Engineering the properties of semiconductors by changing their crystalline phase is a technologically and economically relevant alternative to doping using foreign elements, with strong potential for photonic and electronic applications. Although major advances have been reported recently for crystal-phase engineering of III-V and group IV semiconductor nanowires, interfacing two mismatched crystalline phases in a nanostructure induces several deformation mechanisms, which remain largely unexplored. Here, using state-of-the-art synchrotron X-ray nanobeam diffraction and transmission electron microscopy, subtle twisting and bending is unveiled within an individual GaAs nanowire containing cubic and hexagonal segments. Their role is discussed in accommodating the inter-reticular spacing fluctuations, and their variations are correlated to the nanoscale phase distribution and to the effect of the NW support. This study brings direct evidence of a complex combination of deformation mechanisms in biphasic nanowires, which opens a new path to tune the nanowire properties with appealing perspectives for device engineering in nanophotonics and nanomechanics.

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production.

Ultralong GaAs nanowires were grown by molecular beam epitaxy using the vapor-liquid-solid method. In this ultralong regime we show the existence of two features concerning the growth kinetic and the structural properties. Firstly, we observed a non-classical growth mode, where the axial growth rate is attenuated. Secondly, we observed structural defects at the surface of Wurtzite segments located at the bottom part of the nanowires. We explain these two phenomena as arising from a particular pathway of the group V species, specific to ultralong nanowires. Finally, the optical properties of such ultralong nanowires are studied by photoluminescence experiments.

In this paper we compute and compare the surface energy of various Ga liquid droplets wetting a cylindrical cavity in various configurations. While for some of these configurations the surface energy can be computed explicitely for others numerical computation is needed. Motivated by the results obtained for the cylindrical cavities we explore the case of the more realistic situation, conical cavities. Our results provide a relation between the geometry of the conical cavity and the equilibirum wetting angles of the droplet on the bottom and on the sidewall of the cavity which insure the dewetting of the lateral surface. This is an important result toward the control of the verticality during the nanowire growth by the vapor liquid solid method.

InGaAs lattice matched to InP is a promising material for bottom sub-cell in a 4-junction solar cell designed for concentrated photovoltaics applications. Here we compare the performances of two structures that could replace standard monolithic InGaAs homojuntion. The first one is a stand-alone solar cell realized via epitaxial lift-off (ELO) process on a flexible substrate. The second one is a heterojunction solar cell, kept on its parent InP substrate, composed of an InP emitter and an InGaAs absorber. A third structure made of an homojunction InGaAs solar cell on an InP substrate is used as reference. Under one sun illumination the heterojunction solar cell shows the highest VOC (383 mV) and fill factor. Nevertheless, when performing under concentrated sunlight the structure is limited by a lower VOC increase rate and a high series resistance compared to the ELO cell. Indeed, ELO cell shows a lower VOC (353 mV) than the two other structures under one sun illumination but, when performing under concentration, ELO cell recovers VOC and shows a lower impact of series resistance. Therefore, both ELO and heterojunction solar cell show interesting and complementary behaviors that could be interesting to associate in an ELO-heterojunction solar cell.

The emergence of semiconductor nanowires (NWs) as a new class of functional materials has generated a great interest in the scientific community in the fields of electronics, photonics and energy. In this work, we report on the optical properties of telecom-band emitting InAs/InP quantum rod-nanowires (QR-NWs) grown on silicon substrates by gold catalyst assisted molecular beam epitaxy (MBE). The energies of A and B band transitions in wurtzite InAs QRs are numerically evaluated by finite element method (FEM) as a function of the QR geometry and strain and compared with the experimental results obtained from photoluminescence (PL). Temperature-dependent optical properties of the QR-NWs are studied revealing that the integrated PL intensity keeps up to 30% of its value at 14 K which testify a high stability of the PL intensity. Furthermore, the investigated nanostructure shows a room temperature emission wavelength at 1.55 mu m. These results demonstrate a great promise for telecom-band III-V nanoemitters monolithically grown on silicon.

Nanowire (NW)-based opto-electronic devices require certain engineering in the NW geometry to realize polarized-dependent light sources and photodetectors. We present a growth procedure to produce InAs/InP quantum dot-nanowires (QD-NWs) with an elongated top-view cross-section relying on the vapor-liquid-solid method using molecular beam epitaxy. By interrupting the rotation of the sample during the radial growth sequence of the InP shell, hexagonal asymmetric (HA) NWs with long/short cross-section axes were obtained instead of the usual symmetrical shape. Polarization-resolved photoluminescence measurements have revealed a significant influence of the asymmetric shaped NWs on the InAs QD emission polarization with the photons being mainly polarized parallel to the NW long cross-section axis. A degree of linear polarization (DLP) up to 91% is obtained, being at the state of the art for the reported DLP values from QD-NWs. More importantly, the growth protocol herein is fully compatible with the current applications of HA NWs covering a wide range of devices such as polarized light emitting diodes and photodetectors.

Nanowire heterostructures of the oxide (shell)-semiconducting (core) type are of interest for various applications in energy harvesting, such as electrodes for photocatalysis and in sensors. Their complete synthesis often requires the deposition of the shell and the core in two separate reactors, with the risk of exposing the core to oxidation from atmospheric conditions during transfer. Here, we study the desorption mechanisms and protection efficiency of an arsenic shell, which was purposely deposited on the GaAs core for protection against undesirable oxidation. Using in situ heating in transmission electron microscopy and synchrotron radiation scanning photoelectron microscopy, we explore the morphology, structure, and surface chemistry of GaAs nanowires capped with an arsenic shell, from room temperature to 500 degrees C. A phase transformation from amorphous to polycrystalline arsenic is evidenced at a temperature of about 300 degrees C, alongside the disappearance of the surface oxidation observed at room temperature. At higher temperatures, the arsenic shell desorbs with an activation energy of 2.32 eV, leading to clean facets. These results are helpful to determine pathways toward improving the efficiency of oxidation-protective layers on III-V semiconducting nanostructures.

In this work we show that the incidence angle of group-III element fluxes plays a significant role in the diffusion-controlled growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE). We present a thorough experimental study on the self-assisted growth of GaAs NWs by using a MBE reactor equipped with two Ga cells located at different incidence angles with respect to the surface normal of the substrate, so as to ascertain the impact of such a parameter on the NW growth kinetics. The as-obtained results show a dramatic influence of the Ga flux incidence angle on the NW length and diameter, as well as on the shape and size of the Ga droplets acting as catalysts. In order to interpret the results we developed a semi-empirical analytical model inspired by those already developed for MBE-grown Au-catalyzed GaAs NWs. Numerical simulations performed with the model allow us to reproduce thoroughly the experimental results (in terms of NW length and diameter and of droplet size and wetting angle), putting in evidence that under formally the same experimental conditions the incidence angle of the Ga flux is a key parameter which can drastically affect the growth kinetics of the NWs grown by MBE.

Realizing single photon sources emitting in the telecom band on silicon substrates is essential to reach complementary-metal-oxide-semiconductor (CMOS) compatible devices that secure communications over long distances. In this work, we propose the monolithic growth of needlelike tapered InAs/InP quantum dot-nanowires (QD-NWs) on silicon substrates with a small taper angle and a nanowire diameter tailored to support a single mode waveguide. Such a NW geometry is obtained by a controlled balance over axial and radial growths during the gold-catalyzed growth of the NWs by molecular beam epitaxy. This allows us to investigate the impact of the taper angle on the emission properties of a single InAs/InP QD-NW. At room temperature, a Gaussian far-field emission profile in the telecom O-band with a beam divergence angle θ = 30° is demonstrated using a single InAs QD embedded in a 2° tapered InP NW. Moreover, single photon emission is observed at cryogenic temperature for an off-resonant excitation and the best result, g2(0) = 0.05, is obtained for a 7° tapered NW. This all-encompassing study paves the way for the monolithic growth on silicon of an efficient single photon source in the telecom band based on InAs/InP QD-NWs.

High-quality pseudomorphic InP/In x Ga 1~x As/InP (x"0.85, 0.90) quantum wells have been grown by solid source molecular beam epitaxy using As and P valved cracking cells under standard growth conditions. Photoluminescence measurements at 77 and 300 K were used to characterize the optoelectronic quality of the "lms and the maximum operating wavelength. Promoting a 3D growth mode allows to extend the operating wavelength at 300 K up to &2.3 lm. ( 1999 Elsevier Science B.V. All rights reserved.

The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects. In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.

One obstacle for the development of nanowire (NW) solar cells is the challenge to assess and control their nanoscale electrical properties. In this work a top-cell made of p-n GaAs core/shell NWs grown on a Si(111) substrate by Molecular Beam Epitaxy (MBE) is investigated by high resolution charge collection microscopy. Electron Beam Induced Current (EBIC) analyses of single NWs have validated the formation of a homogeneous radial p-n junction over the entire length of the NWs. The radial geometry leads to an increase of the junction area by 38 times with respect to the NW footprint. The interface between the NWs and the Si(111) substrate does not show any electrical loss, which would have led to a decrease of the EBIC signal. Single NW I-V characteristics present a diodic behavior. A model of the radial junction single NW is proposed and the electrical parameters are estimated by numerical fitting of the I-Vs and of the EBIC map. Solar cells based on NW arrays were fabricated and analyzed by EBIC microscopy, which evidenced the presence of a Schottky barrier at the NW/ITO top contact. Improvement of the top contact quality is achieved by thermal annealing at 400 °C, which strongly reduces the parasitic Schottky barrier.

Photovoltaics is known as one of the most important renewable energy sources converting solar energy directly into electricity. Benefiting from abundancy, cheapness and environmental sustainability, Si has become an indispensable material in this domain. Since the maximum theoretical efficiency of a single junction Si solar cell is only 29.4% [1], 2-terminal Si based tandem cells are studied to overcome this value by using III-V semiconductors as top cell, so as to achieve optimal bandgap matching between sub-cells [2]. The biggest challenge for III-V films on Si consists in the lattice mismatch, a problem which can be solved by replacing thin films with nanowires (NWs), where mismatch-induced strains can be relaxed on the sidewalls thanks to the small lateral dimension and high surface area of NWs. In our project, a top cell consisting of an ordered array of core-shell GaAs/GaAlAs NWs is fabricated by self-catalyzed vapor-liquid-solid (VLS) molecular beam epitaxy (MBE) on a bottom cell of crystalline Si (Figure 1). Considering the epitaxial nature of the NW growth, it is necessary to employ a Si (111) bottom cell as substrate to ensure a high vertical NW yield [3]. Moreover, a low-resistance p/n type tunnel junction must be implemented on the top of the Si (111) bottom cell to electrically connect the two sub-units.

Excitation photoluminescence spectroscopy is usually used to extract the crystal field splitting (ΔCR) and spin orbit coupling (ΔSO) parameters of wurtzite (Wz) InP nanowires (NWs). However, the equations expressing the valence band splitting are symmetric with respect to these two parameters, and a choice ΔCR > ΔSO or ΔCR < ΔSO has to be taken into account in order to assign the numerical values. To solve this issue, polarization resolved micro-photoluminescence was performed on vertically aligned and untapered Wz InP NWs grown on silicon. The experimental results combined with a theoretical model and finite difference time domain calculations allow us to conclude that ΔCR > ΔSO in Wz InP.

Herein, we describe a new technique that allows high-sensitivity near-field imaging spectroscopic analysis of individual quantum constituents in semiconductors. This method employs an optical mask composed of a phase-change material (PCM) and operates at optical telecommunication wavelengths. Superior collection efficiency and spatial resolution are achieved by using an amorphous nanoaperture as a result of the extreme optical contrast between the PCM in amorphous and crystalline phases at visible wavelengths and the good near-infrared transparency of this material. Fine tuning of quantum dot (QD) emission levels via localized strain as a result of the increase in volume of the PCM upon amorphization has also been demonstrated. Both red and blue shifts of the energy levels were predicted to occur beneath the flat and edge regions of the amorphous mask, respectively, using finite element simulations. The viability of localized strain tuning as an approach to nanospectroscopy employing phase changes was confirmed by measurements of the photoluminescence of individual InAs/InP QDs. In addition, the emission levels of two neighboring QDs were matched based on modifying the shift magnitudes and directions via careful adjustment of the indenter size and position.

The nucleation and the structural and optical properties of InP nanowires (NWs) grown on Si(111) by molecular beam epitaxy using the vapor-liquid-solid method with gold-indium droplets as catalyst are investigated as a function of the temperature of the formation of the catalyst droplets and of the NW growth time. It is highlighted a complex behavior of the gold-indium catalyst droplets depending on the temperature. It is then shown than an InP pyramid-like pedestal is formed prior to the NW growth. When the temperature of formation of the catalyst droplets is lower than 550°C, almost only vertically standing pure wurtzite InP NWs are grown on Si(111).