/Ge/Si(111) epitaxy

Abstract In surfactant-mediated epitaxy of Ge on Si(111), islanding can be suppressed at 600°C sample temperature using Sb as a surfactant. However, for growth at higher temperatures (> 620°C) islanding occurs again. The equilibrium growth mode is Stranski-Krastanov growth and only kinetically-limited growth suppresses islanding in surfactant-mediated epitaxy. The thickness of the Stranski-Krastanov wetting-layer in between the islands is thinner in Sb-mediated epitaxy (3 ML Ge) than in pure Ge Si (111) epitaxy (4 ML Ge). The structure of the Stranski-Krastanov layer was analyzed quantitatively by scanning tunneling microscopy (STM). Triangular areas of (1 × 1) symmetry with and without stacking fault are connected by dimers. This structure is similar to the dimer-adatom-stacking-fault Ge(5 × 5) structure on Si(111), however without the adatoms. This dimer-stacking-fault structure is superimposed by a regular (6√3 × 6√3)R30° hexagonal superstructure. This structure is a response to the compressive stress induced by the large covalent radius of the antimony. Trenches of hexagonal arrangement in this structure allow strain relaxation of the (1 × 1) patches by outward expansion towards the trenches.

Design and performance of a beetle-type double-tip scanning tunneling microscope

A combination of a double-tip scanning tunneling microscope with a scanning electron microscope in ultrahigh vacuum environment is presented. The compact beetle-type design made it possible to integrate two independently driven scanning tunneling microscopes in a small space. Moreover, an additional level for coarse movement allows the decoupling of the translation and approach of the tunneling tip. The position of the two tips can be controlled from the millimeter scale down to 50 nm with the help of an add-on electron microscope. The instrument is capable of atomic resolution imaging with each tip.

Investigation of the growth of Co on Cu(111) and Sb/Cu(111) using photoelectron forward scattering

Abstract We recorded the angular anisotropies of photoelectron intensities arising from thin Co layers on a clean and Sb-covered Cu(111) surface. Forward scattering leads to intensity enhancements along close-packed directions and thus, the azimuthal distributions reveal information about the mode of heteroepitaxial growth. For Co on the clean Cu surface we find evidence for the formation of three-dimensional islands even at low Co coverages. At about a nominal coverage of 4 ML, the Co layer undergoes a structural transition from fcc to hcp stacking which can be easily identified by means of symmetry of the angular distributions. The influence of Sb as a surfactant on the growth behaviour has also been studied. Showing the typical effect of segregating to the surface, Sb induces the Co film to grow in a rather flat manner. Accordingly, the fcc→ hcp transition is delayed to a nominal Co coverage beyond 10 ML.

Modification of the conductance of single fullerene molecules by endohedral doping

We use scanning tunneling microscopy to establish controlled contacts to single molecules of endohedrally doped Ce2@C80 fullerenes with C60 as a reference. The stability of the experimental setup allows for the determination of the conductance of Ce2@C80 relative to the conductance of C60. The endohedral doping reduces the conductance of Ce2@C80 by a factor of about five with respect to C60. Ab initio calculations show that the reason for this reduced conductance is the absence of electron orbitals delocalized over the cage of Ce2@C80 in the energy window of the conductance measurement.

On the origin of the kinetic growth instability of homoepitaxy on Si(001)

Abstract The recently observed kinetic growth instability of homoepitaxial layers on Si(001) was investigated by in-situ scanning tunneling microscopy (STM ) experiments. In the step-flow regime the instability consists of straight step bunches, and it vanishes both during two-dimensional (2-D) island growth and at high temperatures. Kinetic Monte Carlo (KMC) simulations were performed to identify the dominating mechanism causing the instability. Strong evidence for the presence of an asymmetric step-edge barrier with the behavior of an inverse Ehrlich–Schwoebel barrier is found. Comparison between the experiments and the simulations reveal that only double atomic height D B steps, which form kinetically in a rather narrow temperature range, develop this type of step-edge barrier.

Resistance and dopant profiling along freestanding GaAs nanowires

Resistance profiles along as-grown GaAs nanowires were measured with a multi-tip scanning tunneling microscope used as a nanoprober. The nanowires were grown in the vapor-liquid-solid growth mode in a two-temperature-step mode and doped with Zn. Using a transport model, the resistance profile was converted to a dopant profile. The dopant distribution along the nanowires was found to correlate with the temperature during different phases of nanowire growth. The nanowire base grown at higher temperature exhibits a decreased dopant concentration. Mechanical stress by intentional bending of a nanowire was shown not to influence nanowire conductance.