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

Visual Understanding of Light Absorption and Waveguiding in Standing Nanowires with 3D Fluorescence Confocal Microscopy

Semiconductor nanowires are promising building blocks for next generation photonics. Indirect proofs of large absorption cross sections have been reported in nanostructures with subwavelength diameters, an effect that is even more prominent in vertically standing nanowires. In this work we provide a three-dimensional map of the light around vertical GaAs nanowires standing on a substrate by using fluorescence confocal microscopy, where the strong long-range disruption of the light path along the nanowire is illustrated. We find that the actual long-distance perturbation is much larger in size than calculated extinction cross sections. While the size of the perturbation remains similar, the intensity of the interaction changes dramatically over the visible spectrum. Numerical simulations allow us to distinguish the effects of scattering and absorption in the nanowire leading to these phenomena. This work provides a visual understanding of light absorption in semiconductor nanowire structures, which is of high interest for solar energy conversion applications

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

Conductive-probe atomic force microscopy as a characterization tool for nanowire-based solar cells

The photonic properties of nanowires advocate for their utilization in next generation solar cells. Compared to traditional devices, the electric scheme is transformed from a single into an ensemble of pn junctions connected in parallel. This new configuration requires new schemes for the characterization. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of this parallel-connected nanowire devices. With C-AFM it is possible to obtain both surface topography and local electrical characterization with nanoscale resolution. We demonstrate topography and current mapping of nanowire forests, combined with current-voltage measurements of the individual nanowire junctions from the ensemble. Our results provide discussion elements on some factors limiting the performance of a nanowire-based solar cell and thereby to provide a path for their improvement

Engineering the Size Distributions of Ordered GaAs Nanowires on Silicon

Reproducible integration of III-V semiconductors on silicon can open new path toward CMOS compatible optoelectronics and novel design schemes in next generation solar cells. Ordered arrays of nanowires could accomplish this task, provided they are obtained in high yield and uniformity. In this work, we provide understanding on the physical factors affecting size uniformity in ordered GaAs arrays grown on silicon. We show that the length and diameter distributions in the initial stage of growth are not much influenced. by the Poissonian fluctuation-induced broadening, but rather are determined by the long incubation stage. We also show that the size distributions are consistent with the double exponential shapes typical for macroscopic nucleation with a large critical length after Which the nanowires grow irreversibly. The size uniformity is dramatically improved by increasing the As-4 flux, suggesting, a new path for obtaining highly uniform arrays of GaAs nanowires on silicon

Plasmonic Waveguide-Integrated Nanowire Laser

Next-generation optoelectronic devices and photonic circuitry will have to incorporate on-chip compatible nanolaser sources. Semiconductor nanowire lasers have emerged as strong candidates for integrated systems with applications ranging from ultrasensitive sensing to data communication technologies. Despite significant advances in their fundamental aspects, the integration within scalable photonic circuitry remains challenging. Here we report on the realization of hybrid photonic devices consisting of nanowire lasers integrated with wafer-scale lithographically designed V-groove plasmonic waveguides. We present experimental evidence of the lasing emission and coupling into the propagating modes of the V-grooves, enabling on-chip routing of coherent and sub diffraction confined light with room temperature operation. Theoretical considerations suggest that the observed lasing is enabled by a waveguide hybrid photonic-plasmonic mode. This work represents a major advance toward the realization of application-oriented photonic circuits with integrated nanolaser sources

Impact of the Ga Droplet Wetting, Morphology, and Pinholes on the Orientation of GaAs Nanowires

Ga-catalyzed growth of GaAs nanowires on Si is a candidate process for achieving seamless III/V integration on IV. In this framework, the nature of silicon's surface oxide is known to have a strong influence on nanowire growth and orientation and therefore important for GaAs nanowire technologies. We show that the chemistry and morphology of the silicon oxide film controls liquid Ga nucleation position and shape; these determine GaAs nanowire growth morphology. We calculate the energies of formation of Ga droplets as a function of their volume and the oxide composition in several nucleation configurations. The lowest energy Ga droplet shapes are then correlated to the orientation of nanowires with respect to the substrate. This work provides the understanding and the tools to control nanowire morphology in self-assembly and pattern growth

The Geode Process I: Hollow Silica Microcapsules as a High Surface Area Substrate for Semiconductor Nanowire Growth

We introduce and demonstrate critical steps toward the Geode process for the bottom-up synthesis of semiconductor nanowires. Central to the process is the design and fabrication of an unconventional, high surface area substrate: the interior surface of hollow silica microcapsules, assembled from silica particles via emulsion templating, and featuring porous walls to enable efficient gas transport. The interior surface of these hollow silica microcapsules is decorated with gold nanoparticles that seed nanowire growth via the vapor-liquid-solid (VLS) mechanism. We demonstrate the production of the necessary microcapsules and show how microcapsule structure and stability upon drying is influenced by the type of silica particles and use of a particle cross-linking agent. Finally, we demonstrate the synthesis of Si nanowires in the microcapsule interior.</div

Coherent Dynamics of Nanowire Force Sensors

We describe the use of grown nanowires as scanning directional force sensors. Two orthogonal flexural modes are used to demonstrate vectorial sensing of electric and magnetic fields. Furthermore, we show that the modes can be strongly coupled by demonstrating Rabi oscillations. These results open the way to implement coherent control and frequency stabilization in nanomechanical force and mass sensors

Towards defect-free 1-D GaAs/AlGaAs heterostructures based on GaAs nanomembranes

We demonstrate the growth of defect-free zinc-blende GaAs nanomembranes by molecular beam epitaxy. Our growth studies indicate a strong impact of As4 re-emission and shadowing in the growth rate of the structures. The highest aspect ratio structures are obtained for pitches around 0.7-1 μm and a gallium rate of 1 Å s(-1). The functionality of the membranes is further illustrated by the growth of quantum heterostructures (such as quantum wells) and the characterization of their optical properties at the nanoscale. This proves the potential of nanoscale membranes for optoelectronic applications.GT, HP, FM and AFiM acknowledge SNF funding through the NCCR QSIT, its director's reserve and project nr 143908 and 134506 (DACH program), as well as CIME and CMI for access to the electron microscopes. Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3). JA and MdlM acknowledge the funding from the Spanish MINECO MAT2014-51480-ERC (e-ATOM), MAT2014-59961-C2-2-R and Severo Ochoa Excellence Program and Generalitat de Catalunya 2014SGR1638. MdlM thanks the CSIC Jae-Predoc program. The STEM and FIB sample preparation works have been conducted partially in the “Laboratorio de Microscopias Avanzadas” at “Instituto de Nanociencia de Aragon – Universidad de Zaragoza”. Authors acknowledge the LMA-INA for offering access to their instruments and expertise. Authors also acknowledge ICN2 for the use of their microscopy facilities. Lastly, we would like to acknowledge Fauzia Jabeen for her valuable help with our MBE system.Peer reviewe