Gold-free Growth of InAs Nanowires:Growth, Structural & Electrical Properties

Semiconductor nanowires are interesting building blocks for a variety of electronic and optoelectronic applications, and they provide an excellent platformto probe fundamental physical effects. For the realization of nanowire based devices, a deep understanding of the growth mechanism and the nanowire properties is required. In this thesis we investigate gold-free growth of InAs(Sb) nanowires and their properties. Nanowires are grown by molecular beam epitaxy on GaAs(111)B substrates. In the first part of this thesis we demonstrate the growth of InAs and InAs1¡xSbx nanowires and show that polytypism can be suppressed by the incorporation of antimony. The electric properties of InAs(Sb) nanowires are studied by electrical measurements and by Raman spectroscopy, and a higher electron mobility is found for defect-free InAs0.65Sb0.35 nanowires compared to InAs nanowires. We also investigate surface passivation using aluminiumoxide. The oxide layer not only serves as passivation layer but it can also be used as gate-dielectric for top-gated field-effect devices. The second part of this thesis is dedicated to the nanowire growth direction and orientation with respect to the substrate. We analyze the existence of tilted nanowires on (111)B substrates, and demonstrate that in most cases they are a result of 3D twinning at the early stages of growth. In addition, also a few unconventional crystalline directions are observed. The ratio of tilted nanowires can be tuned by the growth conditions and substrate preparation. This allows to achieve either all vertical nanowires or a high density of tilted nanowires, whichever is desired for a certain application. Our results also shed light upon the growth mechanism of InAs nanowires, since 3D twinning is associated with the presence of a droplet. Being able to control the formation of tilted nanowires is important, but for certain applications it is also desired to modify the growth direction during growth. For example topological qubits based onMajorana Fermions require junctions and networks. In the third part of this thesis we show a new approach to change growth direction. For this, InAs nanowires are annealed in vacuum in order to create indium droplets. The droplets first formon the top facet of the nanowires and then slide down onto the nanowire side facets. These droplets can act as catalyst-particle, and re-initiation of growth results in L-shaped nanostructures. Merging of these nanostructures constitutes a new approach for the formation of nanowire networks

Electrical control of spins and giant g-factors in ring-like coupled quantum dots

Emerging theoretical concepts for quantum technologies have driven a continuous search for structures where a quantum state, such as spin, can be manipulated efficiently. Central to many concepts is the ability to control a system by electric and magnetic fields, relying on strong spin-orbit interaction and a large g-factor. Here, we present a new mechanism for spin and orbital manipulation using small electric and magnetic fields. By hybridizing specific quantum dot states at two points inside InAs nanowires, nearly perfect quantum rings form. Large and highly anisotropic effective g-factors are observed, explained by a strong orbital contribution. Importantly, we find that the orbital and spin-orbital contributions can be efficiently quenched by simply detuning the individual quantum dot levels with an electric field. In this way, we demonstrate not only control of the effective g-factor from 80 to almost 0 for the same charge state, but also electrostatic change of the ground state spin

Molecular beam epitaxy of InAs nanowires in SiO2 nanotube templates: challenges and prospects for integration of III-Vs on Si

Guided growth of semiconductor nanowires in nanotube templates has been considered as a potential platform for reproducible integration of III-Vs on silicon or other mismatched substrates. Herein, we report on the challenges and prospects of molecular beam epitaxy of InAs nanowires on SiO2/Si nanotube templates. We show how and under which conditions the nanowire growth is initiated by In-assisted vapor-liquid-solid growth enabled by the local conditions inside the nanotube template. The conditions for high yield of vertical nanowires are investigated in terms of the nanotube depth, diameter and V/III flux ratios. We present a model that further substantiates our findings. This work opens new perspectives for monolithic integration of III-Vs on the silicon platform enabling new applications in the electronics, optoelectronics and energy harvesting arena

Microscopic co-existence of superconductivity and magnetism in Ba1-xKxFe2As2

It is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq 0.25) showing that this paradigm is wrong. Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magneto-elastically coupled orthorhombic lattice distortion below the superconducting phase transition.Comment: 4 pages, 4 figure

Wetting of Ga on SiOx and Its Impact on GaAs Nanowire Growth

Ga-assisted growth of GaAs nanowires on silicon provides a path for integrating high-purity III-Vs on silicon. The nature of the oxide on the silicon surface has been shown to impact the overall possibility of nanowire growth and their orientation with the substrate. In this work, we show that not only the exact thickness, but also the nature of the native oxide determines the feasibility of nanowire growth. During the course of formation of the native oxide, the surface energy varies and results in a different contact angle of Ga droplets. We find that, only for a contact angle around 90 degrees (i.e., oxide thickness similar to 0.9 nm), nanowires grow perpendicularly to the silicon substrate. This native oxide engineering is the first step toward controlling the self-assembly process, determining mainly the nanowire density and orientation

Tuning growth direction of catalyst-free InAs(Sb) nanowires with indium droplets

The need for indium droplets to initiate self-catalyzed growth of InAs nanowires has been highly debated in the last few years. Here, we report on the use of indium droplets to tune the growth direction of self-catalyzed InAs nanowires. The indium droplets are formed in situ on InAs(Sb) stems. Their position is modified to promote growth in the or equivalent directions. We also show that indium droplets can be used for the fabrication of InSb insertions in InAsSb nanowires. Our results demonstrate that indium droplets can initiate growth of InAs nanostructures as well as provide added flexibility to nanowire growth, enabling the formation of kinks and heterostructures, and offer a new approach in the growth of defect-free crystals

Tilting Catalyst-Free InAs Nanowires by 3D-Twinning and Unusual Growth Directions

Controlling the growth direction of nanowires is of strategic importance both for applications where nanowire arrays are contacted in parallel and for the formation of more complex nanowire networks. We report on the existence of tilted InAs nanowires on (111)B GaAs. The tilted direction is predominantly the result of a three-dimensional twinning phenomenon at the initial stages of growth, so far only observed in VLS growth. We also find some nanowires growing in (112) and other directions. We further demonstrate how the tilting of nanowires can be engineered by modifying the growth conditions, and outline the procedures to achieve fully vertical or tilted nanowire ensembles. Conditions leading to a high density of tilted nanowires also provide a way to grow nanoscale crosses. This work opens the path toward achieving control over nanowire structures and related hierarchical structures