PhD defense

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    (in french)

    C2N site orsay, salle 44, Orsay Cedex

    Ahmed ALI AHMED

    C2N, , Palaiseau

    PhD defense

  • semin image

    (in french)

    CentraleSupelec, , Gif-sur-Yvette cedex

    Brice DAVIER

    C2N, , Palaiseau

    PhD defense

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    Piezoelectric nitride nanowires for energy harvesting and piezosensing

    C2N site Orsay, Salle 44, Orsay Cedex

    Lu LU

    C2N, C2N, Palaiseau

    PhD defense

    Jury members :

    Maria TCHERNYCHEVA , Directeur de recherche, Université Paris-Sud, Directeur de thèse

    François JULIEN, Directeur de Recherche, Université Paris-Sud, CoDirecteur de thèse

    Joel EYMERY, Directeur de Recherche, CEA-INAC, Examinateur

    Olga BOYKO, Maître de Conférences, L’Institut des NanoSciences de Paris (INSP),Examinateur

    Elie LEFEUVRE, Professeur, Université Paris-Sud, Examinateur

    Guilhem LARRIEU, Chargé de Recherche, LAAS, Rapporteur

    Guylaine POULIN-VITTRANT, Chargé de Recherche, Université de Tours, Rapporteur

    Noëlle GOGNEAU, Chargé de Recherche, C2N, Invitée

    Pascal Chretien, ingerieur de recherche, Génie électrique et électronique de Paris(GEEPS)

                      

    Abstract :

    This PhD work focuses on the study of GaN nanowire-based piezogenerating devices. The main objective is to develop novel devices for mechanical-to-electrical energy conversion for energy harvesting and for detection of transient deformations. The active material of the developed devices consists of a polymer-embedded nanowire membranes containing either molecular beam epitaxy (MBE) grown GaN nanowires or metal-organic chemical vapor deposition (MOCVD) grown GaN microwires.

    Three device architectures are explored, namely a piezogenerator with a rigid matrix, with a flexible matrix and a fully flexible device. Two home-made mechanical excitation set-ups are used to characterize the generators. In particular, tapping mode and continuous compression deformations are applied to explore the devices’ electrical performance in a large frequency range (from 1 Hz to 3 kHz). Based on these extensive experimental investigations, a panoramic summary of the generator transient behavior under various deformation conditions are made. A Schottky diode design and different versions of capacitive design for the piezogeneration are compared, and their equivalent electrical circuits are proposed. The piezogenerators’ working mechanisms are further validated by experimental investigations.

    Finally, a process to fabricate fully flexible generators and sensors is developed and these flexible devices are extensively characterized. In particular, a flexible device composed of a matrix of active pixels is demonstrated.

    For the MBE nanowire-based piezogenerators on a rigid substrate, the best recorded average power output density reaches 22.1 mW/cm3. For the MOCVD microwire based flexible generators, the best recorded average power output density attains 16.5 µW/cm3. The flexible devices show a good sensitivity to ambient vibrations and respond stably to finger tapping deformations. An average energy of about 100 pJ can be delivered by the flexible device under one finger tapping gesture.

    Keywords :  piezogenerator, nitride, nanowire, piezoelectric semiconductors

  • semin image

    (in french)

    C2N site Orsay, salle 44, Orsay Cedex

    Eric BERNUCHON

    C2N, , Orsay

    PhD defense

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    Ultrathin and nanowire-based GaAs solar cells

    Institut Photovoltaïque d'Ile-de-France (IPVF), Amphithéatre, Palaiseau

    Hung-Ling CHEN

    C2N, , Palaiseau

    PhD defense

    Jury members :

    Marko TOPIč , Professeur, University of Ljubljana, Rapporteur

    Nicolas CHAUVIN, Chargé de Recherche, Institut des Nanotechnologies de Lyon, Rapporteur

    Stéphane COLLIN, Chargé de Recherche, Centre de Nanosciences et de Nanotechnologies, Directeur de thèse

    Andrea CATTONI, Chargé de Recherche, Centre de Nanosciences et de Nanotechnologies, Examinateur

    Jean-Jacques GREFFET, Professeur, Institut d'Optique Graduate School, Examinateur

    Henri MARIETTE, Directeur de Recherche, Institut Néel, Examinateur

    Oliver HöHN, Researcher Fraunhofer Institute for Solar Energy Systems ISE, Examinateur

     Abstract

    Confining sunlight in a reduced volume of photovoltaic absorber offers new directions for high-efficiency solar cells. This can be achieved using nanophotonic structures for light trapping, or semiconductor nanowires. First, we have designed and fabricated ultrathin (205 nm) GaAs solar cells. Multi-resonant light trapping is achieved with a nanostructured TiO2/Ag back mirror fabricated using nanoimprint lithography, resulting in a high short-circuit current of 24.6 mA/cm². We obtain the record 1 sun efficiency of 19.9%. A detailed loss analysis is carried out and we provide a realistic pathway toward 25% efficiency using only 200 nm-thick GaAs absorber. Second, we investigate the properties of GaAs nanowires grown on Si substrates and we explore their potential as active absorber. High doping is desired in core-shell nanowire solar cells, but the characterization of single nanowires remains challenging. We show that cathodoluminescence (CL) mapping can be used to determine both n-type and p-type doping levels of GaAs with nanometer scale resolution. n-type III-V semiconductor shows characteristic blueshift emission due to the conduction band filling, while p-type semiconductor exhibits redshift emission due to the dominant bandgap narrowing. The generalized Planck’s law is used to fit the whole spectra and allows for quantitative doping assessment. We also use CL polarimetry to determine selectively the properties of wurtzite and zincblende phases of single nanowires. Finally, we demonstrate successful GaAs nanowire solar cells. These works open new perspectives for next-generation photovoltaics.

     

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    Nitride nanowire light-emitting diode

    C2N site Orsay, salle 44, Orsay Cedex

    Nan GUAN

    C2N, , Orsay

    PhD defense

    Jury members :

    Dr. Jean-Yves Duboz (rapporteur)

    Dr. Jean-Paul Salvestrini (rapporteur)

    Dr. Bernard Gil (examinateur)

    Dr. Jean-Christophe Harmand (examinateur)

    Dr. Christophe Durand (examinateur)

    Dr. François H. Julien (encadrant)

    Dr. Maria Tchernycheva (co-encadrant)

                         

    Abstract :

    Nitride nanowires exhibit outstanding opto-electronic and mechanical properties and are considered as promising materials for light-emitting diodes (LEDs), thanks to their high crystalline quality, non-polar facets, good mechanical flexibility, high aspect ratio, etc.
    This Ph.D. thesis addresses the growth, the device fabrication, the optical and electrical characterizations and the optical simulations of III-nitride NW devices, with a special emphasis on the LED applications.
    First, this thesis presents the growth of m-plane InGaN/GaN quantum wells with different In concentrations in self-assembled core-shell nanowires by metal-organic chemical vapor deposition. Then, by using these nanowires, LED devices based on two different integration strategies (namely, in-plane and vertical integration) are demonstrated.
    The in-plane integration is based on the horizontally dispersed single nanowires. I have proposed a basic integrated photonic platform consisting of a nanowire LED, an optimized waveguide and a nanowire photodetector. I have also developed a nanowire alignment system using dielectrophoresis.
    The vertical integration targets the fabrication of flexible LEDs based on vertical nanowire arrays embedded in polymer membranes. Flexible monochromatic, bi-color, white LEDs have been demonstrated. Their thermal properties have been analyzed.
    The nanowires grown on 2D materials by van der Waals epitaxy are easy to be lifted-off from their native substrate, which should facilitate the fabrication of flexible nanowire devices. With this motivation, in the last part of this thesis, I have investigated the selective area growth of GaN NWs on micro- and nano- scale graphene by molecular beam epitaxy.

  • semin image

    (in french)

    C2N site Orsay, salle 44, Orsay Cedex

    Ivens SABER

    C2N, ,

    PhD defense

  • semin image

    (in french)

    C2N Site Orsay, Salle 44, Orsay Cedex

    Bogdan VYSOTSKYI

    C2N, C2N, Orsay

    PhD defense

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    Growth of InGaN nanowires for photovoltaic and piezoelectric energy harvesting

    C2N Site Orsay, Salle 44, Orsay Cedex

    Martina MORASSI

    C2N, C2N, Palaiseau

    PhD defense

    Membres du jury:

    Lutz Geelhaar PDI, Berlin
    Eva Monroy, CEA Grenoble
    Mathiey Kociak, LPS Orsay
    Silvia Rubini, IOM-CNR Trieste
    Maria Tchernycheva, C2N (supervisor)
    Noelle Gogneau, C2N (co-supervisor)

    Résumé:

    Les matériaux III-nitrures sont des semi-conducteurs à bande interdite directe qui présentent plusieurs propriétés intéressantes pour les applications photovoltaïques et piézoélectriques. En même temps, la croissance épitaxiale de ces matériaux sous forme de nanofil (NF) est de tant plus intéressant, car les NFs nitrures binaires et heterostructurés ont une qualité cristalline supérieure comparée aux homologues 2D et massifs. Dans ce contexte, mon travail est axé sur la croissance par MBE assistée par plasma (PA-MBE) de NFs InGaN/GaN et sur leur caractérisation. Trois sujets principaux ont été abordés: l'étude de la croissance d’heterostructures InGaN axiales par PA-MBE, leur caractérisation optique, et l'étude de la croissance sélective de NFs GaN sur graphène transféré. Ces études m’ont permis d’obtenir un control sur le mode de croissance d’heterostructures InGaN dans une large gamme de teneurs d’In (jusqu'à ~ 40%) et de morphologies, d’étudier leur profil de bande dans la direction axiale, utile pour la conception optimale de la structure p-i-n photovoltaïque, et de démontrer pour la première fois, que l’épitaxie sélective de NFs de GaN sur MCG lithographié est une route possible et très prometteuse pour améliorer leur homogénéité. Ainsi, des tests préliminaires ont montré que la capacité de piézo-conversion des NFs GaN peut être améliorée d'environ 35% lors de l'intégration d’une insertion InGaN riche en In dans leur volume.

    Tous ces résultats constituent une étape décisive dans le contrôle et la compréhension des propriétés de ces nanostructures, et donnent des perspectives très encourageantes pour leur intégration dans des nano-générateurs à haute efficacité

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    Opto-phononic confinement in GaAs/AlAs based resonators

    C2N site Orsay, Amphithéatre, Orsay Cedex

    Fabrice-Roland LAMBERTI

    C2N, C2N, Marcoussis

    PhD defense

    Jury members :

    Rapporteur     M. Pascal Ruello, laboratoire IMMM, Université du Maine, France

    Rapporteur     M. Bruno Gayral, Institut Néel, CEA Grenoble, France

    Examinatrice  Mme. Sarah Benchabane, Femto-ST, France

    Examinateur   M. Alexey Scherbakov, TU Dortmund

    Examinatrice   Mme Angela Vasanelli, laboratoire MPQ, Université Paris Diderot

    Directeur de thèse       M. Paul Voisin, C2N

    Encadrant de thèse   M. Daniel Lanzillotti Kimura, C2N

     

    Abstract :

    Nanophononics is a research field addressing the control and the manipulation of high frequency mechanical vibrations at the nanoscale. Current fabrication techniques enable the realization of nanophononic systems where acoustic phonons interact with confined optical fields, with exciting perspectives for example in the context of high frequency cavity optomechanics.The work carried out in this thesis addresses the conception and the experimental characterization of novel opto-phononic resonators. We will first present a novel confinement method for high frequency mechanical vibrations, based on the adiabatic localization of longitudinal acoustic phonons. We will then present the three-dimensional confinement of light and hypersound in micropillar optomechanical platforms operating at unprecedently high mechanical frequencies (20 GHz). This theoretical study was carried out through finite element simulations and demonstrates the potential of these systems for future high frequency cavity optomechanics experiments. Finally, we will present our experimental work on the measurement of confined high frequency phonons in micropillar systems through Raman scattering spectroscopy. Based on these results I will discuss some future perspectives.

    Keywords : Nanophononics, Raman scattering spectroscopy, Optomechanics, superlattices, adiabatic cavity, micropillars.