Search

The integration of transition-metal dichalcogenides (TMDs) with non-planar substrates such as nanopillars provides a way to spatially modify the optical properties mainly through the localized strain. Similar studies to date have utilized insulating SiO 2 nanopillars. Here, we combine monolayer MoS 2 with free standing GaAs nanowires (NWs), in views of coupling their semiconducting properties. We find that monolayer MoS 2 exhibits three different configurations: pierced, wrapped and tent-like. We demonstrate how to identify the configurations by optical microscopy and elucidate the impact on the vibrational and luminescence characteristics by confocal spectroscopy mapping. In particular, we highlight the increase of intensity and shift due to the photonic properties of nanowires and increase in dielectric screening associated with the GaAs NW. This work signifies the first step towards the use of vertical III-V NW arrays as a versatile platform for spatially engineering the optical properties of TMDs.

Zinc phosphide (Zn3P2) has been lauded as a promising solar absorber material due to its functional properties and the abundance of zinc and phosphorous. In the last 4 decades, there has not been any significant improvement in the efficiencies of Zn3P2-based solar cells. This is vastly due to the limited understanding of how to tune its optoelectronic properties. Recently, significant progress has been made in the growth and charac-terization of the material, which has shed light on its potential. In this study, we report an energy conversion efficiency as high as 4.4% for a solar cell based on a polycrystalline Zn3P2/InP heterojunction. This device presents an open circuit voltage of 528.8 mV, 7.5% higher than the Zn3P2 homojunction record. We investigate the dominant recombination mechanisms within the film using different techniques to identify the key factors underpinning our device efficiency. In particular, we pinpoint that reduced carrier collection at the front of Zn3P2 is responsible for the reduced collection of high-energy photons. This allows us to suggest the design rules for next-generation Zn3P2-based heterojunction solar cells, which should allow us to go beyond the current con-version values.

Selective area epitaxy (SAE) provides the path for scalable fabrication of semiconductor nanostructures in a device-compatible configuration. In the current paradigm, SAE is understood as localized epitaxy, and is modelled by combining planar and self-assembled nanowire growth mechanisms. Here we use GaAs SAE as a model system to provide a different perspective. First, we provide evidence of the significant impact of the annealing stage in the calculation of the growth rates. Then, by elucidating the effect of geometrical constraints on the growth of the semiconductor crystal, we demonstrate the role of adatom desorption and resorption beyond the direct-impingement and diffusion-limited regime. Our theoretical model explains the effect of these constraints on the growth, and in particular why the SAE growth rate is highly sensitive to the pattern geometry. Finally, the disagreement of the model at the largest pitch points to non-negligible multiple adatom recycling between patterned features. Overall, our findings point out the importance of considering adatom diffusion, adsorption and desorption dynamics in designing the SAE pattern to create pre-determined nanoscale structures across a wafer. These results are fundamental for the SAE process to become viable in the semiconductor industry.

Strain is a commonly used tool to tune the optoelectronic properties of semiconductors. It is especially effective for transition metal dichalcogenides (TMDs), which can withstand extreme strain up to 10%. To date, localised strain fields have been applied by transferring TMDs flakes onto a patterned SiO2 substrate. Here, we present a novel approach, where the strain in MoS2 monolayer is induced by an array of homoepitaxially grown GaAs nanomembranes. This represents a first step towards the integration of TMD monolayers with III–V semiconductor nanostructures, which is essential to develop scalable nanophotonic platforms. The strain imposed by the nanomembrane lifts the degeneracy of the exciton states, leading to linearly polarised emission. The principal axis of the linear polarisation of the emission is strictly determined by the orientation of the nanomembranes. This result is fully consistent with the expected broken crystal symmetry resulting from the imposed uniaxial strain.

Selective area epitaxy constitutes a mainstream method to obtain reproducible nanomaterials. As a counterpart, self-assembly allows their growth without costly substrate preparation, with the drawback of uncontrolled positioning. We propose a mixed approach in which self-assembly is limited to reduced regions on a patterned silicon substrate. While nanowires grow with a wide distribution of diameters, we note a mostly binary occurrence of crystal phases. Self-catalyzed GaAs nanowires form in either a wurtzite or zincblende phase in the same growth run. Quite surprisingly, thicker nanowires are wurtzite and thinner nanowires are zincblende, while the common view predicts the reverse trend. We relate this phenomenon to the influx of Ga adatoms by surface diffusion, which results in different contact angles of Ga droplets. We demonstrate the wurtzite phase of thick GaAs NWs up to 200 nm in diameter in the Au-free approach, which has not been achieved so far to our knowledge.

International audience; Nowadays the realization of market-competitive devices based on nanomaterials is a major challenge. Optimization of the device performance requires deep understanding of the physical phenomena at the nanoscale. In this context, electron beambased techniques become indispensable. Several instruments have been developed in the past decades to provide versatile diagnostic solutions for improving materials, designs and device fabrication. These characterization techniques applied to nanostructured semiconductors can help filling the gap between material science and engineering by bringing in light important physical parameters. In this Chapter, the family of electron beam-based techniques is briefly introduced. First, the electron beam/matter interaction is described both in physical and operational terms. In particular, different phenomena occurring when a flux of electron collides with a semiconductor material are discussed. Then, two main electron beam scanning techniques are discussed in the following sections: electron beam induced current microscopy and cathodoluminescence. After a short description of the fundamentals, for each technique a bibliographic review is presented to illustrate its applications to analyses of semiconductor nanowires.

Organometallic perovskites are one of the most investigated materials for high-efficiency thin-film devices to convert solar energy and supply energy. In particular, methylammonium lead iodide has been used to realize thin-film perovskite solar cells, achieving an efficiency higher than 20%. Different fabrication procedures based on the spin-coating technique have been proposed, which do not ensure homogenous morphologies. In this work, we present a scalable process to fabricate methylammonium lead iodide thin films directly on conductive substrates, consisting of electrodeposition and two subsequent chemical conversions. A thorough investigation of the morphological, structural and compositional properties of the layer is performed after each fabrication step. It is demonstrated that this method allows fine control of the thickness of the layer by tuning the cell parameters during the electrodeposition step. X-ray diffraction patterns and energy-dispersive X-ray analysis indicate the achievement of high-purity methylammonium lead iodide layers. Micro-Raman analyses were used to demonstrate the formation of methylammonium lead iodide. Finally, ultraviolet-visible absorption spectra were acquired to determine the optical band edge of the layer ( 1.56 eV) and the absorbance of methylammonium lead iodide as a function of the film thickness. As expected, the material exploits excellent optical properties, achieving an absorption ≥ 99.9% in the entire visible range for a layer thickness of 1.3 µm. The results presented here pave the way for the application of cost-friendly solution-based processes to fabricate high-quality perovskite solar cells.

Large-scale deployment of thin-film photovoltaics will be facilitated through earth-abundant components. Herein, selective area epitaxy and lateral overgrowth epitaxy are explored for the growth of zinc phosphide (Zn3P2), a promising earth-abundant absorber. The ideal growth conditions are elucidated, and the nucleation of single-crystal nanopyramids that subsequently evolve towards coalesced thin-films is demonstrated. The zinc phosphide pyramids exhibit room temperature bandgap luminescence at 1.53 eV, indicating a high-quality material. The electrical properties of zinc phosphide and the junction with the substrate are assessed by conductive atomic force microscopy on n-type, p-type and intrinsic substrates. The measurements are consistent with the p-type characteristic of zinc phosphide. Overall, this constitutes a new, and transferrable, approach for the controlled and tunable growth of high-quality zinc phosphide, a step forward in the quest for earth-abundant photovoltaics.

International audience; Axial p-n and p-in junctions in GaAs 0.7 P 0.3 nanowires are demonstrated and analysed using electron beam induced current microscopy. Organized self-catalysed nanowire arrays are grown by molecular beam epitaxy on nanopatterned Si substrates. The nanowires are doped using Be and Si impurities to obtain p-and n-type conductivity, respectively. A method to determine the doping type by analysing the induced current in the vicinity of a Schottky contact is proposed. It is demonstrated that for the applied growth conditions using Ga as a catalyst, Si doping induces an n-type conductivity contrary to the GaAs self-catalysed nanowire case, where Si was reported to yield a p-type doping. Active axial nanowire p-n junctions having a homogeneous composition along the axis are synthesized and the carrier concentration and minority carrier diffusion lengths are measured. To the best of our knowledge, this is the first report of axial p-n junctions in self-catalyzed GaAsP nanowires.

Both Kelvin Probe Force Microscopy and Scanning Electron Microscopy enable assessment of the effect of electrical bias on the surface potential of the layers of a solar cell. We report on a comprehensive comparison of surface potential measurements on an interdigitated back contact solar cell using these two techniques. Measurements under different values of electrical biases are performed on and between the metallic contacts. They show a good agreement between the surface potential obtained with Kelvin Probe Force Microscopy and the Scanning Electron Microscopy signal. In order to provide an accurate comparison, the scanned areas are adjacent to each other and accurate repositioning is achieved thanks to a nano-indentation between the contacts. We show that measurements under reverse bias are of interest to locate nano-defects and measurements under forward bias are relevant to identify local series resistance issues. We suggest that a setup combining Scanning Electron Microscopy and Kelvin Probe Force Microscopy under different values of the electrical bias should be valuable since the former is a high throughput technique enabling measurements on large scan areas, while the latter is a quantitative, low noise, and unintrusive local technique.

Today, the record in photovoltaic (PV) 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, especially the expensive III-V substrates. As an alternative, a hybrid solar cell was proposed by LaPierre et al.1 The cell geometry, which combines a 2D Si bottom-cell with a nanowire (NW) top-cell in a tandem device, presents a theoretical efficiency record of 34% when the top-cell band gap lies around 1.7 eV[1],[2]. In this work, we report the elaboration, nanoscale characterization and device fabrication of solar cells based on axial junction GaAsP NWs. Organized GaAsP NWs were grown on patterned SiO2/Si(111) substrates by MBE. Junction was axially created during the growth by incorporating different doping impurities (Be for p- and Si for n-doping). In-situ surface passivation using a radial GaP shell was applied to reduce non-radiative recombinations on surface states[3]. Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases were used to probe the electrical properties and the generation patterns of individual NWs. The doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. Macroscopic devices based on NW arrays were fabricated by dielectric encapsulation and ITO contacting. Top view EBIC analyses were applied to probe the device homogeneity. References [1] R.R. LaPierre et al., J. Appl. Phys. 110 (2011), 014310. [2] S. Bu et al., Appl. Phys. Lett. 102 (2013), 031106. [3] C. Himwas et al., Nanotechnology. 28 (2017), 495707.

L'objectif de cette these est de realiser l'integration monolithique de nanofils (NFs) a base de l’alliage Al0.2Ga0.8As sur des substrats de Si par epitaxie par jets moleculaires via la methode vapeur-liquide-solide (VLS) auto-assistee et de developper une cellule solaire tandem (TSC) a base de ces NFs.Pour atteindre cet objectif, nous avons tout d'abord etudie la croissance de NFs GaAs, etape cle pour le developpement des NFs p-GaAs/p.i.n-Al0.2Ga 0.8As coeur/coquille, qui devraient constituer la cellule superieure de la TSC. Nous avons montre, en particulier, l'influence de l'angle d'incidence du flux de Ga sur la cinetique de croissance des NFs GaAs. Un modele theorique et des simulations numeriques ont ete realisees pour expliquer ces resultats experimentaux.Nous avons ensuite utilise le savoir-faire acquis pour faire croitre des NFs p-GaAs/p.i.n-Al0,2Ga0,8As coeur/coquille sur des substrats de Si prets pour l'emploi. Les caracterisations EBIC realisees sur ces NFs ont montre qu'ils sont des candidats potentiels pour la realisation d’une cellule photovoltaique. Nous avons ensuite fait croitre ces NFs sur des substrats de Si patternes afin d'obtenir des reseaux reguliers de ces NFs. Nous avons developpe un protocole, base sur un pre-traitement thermique, qui permet d'obtenir des rendements eleves de NFs verticaux (80-90 %) sur une surface patternee de 0,9 x 0,9 mm2.Enfin, nous avons consacre une partie de notre travail a definir le procede de fabrication optimal pour la TSC, en concentrant notre attention sur le developpement de la jonction tunnel de la TSC, l'encapsulation des NFs et le contact electrique superieur du reseau de NFs.

International audience; In this work, nanoscale electrical and optical properties of n-GaN nanowires (NWs) containing GaN/AlN multiple quantum discs (MQDs) grown by molecular beam epitaxy (MBE) are investigated by means of single wire I(V) measurements, electron-beam induced current microscopy (EBIC) and cathodoluminescence (CL) analysis. A strong impact of non-intentional AlN and GaN shells on the electrical resistance of individual NWs is put in evidence. The EBIC mappings reveal the presence of two regions with internal electric fields oriented in opposite directions: one in the MQDs region and the other in the adjacent bottom GaN segment. These fields are found to co-exist under zero bias, while under an external bias either one or the other dominates the current collection. In this way EBIC maps allow to locate the current generation within the wire under different bias conditions and to give the first direct evidence of carrier collection from AlN/GaN MQDs. The NWs have been further investigated by photoluminescence (PL) and cathodoluminescence (CL) analyses at low temperature. CL mappings show that the near band edge emission of GaN from the bottom part of the NW is blue-shifted due to the presence of the radial shell. In addition, it is observed that CL intensity drops in the central part of the NWs. Comparing the CL and EBIC maps, this decrease of the luminescence intensity is attributed to an efficient charge splitting effect due to the electric fields in the MQDs region and in the GaN base.

International audience; The performance of core-shell InGaN/GaN nanowire light emitting diodes can be limited by wire-to-wire electrical inhomogeneities. Here we investigate an array of core-shell InGaN/GaN nanowires which are morphologically identical, but present electrical dissimilarities in order to understand how the nanoscale phenomena observed in individual nanowires affect the working performance of the whole array. The light emitting diode shows a low number of nanowires (̴ 20%) producing electroluminescence under operating conditions. This is related to a presence of a potential barrier at the interface between the nanowire core and the radially grown n-doped layer, which differently affects the electrical properties of the nanowires although they are morphologically identical. The impact of the potential barrier on the performance of the nanowire array is investigated by correlating multi-scanning techniques, namely electron beam induced current microscopy, electroluminescence mapping and cathodoluminescence analysis. It is found that the main cause of inhomogeneity in the array is related to a non-optimized charge injection into the active region, which can be overcome by changing the contact architecture so that the electrons become injected directly in the n-doped underlayer. The light emitting diode with so-called "front-n-contacting" is developed leading to an increase of the yield of emitting nanowires from 20% to 65%.

International audience; A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane trans- ferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p−n junction nanowires containing InGaN/ GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

International audience; 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 with 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.

The development of photovoltaics as a serious means of producing renewable energy has accelerated greatly in the last ten years, with prices for silicon-based solar cell systems dropping dramatically in the last few years. The next great opportunity for photovoltaics following this competitiveness in prices will be to enhance the cell and panel efficiencies. It is quite generally seen that the most viable platform on which this should be realized will be as augmented silicon solar cells, in which a top cell will be combined with the silicon bottom cell in a tandem configuration, by which the efficiency can be enhanced by a factor from 20% to 50%, depending on details of the approach. In this paper, we report on the status of one such approach, namely, with a top cell comprising III-V nanowires, connected to the bottom silicon cell in a two-terminal or four-terminal configuration. Among the most important opportunities, we show that a substrate-free growth, called Aerotaxy, offers a radical reduction in the total price picture. Besides the description of the key technical approaches, we also discuss the environmental issues.

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.

Semiconductor nanowires are key materials for next-generation solar cells and light-emitting diodes (LEDs), which can improve the performance and bring new functionalities. However, the performance of nanowire devices today is still below theoretical predictions. Standard macroscopic characterizations averaged over millions of nanowires do not provide all the information necessary to optimize the device efficiency. Nanoscale analyses of individual nanowire properties are required in order to evaluate the material quality, to assess the wire-to-wire homogeneity and to detect eventual electrical or optical failures. In this chapter, nanoscale characterizations of nanowire LEDs and solar cells are reviewed. Electron beam-induced current microscopy, photo- and cathodoluminescence, and atomic probe tomography are applied to single nanowires and to macroscopic nanowire devices to address the compositional homogeneity, to analyze defects, to estimate the doping, to measure the minority carrier diffusion lengths and to assess the nonradiative recombination.

session III-V, Multijonctions, Nanofils; National audience

International audience; We report the analysis of the electrical properties of In x-1 Ga x P nanowires (NWs) grown by template-assisted selective epitaxy (TASE). The individual NW properties are investigated by means of electron beam induced current microscopy (EBIC) and current-voltage curves acquired on single nano-objects. First, a set of samples containing n-doped InGaP NWs grown on a p-doped Si substrate are investigated. The electrical activity of the hetero-junction between the NWs and the substrate is demonstrated and the material parameters are analyzed, namely the n-doping level is determined in relation with the dopant flow used during the growth. These results were used to design and elaborate InGaP NWs containing a p-n homo-junction. The electrical activity of the homo-junction is evidenced using EBIC mapping on single NWs and material parameters (namely, the doping and the minority carrier diffusion lengths) for the p-and n-doped InGaP segments are estimated. Finally, the first proof of a photovoltaic effect from the NW homo-junctions is obtained by photocurrent measurements of a contacted NW array under white light irradiation.

In the present work we investigate compact anodic oxides formed on titanium by a broad range of anodizing parameters (electrolyte, applied voltage, substrate pre-treatment) and their influence on the performance of such oxides in photocurrent response and photocatalytic properties (photodegradation of AO7). In general, crystallinity and ion inclusion from the electrolyte are crucial. As-grown compact layers of partial crystallinity show increased efficiencies in both applications; interestingly, phosphate ions have a detrimental effect, whereas sulfate ions do not. After thermal annealing (crystallization) the photoresponse significantly increases: except for a crystallization of the anodic layer, annealing causes a rutile oxide layer growth at the oxide/metal interface - this is found to additionally affect the photoresponse. Results overall demonstrate that higher anodization voltage, exclusion of phosphates from the electrolyte, substrate pre-treatment and annealing are of major importance for achieving an enhanced photoresponse on compact anodic layers.

III-V semiconductors outperform Si in many optoelectronics applications due to their high carrier mobility, efficient light emission and absorption processes, and the possibility to engineer their band gap through alloying. However, complementing Si technology with III-V semiconductors by integration on Si(100) remains a challenge still today. Vertical nanospades (NSPDs) are quasi-bi-crystal III-V nanostructures that grow on Si(100). Here, we showcase the potential of these structures in optoelectronics application by demonstrating InGaAs heterostructures on GaAs NSPDs that exhibit bright emission in the near-infrared region. Using cathodoluminescence hyperspectral imaging, we are able to study light emission properties at a few nanometers of spatial resolution, well below the optical diffraction limit. We observe a symmetric spatial luminescence splitting throughout the NSPD. We correlate this characteristic to the structure's crystal nature, thus opening new perspectives for dual wavelength light-emitting diode structures. This work paves the path for integrating optically active III-V structures on the Si(100) platform.

Semiconductor nanowires (NWs) have been investigated for decades, but their application into commercial products is still difficult to achieve, with triggering causes related to the fabrication cost and structure complexity. Dopant control at the nanoscale greatly narrows their exploitation as components for device integration. In this context, doping appears the truly last missing piece of the puzzle for III–V NWs, for them to become commercially exploitable. In this paper, we review the doping of bottom up III–V NW arrays grown by molecular beam epitaxy and metal-organic vapor phase epitaxy, aiming to link materials science challenges with the critical aspect of device design. First, the doping methods and mechanisms are described, highlighting the differences between self-assembled and ordered NW arrays. Then, a brief overview of the available tools for investigating the doping is offered to understand the common strategies used for doping characterization. Both aspects are crucial to discuss the recent advancements in reproducibility and up-scalability, which are discussed in view of large area fabrication for industrial production. Finally, the most common doping-related challenges are presented together with the latest solutions to achieve high performing NW-based devices. On this basis, we believe that new insights and innovative findings discussed herein will open the low dimensional materials era, on the premise of multidisciplinary collaborative works of all the sectors involved in the design and optimization of commercial products.

International audience

Single nitride nanowire core/shell n-p photodetectors are fabricated and analyzed. Nanowires consisting of an n-doped GaN stem, a radial InGaN/GaN multiple quantum well system and a p-doped GaN external shell were grown by catalyst-free metal–organic vapour phase epitaxy on sapphire substrates. Single nanowires were dispersed and the core and the shell regions were contacted with a metal and an ITO deposition, respectively, defined using electron beam lithography. The single wire photodiodes present a response in the visible to UV spectral range under zero external bias. The detector operation speed has been analyzed under different bias conditions. Under zero bias, the −3 dB cut-off frequency is ~200 Hz for small light modulations. The current generation was modeled using non-equilibrium Green function formalism, which evidenced the importance of phonon scattering for carrier extraction from the quantum wells.