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In the pursuit of magneto-electronic systems nonstoichiometric magnetic elements commonly introduce disorder and enhance magnetic scattering. We demonstrate the growth of (EuIn)As shells, with a unique crystal structure comprised of a dense net of Eu inversion planes, over InAs and InAs 1- x Sb x core nanowires. This is imaged with atomic and elemental resolution which reveal a prismatic configuration of the Eu planes. The results are supported by molecular dynamics simulations. Local magnetic and susceptibility mappings show magnetic response in all nanowires, while a subset bearing a DC signal points to ferromagnetic order. These provide a mechanism for enhancing Zeeman responses, operational at zero applied magnetic field. Such properties suggest that the obtained structures can serve as a preferred platform for time-reversal symmetry broken one-dimensional states including intrinsic topological superconductivity.

The cross-sectional dimensions of nanowires set the quantization conditions for the electronic subbands they host. These can be used as a platform to realize one-dimesional topological superconductivity. Here we develop a protocol that forces such nanowires to kink and change their growth direction. Consequently, a thin rectangular nanoplate is formed, which gradually converges into a very thin square tip. We characterize the resulting tapered nanowires structurally and spectroscopically by scanning and transmission electron microscopy and scanning tunneling microscopy and spectroscopy and model their growth. A unique structure composed of ordered rows of atoms on the (110) facet of the nanoflag is further revealed by atomically resolved topography and modeled by simulations. We discuss possible advantages tapered InAs nanowires offer for Majorana zero-mode realization and manipulation.

We study the role of gold droplets in the initial stage of nanowire growth via the vapor-liquid-solid method. Apart from serving as a collections center for growth species, the gold droplets carry an additional crucial role that necessarily precedes the nanowire emergence, that is, they assist the nucleation of nanocraters with strongly faceted {111}B side walls. Only once these facets become sufficiently large and regular, the gold droplets start nucleating and guiding the growth of nanowires. We show that this dual role of the gold droplets can be detected and monitored by high-energy electron diffraction during growth. Moreover, gold-induced formation of craters and the onset of nanowires growth on the {111}B facets inside the craters are confirmed by the results of Monte Carlo simulations. The detailed insight into the growth mechanism of inclined nanowires will help to engineer new and complex nanowire-based device architectures.

It was recently shown that in situ epitaxial aluminum coating of indium arsenide nanowires is possible and yields superior properties relative to ex-situ evaporation of aluminum ( Nat. Mater. 2015 , 14 , 400 - 406 ). We demonstrate a robust and adaptive epitaxial growth protocol satisfying the need for producing an intimate contact between the aluminum superconductor and the indium arsenide nanowire. We show that the (001) indium arsenide substrate allows successful aluminum side-coating of reclined indium arsenide nanowires that emerge from (111)B microfacets. A robust, induced hard superconducting gap in the obtained indium arsenide/aluminum core/partial shell nanowires is clearly demonstrated. We compare epitaxial side-coating of round and hexagonal cross-section nanowires and find the surface roughness of the round nanowires to induce a more uniform aluminum profile. Consequently, the extended aluminum grains result in increased strain at the interface with the indium arsenide nanowire, which is found to induce dislocations penetrating into round nanowires only. A unique feature of proposed growth protocol is that it supports in situ epitaxial deposition of aluminum on all three arms of indium arsenide nanowire intersections in a single growth step. Such aluminum coated intersections play a key role in engineering topologically superconducting networks required for Majorana based quantum computation schemes.

Quantized conductance in nanowires can be observed at low temperature in transport measurements; however, the observation of sub-bands at room temperature is challenging due to temperature broadening. So far, conduction band splitting at room temperature has not been observed in III-V nanowires mainly due to the small energetic separations between the sub-bands. We report on the measurement of conduction sub-bands at room temperature, in single InAs nanowires, using Kelvin probe force microscopy. This method does not rely on charge transport but rather on measurement of the nanowire Fermi level position as carriers are injected into a single nanowire transistor. As there is no charge transport, electron scattering is no longer an issue, allowing the observation of the sub-bands at room temperature. We measure the energy of the sub-bands in nanowires with two different diameters, and obtain excellent agreement with theoretical calculations based on an empirical tight-binding model.

Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an unintentional p-type doping. Magneto-optical studies of such a GaAs/AlAs core-multishell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/shell interface due to the presence of ionized carbon acceptors in the 1 nm GaAs layer in the shell. Microphotoluminescence in high magnetic field shows a clear signature of avoided crossings of the n = 0 Landau level emission line with the n = 2 Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large two-dimensional hole density in the structure.

Molecular beam epitaxy growth of merging InAs nanowire intersections, that is, a first step toward the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection, a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed low-temperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.

Magneto-photoluminescence measurements of individual zinc-blende GaAs/AlAs core/shell nanowires are reported. At low temperature, a strong emission line at 1.507 eV is observed under low power (nW) excitation. Measurements performed in high magnetic field allowed us to detect in this emission several lines associated with excitons bound to defect pairs. Such lines were observed before in epitaxial GaAs of very high quality, as reported by Kunzel and Ploog. This demonstrates that the optical quality of our GaAs/AlAs core/shell nanowires is comparable to the best GaAs layers grown by molecular beam epitaxy. Moreover, strong free exciton emission is observed even at room temperature. The bright optical emission of our nanowires in room temperature should open the way for numerous optoelectronic device applications.

The coupling of excitons to surface plasmon polaritons (SPPs) in Au- and Al-coated GaAs/AlAs/GaAs core-shell nanowires, possessing diameters of ~100 nm, was probed using time-resolved cathodoluminescence (CL). Excitons were generated in the metal coated nanowires by injecting a pulsed high-energy electron beam through the thin metal films. The Purcell enhancement factor (FP) was obtained by direct measurement of changes in the temperature-dependent radiative lifetime caused by the nanowire exciton-SPP coupling and compared with a model that takes into account the dependence of FP on the distance from the metal film and the thickness of the film covering the GaAs nanowires.

Incorporation of catalyst atoms during the growth process of semiconductor nanowires reduces the electron mean free path and degrades their electronic properties. Aberration-corrected scanning transmission electron microscopy (STEM) is now capable of directly imaging single Au atoms within the dense matrix of a GaAs crystal, by slightly tilting the GaAs lattice planes with respect to the incident electron beam. Au doping values in the order of 10(17-18) cm(3) were measured, making ballistic transport through the nanowires practically inaccessible.

Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.

We report on observation of coherent electron transport in suspended high-quality InAs nanowire-based devices. The InAs nanowires were grown by low-temperature gold-assisted vapor-liquid-solid molecular-beam-epitaxy. The high quality of the nanowires was achieved by removing the typically found stacking faults and reducing possibility of Au incorporation. Minimizing substrate-induced scattering in the device was achieved by suspending the nanowires over predefined grooves. Coherent transport involving more than a single one-dimensional mode transport was observed in the experiment and manifested by Fabry-Perot conductance oscillations. The length of the Fabry-Perot interferometer, deduced from the period of the conductance oscillations, was found to be close to the physical length of the device. The high oscillations visibility imply nearly ballistic electron transport through the nanowire.

The growth of wurtzite GaAs and InAs nanowires with diameters of a few tens of nanometers with negligible intermixing of zinc blende stacking is reported. The suppression of the number of stacking faults was obtained by a procedure within the vapor-liquid-solid growth, which exploits the theoretical result that nanowires of small diameter ( approximately 10 nm) adopt purely wurtzite structure and are observed to thicken (via lateral growth) once the axial growth exceeds a certain length.

Stacking-faults-free zinc blende GaAs nanowires have been grown by molecular beam epitaxy using the vapor-liquid-solid gold assisted growth method. Two different approaches were used to obtain continuous low supersaturation in the vicinity of the growing wires. A double distribution of gold droplets on the (111)B surface in the first case, and a highly terraced (311)B growth surface in the second case both avoided the commonly observed transition to wurtzite structure.