MBE Growth of Al/InAs and Nb/InAs Superconducting Hybrid Nanowire Structures

We report on \textit{in situ} growth of crystalline Al and Nb shells on InAs nanowires. The nanowires are grown on Si(111) substrates by molecular beam epitaxy (MBE) without foreign catalysts in the vapor-solid mode. The metal shells are deposited by electron-beam evaporation in a metal MBE. High quality supercondonductor/semiconductor hybrid structures such as Al/InAs and Nb/InAs are of interest for ongoing research in the fields of gateable Josephson junctions and quantum information related research. Systematic investigations of the deposition parameters suitable for metal shell growth are conducted. In case of Al, the substrate temperature, the growth rate and the shell thickness are considered. The substrate temperature as well as the angle of the impinging deposition flux are explored for Nb shells. The core-shell hybrid structures are characterized by electron microscopy and x-ray spectroscopy. Our results show that the substrate temperature is a crucial parameter in order to enable the deposition of smooth Al layers. Contrary, Nb films are less dependent on substrate temperature but strongly affected by the deposition angle. At a temperature of 200{\deg}C Nb reacts with InAs, dissolving the nanowire crystal. Our investigations result in smooth metal shells exhibiting an impurity and defect free, crystalline superconductor/InAs interface. Additionally, we find that the superconductor crystal structure is not affected by stacking faults present in the InAs nanowires.Comment: 8 pages, 10 figures, 1 tabl

Effect of gallium termination on InGaAs wetting layer properties in droplet epitaxy InGaAs quantum dots

Self-assembled quantum dots based on III-V semiconductors have excellent properties for applications in quantum optics. However, the presence of a 2D wetting layer which forms during the Stranski-Krastanov growth of quantum dots can limit their performance. Here, we investigate wetting layer formation during quantum dot growth by the alternative droplet epitaxy technique. We use a combination of photoluminescence excitation spectroscopy, lifetime measurements, and transmission electron microscopy to identify the presence of an InGaAs wetting layer in these droplet epitaxy quantum dots, even in the absence of distinguishable wetting layer photoluminescence. We observe that increasing the amount of Ga deposited on a GaAs (100) surface prior to the growth of InGaAs quantum dots leads to a significant reduction in the emission wavelength of the wetting layer to the point where it can no longer be distinguished from the GaAs acceptor peak emission in photoluminescence measurements

Semiconductor membranes for electrostatic exciton trapping in optically addressable quantum transport devices

Combining the capabilities of gate defined quantum transport devices in GaAs-based heterostructures and of optically addressed self-assembled quantum dots could open broad perspectives for new devices and functionalities. For example, interfacing stationary solid-state qubits with photonic quantum states would open a new pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220 nm thick membrane. We show that mobilities over $1 \times 10^{6}$ cm$^{2}$V$^{-1}$s$^{-1}$ can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single photon sources, spin photon interfaces and eventually quantum network nodes in GaAs quantum wells, realized entirely with a top-down fabrication process.Comment: v2: added missing acknowledgement. v3: fixed typos in acknolwedgemen

Fully in situ Nb/InAs-nanowire Josephson junctions by selective-area growth and shadow evaporation

Josephson junctions based on InAs semiconducting nanowires and Nb superconducting electrodes are fabricated in situ by a special shadow evaporation scheme for the superconductor electrode. Compared to other metallic superconductors such as Al, Nb has the advantage of a larger superconducting gap which allows operation at higher temperatures and magnetic fields. Our junctions are fabricated by shadow evaporation of Nb on pairs of InAs nanowires grown selectively on two adjacent tilted Si (111) facets and crossing each other at a small distance. The upper wire relative to the deposition source acts as a shadow mask determining the gap of the superconducting electrodes on the lower nanowire. Electron microscopy measurements show that the fully in situ fabrication method gives a clean InAs/Nb interface. A clear Josephson supercurrent is observed in the current–voltage characteristics, which can be controlled by a bottom gate. The large excess current indicates a high junction transparency. Under microwave radiation, pronounced integer Shapiro steps are observed suggesting a sinusoidal current–phase relation. Owing to the large critical field of Nb, the Josephson supercurrent can be maintained to magnetic fields exceeding 1 T. Our results show that in situ prepared Nb/InAs nanowire contacts are very interesting candidates for superconducting quantum circuits requiring large magnetic fields

InAs nanowires with Alx_xGa1−x_{1−x}Sb shells for band alignment engineering

InAs nanowires surrounded by AlxGa1−xSb shells exhibit a change in the band alignment from a broken gap for pure GaSb shells to a staggered type II alignment for AlSb. These different band alignments make InAs/AlxGa1−xSb core–shell nanowires ideal candidates for several applications such as TFETs and passivated InAs nanowires. With increasing the Al content in the shell, the axial growth is simultaneously enhanced changing the morphological characteristics of the top region. Nonetheless, for Al contents ranging from 0 to 100 % conformal overgrowth of the InAs nanowires was observed. AlGaSb shells were found to have a uniform composition along the nanowire axis. High Al content shells require an additional passivation with GaSb to prevent complete oxidation of the AlSb. Irrespective of the lattice mismatch being 1.2% between InAs and AlSb, the shell growth was found to be coherent

Si substrate preparation for the VS and VLS growth of InAs nanowires

The growth of self-catalyzed InAs nanowires on Si(111) substrates via vapour–solid (VS) and vapour–liquid–solid (VLS) growth mechanisms is investigated using molecular beam epitaxy. For both mechanisms, the substrate preparation plays a crucial role. In this context, the required thin oxide layer for the VS growth of the nanowires is obtained by treating the HF-cleaned Si substrate with hydrogen peroxide. For the VLS growth, Ga is predeposited on the unprocessed Si substrate. The Ga forms droplets, which etch the native oxide and create the necessary pinholes. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Crystal Phase Selective Growth in GaAs/InAs Core–Shell Nanowires

We present a novel type of core–shell nanowires in which only certain parts of the core are covered by the shell. This is achieved by the crystal phase selective growth of the InAs shell on zinc blende GaAs nanowires with controlled wurtzite inclusions. The shell grows on the zinc blende phase, but its growth is hindered on the wurtzite crystal phase. Nucleation of InAs occurs exclusively on the zinc blende GaAs regions. The wurtzite segments are placed inside self-catalyzed GaAs nanowires by partially consuming and refilling the Ga droplet. The crystal phase selective growth of InAs on the side facets of the GaAs nanowires is explained by the local environment of each new In atom. Because of unbalanced neighbors on the wurtzite side facets, the growth of a highly lattice mismatched material is hindered. This happens not only on the wurtzite segments, but also on regions being characterized by a high density of twins