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We analyze fluctuation of the layer thicknesses and its influence on the strain state of (In,Ga)As/(Al,Ga)As micro-tubes containing quantum well structures. In those structures a curved high-mobility two-dimensional electron gas (HM2DEG) is established. The layer thickness fluctuation studied by atomic force microscopy, x-ray scattering, and spatially resolved cathodoluminescence spectroscopy occurs on two different lateral length scales. On the shorter length scale of about 0.01~$\mu$m, we found from atomic force micrographs and the broadening of the satellite maxima in x-ray diffraction curves a very small value of the mean square roughness of 0.1~nm. However, on a longer length scale of about 1.0~$\mu$m, step bunching during epitaxial growth resulted in layer thickness inhomogeneities of up to 2~nm. The resulting fluctuation of the strain in the micro-tubes leads to a local variation of the chemical potential, which results in the fluctuation of the carrier density as well. This leads to a phase cancelation of the Shubnikov-de-Haas oscillations in the curved HM2DEG and a reduction of the single-electron scattering time, while the electron mobility in the structures remains high. The estimated fluctuation of the carrier density agrees well with the energy fluctuation measured in the cathodoluminescence spectra of the free-electron transition of the quantum well.

Co$_{2}$TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the \textit{L$2_1$} and \textit{B}2 structures and considering the influence of non--stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13~nm these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found \textit{L$2_1$} and \textit{B}2 ordered regions, however the [Co]/[Ti]--ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic \textit{B}2--ordering with the best order parallel to the axes of the columns.

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al deficient core and an Al rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies, but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2019), copyright (C) American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.9b01521, the supporting information is available (free of charge) under the same link

Several ten $\mu$m GaN have been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates have been covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features allow for an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear, and the luminescence of this part of the GaN layer is dominated by the donor-bound exciton. For templates consisting of both a thin AlN buffer and GaN nanostructures, a silicon incorporation into the GaN grown by HVPE is not observed. Moreover, the growth mode of the (high-T) HVPE step depends on the specific structure of the AlN/GaN template, where in a first case, the epitaxy is dominated by the formation of slowly growing facets, while in a second case, the epitaxy proceeds directly along the c-axis., Comment: 19 pages, 11 figures

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

We have fabricated high-mobility, two-dimensional electron gases in a GaAs quantum well on cylindrical surfaces, which allows to investigate the magnetotransport behavior under varying magnetic fields along the current path. A strong asymmetry in the quantum Hall effect appears for measurements on both sides of the conductive path. We determined the strain at the position of the quantum well. We observe ballistic transport in 8-micrometers-wide collimating structures., Comment: 6 pages, 3 figures; Proceedings of the International Symposium on Mesoscopic Superconductivity and Spintronics, Atsugi, Japan, February 2006, edited by H. Takayanagi and J. Nitta (World Scientific, in press)

A giant asymmetry in the magnetoresistance was revealed in high-mobility, two-dimensional electron gas on a cylindrical surface. The longitudinal resistance along the magnetic-field gradient impressed by the surface curvature was found to vanish if measured along one of the edges of the curved Hall bar. If the external magnetic field is reversed, then the longitudinal resistance vanishes at the opposite edge of the Hall bar. This asymmetry is analyzed quantitatively in terms of the Landauer-Buettiker formalism., Comment: 7 pages, 3 figures; to appear in PRB