Improved success rate and stability for phase retrieval by including randomized overrelaxation in the hybrid input output algorithm

In this paper, we study the success rate of the reconstruction of objects of finite extent given the magnitude of its Fourier transform and its geometrical shape. We demonstrate that the commonly used combination of the hybrid input output and error reduction algorithm is significantly outperformed by an extension of this algorithm based on randomized overrelaxation. In most cases, this extension tremendously enhances the success rate of reconstructions for a fixed number of iterations as compared to reconstructions solely based on the traditional algorithm. The good scaling properties in terms of computational time and memory requirements of the original algorithm are not influenced by this extension.Comment: 14 pages, 8 figure

Scanning X-ray nanodiffraction: from the experimental approach towards spatially resolved scattering simulations

An enhancement on the method of X-ray diffraction simulations for applications using nanofocused hard X-ray beams is presented. We combine finite element method, kinematical scattering calculations, and a spot profile of the X-ray beam to simulate the diffraction of definite parts of semiconductor nanostructures. The spot profile could be acquired experimentally by X-ray ptychography. Simulation results are discussed and compared with corresponding X-ray nanodiffraction experiments on single SiGe dots and dot molecules

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

Non-destructive detection of cross-sectional strain and defect structure in an individual Ag five-fold twinned nanowire by 3D electron diffraction mapping

Coherent x-ray diffraction investigations on Ag five-fold twinned nanowires (FTNWs) have drawn controversial conclusions concerning whether the intrinsic 7.35° angular gap could be compensated homogeneously through phase transformation or inhomogeneously by forming disclination strain field. In those studies, the x-ray techniques only provided an ensemble average of the structural information from all the Ag nanowires. Here, using three-dimensional (3D) electron diffraction mapping approach, we non-destructively explore the cross-sectional strain and the related strain-relief defect structures of an individual Ag FTNW with diameter about 30 nm. The quantitative analysis of the fine structure of intensity distribution combining with kinematic electron diffraction simulation confirms that for such a Ag FTNW, the intrinsic 7.35° angular deficiency results in an inhomogeneous strain field within each single crystalline segment consistent with the disclination model of stress-relief. Moreover, the five crystalline segments are found to be strained differently. Modeling analysis in combination with system energy calculation further indicates that the elastic strain energy within some crystalline segments, could be partially relieved by the creation of stacking fault layers near the twin boundaries. Our study demonstrates that 3D electron diffraction mapping is a powerful tool for the cross-sectional strain analysis of complex 1D nanostructures

X-ray diffraction from single GaAs nanowires

Die Entwicklung neuartiger optischer Komponenten für harte Röntgenstrahlung hat es in den letzten Jahren ermöglicht, hoch intensive, kohärente Röntgenstrahlen mit Durchmessern von 100nm und darunter zu erzeugen. Zusammen mit der Entwicklung neuer experimenteller Aufbauten sind so nun zerstörungsfreie Röntgenbeugungsexperimente an einzelnen Nanoobjekten möglich. In der vorliegenden Arbeit werden eine Reihe von Aspekten beim epitaktischen Wachstum von Halbleiter-Nanodrähten mittels Röntgenbeugung untersucht. Besonderes Augenmerk liegt dabei auf der Anwendung neuartiger Methoden der "Nanobeugung", um einzelne Nanodrähte zu untersuchen. In einem ersten Schritt wird die Methode der kohhärenten Röntgenbeugung benutzt, um gleichzeitig die Gitterparameter und die 3-dimensionale Form einzelner Galliumarsenid Nanodrähte zu bestimmen, die mittels metallorganischer Gasphasenepitaxie gewachsen wurden. Auf Grund einer hohen Dichte von Rotationszwillingen in der Zinkblende-Struktur des Kristallgitters kommt es zu einer systematischen Abweichung der Gitterparameter im Vergleich zu GaAs Volumenkristallen. In einem zweiten Beispiel wird insbesondere das Anfangsstadium im selbstassistierten Wachstum von GaAs Nanodrähten auf Silizium (1 1 1) Oberflächen untersucht. Diese mittels Molekularstrahlepitaxie erzeugten GaAs Drähte wachsen vorwiegend in der kubischen Zinkblende-Struktur. Jedoch finden sich Abschnitte der hexagonalen Wurtzit-Struktur kurz oberhalb der Grenzfläche der Drähte zum Substrat, deren exakte Position mittels Nanobeugung bestimmt werden konnte. Da das Kristallgitter von GaAs einen um 4% größeren Gitterparameter besitzt als Silizium, kommt es zu Verspannungen an der Grenzfläche, welche durch den Einschluss von Versetzungen an der Grenzfläche abgebaut werden. Während bei Nanodrähten mit Durchmessern über 100nm der Abbau der Verspannung komplett durch diese Versetzungen erfolgt, verhindert eine raue Grenzfläche zum Substrat bei Drahtdurchmessern oberhalb von 100nm eine vollständige Relaxation, so dass ein Teil der Verspannung elastisch entlang der Wachstumsrichtung abgebaut wird. Zuletzt werden erste experimentelle Ergebnisse zur Relaxation in GaAs - InAs "core-shell" Nanodraht Heterostrukturen dargestellt und behandelt. In diesem System führt ein unvollständiger plastischer Abbau der Verspannung an der Grenzfläche zu einer elastischen Wechselwirkung zwischen GaAs Kern und InAs Hülle, welche eine signifikante Verzerrung des Kristallgitters im GaAs Kern mit ansteigender Dicke der InAs Hülle hervorruft.In recent years, developments in x-ray focussing optics have allowed to produce highly intense, coherent x-ray beams with spot sizes in the range of 100nm and below. Together with the development of new experimental stations, x-ray diffraction techniques can now be applied to study single nanometer-sized objects. In the present work, x-ray diffraction is applied to study different aspects of the epitaxial growth of GaAs nanowires. Besides conventional diffraction methods, which employ x-ray beams with dimensions of several tens of µm, special emphasis lies on the use of nanodiffraction methods which allow to study single nanowires in their as-grown state without further preparation. In particular, coherent x-ray diffraction is applied to measure simultaneously the 3-dimensional shape and lattice parameters of GaAs nanowires grown by metal-organic vapor phase epitaxy. It is observed that due to a high density of zinc-blende rotational twins within the nanowires, their lattice parameter deviates systematically from the bulk zinc-blende phase. In a second step, the initial stage in the growth of GaAs nanowires on Si (1 1 1) surfaces is studied. This nanowires, obtained by Ga-assisted growth in molecular beam epitaxy, grow predominantly in the cubic zinc-blende structure, but contain inclusions of the hexagonal wurtzite phase close to their bottom interface. Using nanodiffraction methods, the position of the different structural units along the growth axis is determined. Because the GaAs lattice is 4% larger than silicon, these nanowires release their lattice mismatch by the inclusion of dislocations at the interface. Whereas NWs with diameters below 50nm are free of strain, a rough interface structure in nanowires with diameters above 100nm prevents a complete plastic relaxation, leading to a residual strain at the interface that decays elastically along the growth direction. Finally, measurements on GaAs-core / InAs-shell nanowire heterostructures are presented. In this system, a saturation of the dislocation density at the core-shell interface causes residual stresses at the heterojunction and significant strain in the GaAs core, increasing with the thickness of the InAs shell

X-ray Scattering Study of Nanodot Pattern Formation on Semiconductor Surfaces by Low Energy Ion Beam Sputtering

ISBN : 978-1-61668-209-