Anton Davydok

Individual GaAs nanorods imaged by coherent X‐ray diffraction

Using scanning X-ray diffraction microscopy with a spot size of 220 x 600 nm, it was possible to inspect individual GaAs nanorods grown seed-free through circular openings in a SiN(x) mask in a periodic array with 3 microm spacing on GaAs[111]B. The focused X-ray beam allows the determination of the strain state of individual rods and, in combination with coherent diffraction imaging, it was also possible to characterize morphological details. Rods grown either in the centre or at the edge of the array show significant differences in shape, size and strain state.

Strain accommodation in Ga-assisted GaAs nanowires grown on silicon (111)

We study the mechanism of lattice parameter accommodation and the structure of GaAs nanowires (NWs) grown on Si(111) substrates using the Ga-assisted growth mode in molecular beam epitaxy. These nanowires grow preferentially in the zincblende structure, but contain inclusions of wurtzite at the base. By means of grazing incidence x-ray diffraction and high-resolution transmission electron microscopy of the NW-substrate interface, we show that the lattice mismatch between the NW and the substrate is released immediately after the beginning of NW growth through the inclusion of misfit dislocations, and no pseudomorphic growth is obtained for NW diameters down to 10 nm. NWs with a diameter above 100 nm exhibit a rough interface towards the substrate, preventing complete plastic relaxation. Consequently, these NWs exhibit a residual compressive strain at their bottom. In contrast, NWs with a diameter of 50 nm and below are completely relaxed because the interface is smooth.

Structural polytypism and residual strain in GaAs nanowires grown on Si(111) probed by single‐nanowire X‐ray diffraction

The structural composition, phase arrangement and residual strain of individual GaAs nanowires (NWs) grown on Si(111) have been investigated using NW-resolved high-resolution X-ray diffraction employing a focused synchrotron beam. It is found that even neighbouring NWs grown on the same sample under the same growth conditions differ significantly in their phase structure, most of them exhibiting small wurtzite segments embedded between larger zincblende sections. Moreover, using structurally sensitive Bragg reflections, residual strain is observed in the zincblende sections of the NWs, likely caused by an incomplete relaxation at the substrate interface.

Role of Liquid Indium in the Structural Purity of Wurtzite InAs Nanowires That Grow on Si(111)

InAs nanowires that grow catalyst-free along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase highly challenging. In this work, we explore the dynamic relation between the growth conditions and the structural composition of the nanowires using time-resolved X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy. A spontaneous buildup of liquid indium is directly observed in the beginning of the growth process and associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. The highly arsenic-rich growth conditions that we used limited the existence of the liquid indium to a short time interval, which is defined as the nucleation phase. After their nucleation, the nanowires grow in the absence of liquid indium, and with a highly defective wurtzite structure. Complementary ex-situ diffuse X-ray scattering measurements and modeling revealed that this structural degradation is due to the formation of densely spaced stacking faults. Thus, high wurtzite phase purity is associated with the presence of liquid indium. This finding implies that pure wurtzite nanowires may be obtained only if the growth is performed under the continuous presence of liquid indium at the growth interface, that is, in the vapor-liquid-solid mode.

Lattice parameter accommodation between GaAs(111) nanowires and Si(111) substrate after growth via Au-assisted molecular beam epitaxy

Using out-of-plane and in-plane X-ray diffraction techniques, we have investigated the structure at the interface between GaAs nanowires [NWs] grown by Au-assisted molecular beam epitaxy and the underlying Si(111) substrate. Comparing the diffraction pattern measured at samples grown for 5, 60, and 1,800 s, we find a plastic strain release of about 75% close to the NW-to-substrate interface even at the initial state of growth, probably caused by the formation of a dislocation network at the Si-to-GaAs interface. In detail, we deduce that during the initial stage, zinc-blende structure GaAs islands grow with a gradually increasing lattice parameter over a transition region of several 10 nm in the growth direction. In contrast, accommodation of the in-plane lattice parameter takes place within a thickness of about 10 nm. As a consequence, the ratio between out-of-plane and in-plane lattice parameters is smaller than the unity in the initial state of growth. Finally the wurtzite-type NWs grow on top of the islands and are free of strain.

In situ three-dimensional reciprocal-space mapping during mechanical deformation

Mechanical deformation of a SiGe island epitaxically grown on Si(001) was studied by a specially adapted atomic force microscope and nanofocused X-ray diffraction. The deformation was monitored during in situ mechanical loading by recording three-dimensional reciprocal-space maps around a selected Bragg peak. Scanning the energy of the incident beam instead of rocking the sample allowed the safe and reliable measurement of the reciprocal-space maps without removal of the mechanical load. The crystal truncation rods originating from the island side facets rotate to steeper angles with increasing mechanical load. Simulations of the displacement field and the intensity distribution, based on the finite-element method, reveal that the change in orientation of the side facets of about 25° corresponds to an applied pressure of 2-3 GPa on the island top plane.

Grazing-incidence X-ray diffraction of single GaAs nanowires at locations defined by focused ion beams.

The crystalline structure of single free-standing GaAs nanowires, grown by molecular beam epitaxy on a GaAs substrate at specific positions defined by focused ion beams, and the substrate regions close to the Au-implanted regions are investigated through grazing-incidence X-ray diffraction.

In situ doping of catalyst-free InAs nanowires with Si: Growth, polytypism, and local vibrational modes of Si

Growth and structural aspects of the in situ doping of InAs nanowires with Si have been investigated. The nanowires were grown catalyst-free on Si(111) substrates by molecular beam epitaxy. The supply of Si influenced the growth kinetics, affecting the nanowire dimensions, but not the degree of structural polytypism, which was always pronounced. As determined by Raman spectroscopy, Si was incorporated as substitutional impurity exclusively on In sites, which makes it a donor. Previously unknown Si-related Raman peaks at 355 and 360 cm−1 were identified, based on their symmetry properties in polarization-dependent measurements, as the two local vibrational modes of an isolated Si impurity on In site along and perpendicular, respectively, to the c-axis of the wurtzite InAs crystal.

Alloy formation during molecular beam epitaxy growth of Si-doped InAs nanowires on GaAs[111]B

An investigation of the influence of Si supply on the molecular beam epitaxy growth process and morphology of InAs nanowires grown on GaAs[111]B substrates is presented.

Structure investigation of individual GaAs nanorods by X-ray coherent diffraction imaging

The structure of a natural bone has well oriented hydroxyapatite crystals and fibrous collagens. Although very different materials have been used for bone replacement and substitutes, mechanical and biological performances of these substances still are not very close to those of natural bones. Scientific researches about this subject have been going on with increasing interests [1]. The aim of this study is to prepare novel biocompatible bone nanocomposites to use in veterinary orthopedic surgery and bone tissue engineering as bone filling materials. Sprague Dawley / Wistar Rats and New Zealand White Rabbits’ femur, tibia and calvairum bone samples were used after sterilization and autoclave application and turned into micropowder forms by using a grinder. Molecular structure of hydroxyapatite has been confirmed and controlled and then the crystallite sizes, crystallite morphologies and distributions in the micropowder contents were investigated by FTIR and SWAXS methods. Moreover, titanium and zirconium oxide nanopowders that can modify cell response and dental restorative Eco-Flow (Ivoclar Vivadent AG) were systematically mixed with micropowders and stirred for 20 minutes and then the prepared forms have been poured into a special matrix having four cells. Hardening of the samples was also completed by using visible light. FTIR and SWAXS measurements have been also carried out to characterize the shapes, sizes, distributions and molecular contents of the nanostructured aggregations. Mechanical testing of the samples was also carried out to compare mechanical properties of the composites including different bones and metallic nanopowders.