Real structure of lattice matched GaAs-Fe3Si core-shell nanowires

GaAs nanowires and GaAs-Fe3Si core-shell nanowire structures were grown by molecular-beam epitaxy on oxidized Si(111) substrates and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Ga droplets were formed on the oxide surface, and the semiconducting GaAs nanowires grew epitaxially via the vapor-liquid-solid mechanism as single-crystals from holes in the oxide film. We observed two stages of growth of the GaAs nanowires, first the regular growth and second the residual growth after the Ga supply was finished. The magnetic Fe3Si shells were deposited in an As-free chamber. They completely cover the GaAs cores although they consist of small grains. High-resolution TEM micrographs depict the differently oriented grains in the Fe3Si shells. Selected area diffraction of electrons and XRD gave further evidence that the shells are textured and not single crystals. Facetting of the shells was observed, which lead to thickness inhomogeneities of the shells.Comment: 15 pages, 8 figure

Application of electron tomography for comprehensive determination of III-V interface properties

We present an electron tomography method for the comprehensive characterization of buried III-V semiconductor interfaces that is based on chemical-sensitive high-angle annular dark-field scanning transmission electron microscopy. For this purpose, an (Al,Ga)As/GaAs multi-layer system grown by molecular beam epitaxy is used as a case study. Isoconcentration surfaces are exploited to obtain topographic height maps of 120 nm × 120 nm area, revealing the interface morphology. By applying the height-height correlation function, we are able to determine important interface properties like root mean square roughness and lateral correlation length of various interfaces of the (Al,Ga)As/GaAs system characterized by different Al concentrations. Height-difference maps based on isosurfaces corresponding to 30% and 70% of the total compositional difference at the interfaces are used to create topographic maps of the interface width and to calculate an average interface width. This methodology proves differences in the properties of direct and inverted interfaces and allows the observation of interfacial anisotropies. © 202

Diffraction at GaAs/Fe3_{3}Si core/shell nanowires: the formation of nanofacets

GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit nanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes. Density functional theory reveals that the Si-terminated Fe$_3$Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30{\deg} compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1$\overline{1}$0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si shells and GaAs cores occurring at increased growth temperatures.Comment: 15 pages, 5 figure

Electron Tomography of Pencil-Shaped GaN/(In,Ga)N Core-Shell Nanowires

The three-dimensional structure of GaN/(In,Ga)N core-shell nanowires with multi-faceted pencil-shaped apex is analyzed by electron tomography using high-angle annular dark-field mode in a scanning transmission electron microscope. Selective area growth on GaN-on-sapphire templates using a patterned mask is performed by molecular beam epitaxy to obtain ordered arrays of uniform nanowires. Our results of the tomographic reconstruction allow the detailed determination of the complex morphology of the inner (In,Ga)N multi-faceted shell structure and its deviation from the perfect hexagonal symmetry. The tomogram unambiguously identifies a dot-in-a-wire configuration at the nanowire apex including the exact shape and size, as well as the spatial distribution of its chemical composition. © 2019, The Author(s)

Nucleation chronology and electronic properties of In(As,Sb,P) graded composition quantum dots grown on InAs(100) substrate

We provide a detailed study of nucleation process, characterization, electronic and optical properties of graded composition quantum dots (GCQDs) grown from In-As-Sb-P composition liquid phase on an InAs(100) substrate in the Stranski-Krastanov growth mode. Our GCQDs exhibit diameters from 10 to 120 nm and heights from 2 to 20 nm with segregation profiles having a maximum Sb content of approximately 20% at the top and a maximum P content of approximately 15% at the bottom of the GCQDs so that hole confinement is expected in the upper parts of the GCQDs. Using an eight-band k · p model taking strain and built-in electrostatic potentials into account, we have computed the hole ground state energies and charge densities for a wide range of InAs1-x-ySbxPy GCQDs as close as possible to the systems observed in experiment. Finally, we have obtained an absorption spectrum for an ensemble of GCQDs by combining data from both experiment and theory. Excellent agreement between measured and simulated absorption spectra indicates that such GCQDs can be grown following a theory-guided design for application in specific devices

Nucleation chronology and electronic properties of In(As,Sb,P) graded composition quantum dots grown on InAs(100) substrate

We provide a detailed study of nucleation process, characterization, electronic and optical properties of graded composition quantum dots (GCQDs) grown from In-As-Sb-P composition liquid phase on an InAs(100) substrate in the Stranski-Krastanov growth mode. Our GCQDs exhibit diameters from 10 to 120 nm and heights from 2 to 20 nm with segregation profiles having a maximum Sb content of approximately 20% at the top and a maximum P content of approximately 15% at the bottom of the GCQDs so that hole confinement is expected in the upper parts of the GCQDs. Using an eight-band k · p model taking strain and built-in electrostatic potentials into account, we have computed the hole ground state energies and charge densities for a wide range of InAs1-x-ySbxPy GCQDs as close as possible to the systems observed in experiment. Finally, we have obtained an absorption spectrum for an ensemble of GCQDs by combining data from both experiment and theory. Excellent agreement between measured and simulated absorption spectra indicates that such GCQDs can be grown following a theory-guided design for application in specific devices

In Situ Transmission Electron Microscopy of Disorder–Order Transition in Epitaxially Stabilized FeGe2

Isothermal crystallization of amorphous Ge deposited on a cubic Fe3Si/GaAs(001) substrate is performed by in situ annealing within a transmission electron microscope. It was found that the formation of epitaxially aligned tetragonal FeGe2 is associated with a disorder–order phase transition mainly consisting of a rearrangement of the Fe/vacancy sublattice from a random distribution to alternating filled and empty layers. Additionally, atomically resolved high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy demonstrated that the vertical lattice spacing of the Ge sublattice reduces across vacancy layers, indicating that strain minimization plays a role in the phase transition process. Crystallization and ordering are both found to proceed layer-by-layer and with square-root-shaped kinetics with a smaller transition rate for the latter