Bernhard Mandl

Growth Mechanism of Self-Catalyzed Group III−V Nanowires

Group III−V nanowires offer the exciting possibility of epitaxial growth on a wide variety of substrates, most importantly silicon. To ensure compatibility with Si technology, catalyst-free growth schemes are of particular relevance, to avoid impurities from the catalysts. While this type of growth is well-documented and some aspects are described, no detailed understanding of the nucleation and the growth mechanism has been developed. By combining a series of growth experiments using metal−organic vapor phase epitaxy, as well as detailed in situ surface imaging and spectroscopy, we gain deeper insight into nucleation and growth of self-seeded III−V nanowires. By this mechanism most work available in literature concerning this field can be described.

Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.

X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor

For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si–metal–oxide semiconductor field-effect transistor.

Wurtzite-zincblende superlattices in InAs nanowires using a supply interruption method

Crystal phase control in single III-V semiconductor nanowires has emerged recently as an important challenge and possible complement to conventional bandgap engineering in single material systems. Here we investigate a supply interruption method for precise crystal phase control in single nanowires. The nanowires are grown by metalorganic vapor phase epitaxy using gold particles as seeds and are analyzed by transmission electron microscopy. It is observed that wurtzite segments with controlled length and position can be inserted on demand into a pure InAs zincblende nanowire. The interface between wurtzite and zincblende segments can be made atomically sharp and the segments can be made only a few bilayers in thickness. The growth mechanisms, applicability and limitations of the technique are presented and discussed.

Structural Investigations of Core-shell Nanowires Using Grazing Incidence X-ray Diffraction.

The fabrication of core-shell structures is crucial for many nanowire device concepts. For the proper tailoring of their electronic properties, control of structural parameters such as shape, size, diameter of core and shell, their chemical composition, and information on their strain fields is mandatory. Using synchrotron X-ray diffraction studies and finite element simulations, we determined the chemical composition, dimensions, and strain distribution for series of InAs/InAsP core-shell wires grown on Si(111) with systematically varied growth parameters. In particular we detect initiation of plastic relaxation of these structures with increasing shell thickness and/or increasing phosphorus content. We establish a phase diagram, defining the region of parameters leading to pseudomorphic nanowire growth. This is important to avoid extended defects which are detrimental for their electronic properties.

Evidence of stacking‐fault distribution along an InAs nanowire using micro‐focused coherent X‐ray diffraction

InAs nanowire samples grown by metal-organic chemical vapor deposition present a significant amount of wurtzite structure, while the zincblende lattice is known to be the stable crystal structure for the bulk material. The question of the wurtzite distribution in the sample is addressed using phase-sensitive coherent X-ray diffraction with a micro-focused beam at a synchrotron source. The simultaneous investigation of the wurtzite 10\bar{1}0, 20\bar{2}0 and 30\bar{3}0 reflections performed on a bunch of single wires shows unambiguously that the wurtzite contribution is a result of stacking faults distributed along the wire. Additional simulations lead to adjustments of the wire structural parameters, such as the wurtzite content, the strain distribution, the wire diameters and their respective orientations.

Characterizing the geometry of InAs nanowires using mirror electron microscopy

Mirror electron microscopy (MEM) imaging of InAs nanowires is a non-destructive electron microscopy technique where the electrons are reflected via an applied electric field before they reach the specimen surface. However strong caustic features are observed that can be non-intuitive and difficult to relate to nanowire geometry and composition. Utilizing caustic imaging theory we can understand and interpret MEM image contrast, relating caustic image features to the properties and parameters of the nanowire. This is applied to obtain quantitative information, including the nanowire width via a through-focus series of MEM images.

New Flexible Toolbox for Nanomechanical Measurements with Extreme Precision and at Very High Frequencies.

We show that the principally two-dimensional (2D) scanning tunneling microscope (STM) can be used for imaging of 1D micrometer high free-standing nanowires. We can then determine nanowire megahertz resonance frequencies, image their top-view 2D resonance shapes, and investigate axial stress on the nanoscale. Importantly, we demonstrate the extreme sensitivity of electron tunneling even at very high frequencies by measuring resonances at hundreds of megahertz with a precision far below the angstrom scale.

InAs film grown on Si(111) by Metalorganic Vapor Phase Epitaxy

We report the successful growth of high quality InAs films directly on Si( 111) by Metal Organic Vapor Phase Epitaxy. A nearly mirror-like and uniform InAs film is obtained at 580 C for a thickness of 2 mu m. We measured a high value of the electron mobility of 5100 cm(2)/Vs at room temperature. The growth is performed using a standard two-step procedure. The influence of the nucleation layer, group V flow rate, and layer thickness on the electrical and morphological properties of the InAs film have been investigated. We present results of our studies by Atomic Force Microscopy, Scanning Electron Microscopy, electrical Hall/van der Pauw and structural X-Ray Diffraction characterization.

X-ray diffraction strain analysis of a single axial InAs1–xPx nanowire segment

Strain analysis of an axial InAs1–xPx hetero-segment in an InAs nanowire using nano-focused X-ray diffraction is presented.