Achieving quantum confinement by bottom-up growth of nanowires has so far been limited to the ability of obtaining stable metal droplets of radii around 10 nm or less. This is within reach for gold-assisted growth. Because of the necessity to maintain the group III droplets during growth, direct synthesis of quantum sized structures becomes much more challenging for self-assisted III−V nanowires. In this work, we elucidate and solve the challenges that involve the synthesis of galliumassisted quantum-sized GaAs nanowires. We demonstrate the existence of two stable contact angles for the gallium droplet on top of GaAs nanowires. Contact angle around 130°fosters a continuous increase in the nanowire radius, while 90°allows for the stable growth of ultrathin tops. The experimental results are fully consistent with our model that explains the observed morphological evolution under the two different scenarios. We provide a generalized theory of self-assisted III−V nanowires that describes simultaneously the droplet shape relaxation and the NW radius evolution. Bistability of the contact angle described here should be the general phenomenon that pertains for any vapor−liquid−solid nanowires and significantly refines our picture of how nanowires grow. Overall, our results suggest a new path for obtaining ultrathin one-dimensional III−V nanostructures for studying lateral confinement of carriers. KEYWORDS: III−V semiconductors on silicon, nanowires, nanoneedles, regular arrays, self-assisted growth, droplet size engineering, crystal structure, growth modeling F undamental physical properties and applications of semiconductor nanowires (NWs) may benefit from quantum confinement due to size-dependent modulation of the density of states as well as modification of the photon energy for the allowed optical transitions.1 In GaAs, quantum confinement occurs for a size below 25 nm.2,3 In addition to quantum confinement, the optical properties can be engineered by modifying the NW shape. For example, progressively tapered nanowires have exhibited adiabatic outcoupling in the emission of quantum dots, thereby enhancing their brightness. 4 A similar design may improve the light extraction or absorption in different optoelectronic devices including light emitting diodes, solar cells, and optical biosensors.5−11 Quantum confinement in the core of radial NW heterostructures allows for the fabrication of high quality NW-based quantum dots, which are a perfect platform for delicate quantum transport experiments. 12,13 It is admittedly challenging to obtain very thin (quantumconfined) III−V NWs directly by the vapor−liquid−solid (VLS) growth technique. There are several reasons for that (see ref 14 for a review), including the Gibbs−Thomson effect of elevation of chemical potential due to the curvature of the droplet surface and difficulties in obtaining very small droplets on the substrate surface. Thin tips of VLS GaAs NWs with a stable radius down to 5 nm have previously been grown by hydride vapor phase epitaxy 15,16 (a te...

Topological qubits based on Majorana Fermions have the potential to revolutionize the emerging field of quantum computing by making information processing significantly more robust to decoherence. Nanowires are a promising medium for hosting these kinds of qubits, though branched nanowires are needed to perform qubit manipulations. Here we report a gold-free templated growth of III-V nanowires by molecular beam epitaxy using an approach that enables patternable and highly regular branched nanowire arrays on a far greater scale than what has been reported thus far. Our approach relies on the lattice-mismatched growth of InAs on top of defect-free GaAs nanomembranes yielding laterally oriented, low-defect InAs and InGaAs nanowires whose shapes are determined by surface and strain energy minimization. By controlling nanomembrane width and growth time, we demonstrate the formation of compositionally graded nanowires with cross-sections less than 50 nm. Scaling the nanowires below 20 nm leads to the formation of homogeneous InGaAs nanowires, which exhibit phase-coherent, quasi-1D quantum transport as shown by magnetoconductance measurements. These results are an important advance toward scalable topological quantum computing.

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) 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 Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). 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 magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.

III-V nanowires are candidate building blocks for next generation electronic and optoelectronic platforms. Low bandgap semiconductors such as InAs and InSb are interesting because of their high electron mobility. Fine control of the structure, morphology, and composition are key to the control of their physical properties. In this work, we present how to grow catalyst-free InAs1-xSbx nanowires, which are stacking fault and twin defect-free over several hundreds of nanometers. We evaluate the impact of their crystal phase purity by probing their electrical properties in a transistor-like configuration and by measuring the phonon-plasmon interaction by Raman spectroscopy. We also highlight the importance of high-quality dielectric coating for the reduction of hysteresis in the electrical characteristics of the nanowire transistors. High channel carrier mobilities and reduced hysteresis open the path for high-frequency devices fabricated using InAs1-xSbx nanowires.

Ga-assisted growth of GaAs nanowires on silicon provides a path for integrating highpurity 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°( i.e., oxide thickness ∼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.