Off-axis electron holography (EH) is a powerful method for mapping projected electric potentials, such as built-in potentials in semiconductor devices, in two dimensions (2D) at nanometer resolution. However, not well-defined thickness profiles, surface effects, and composition changes of the sample under investigation complicate the interpretation of the projected potentials. Here, we demonstrate how these problems can be overcome by combining EH with tomographic techniques, that is, electron holographic tomography (EHT), reconstructing electric potentials in 3D. We present EHT reconstructions of an n-type MOSFET including its dopant-related built-in potentials inside the device, as well as of a GaAs/AlGaAs core-multishell nanowire containing a 5 nm thick quantum well tube.

Vertically-aligned CdTe nanowire (NWs) were grown for the first time by metalorganic vapor phase epitaxy, using diisopropyl-telluride and dimethylcadmium as precursors, and Au nanoparticles as metal catalysts. The NWs were grown between 485 and 515, °C on (111)B-GaAs substrates, the latter overgrown with a 2-μm thick CdTe epilayer. To favor the Au-catalyst assisted process against planar deposition of CdTe, an alternate precursors flow process was adopted during NW self-assembly. Field emission electron microscopy observations and X-ray energy dispersive analyses of CdTe NWs revealed the presence of Au-rich droplets at their tips, the contact-angle between Au-droplets and NWs being, ~130°. The NW height increases exponentially with the growth temperature, indicating that the Au-catalyzed process is kinetics-limited (activation energy: ~57, kcal/mol), but no tapering is observed. Low temperature cathodoluminescence spectra recorded from single NWs evidenced a band-edge emission typical of zincblend CdTe, and a dominant (defects-related) emission band at 1.539 eV.

GaAs, AlGaAs and GaAs-AlGaAs core-shell nanowires (NWs) may find potential applications in the fields of photo-detectors and solar cells. Here we present their self-assembly by Au-catalysed metalorganic vapour phase epitaxy and related physical properties. While AlGaAs NWs show an inherent tapered morphology and unintentional core-shell compositional structure, under selected conditions nearly cylindrical GaAs NWs can be grown; the latter have been used to synthesise straight uniform GaAs-AlGaAs core-shell NWs, showing intense luminescence from both core and shell materials. The dominant GaAs core emission red-shifts with decreasing the precursors V:III ratio, an effect ascribed to the build-up of a space-charge induced electric field at the GaAs-AlGaAs hetero-interface, in turn due to unintentional C and Si doping of GaAs and AlGaAs, respectively.

The core photoluminescence emission of MOVPE-grown GaAs-Al0.33Ga0.67As core-shell nanowires is studied as function of the relevant geometrical parameter of these nanostructures, namely the shell-thickness to core-radius ratio hs/Rc. The energy of the dominant emission peak was compared with values of the GaAs heavy- and light-hole excitons redshifted by a uniaxial tensile strain, the latter calculated assuming perfect coherence at the core/shell interface and elastic energy equilibrium within the nanowires. Good agreement is obtained for hs/Rc < 1, the intrinsic strain-free excitonic emission being identified at 1.510 eV, and further ascribed to bound heavy-hole excitons. For hs/Rc > 1 increasingly larger redshifts (up to ∼9 meV in excess of values calculated based on our elastic strain model) are observed, and tentatively ascribed to shell-dependent exciton localization effects. Experimental and calculated bound exciton peak energies for GaAs-Al0.33Ga0.67As core-shell nanowires as function of their shell-thickness to core-radius ratio hs/Rc.

We report on the first Au-catalyzed growth of CdTe nanowires by metalorganic vapor phase epitaxy. The nanowires were obtained by a separate precursors flow process in which (i) di-isopropyltelluride (iPr2Te) was first flowed through the reactor to ensure the formation of liquid Au−Te alloy droplets, and (ii) after purging with pure H2 to remove unreacted iPr2Te molecules from the vapor and the growth surface, (iii) dimethylcadmium (Me2Cd) was supplied to the vapor so that Cd atoms could enter the catalyst droplets, leading to nanowire self-assembly. CdTe nanowires were grown between 485 and 515 °C on (111)B-GaAs substrates, the latter preliminary deposited with a 2 μm thick (111)-oriented CdTe buffer layer onto which Au nanoparticles were provided. As-grown CdTe nanowires were vertical ([111]-aligned) straight segments of constant diameter and showed an Au-rich nanodroplet at their tips, the contact angle between the droplets and the nanowires being ∼130°. The nanowire axial growth rate appeared kinetics-limited with an activation energy ∼57 kcal/mol. However, the growth rate turned independent from the nanowire diameter. Present data are interpreted by a theoretical model explaining the nanowire growth through the diffusion transport of Te adatoms under the assumption that their growth occurs during the Me2Cd-flow process step. Low-temperature cathodoluminescence spectra recorded from single nanowires showed a well-resolved band-edge emission typical of zincblend CdTe along with a dominant band peaked at 1.539 eV.

High (>350 °C) temperature DC-sputtering deposition of ZnO:Al thin films onto single-crystal (00.1) oriented Al2O3 (sapphire) substrates is reported, using a ultrahigh-density, low-resistivity and low-cost composite ceramic target produced by slip-casting (pressureless) sintering of ZnO-Al2O3 (AZO) powders. The original combination of high-angle theta-2 theta(Bragg-Brentano geometry) X-ray diffraction with low angle theta-2 theta X-ray reflectivity (XRR) techniques allows us to define the AZO target composition and investigate the structural properties and surface/interface roughness of as-sputtered ZnO:Al films; besides, the growth dynamics of ZnO:Al is unambiguously determined. The target turned out composed of the sole wurtzite ZnO and spinel ZnAl2O4 phases. X-ray diffraction analyses revealed highly (00.1)-oriented (epitaxial) ZnO:Al films, the material mean crystallite size being in the 13-20 nm range and increasing with temperature between 350 °C and 450 °C, while the film growth rate (determined via XRR measurements) decreases appreciably. XRR spectra also allowed to determine rms surface roughness <1 nm for present films and showed ZnO:Al density changes by only a few percent between 350 °C and 450 °C. The latter result disproves the often-adopted Thornton model for the description of the sputter-grown ZnO films and instead points out toward a reduction of the sticking coefficients of impinging species, as the main origin of film growth rate and grain size dependence with temperature. ZnO:Al films appear highly transparent, with visible-NIR integrated transmittance >97%.

The direct chemical vapor deposition of WS2 by W(CO)(6) and elemental sulfur as precursors onto epitaxial-graphene on SiC and CVD-graphene transferred on SiO2/Si substrate is presented. This methodology allows the epitaxial growth of continuous WS2 films with a homogeneous and narrow photoluminescence peak without inducing stress or structural defects in the graphene substrates. The control of the WS2 growth dynamics for providing the localized sulfide deposition by tuning the surface energy of the graphene substrates is also demonstrated. This growth methodology opens the way towards the direct bottom-up fabrication of devices based on TMDCs/graphene van der Waals heterostructures.

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa 1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

We investigated by means of transmission electron microscopy (TEM) the final growth stage of GaAs/AlGaAs core-shell nanowires (NWs) self-assembled by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). TEM observations and energy dispersive x-ray spectroscopy revealed the presence of an AlGaAs tapered region of varying chemical composition nearby the NW extreme end (i.e. between the core-shell NW trunk and the Au nanoparticle catalyst). Our findings evidence that this region exhibits an unintentional AlyGa1−yAs/AlxGa1−xAs core-shell structure, a result of the combined axial (vapor-liquid-solid, VLS) self-assembly and conventional (vapor-solid, VS) overgrowth of the material. While the VS-grown AlxGa1−xAs alloy retains the Al composition (x=0.3) of the AlGaAs shell along the NW trunk, the central AlyGa1−yAs section is made of an Al-rich (y≈0.8–0.9) alloy segment formed during AlGaAs shell overgrowth, followed by a graded-alloy segment formed upon deposition of the terminating GaAs cap layer, the latter segment due to the effect of the Al reservoir left in the Au catalyst nanoparticle (NP).

Electron holographic tomography (EHT), the combination of off-axis electron holography with electron tomography, is a technique, which can be applied to the quantitative 3-dimensional (3D) mapping of electrostatic potential at the nanoscale. Here, we show the results obtained in the EHT investigation of GaAs and GaAs-AlGaAs core-shell nanowires grown by Au-catalysed metalorganic vapor phase epitaxy. The unique ability of EHT of disentangling the materials mean inner potential (MIP) from the specimen projected thickness allows reconstruction of the nanowire 3D morphology and inner compositional structure as well as the measurement of the MIP.

We report on the effects of changing the surface densities of MOVPE-grown free-standing GaAs-AlGaAs core-shell nanowires on the resulting nanostructure size and their photoluminescence (PL) properties. It is demonstrated that decreasing the local density of GaAs nanowires within the array leads to an increase of the overgrown AlGaAs shell thickness and to a substantial redshift of the nanostructure excitonic emission. Application of a vapor mass-transport limited growth model of the AlGaAs shell allows explaining the dependence of shell growth rate on nanowire density. The observed redshift of the nanowire PL emission is then experimentally correlated with these density-induced changes of the nanostructure size, namely with the nanowire shell-thickness to core-radius ratio hs/Rc. To account for a possible contribution of the nanostructure built-in elastic strain to the energy shift of the peak excitonic emission, the strain field in present core-shell nanowires was calculated as function of the nanostructure relevant geometrical parameters, based on a uniaxial elastic energy equilibrium model, and its effect on valence and conduction band shifts of the GaAs core evaluated by means of the Pikus-Bir Hamiltonian. Good agreement is obtained for h(s)/R-c<1, the strain-free excitonic emission being identified at 1.510 eV and ascribed to bound heavy-hole excitons. For h(s)/R-c>1 increasingly larger redshifts (up to similar to 9 meV in excess of values calculated based on the elastic strain model) are observed, and tentatively ascribed to shell-dependent exciton localization effects.

We report on the self-assembly by Au-catalyzed metalorganic vapor phase epitaxy (MOVPE) of GaAs-based nanowires (NWs) and their applications to novel and efficient nano-devices. The growth of GaAs and GaAs-AlGaAs core-shell NWs is presented as case study, focusing on the dependence of their structural, optical and electrical properties on MOVPE conditions. MSM diodes fabricated using as-grown core-shell NWs are reported, along with their photoelectric performances. These devices show potentials for applications as fast photo-detectors and efficient solar cells.

The inner composition, defect content and morphology of AlGaAs nanowires (NWs) grown on (111)B-GaAs by Au-catalyzed MOVPE is reported. The NWs grow tapered with their [111] axis normal to the substrate. The Raman spectra of single AlGaAs NWs were measured in non-resonant conditions with sub-μ-meter spatial resolution, allowing determination of the Al content. NWs consist of GaAs for TG<475°C, but show a two-fold compositional structure for TG>475°C, namely an AlxGa1-xAs core surrounded by an AlyGa1-yAs (y<x) shell, ascribed to the combination of Au-catalyzed (axial) and conventional (sidewall) growth. The cross-sectional shape of AlGaAs NWs changes from triangular (for TG=500÷525°C) to almost hexagonal (for TG=550°C), due to an exchange between {211} and {110} planes as the slowest to grow. The NWs have free-electron concentrations ∼10^18 cm^-3, due to Si contamination of the Al source.

Group III-V compound semiconductor nanowires with radial modulation of the materials composition and/or doping in the form of core-shell and core-multishell nanowire heterostructures show promise as novel and high-performance nano-scale light emitting diodes, lasers, photodetectors and solar cells. Strict control over the growth of such radially heterostructured nanowires is, however, necessary. We report the experimental dependence of AlGaAs shell growth by metalorganic vapor phase epitaxy (MOVPE) around free-standing Au-catalysed GaAs nanowires on the relevant sizes and densities of the nanostructures. A model based on (i) the vapor mass transport of group III species, and (ii) perfect conformality between the nanowires and the substrate of AlGaAs deposition is proposed and validated, describing the observed MOVPE growth dynamics of the shell material around dense ensembles of GaAs nanowires. We predict the complex (non-linear) dependence of the shell growth rate on the initial GaAs nanowire diameters (i.e., initial Au catalyst nanoparticle size), heights, local densities on the substrate, and deposition time, which is in very good agreement with experimental data; in particular, a monotonic decrease of AlGaAs shell thickness is expected and observed with increasing nanowire density.

In this work, we report on the microstructural and morphological characterization of III-V semiconductor nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy. As-grown dense (10^8-10^9 cm^-2) arrays of few-micron long vertically-aligned (i.e. parallel to the <111> crystallographic axis) GaAs, AlxGa1-xAs and core-shell GaAs-AlxGa1-xAs NWs were investigated, carrying out HRXRD measurements on different (hkl) reflections and by recording reciprocal space maps (RSMs) around the materials (111) reciprocal lattice points (relps). We show that NW diffraction peaks are visible in the RSM by means of characteristic halos. In the case of GaAs NWs, the halo is located at the (111) relp indicating that the NWs are grown along the <111> direction and parallel to the <111> axis of the GaAs substrate. On the contrary, for AlxGa1-xAs NWs or intentional core-shell GaAs-AlxGa1-xAs NWs the halo is displaced (along the momentum transfer normal to the surface, Qz) with respect to the GaAs (111) relp due to the elastic lattice strain associated with the compositional variation, e.g. the Al molar fraction in the AlxGa1-xAs alloy, within the nanostructures.

In this work we present new results on the morphological and microstructural properties of GaAs-AlxGa1-xAs (x≈0.24) core-shell nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). Optimized growth conditions allowed us to fabricate highly-dense arrays of vertically-aligned (i.e., along the <111> crystallographic orientation) NWs. The NW arrays were investigated by Helium Ion microscopy (HeIM) and X-ray double- and triple-axis measurements and reciprocal space mapping (RSM). We demonstrate that these techniques can be employed in order to correlate some intrinsically local morphological information with statistically relevant (i.e. averaged over millions-to-billions of NWs) data on the NW structural properties.

The different approaches being currently explored for the fabrication of free-standing quasi-1D nanostructures of III-V compound semiconductors utilizing the MOVPE technology are reviewed: major limitations and advantages are discussed. In particular, we focus on the self-assembly of semiconductor nanowires by the so-called metal-catalyst assisted ― or VLS ― mechanism. The latter is currently considered a most promising technology for the realization of high quality quasi-1D nanostructures. Examples of this approach are given based on results obtained in the author’s laboratory using low pressure MOVPE to growth nanowire structures of III-As compounds.

We report on the development of electron holographic tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as x-ray tomography.

Electron holography at medium resolution simultaneously probes projected electrostatic and magnetostatic potentials as well as elastic and inelastic attenuation coefficients with a spatial resolution of a few nanometers. In this work, we derive how the elastic and inelastic attenuation can be disentangled. Using that result, we perform the first three dimensional tomographic reconstruction of potential and (in) elastic attenuation in parallel. The technique can be applied to distinguish between functional potentials and composition changes in nanostructures, as demonstrated using the example of a GaAs-Al0.33Ga0.67As core-shell nanowire.

We investigate the impact of a quasi-crystalline two-dimensional (2D) surface on the lateral epitaxy of one-dimensional (1D) nanocrystals. The quasi-2D surface was formed by locally conditioning a GaN crystalline lattice at and below surface using a low-dose focused Ga ion beam ranging from 9 to 354 ions/pulse. Short ion pulses/site are used to create the 2D arrays of sub-10-nm circular disks modulating the GaN lattice to a depth of about 30 nm. Impact of this localized lattice modulation was investigated on lateral epitaxy of 1D ZnO nanocrystal and electronic structure of formed heterojunctions. High resolution transmission electron microscopy (HRTEM) below the GaN surface reveals direct evidence of a “skin effect” that influences the surface epitaxy of low dimensional nanocrystals. We define this effect as the crystallinity of the top 10 nm of the substrate that is found to be a key factor in occurrence of lateral epitaxy. HRTEM shows that lateral epitaxy stops, if the thin skin layer is disrupted. However, if the lattice structure of this layer rebounds, the lateral epitaxy occurs. Results indicate that, beyond the skin depth of about 10 nm, the disorder of the subsurface lattice does not impact the structure of the overgrown nanocrystals. These findings suggest the possibility of surface engineering for enabling spatially controlled modulation of the electronic structure of low-dimensional nanocrystals on a scalable fashion.

The electronic transport and gating characteristics in GaAs and Ge nanowires (NWs) are altered significantly following either indirect or direct exposure to focused Ga+ ion beam (FIB), such as that used to produce Pt electrical contacts to NWs. While these results challenge the assumptions made in some previously reported work relating to the electronic properties of semiconductor NWs using FIB-assisted production of contacts and/or their leads, local electron beam induced deposition (EBID) is shown to be a reliable and facile route for producing robust electrical contacts to individual vapor phase-grown NWs in a manner that enables study of their actual carrier transport properties.

We analyze the role of overlap function, oscillator strength, and joint optical density of states in causing GaAs/AlGaAs nanowires to absorb nearly two orders of magnitude more light compared to the same material in bulk.

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

We report on the growth of GaAs-AlGaAs core-multishell nanowire quantum heterostructures by metalorganic vapor phase epitaxy, and their photoluminescence (PL) properties. Dense arrays of vertically-aligned GaAs nanowires were fabricated onto (111)B-GaAs wafers by Au-catalyzed self-assembly, and radially overgrown by two AlGaAs shells between which a few-nm thin GaAs shell was introduced to form a quantum well tube (QWT). Besides the GaAs nanowire core emission band peaked at around 1.503 eV, 7K PL spectra showed an additional broad peak in the 1.556-1.583 eV energy interval, ascribed to the transition between electron and hole confined states within the QWT. The emission blue-shifts with the shrinkage of as-grown GaAs well tubes, as the nanowire local (on the substrate) density and height change.

We report on the photoluminescence (PL) of GaAs-Al0.32Ga0.68As core-shell nanowires grown by MOVPE, and its dependence on the precursors V:III molar ratio utilized in the vapor during growth. It is shown that the PL emission of the GaAs nanowire core red-shifts with decreasing the V:III ratio from 30:1 to 4:1, an effect tentatively ascribed to the build-up of a space-charge region at the core-shell hetero-interface, the latter associated to the unintentional incorporation of impurities, namely C in GaAs and Si in AlGaAs.

The photoconduction properties of individual GaAs/AlGaAs core-shell nanowires under uniform and local optical excitation are investigated, allowing the external quantum efficiency, the polarization anisotropy, and the role of the nanocontacts to be valuated.

Conductivity and photoconductivity properties of individual GaAs/AlGaAs core–shell nanowires (NWs) are reported. The NWs were grown by Au-assisted metalorganic vapor phase epitaxy, and then dispersed on a substrate where electrical contacts were defined on the individual NWs by electron beam induced deposition. Under dark conditions, the carrier transport along the NW is found to be limited by Schottky contacts, and influenced by the presence of an oxide layer. Nonetheless, under illumination, the GaAs/AlGaAs core–shell NW shows a significant photo-current, much higher than the bare GaAs NW. The spatial dependence of the photo-current within the single core–shell NW, evaluated by a mapping technique, confirms the blocking behavior of the contacts. Moreover, local spectral measurements were performed which allow one to discriminate the contribution of carriers photo-generated in the core and in the shell.

High-speed metal-semiconductor-metal (MSM) photodetectors based on Schottky-contacted core/shell GaAs/AlGaAs and bare GaAs nanowires were fabricated and characterized. The measured core/shell temporal response has a about 10 ps full-width at half-maximum and an estimated corrected value less than 5 ps. The bare GaAs devices exhibit a slower response (ca. 35 ps) along with a slow decaying persistent photocurrent (ca. 80 s). The core/shell devices exhibit significantly improved dc and high-speed performance over bare nanowires and comparable performance to planar MSM photodetectors. The picosecond temporal response, coupled with picoampere dark current, demonstrate the potential for core/shell nanowires in high-speed imaging arrays and on-chip optical interconnects.

We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy (varying from about 0.1 to about 0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.

The stoichiometry of single ternary III-V semiconductor nanowires was analyzed by Raman spectroscopy. Freestanding AlxGa1-xAs nanowires were obtained through Metal-Organic Vapor Phase Epitaxy (MOVPE) by the Vapor Liquid Solid (VLS) mechanism on GaAs substrates. The as-grown nanowires were studied with a scanning confocal Raman spectrometer (spatial resolution ca. 300 nm). They were located by Rayleigh imaging and selected for Raman measurements. The acquired spectra exhibit 2-mode behavior. The stoichiometry of individual nanowires was determined based on the frequencies of the GaAs- and AlAs-like TO and LO peaks with an accuracy of <10%. Measurements at different positions along the axis revealed variations of the composition within single nanowires. This non-uniformity indicates that the nanowires possess an internal structure.

tWe demonstrate the possibility of fast and efficient solvent-free functionalization of buckypaper(BP) mats prefabricated from oxidized multiwalled carbon nanotubes (MWCNTs-ox), by using threerepresentative amines of different structure: one monofunctional aliphatic amine, octadecylamine(ODA), one monofunctional aromatic amine, 1-aminopyrene (AP), and one aromatic diamine, 1,5-diaminonaphthalene (DAN). The functionalization procedure, which relies on the formation of amidebonds with carboxylic groups of MWCNTs-ox, is performed at 150–180◦C under reduced pressure andtakes about 4 h including auxiliary degassing. The amine-treated BP samples (BP-ODA, BP-AP and BP-DAN, respectively) were characterized by means of a variety of analytical techniques such as Fourier-transforminfrared and Raman spectroscopy, thermogravimetric and differential thermal analysis, scanning andtransmission electron microscopy, scanning helium ion microscopy, and atomic force microscopy. Thehighest amine content was found for BP-ODA, and the lowest one was observed for BP-DAN, with a possi-ble contribution of non-covalently bonded amine molecules in all three cases. Despite of some differencesin spectral and morphological characteristics for amine-functionalized BP samples, they have in commona dramatically increased stability in water as compared to pristine BP and, on the other hand, a relativelyinvariable electrical conductivity.

We investigated by means of transmission electron microscopy (TEM) GaAs-AlGaAs core-shell nanowires (NWs) grown by Au-catalyzed metalorganic vapor phase epitaxy under As-rich vapor conditions. The structural analysis reveals that the NWs exhibit zincblende phase only and a very low density of twin defects along the NW trunk; a higher amount of twins around the NW axial (growth) direction occur instead within a tapered region closeby the NW tip. We correlate this finding to the Alrich composition of the tapered region, as deduced by scanning-TEM (STEM) mass contrast analysis, this region being the effect of residual Au-assisted self-assembly of the material during shell overgrowth. In addition a few percent of NWs show branching during their growth, and we determine the crystal properties and defects of branched NWs. We propose that this phenomenon may be induced by Au nanoparticle instabilities during NW growth and further demonstrate that twin defects occur at the NW branching points.

We demonstrate spatial probing of carrier transport within GaAs/AlGaAs coreshell nanowires with nanometer lateral resolution and subsurface sensitivity by energy-variable electron beam induced current imaging. Carrier drift that evolves with applied electric field is distinguished from a coupled drift-diffusion length. Along with simulation of injected electron trajectories, combining beam energy tuning with precise positioning for selective probing of core and shell reveals axial position- and bias-dependent differences in carrier type and transport along parallel conduction channels. These results indicate how analysis of transport within heterostructured nanomaterials is no longer limited to nonlocal or surface measurements.

III-V semiconductor compound based nanowires (NWs) are expected to impact the fields of nano-electronic, nano-photonic, and photovoltaic devices. Self-assembly of crystal-phase controlled and high optical quality III-V NWs has been demonstrated. However, important physical and technological issues, such as carrier transport properties and reproducible incorporation of high dopant concentrations in NW materials, remain to be addressed for enabling robust nano-devices fabrication. In this work, we show the use of a multi-probe scanning tunneling microscope for the rapid electrical characterization of free-standing GaAs NWs, without any need for post-growth sample processing and contact fabrication. In particular, 2-probe I-V measurements were performed along the axis of a single 60-nm diameter unpassivated GaAs NW, and its resistance profile determined, obtaining high (in the range of GOhm) resistance values. Due to its reduced radial dimension, the NW is expected to be completely depleted. Analysis of the NW resistance profile reveals instead, that carrier transport is mediated by the NW surface states. Finally, by using the substrate as a reference electrode and placing the other three STM-tips along the NWs, we demonstrate a 4-point probe geometry that can be used for the electrical characterization of highly doped NWs.

We report the hot photoexcited electron transfer across the coaxial interface of a cylindrical core-shell nanowire. Modulation of the transfer rates, manifested as a large tunability of the voltage onset of negative differential resistance and of voltage-current phase, is achieved using three different modes. The coupling of electrostatic gating, incident photon energy, and the incident photon intensity to transfer rates is facilitated by the combined influences of geometric confinement and heterojunction shape on hot-electron transfer, and by electron-electron scattering rates that can be altered by varying the incident photon flux, with evidence of weak electron-phonon scattering. Dynamic manipulation of this transfer rate permits the introduction and control of a continuously adjustable phase delay of up to about 130° within a single nanometer-scale device element.

We report on the Au-catalysed synthesis of GaAs nanowires on hetero-structured GaAs/(111)Si substrates by metalorganic vapour phase epitaxy. It is demonstrated that the deposition of a 40-50 nm thin GaAs epilayer onto Si guarantees a high percentage of straight and vertically-aligned GaAs nanowires. GaAs epilayers were grown at 400 °C and subsequently annealed at 700 °C. Growth experiments performed on 4°-miscut and exactly-oriented (111)Si substrates show that a higher yield (close to 90%) of vertical nanowires is obtained using miscut substrates, an effect ascribed to the smoother surface morphology of GaAs epilayers on these substrates. Comparison between the cross-sectional shape of nanowires grown on GaAs/(111)Si heterosubstrates and those on (111)A-GaAs and (111)B-GaAs substrates demonstrates that both GaAs epilayers and over-grown nanowires are (111)B-oriented.

With ongoing instrumental improvements Raman spectroscopy (RS) is advancing into the study of surface vibrational modes of semiconductors. On compound semiconductors, like the IIIV's, two distinct types of surface modes occur, microscopic modes being vibrations confined to the surface and macroscopic modes penetrating much deeper into the bulk, dependent on the electromagnetic boundary conditions. Both mode types depend on surface properties, the microscopic ones on the atomic scale, and the macroscopic ones on the scale of the surface polariton wavelength. While the former one delivers surface atomic structure information, the latter one may be useful for instance to study the morphology of surfaces, such as the shape of semiconductor nanowires. We discuss Raman spectra obtained on the atomically well-defined III-V(110) model surfaces and recent results obtained on isolated III-V nanowires. The comparison of both gives insight in the capabilities of Raman scattering from surface phonons: The contributions of surface phonons, surface resonances, and macroscopic modes (Fuchs-Kliewer modes, surface phonon polaritons) to the Raman spectra become evident.