Wetting of Ga on SiO_<i>x</i> 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° (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

Three-Dimensional Magneto-Photoluminescence as a Probe of the Electronic Properties of Crystal-Phase Quantum Disks in GaAs Nanowires

Crystal-phase engineering has emerged as a novel method of bandgap engineering, made feasible by the high surface-to-volume ratio of nanowires. There remains intense debate about the exact characteristics of the band structure of the novel crystal phases, such as wurtzite GaAs, obtained by this approach. We attack this problem via a low-temperature angle-dependent magneto-photoluminescence study of wurtzite/zinc-blende quantum disks in single GaAs nanowires. The exciton diamagnetic coefficient is proportional to the electron–hole correlation length, enabling a determination of the spatial extent of the exciton wave function in the plane and along the confinement axis of the crystal-phase quantum disks. Depending on the disk nature, the diamagnetic coefficient measured in Faraday geometry ranges between 25 and 75 μeV/T². For a given disk, the diamagnetic coefficient remains constant upon rotation of the magnetic field. Along with our envelope function calculation accounting for excitonic effects, we demonstrate that the electron effective mass in wurtzite GaAs quantum disks is heavy, mostly isotropic and results from mixing of the two lower-energy conduction bands with Γ₇ and Γ₈ symmetries. Finally, we discuss the implications of the results of the angle dependent magneto-luminescence for the likely symmetry of the exciton states. This work provides important insight in the band structure of wurtzite GaAs for future nanowire-based polytypic bandgap engineering

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs_0.65Sb_0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs_0.65Sb_0.35 nanowires to date, exceeding 16,000 cm² V^–1 s^–1 at 10 K

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

La Lanterne : journal politique quotidien

25 juillet 18991899/07/25 (N1552,A16)

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

Plastic and Elastic Strain Fields in GaAs/Si Core–Shell Nanowires

Thanks to their unique morphology, nanowires have enabled integration of materials in a way that was not possible before with thin film technology. In turn, this opens new avenues for applications in the areas of energy harvesting, electronics, and optoelectronics. This is particularly true for axial heterostructures, while core–shell systems are limited by the appearance of strain-induced dislocations. Even more challenging is the detection and understanding of these defects. We combine geometrical phase analysis with finite element strain simulations to quantify and determine the origin of the lattice distortion in core–shell nanowire structures. Such combination provides a powerful insight in the origin and characteristics of edge dislocations in such systems and quantifies their impact with the strain field map. We apply the method to heterostructures presenting single and mixed crystalline phase. Mixing crystalline phases along a nanowire turns out to be beneficial for reducing strain in mismatched core–shell structures