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The coupling of a prescribed number of site-controlled pyramidal quantum dots (QDs) with photonic crystal (PhC) cavities was studied by polarization and power-dependent photoluminescence measurements. The energy of the cavity mode could be readily tuned, making use of the high spectral uniformity of the QDs and designing PhC cavities with different hole radii. Efficient coupling of the PhC cavity modes both to the ground state and to the excited state transitions of the QDs was observed, whereas no evidence for far off-resonant coupling was found. [ABSTRACT FROM AUTHOR]

We fabricate dual-gated electric double layer (EDL) field effect transistors based on InAs nanowires gated with an ionic liquid, and we perform electrical transport measurements in the temperature range from room temperature to 4.2 K. By adjusting the spatial distribution of ions inside the ionic liquid employed as gate dielectric, we electrostatically induce doping in the nanostructures under analysis. We extract low-temperature carrier concentration and mobility in very different doping regimes from the analysis of current-voltage characteristics and transconductances measured exploiting global back-gating. In the liquid gate voltage interval from -2 to 2 V, carrier concentration can be enhanced up to two orders of magnitude. Meanwhile, the effect of the ionic accumulation on the nanowire surface turns out to be detrimental to the electron mobility of the semiconductor nanostructure: the electron mobility is quenched irrespectively to the sign of the accumulated ionic species. The reported results shine light on the effective impact on crucial transport parameters of EDL gating in semiconductor nanodevices and they should be considered when designing experiments in which electrostatic doping of semiconductor nanostructures via electrolyte gating is involved.

We have studied the effects of optical-frequency light on proximitized InAs/Al Josephson junctions based on highly n-doped InAs nanowires at varying incident photon flux and at three different photon wavelengths. The experimentally obtained IV curves were modeled using a resistively shunted junction model which takes scattering at the contact interfaces into account. Despite the fact that the InAs weak link is photosensitive, the Josephson junctions were found to be surprisingly robust, interacting with the incident radiation only through heating, whereas above the critical current our devices showed non-thermal effects resulting from photon exposure. Our work indicates that Josephson junctions based on highly-doped InAs nanowires can be integrated in close proximity to photonic circuits. The results also suggest that such junctions can be used for optical-frequency photon detection through thermal processes by measuring a shift in critical current.

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 analyze the benefits and shortcomings of a thermal control in nanoscale electronic conductors by means of the contact heating scheme. Ideally, this straightforward approach allows one to apply a known thermal bias across nanostructures directly through metallic leads, avoiding conventional substrate intermediation. We show, by using the average noise thermometry and local noise sensing technique in InAs nanowire-based devices, that a nanoscale metallic constriction on a SiO 2 substrate acts like a diffusive conductor with negligible electron-phonon relaxation and non-ideal leads. The non-universal impact of the leads on the achieved thermal bias-which depends on their dimensions, shape and material composition-is hard to minimize, but is possible to accurately calibrate in a properly designed nano-device. Our results allow to reduce the issue of the thermal bias calibration to the knowledge of the heater resistance and pave the way for accurate thermoelectric or similar measurements at the nanoscale.

We present a comprehensive electrical characterization of an InAs/InP nanowire (NW) heterostructure, comprising of two InP barriers forming a quantum dot (QD), two adjacent lead segments and two metallic contacts. We demonstrate how to extract valuable quantitative information of the QD. The QD shows very regular Coulomb blockade resonances over a large gate voltage range. By analyzing the resonance line shapes, we map the evolution of the tunnel couplings from the few to the many electron regime, with electrically tunable tunnel couplings from <1 μeV to >600 μeV, and a transition from the temperature to the lifetime broadened regime. The InP segments form tunnel barriers with almost fully symmetric tunnel couplings and a barrier height of ∼350 meV. All of these findings can be understood in great detail based on the deterministic material composition and geometry. Our results demonstrate that integrated InAs/InP QDs provide a promising platform for electron tunneling spectroscopy in InAs NWs, which can readily be contacted by a variety of superconducting materials to investigate subgap states in proximitized NW regions, or be used to characterize thermoelectric nanoscale devices in the quantum regime.

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.

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.

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 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.

We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 microm thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.