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High-quality heteroepitaxial two-dimensional (2D) InSb layers are very difficult to realize because of the large lattice mismatch with other widespread semiconductor substrates. A way around this problem is to grow free-standing 2D InSb nanostructures on nanowire (NW) stems, thanks to the capability of NWs to efficiently relax elastic strain along the sidewalls when lattice-mismatched semiconductor systems are integrated. In this work, we optimize the morphology of free-standing 2D InSb nanoflags (NFs). In particular, robust NW stems, optimized growth parameters, and the use of reflection high-energy electron diffraction (RHEED) to precisely orient the substrate for preferential growth are implemented to increase the lateral size of the 2D InSb NFs. Transmission electron microscopy (TEM) analysis of these NFs reveals defect-free zinc blend crystal structure, stoichiometric composition, and relaxed lattice parameters. The resulting NFs are large enough to fabricate Hall-bar contacts with suitable length-to-width ratio enabling precise electrical characterization. An electron mobility of ∼29 500 cm 2 /(V s) is measured, which is the highest value reported for free-standing 2D InSb nanostructures in literature. We envision the use of 2D InSb NFs for fabrication of advanced quantum devices., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The nanowire platform offers great opportunities for improving the quality and range of applications of semiconductor quantum wells and dots. Here, we present the self-catalyzed growth of InAs/InSb/InAs axial heterostructured nanowires with a single defect-free InSb quantum dot, on Si substrates, by chemical beam epitaxy. A systematic variation of the growth parameters for the InAs top segment has been investigated and the resulting nanowire morphology analyzed. We found that the growth temperature strongly influences the axial and radial growth rates of the top InAs segment. As a consequence, we can reduce the InAs shell thickness around the InSb quantum dot by increasing the InAs growth temperature. Moreover, we observed that both axial and radial growth rates are enhanced by the As line pressure as long as the In droplet on the top of the nanowire is preserved. Finally, the time evolution of the diameter along the entire length of the nanowires allowed us to understand that there are two In diffusion paths contributing to the radial InAs growth and that the interplay of these two mechanisms together with the total length of the nanowires determine the final shape of the nanowires. This study provides insights in understanding the growth mechanisms of self-catalyzed InSb/InAs quantum dot nanowires, and our results can be extended also to the growth of other self-catalyzed heterostructured nanowires, providing useful guidelines for the realization of quantum structures with the desired morphology and properties.

Research interest in indium antimonide (InSb) has increased significantly in recent years owing to its intrinsic properties and the consequent opportunities to implement next-generation quantum devices. Hence, the precise, reproducible control over morphology and crystalline quality becomes of paramount importance for a practical quantum-device technology. Here, we investigate the growth of InSb nanostructures with different morphologies on InAs stems without pre-growth efforts (patterning). InSb nanostructures such as nanowires (1D), nanoflags (2D) and nanocubes (3D) have been realized by means of Au-assisted chemical beam epitaxy by tailoring the growth parameters like growth temperature, precursor fluxes, sample rotation and substrate orientation. Through morphological and crystallographic characterization, all the as-grown InSb 2D nanostructures are found to be single-crystalline with zinc blende structure, free from any defects such as stacking faults and twin planes. The existence of two families of 2D nanostructures, characterised by an aperture angle at the base of 145° and 160°, is observed and modelled. This study provides useful guidelines for the controlled growth of high-quality InSb nanostructures with different shape.

We report results on the control of barrier transparency in InAs/InP nanowire quantum dots via the electrostatic control of the device electron states. Recent works demonstrated that barrier transparency in this class of devices displays a general trend just depending on the total orbital energy of the trapped electrons. We show that a qualitatively different regime is observed at relatively low filling numbers, where tunneling rates are rather controlled by the axial configuration of the electron orbital. Transmission rates versus filling are further modified by acting on the radial configuration of the orbitals by means of electrostatic gating, and the barrier transparency for the various orbitals is found to evolve as expected from numerical simulations. The possibility to exploit this mechanism to achieve a controlled continuous tuning of the tunneling rate of an individual Coulomb blockade resonance is discussed.

Random assemblies of vertically aligned core-shell GaAs-AlGaAs nanowires displayed an optical response dominated by strong oscillations of the reflected light as a function of the incident angle. In particular, angle-resolved specular reflectance measurements showed the occurrence of periodic modulations in the polarization-resolved spectra of reflected light for a surprisingly wide range of incident angles. Numerical simulations allowed for identifying the geometrical features of the core-shell nanowires leading to the observed oscillatory effects in terms of core and shell thickness as well as the tapering of the nanostructure. The present results indicate that randomly displaced ensembles of nanoscale heterostructures made of III-V semiconductors can operate as optical metamirrors, with potential for sensing applications.

We demonstrate high-temperature thermoelectric conversion in InAs/InP nanowire quantum dots by taking advantage of their strong electronic confinement. The electrical conductance G and the thermopower S are obtained from charge transport measurements and accurately reproduced with a theoretical model accounting for the multilevel structure of the quantum dot. Notably, our analysis does not rely on the estimate of cotunnelling contributions, since electronic thermal transport is dominated by multilevel heat transport. By taking into account two spin-degenerate energy levels we are able to evaluate the electronic thermal conductance K and investigate the evolution of the electronic figure of merit ZT as a function of the quantum dot configuration and demonstrate ZT ≈ 35 at 30 K, corresponding to an electronic efficiency at maximum power close to the Curzon-Ahlborn limit.

The steady-state chemical composition of the In/Au alloy nanoparticles (NPs) during isothermal growth of Au-assisted InAs and InP nanowires (NWs) is different for the two materials. Therefore, when switching from one material to the other, to grow axial NW heterostructures, transient effects dominate during the time period of the NP reconfiguration. As a consequence, the precise control of the thickness of thin InP and InAs segments, which is fundamental for the realization of quantum dot (QD) structures and superlattices, can be very challenging. In this work, we present a study of the thickness/diameter dependence of two InP barriers and of the InAs short segment in between (QD), inserted into InAs NWs grown by means of Au-assisted chemical beam epitaxy. We found a broad variability of the InP segment thickness within the same as-grown sample, resulting in InAs NWs with asymmetric and non-homogeneous InP barriers. We explain the results by considering the NP reconfiguration dynamics which dominates at the early stages of the growth in both growth sequences. Moreover, we propose a strategy to control the growth rate and the dynamics of the barriers, by forcing the NP reconfiguration before starting the InP growth. This allows for the realization of InAs/InP NW heterostructures of different diameters, all having symmetric InP barriers with well controlled thickness, which are crucial parameters for the realization of advanced electronic quantum devices.

The possibility to expand the range of material combinations in defect-free heterostructures is one of the main motivations for the great interest in semiconductor nanowires. However, most axial nanowire heterostructures suffer from interface compositional gradients and kink formation, as a consequence of nanoparticle-nanowire interactions during the metal-assisted growth. Understanding such interactions and how they affect the growth mode is fundamental to achieve a full control over the morphology and the properties of nanowire heterostructures for device applications. Here we demonstrate that the sole parameter affecting the growth mode (straight or kinked) of InP segments on InAs nanowire stems by the Au-assisted method is the nanoparticle composition. Indeed, straight InAs-InP nanowire heterostructures are obtained only when the In/Au ratio in the nanoparticles is low, typically smaller than 1.5. For higher In content, the InP segments tend to kink. Tailoring the In/Au ratio by the precursor fluxes at a fixed growth temperature enables us to obtain straight and radius-uniform InAs-InP nanowire heterostructures (single and double) with atomically sharp interfaces. We present a model that is capable of describing all the experimentally observed phenomena: straight growth versus kinking, the stationary nanoparticle compositions in pure InAs and InAs-InP nanowires, the crystal phase trends, and the interfacial abruptness. By taking into account different nanowire/nanoparticle interfacial configurations (forming wetting or nonwetting monolayers in vertical or tapered geometry), our generalized model provides the conditions of nanoparticle stability and abrupt heterointerfaces for a rich variety of growth scenarios. Therefore, our results provide a powerful tool for obtaining high quality InAs-InP nanowire heterostructures with well-controlled properties and can be extended to other material combinations based on the group V interchange.

Subwavelength nanostructured surfaces are realized with self-assembled vertically-aligned InAs nanowires, and their functionalities as optical reflectors are investigated. In our system, polarization-resolved specular reflectance displays strong modulations as a function of incident photon energy and angle. An effective-medium model allows one to rationalize the experimental findings in the long wavelength regime, whereas numerical simulations fully reproduce the experimental outcomes in the entire frequency range. The impact of the refractive index of the medium surrounding the nanostructure assembly on the reflectance was estimated. In view of the present results, sensing schemes compatible with microfluidic technologies and routes to innovative nanowire-based optical elements are discussed., Competing Interests: The authors declare no conflict of interest.

Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

Au-catalyzed III-V nanowire heterostructures based on the group III interchange usually grow straight only in one of the two growth sequences, whereas the other sequence produces kinked geometries; thus, the realization of double heterostructures remains challenging. Here, we investigate the growth of Au-assisted InAs-GaAs and GaAs-InAs axial nanowire heterostructures. A detailed study of the heterostructure morphology as a function of growth parameters and chemical composition of the catalyst nanoparticle is performed by means of scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray analysis. Our results clearly demonstrate that the nanoparticle composition, rather than other growth parameters, as postulated so far, controls the growth mode and the resulting nanowire morphology. Although GaAs easily grows straight on InAs, straight growth of InAs on GaAs is achieved only if the nanoparticle composition is properly tuned. We find that straight InAs segments on GaAs require high group III-to-Au ratios in the nanoparticle (greater than 0.8); otherwise, the droplet wets the sidewalls and the nanowire kinks. We discuss the observed behavior within a theoretical model that relates the nanoparticle stability to the group III-to-Au ratio. Based on this finding, we demonstrate the growth of straight nanowire heterostructures for both sequences. The proposed strategy can be extended to other III-V nanowire heterostructures based on the group III interchange, allowing for straight morphology regardless of the growth sequence, and ultimately for designing nanowire heterostructures with the required properties for different applications.

In this article we demonstrate type-II band alignment at the wurtzite/zinc-blende hetero-interface in InAs polytype nanowires using resonance Raman measurements. Nanowires were grown with an optimum ratio of the above mentioned phases, so that in the electronic band alignment of such NWs the effect of the difference in the crystal structure dominates over other perturbing effects (e.g. interfacial strain, confinement of charge carriers and band bending due to space charge). Experimental results are compared with the band alignment obtained from density functional theory calculations. In resonance Raman measurements, the excitation energies in the visible range probe the band alignment formed by the E 1 gap of wurtzite and zinc-blende phases. However, we expect our claim to be valid also for band alignment near the fundamental gap at the heterointerface.

We investigate light emission from nanoscale point-sources obtained in hybrid metal-GaAs nanowires embedding two sharp axial Schottky barriers. Devices are obtained via the formation of Ni-rich metallic alloy regions in the nanostructure body thanks to a technique of controlled thermal annealing of Ni/Au electrodes. In agreement with recent findings, visible-light electroluminescence can be observed upon suitable voltage biasing of the junctions. We investigate the time-resolved emission properties of our devices and demonstrate an electrical modulation of light generation up to 1 GHz. We explore different drive configurations and discuss the intrinsic bottlenecks of the present device architecture. Our results demonstrate a novel technique for the realization of fast subwavelength light sources with possible applications in sensing and microscopy beyond the diffraction limit.

We demonstrate localization and field-effect spatial control of the plasmon resonance in semiconductor nanostructures, using scattering-type scanning near-field optical microscopy in the mid-infrared region. We adopt InAs nanowires embedding a graded doping profile to modulate the free carrier density along the axial direction. Our near-field measurements have a spatial resolution of 20 nm and demonstrate the presence of a local resonant feature whose position can be controlled by a back-gate bias voltage. In the present implementation, field-effect induces a modulation of the free carrier density profile yielding a spatial shift of the plasmon resonance of the order of 100 nm. We discuss the relevance of our electrically tunable nanoplasmonic architectures in view of innovative optoelectronic devices concepts.

The control of orbitals and spin states of single electrons is a key ingredient for quantum information processing and novel detection schemes and is, more generally, of great relevance for spintronics. Coulomb and spin blockade in double quantum dots enable advanced single-spin operations that would be available even for room-temperature applications with sufficiently small devices. To date, however, spin operations in double quantum dots have typically been observed at sub-kelvin temperatures, a key reason being that it is very challenging to scale a double quantum dot system while retaining independent field-effect control of individual dots. Here, we show that the quantum-confined Stark effect allows two dots only 5 nm apart to be independently addressed without the requirement for aligned nanometre-sized local gating. We thus demonstrate a scalable method to fully control a double quantum dot device, regardless of its physical size. In the present implementation we present InAs/InP nanowire double quantum dots that display an experimentally detectable spin blockade up to 10 K. We also report and discuss an unexpected re-entrant spin blockade lifting as a function of the magnetic field intensity.

The pressure-dependent phonon modes of InAs nanowires have been investigated by Raman spectroscopy under high pressure up to ∼58 GPa. X-ray diffraction measurements show that InAs nanowires at 21 GPa exhibit a phase transition from a wurtzite to an orthorhombic crystal structure, with a corresponding drastic change in the first-order Raman spectra. In the low-pressure regime, a linear increase in phonon frequencies is observed, whereas splitting between longitudinal and transversal optical phonon modes decreases as a function of applied pressure. The calculated mode Grüneisen parameters and Born's transverse effective charge indicate that the wurtzite InAs nanowires exhibit a more covalent nature under compression.

The crystal structure of GaP nanowires grown by Au-assisted chemical beam epitaxy was investigated as a function of group V flux and growth temperature. By increasing the tertiarybutyl phosphine flux we obtained nanowires with a stacking defect-free wurtzite crystal structure. Variation of growth temperature also had a profound impact on the crystal structure. Lowering the growth temperature from 600 to 560 °C and keeping constant both triethylgallium and tertiarybutyl phosphine precursor fluxes, the crystal structure of GaP NWs was drastically improved from a highly defective intergrowth of zinc-blende and wurtzite to a wurtzite crystal structure free of stacking defects. These results are compared to current literature on GaP NW growth, and we suggest that the low V/III ratio is the key ingredient for the high crystal quality of our GaP nanowires.

We report on the development of an innovative class of nanowire-based Terahertz (THz) detectors in which the metamaterial properties of an antenna have been imported in the detection scheme of an overdamped plasma-wave field-effect transistor making its response resonant to THz radiation. Responsivities of ~105 V/W at 0.3 THz, with noise equivalent power levels ≈ 10(-10) W/√Hz, detectivities ~2 · 10(8) cm√Hz/W and quantum efficiencies ~1.2 · 10(-5) are reached at room-temperature. The resonant nature of the detection scheme provided by the four-leaf-clover-shaped geometry and the possibility to extend this technology to large multi-pixel arrays opens the path to demanding applications for ultra-sensitive metrology, spectroscopy and biomedicine.

Millivolt range thermovoltage is demonstrated in single InAs nanowire based field effect transistors. Thanks to a buried heating scheme, we drive both a large thermal bias ΔT > 10 K and a strong field-effect modulation of electric conductance on the nanostructures. This allows the precise mapping of the evolution of the Seebeck coefficient S as a function of the gate-controlled conductivity σ between room temperature and 100 K. Based on these experimental data a novel estimate of the electron mobility is given. This value is compared with the result of standard field-effect based mobility estimates and discussed in relation to the effect of charge traps in the devices.

We report on a major modification of the fundamental electronic band structure of AlAs when grown as a nanoscaled wurtzite crystal. Resonant Raman spectra of individual AlAs-GaAs core-shell nanowires display a resonance between 1.83 and 2.18 eV for the AlAs E₁(TO) phonon mode. Our findings substantiate the lowest conduction band of wurtzite AlAs to comprise Γ₈ symmetry and a low effective mass in agreement with calculations reported recently. The electronic resonance falls below the X, L, and Γ valleys known for AlAs in the zincblende phase. This result points toward a direct nature of wurtzite AlAs and is expected to apply more generally to semiconductors that in the bulk phase exhibit L valleys at lower energies than the conduction band at the Γ point.

Very robust voltage-controlled spin transitions in few-electron quantum dots are demonstrated. Two lateral-gate electrodes patterned on opposite sides of an InAs/InP nanowire are used to apply a transverse electric field and tune orbital energy separation down to level-pair degeneracy. Transport measurements in this regime allow us to demonstrate the breakdown of the standard alternate up/down spin filling scheme and unambiguously show singlet-triplet spin transitions. The strong confinement of the present devices leads to a large energy gain for the observed anomalous spin configurations that exceeds 4 meV. As a consequence, this behavior is well visible even at temperatures exceeding T = 20 K.

We report the growth of InAs/InAs(1-x)Sb(x) single and double heterostructured nanowires by Au-assisted chemical beam epitaxy. The InAs(1-x)Sb(x) nanowire segments have been characterized in a wide range of antimony compositions. Significant lateral growth is observed at intermediate compositions (x ~ 0.5), and the nucleation and step-flow mechanism leading to this lateral growth has been identified and described. Additionally, CuPt ordering of the alloy has been observed with high resolution transmission electron microscopy, and it is correlated to the lateral growth process. We also show that it is possible to regrow InAs above the InAsSb alloy segment, at least up to an intermediate antimony composition. Such double heterostructures might find applications both as mid-infrared detectors and as building blocks of electronic devices taking advantage of the outstanding electronic and thermal properties of antimonide compound semiconductors.

We investigated the transport properties of lateral gate field effect transistors (FET) that have been realized by employing, as active elements, (111) B-oriented InAs nanowires grown by chemical beam epitaxy with different Se-doping concentrations. On the basis of electrical measurements, it was found that the carrier mobility increases from 103 to 104 cm2/(V × sec) by varying the ditertiarybutyl selenide (DtBSe) precursor line pressure from 0 to 0.4 Torr, leading to an increase of the carrier density in the transistor channel of more than two orders of magnitude. By keeping the DtBSe line pressure at 0.1 Torr, the carrier density in the nanowire channel measures ≈ 5 × 1017 cm-3 ensuring the best peak transconductances (> 100 mS/m) together with very low resistivity values (70 Ω × μm) and capacitances in the attofarad range. These results are particularly relevant for further optimization of the nanowire-FET terahertz detectors recently demonstrated.PACS: 73.63.-b, 81.07.Gf, 85.35.-p.

The growth of semiconductor nanowires (NWs) has recently opened new paths to silicon integration of device families such as light-emitting diodes, high-efficiency photovoltaics, or high-responsivity photodetectors. It is also offering a wealth of new approaches for the development of a future generation of nanoelectronic devices. Here we demonstrate that semiconductor nanowires can also be used as building blocks for the realization of high-sensitivity terahertz detectors based on a 1D field-effect transistor configuration. In order to take advantage of the low effective mass and high mobilities achievable in III-V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly doped with selenium to control the charge density and to optimize source-drain and contact resistance. The detection mechanism exploits the nonlinearity of the transfer characteristics: the terahertz radiation field is fed at the gate-source electrodes with wide band antennas, and the rectified signal is then read at the output in the form of a DC drain voltage. Significant responsivity values (>1 V/W) at 0.3 THz have been obtained with noise equivalent powers (NEP) < 2 × 10(-9) W/(Hz)(1/2) at room temperature. The large existing margins for technology improvements, the scalability to higher frequencies, and the possibility of realizing multipixel arrays, make these devices highly competitive as a future solution for terahertz detection., (© 2011 American Chemical Society)

We present a novel technique for the manipulation of the energy spectrum of hard-wall InAs/InP nanowire quantum dots. By using two local gate electrodes, we induce a strong transverse electric field in the dot and demonstrate the controlled modification of its electronic orbitals. Our approach allows us to dramatically enhance the single-particle energy spacing between the first two quantum levels in the dot and thus to increment the working temperature of our InAs/InP single-electron transistors. Our devices display a very robust modulation of the conductance even at liquid nitrogen temperature, while allowing an ultimate control of the electron filling down to the last free carrier. Potential further applications of the technique to time-resolved spin manipulation are also discussed.

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.

During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire-tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor-liquid-solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor-solid-solid (VSS) mode.

We report the Au-assisted chemical beam epitaxy growth of defect-free zincblende InSb nanowires. The grown InSb segments are the upper sections of InAs/InSb heterostructures on InAs(111)B substrates. We show, through HRTEM analysis, that zincblende InSb can be grown without any crystal defects such as stacking faults or twinning planes. Strain-map analysis demonstrates that the InSb segment is nearly relaxed within a few nanometers from the interface. By post-growth studies we have found that the catalyst particle composition is AuIn(2), and it can be varied to a AuIn alloy by cooling down the samples under TDMASb flux.