Daniel Rosenbach

Crystal Phase Transformation in Self-Assembled InAs Nanowire Junctions on Patterned Si Substrates

We demonstrate the growth and structural characteristics of InAs nanowire junctions evidencing a transformation of the crystalline structure. The junctions are obtained without the use of catalyst particles. Morphological investigations of the junctions reveal three structures having an L-, T-, and X-shape. The formation mechanisms of these structures have been identified. The NW junctions reveal large sections of zinc blende crystal structure free of extended defects, despite the high stacking fault density obtained in individual InAs nanowires. This segment of zinc blende crystal structure in the junction is associated with a crystal phase transformation involving sets of Shockley partial dislocations; the transformation takes place solely in the crystal phase. A model is developed to demonstrate that only the zinc blende phase with the same orientation as the substrate can result in monocrystalline junctions. The suitability of the junctions to be used in nanoelectronic devices is confirmed by room-temperature electrical experiments.

Electrical resistance of individual defects at a topological insulator surface

Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone. Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering. Here we use scanning tunnelling potentiometry to analyse the resistance of different kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator thin film. We find the largest localized voltage drop to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analysed by means of a resistor network model. The effect resulting from the voids is found to be small compared with the other defects.

Simultaneous Integration of Different Nanowires on Single Textured Si (100) Substrates

By applying a texturing process to silicon substrates, we demonstrate the possibility to integrate III-V nanowires on (100) oriented silicon substrates. Nanowires are found to grow perpendicular to the {111}-oriented facets of pyramids formed by KOH etching. Having control of the substrate orientation relative to the incoming fluxes enables not only the growth of nanowires on selected facets of the pyramids but also studying the influence of the fluxes on the nanowire nucleation and growth. Making use of these findings, we show that nanowires with different dimensions can be grown on the same sample and, additionally, it is even possible to integrate nanowires of different semiconductor materials, for example, GaAs and InAs, on the very same sample.

Evaluation of lattice dynamics, infrared optical properties and visible emissions of hexagonal GeO2 films prepared by liquid phase deposition

GeO2 films with thicknesses from 10 to 22 μm have been deposited on polished Si(100) substrates using liquid phase deposition. The effects of deposition time on the thickness, crystallographic and vibrational properties, as well as optical characteristics have been investigated by X-ray diffraction (XRD), scanning electron microscopy, Raman scattering and infrared (IR) reflectance/transmittance. The XRD analysis indicates the presence of the pure hexagonal GeO2 phase, and the space group P3121, in the films, which is confirmed by the frequencies and intensities of the Raman- and IR-active phonon modes. Furthermore, the optical constants n and k in the mid- and far-IR regions for the α-GeO2 films have been extracted using IR variable angle spectroscopic ellipsometry. Interestingly, we observe that the refractive index n in certain mid- and far-IR regions is below unity (n < 1), which explains the abnormal phenomena observed in the IR transmittance spectra. Based on this unique characteristic, the GeO2 films can be used as attenuated total reflection mid- and far-infrared hollow waveguides for infrared waves. In addition, the photoluminescence spectra were collected in the visible range to investigate the related defects in the GeO2 films. The emission band near 2.1 eV originates from the X3Ge–GeX2 related defects, while the bands located at around 2.4 and 2.8 eV are related to the Ge–O related defects, which are dominant in the as-deposited GeO2 films. These findings provide further information for developing GeO2-based optical devices.

Phase-coherent transport in topological insulator nanocolumns and nanoribbons

The transport properties of topological insulator nanostructures prepared by selective-area molecular beam epitaxy is investigated. For the nanocolumn structures based on Sb2Te3/Bi2Te3-heterostructures pronounced universal conductance fluctuations are observed in the magnetoconductance, indicating phase-coherent transport. Furthermore, angle-dependent measurements indicate that the phase coherent loops are mainly oriented parallel to the substrate plane. Measurements on nanoribbons based on (Bi0.57Sb0.43)2Te3 revealed a resistance dip due to weak antilocalization as well as universal conductance fluctuations. Here, we also found indications, that the phase-coherent loops are predominantly oriented parallel to the quintuple layers forming the topological insulator.