[Show abstract][Hide abstract] ABSTRACT: Using scanning tunneling microscopy, we evaluate the surface structure and morphology down to the atomic scale for micrometers along Au-free grown InAs nanowires (NWs) free from native oxide. We find that removal of the native oxide (which covers the NWs upon exposure to the ambient air) using atomic hydrogen does not alter the underlying step structure. Imaging with sub-nanometer resolution along the NWs, we find an extremely low tapering (diameter change along the NW) of 1.7 ± 0.5 Å μm(-1). A surface morphology with monolayer high islands, whose shape was influenced by stacking faults, was found to cover the NWs and was attributed to the decomposed native oxide. The appearance of point defects in the form of As-vacancies at the surface is analyzed and we set limits to the amount of carbon impurities in the NWs.
[Show abstract][Hide abstract] ABSTRACT: The spatial strain distribution in and around a single axial InAs
hetero-segment in an InAs nanowire was analyzed using nano-focused X-ray diffraction. In connection with finite-element-method simulations a detailed quantitative picture of the nanowire's inhomogeneous strain state was achieved. This allows for a detailed understanding of how the variation of the nanowire's and hetero-segment's dimensions affect the strain in its core region and in the region close to the nanowire's side facets. Moreover, ensemble-averaging high-resolution diffraction experiments were used to determine statistical information on the distribution of wurtzite and zinc-blende crystal polytypes in the nanowires.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this work, the nucleation and growth of InAs nanowires on patterned SiO(2)/Si(111) substrates is studied. It is found that the nanowire yield is strongly dependent on the size of the etched holes in the SiO(2), where openings smaller than 180 nm lead to a substantial decrease in nucleation yield, while openings larger than ≈500nm promote nucleation of crystallites rather than nanowires. We propose that this is a result of indium particle formation prior to nanowire growth, where the size of the indium particles, under constant growth parameters, is strongly influenced by the size of the openings in the SiO(2) film. Nanowires overgrowing the etched holes, eventually leading to a merging of neighboring nanowires, shed light into the growth mechanism.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this work we demonstrate experimentally the dependence of InSb crystal structure on the ratio of Sb to In atoms at the growth front. Epitaxial InSb wires are grown by a self-seeded particle assisted growth technique on several different III-V substrates. Detailed investigations of growth parameters and post-growth energy dispersive x-ray spectroscopy indicate that the seed particles initially consist of In and incorporate up to 20 at.% Sb during growth. By applying this technique we demonstrate the formation of zinc-blende, 4H and wurtzite structure in the InSb wires (identified by transmission electron microscopy and synchrotron x-ray diffraction), and correlate this sequential change in crystal structure to the increasing Sb/In ratio at the particle-wire interface. The low ionicity of InSb and the large diameter of the wire structures studied in this work are entirely outside the parameters for which polytype formation is predicted by current models of particle seeded wire growth, suggesting that the V/III ratio at the interface determines crystal structure in a manner well beyond current understanding. These results therefore provide important insight into the relationship between the particle composition and the crystal structure, and demonstrate the potential to selectively tune the crystal structure in other III-V compound materials as well.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Recent investigations of core–shell nanowires using synchrotron radiation techniques deduced the average structural parameters of heterostructure core–shell nanowires. Here, we report on first results and discuss the problems arising when measuring such complex nanostructures by using nanofocusing X-ray techniques. InAs/InAsP core–shell nanowires exhibit a certain bending, the origin of which is described using finite element simulations assuming a displacement of the core, and a gradient in the chemical composition of the wire’s shell.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 02/2010; 268(3):316-319. DOI:10.1016/j.nimb.2009.09.043 · 1.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The cross section of an InAs nanowire with a diameter of 150 nm epitaxially grown on a 111-oriented InP substrate was characterized using a combination of x-ray scanning microdiffraction and coherent diffraction imaging. Using an x-ray beam focused by a Fresnel zone plate, we were able to scan in real space and hence localize single nanowires on the substrate in the as-grown epitaxial state. For one single nanowire, the three-dimensional coherent intensity distribution in reciprocal space was mapped around the (111) InAs reflection. Using phase retrieval algorithms, the cross section of the wire was reconstructed with a spatial resolution of 8 nm along one direction.
Physical Review B 03/2009; 79(12). DOI:10.1103/PhysRevB.79.125324 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present x-ray diffraction based methods to quantitatively determine the wurtzite content of nanowire ensembles and to investigate the effect of twinning. An increased lattice constant in growth direction is found for all investigated InAs and InP nanowire samples. This increase is independent of the wurtzite content. Using x-ray pole figures we find that twinning is present in GaAs/Si branched nanowires, which leads to 60° rotations of the lattice.
[Show abstract][Hide abstract] ABSTRACT: 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 , and 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.
[Show abstract][Hide abstract] ABSTRACT: 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 degC for a thickness of 2 mum. 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.
Journal of Physics Conference Series 03/2008; 100(4). DOI:10.1088/1742-6596/100/4/042017
[Show abstract][Hide abstract] ABSTRACT: InAs nanowires have been grown without the use of Au or other metal particles as catalyst by metal-organic vapor phase epitaxy. The nanowires growth is initiated by a thin layer of SiOx. The wires exhibit a non-tapered shape with a hexagonal cross section. In addition to InAs also InAs1-xPx wires are grown and the incorporation of P is studied by photoluminescence.
[Show abstract][Hide abstract] ABSTRACT: III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.