Impact of Porosity and Boundary Scattering on Thermal Transport in Diameter-Modulated Nanowires.

We study the thermal conductivity of diameter-modulated Si nanowires to understand the impact of different nanoscale transport mechanisms as a function of nanowire morphology. Our investigation couples transient suspended microbridge measurements of diameter-modulated Si nanowires synthesized via vapor-liquid-solid growth and dopant-selective etching with predictive Boltzmann transport modeling. We show that the presence of a low thermal conductivity phase (i.e., porosity) dominates the reduction in effective thermal conductivity and is supplemented by increased phonon-boundary scattering. The relative contributions of both mechanisms depend on the details of the nanoscale morphology. Our findings provide valuable insights into the factors that govern thermal conduction in complex nanoscale materials.

3D Ordering at the Liquid-Solid Polar Interface of Nanowires.

The nature of the liquid-solid interface determines the characteristics of a variety of physical phenomena, including catalysis, electrochemistry, lubrication, and crystal growth. Most of the established models for crystal growth are based on macroscopic thermodynamics, neglecting the atomistic nature of the liquid-solid interface. Here, experimental observations and molecular dynamics simulations are employed to identify the 3D nature of an atomic-scale ordering of liquid Ga in contact with solid GaAs in a nanowire growth configuration. An interplay between the liquid ordering and the formation of a new bilayer is revealed, which, contrary to the established theories, suggests that the preference for a certain polarity and polytypism is influenced by the atomic structure of the interface. The conclusions of this work open new avenues for the understanding of crystal growth, as well as other processes and systems involving a liquid-solid interface.

GaAs nanoscale membranes: prospects for seamless integration of III-Vs on silicon.

The growth of compound semiconductors on silicon has been widely sought after for decades, but reliable methods for defect-free combination of these materials have remained elusive. Recently, interconnected GaAs nanoscale membranes have been used as templates for the scalable integration of nanowire networks on III-V substrates. Here, we demonstrate how GaAs nanoscale membranes can be seamlessly integrated on silicon by controlling the density of nuclei in the initial stages of growth. We also correlate the absence or presence of defects with the existence of a single or multiple nucleation regime for the single membranes. Certain defects exhibit well-differentiated spectroscopic features that we identify with cathodoluminescence and micro-photoluminescence techniques. Overall, this work presents a new approach for the seamless integration of compound semiconductors on silicon.

Bottom-Up Masking of Si/Ge Surfaces and Nanowire Heterostructures via Surface-Initiated Polymerization and Selective Etching.

The fully bottom-up and scalable synthesis of complex micro/nanoscale materials and functional devices requires masking methods to define key features and direct the deposition of various coatings and films. Here, we demonstrate selective coaxial lithography via etching of surfaces (SCALES), an enabling bottom-up process to add polymer masks to micro/nanoscale objects. SCALES is a three-step process, including (1) bottom-up synthesis of compositionally modulated structures, (2) surface-initiated polymerization of a conformal mask, and (3) selective removal of the mask only from regions whose underlying surface is susceptible to an etchant. We demonstrate the key features of and characterize the SCALES process with a series of model Si/Ge systems: Si and Ge wafers, Si and Ge nanowires, and Si/Ge heterostructure nanowires.

Questioning liquid droplet stability on nanowire tips: from theory to experiment.

Liquid droplets sitting on nanowire (NW) tips constitute the starting point of the vapor-liquid-solid method of NW growth. Shape and volume of the droplet have been linked to a variety of growth phenomena ranging from the modification of growth direction, NW orientation, crystal phase, and even polarity. In this work we focus on numerical and theoretical analysis of the stability of liquid droplets on NW tips, explaining the peculiarity of this condition with respect to the wetting of planar surfaces. We highlight the role of droplet pinning at the tip in engineering the contact angle. Experimental results on the characteristics of In droplets of variable volume sitting on the tips or side facets of InAs NWs are also provided. This work contributes to the fundamental understanding of the nature of droplets contact angle at the tip of NWs and to the improvement of the engineering of such nanostructures.

Contactless Electrical and Structural Characterization of Semiconductor Nanowires with Axially Modulated Doping Profiles.

Efficient characterization of semiconductor nanowires having complex dopant profiles or heterostructures is critical to fully understand these materials and the devices built from them. Existing electrical characterization techniques are slow and laborious, particularly for multisegment nanowires, and impede the statistical understanding of highly variable samples. Here, it is shown that electro-orientation spectroscopy (EOS)-a high-throughput, noncontact method for statistically characterizing the electrical properties of entire nanowire ensembles-can determine the conductivity and dimensions of two distinct segments in individual Si nanowires with axially encoded dopant profiles. This analysis combines experimental measurements and computational simulations to determine the electrical conductivity of the nominally undoped segment of two-segment Si nanowires, as well as the ratio of the segment lengths. The efficacy of this approach is demonstrated by comparing results generated by EOS with conventional four-point-probe measurements. This work provides new insights into the control and variability of semiconductor nanowires for electronic applications and is a critical first step toward the high-throughput interrogation of complete nanowire-based devices.

Tuning adatom mobility and nanoscale segregation by twin formation and polytypism.

Nanoscale variations in the composition of an Al x Ga1-x As shell around a GaAs nanowire affect the nanowire functionality and can lead to the formation of localized quantum emitters. These composition fluctuations can be the consequence of variations of crystal phase and/or nanoscale adatom mobility. By applying electron-microscopy-related techniques we correlate the optical, compositional and structural properties at the nanoscale on the same object. The results indicate a clear correlation between the twin density in the nanowire and the quantum-emitter density as well as a significant redshift in the emission. We propose that twinning increases nanoscale segregation effects in ternary alloys. An additional redshift in the emission can be explained by the staggered band alignment between wurtzite and zinc-blende phases. This work opens new avenues in the achievement of homogeneous ternary and quaternary alloys in nanowires and in the engineering of the segregation effects at the nanoscale.

Optimizing the yield of A-polar GaAs nanowires to achieve defect-free zinc blende structure and enhanced optical functionality.

Compound semiconductors exhibit an intrinsic polarity, as a consequence of the ionicity of their bonds. Nanowires grow mostly along the (111) direction for energetic reasons. Arsenide and phosphide nanowires grow along (111)B, implying a group V termination of the (111) bilayers. Polarity engineering provides an additional pathway to modulate the structural and optical properties of semiconductor nanowires. In this work, we demonstrate for the first time the growth of Ga-assisted GaAs nanowires with (111)A-polarity, with a yield of up to ∼50%. This goal is achieved by employing highly Ga-rich conditions which enable proper engineering of the energies of A and B-polar surfaces. We also show that A-polarity growth suppresses the stacking disorder along the growth axis. This results in improved optical properties, including the formation of AlGaAs quantum dots with two orders or magnitude higher brightness. Overall, this work provides new grounds for the engineering of nanowire growth directions, crystal quality and optical functionality.

Anisotropic-Strain-Induced Band Gap Engineering in Nanowire-Based Quantum Dots.

Tuning light emission in bulk and quantum structures by strain constitutes a complementary method to engineer functional properties of semiconductors. Here, we demonstrate the tuning of light emission of GaAs nanowires and their quantum dots up to 115 meV by applying strain through an oxide envelope. We prove that the strain is highly anisotropic and clearly results in a component along the NW longitudinal axis, showing good agreement with the equations of uniaxial stress. We further demonstrate that the strain strongly depends on the oxide thickness, the oxide intrinsic strain, and the oxide microstructure. We also show that ensemble measurements are fully consistent with characterizations at the single-NW level, further elucidating the general character of the findings. This work provides the basic elements for strain-induced band gap engineering and opens new avenues in applications where a band-edge shift is necessary.

Template-Assisted Scalable Nanowire Networks.

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.

Bistability of Contact Angle and Its Role in Achieving Quantum-Thin Self-Assisted GaAs nanowires.

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 gallium-assisted 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.

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.

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 As4 flux, suggesting a new path for obtaining highly uniform arrays of GaAs nanowires on silicon.

Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires.

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation of these nanostructures into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5 K-300 K. We directly determine an ionization energy of 6.7 ± 0.5 meV (T = 52 K) for the Si donors within the AlGaAs shell that create the modulation doping structure. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360 ± 380 cm2 V-1 s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220 ± 130 cm2 V-1 s-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.

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 subdiffraction 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.

Vectorial scanning force microscopy using a nanowire sensor.

Self-assembled nanowire (NW) crystals can be grown into nearly defect-free nanomechanical resonators with exceptional properties, including small motional mass, high resonant frequency and low dissipation. Furthermore, by virtue of slight asymmetries in geometry, a NW's flexural modes are split into doublets oscillating along orthogonal axes. These characteristics make bottom-up grown NWs extremely sensitive vectorial force sensors. Here, taking advantage of its adaptability as a scanning probe, we use a single NW to image a sample surface. By monitoring the frequency shift and direction of oscillation of both modes as we scan above the surface, we construct a map of all spatial tip-sample force derivatives in the plane. Finally, we use the NW to image electric force fields distinguishing between forces arising from the NW charge and polarizability. This universally applicable technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.

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 〈11-2〉 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.

Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping.

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

From Twinning to Pure Zincblende Catalyst-Free InAs(Sb) Nanowires.

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.

Nonstoichiometric Low-Temperature Grown GaAs Nanowires.

The structural and electronic properties of nonstoichiometric low-temperature grown GaAs nanowire shells have been investigated with scanning tunneling microscopy and spectroscopy, pump-probe reflectivity, and cathodoluminescence measurements. The growth of nonstoichiometric GaAs shells is achieved through the formation of As antisite defects, and to a lower extent, after annealing, As precipitates. Because of the high density of atomic steps on the nanowire sidewalls, the Fermi level is pinned midgap, causing the ionization of the subsurface antisites and the formation of depleted regions around the As precipitates. Controlling their incorporation offers a way to obtain unique electronic and optical properties that depart from the ones found in conventional GaAs nanowires.