This study is devoted to the confirmation of spontaneous doping of GaN nanowires grown on vicinal SiC/Si hybrid substrates by electron beam induced current mapping. GaN nanowires (NWs) were grown on singular and vicinal SiC/Si substrates by molecular beam epitaxy with nitrogen plasma activation. The morphological properties of the NWs were studied by scanning electron microscopy. The electrophysical properties of the obtained nanostructures were studied by electron beam induced current mapping. By electron beam induced current mapping, we confirmed the spontaneous doping of the GaN NWs grown on vicinal SiC/Si wafers. It was also shown that the GaN NWs grown on singular SiC/Si substrates did not exhibit an induced current signal, indicating that they were not doped

Данная работа посвящена подтверждению спонтанного легирования GaN нитевидных нанокристаллов, выращенных на вицинальных гибридных подложках SiC/Si, методом картирования тока, наведенного электронным пучком. Нитевидные нанокристаллы (ННК) GaN выращивались на сингулярных и вицинальных подложках SiC/Si методом молекулярно-пучковой эпитаксии с плазменной активацией азота. Морфологические свойства нитевидных нанокристаллов исследуются методами растровой электронной микроскопии. Электрофизические свойства выращенных наноструктур исследуются методом картирования тока, наведенного электронным пучком. Методом картирования тока, наведенного электронным пучком, нами было подтверждено спонтанное легирование GaN ННК, выращенных на вицинальных пластинах SiC/Si. В свою очередь, было показано, что выращенные на сингулярных подложках SiC/Si GaN ННК не демонстрирует сигнала наведённого тока, что указывает на отсутствие легирования в таком ННК

The electrical properties of passivated and non-passivated axial p–i–n junctions in GaAsP nanowires are investigated using electron-beam induced current microscopy. Organized self-catalyzed p–i–n nanowires having different segment lengths are grown by molecular-beam epitaxy on nanopatterned Si substrates. The nanowires are in situ passivated with a GaP shell. The position of the junction is found to be dependent on the length of the upper Be-doped segment evidencing the diffusion of Be atoms from the upper segment to the bottom part of the nanowire. Comparison between non-passivated and passivated nanowires shows a strong enhancement of the collection region after passivation. The results also prove the existence of a p-doped shell around the nanowires formed due to a parasitic radial growth. This shell is depleted in non-passivated nanowires; however, it becomes electrically active after surface passivation modifying the carrier collection pattern.

We analyse the electrical and optical properties of single GaN nanowire p–n junctions grown by plasma‐assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p–n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p–n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 1018 cm−3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

We report the characterization of GaAsP nanowires in view of their photovoltaic applications. The nanowires containing a p-i-n junction were elaborated by molecular beam epitaxy using Be and Si as p- and n- doping impurities. Electron beam induced current microscopy was used to analyze the electrical activity of single nanowires and of nanowire arrays. Nanowire array devices were fabricated by encapsulating the nanowires in a transparent polymer and contacting them with indium tin oxide electrode. Electron beam induced current microscopy in top view configuration is used to probe the electrical activity and homogeneity of the device. A Schottky barrier at the nanowire/ITO interface induced wireto-wire inhomogeneity.

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.

In-rich InGaN/GaN nanowires (NWs) are key optoelectronic materials, which can close the green gap of the light emitting diodes and can be used in efficient high-bandgap solar cells for integration in tandem devices. Realization of these devices requires as a first step the optimization of the NW structure and their electrical parameters. Electron Beam Induced Current (EBIC) microscopy is well suited to probe nanoscale devices with a high resolution and to extract the material parameters. Here, we analyze the electrical properties of axial GaN and InGaN/GaN n-p and p-n junction NWs using EBIC microscopy. III-N NWs were grown on Si(111) substrates by molecular beam epitaxy using Mg as a p-dopant and Si as an n-dopant. The growth conditions were adjusted to optimize the doping order with an abrupt axial junction without a parasitic radial overgrowth. From the EBIC analysis of the GaN p-n junctions, the doping level and the minorities carrier diffusion lengths were extracted. Next, a p-GaN/i-InGaN/n-GaN junction containing an In-rich InGaN segment [1] was grown yielding a flat and strong EBIC signal in the InGaN NW portion. NW arrays were then contacted and their behavior under visible light was analyzed. [1] Morassi et al., Cryst. Growth Des., 2545, 18 2018

Axial p–n and p–i–n junctions in GaAs0.7P0.3 nanowires are demonstrated and analyzed using electron beam induced current microscopy. Organized self-catalyzed 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 analyzing 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-catalyzed 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.

We report on the structural and optical properties of GaAs0.7P0.3/GaP core-shell nanowires (NWs) for future photovoltaic applications. The NWs are grown by self-catalyzed molecular beam epitaxy. Scanning transmission electron microscopy (STEM) analyses demonstrate that the GaAsP NW core develops an inverse-tapered shape with a formation of an unintentional GaAsP shell having a lower P content. Without surface passivation, this unintentional shell produces no luminescence because of strong surface recombination. However, passivation of the surface with a GaP shell leads to the appearance of a secondary peak in the luminescence spectrum arising from this unintentional shell. The attribution of the luminescence peaks is confirmed by correlated cathodoluminescence and STEM analyses of the same NW.

In this work, we fabricate and characterize piezogenerators based on GaN nanowire (NW) arrays. We integrate GaN NWs grown by either Plasma Assisted Molecular Beam Epitaxy (PA-MBE) or Metal Organic Chemical Vapor Deposition (MOCVD) techniques into a polymeric matrix to explore piezogeneration of rigid and flexible devices. Both types of devices show high sensitivity to external forces and mechanical robustness. With an enhanced mechanic-electrical conversion efficiency, these devices are good candidates for energy harvesting and force sensing applications.

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.

Recently semiconductor nanowires (NWs) have emerged as promising materials for solar cells. Thanks to the strain relaxation by the free lateral surface, III-V NWs can be grown on lattice-mismatched substrates without forming dislocations. This is particularly advantageous for III-V on Si tandem solar cells. In addition, NW arrays have very attractive optical properties such as a small optical reflectance and enhanced light diffusion, leading to an increased absorption in comparison to thin films. Today, NW solar cells have been demonstrated with a record efficiency of 15.3% for GaAs bottom-up NWs [1] and 17.8% for InP top-down NWs [2]. However, their efficiency remains below the typical values for planar devices. To understand the origin of this limitation and to boost the efficiency, it is necessary to characterize the electrical properties of NW-based solar cells down to the nanoscale level. This is a challenging task since the standard macroscopic characterization tools can only provide average parameters. Electron Beam Induced Current (EBIC) microscopy has been widely used to characterize optoelectronic devices, and in particular solar cells, for more than thirty years. In 2D solar cells, EBIC was used to probe the electrical activity of the device with a high resolution, to extract material parameters and to detect failures induced by material defects. This technique offers a nanoscale resolution and therefore is ideally suited for nanostructured solar cell characterization.

Malgré le potentiel des nanofils (NFs) semi-conducteurs pour des applications photovoltaïques, la performance des cellules solaires à NFs reste toujours en deçà de celle des dispositifs à base de couches bidimensionnelles. Pour augmenter l’efficacité de conversion, l’analyse de leurs propriétés jusqu’à l’échelle nanométrique est nécessaire afin de comprendre l’origine des pertes de conversion et trouver des solutions adéquates pour les éliminer. Aujourd’hui avec l’arrivée massive des nanotechnologies il est devenu possible de caractériser des nanofils uniques avec une résolution nanométrique. Dans ce travail de thèse nous analysons des nanofils de semiconducteur III/V avec la microscopie EBIC dans la perspective d’extraire puis d’optimiser leurs paramètres électriques et ainsi améliorer le rendement photovoltaïque. Tout d’abord, des NFs de GaAsP élaborés par épitaxie par jets moléculaires (EJM) ont été caractérisés par des mesures EBIC. Sur des nanofils individuels, mais aussi sur des ensembles de NFs. Les études EBIC nous ont permis tout d’abord de déterminer le type de dopage dans les NFs ainsi que de quantifier la concentration des électrons et des trous. En second lieu, la diffusion des atomes de Be et l’existence d’une coquille parasite autour du coeur des NFs ont été mis en évidence. La seconde partie du travail a été consacré à l’étude de NFs de nitrures crus par EJM assisté par plasma. Les NFs de GaN et d’InGaN ont été étudiés par EBIC, photo- et cathodo-luminescence. La corrélation entre les résultats de ces trois mesures a permis d’extraire la concentration du dopage Mg et Si dans les NFs. L’analyse des NFs de GaN/InGaN a montré que ces nanostructures sont prometteuses pour des applications photovoltaïques.