PhD defense

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    Subwavelength silicon photonic nanostructures for applications in the near-IR and mid-IR

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Thi Thuy Duong DINH

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Philippe DELAYE. Directeur de recherche, Institut d’Optique. (Universite Paris-Saclay) Président

    Gonzalo WANGÜEMERT-PÉREZ. Professor, Málaga University Rapporteur & Examinateur

    Emilien PEYTAVIT. Chargé de recherche, Laboratoire IEMN Rapporteur & Examinateur

    Loïc BODIOU. Maitre de conferences, Universite de Rennes 1 Examinateur

    Joan RAMIREZ Cadre scientifique, Nokia Bell Labs/ III-V lab Examinateur

    Carlos ALONSO RAMOS Chargé de recherche, C2N-Université Paris-Saclay Directeur de thèse

    Abstract :

    Silicon photonics holds the promise for large-scale and low-cost production of highperformance optoelectronic circuits. Driven by the impressive technology development in the recent years, silicon photonics is expanding its frontiers towards new applications beyond datacom, including among others, sensing, radio-over-fiber and quantum. Aiming to meet the requirements of these new applications, the Si photonics community is exploring alternative wavelength ranges and physical phenomena, with a particular interest in the midinfrared (2-20 μm wavelength), and Kerr

    nonlinearities. Silicon photonics holds the promise for large-scale and low-cost production of high-performance optoelectronic circuits. Driven by the impressive technology development in the recent years, silicon photonics is expanding its frontiers towards new applications beyond datacom, including among others, sensing, radio-over-fiber and quantum. Aiming to meet the requirements of these new applications, the Si photonics community is exploring alternative wavelength ranges and physical phenomena, with a particular interest in the mid-infrared (2-20 μm wavelength), and Kerr nonlinearities.

     

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    Understand and overcome the limitations of silicon/amorphous silicon (a-Si:H) nanowire solar cells

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre, Palaiseau

    Tiphaine MATHIEU-PENNOBER

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Anna Fontcuberta-i-Morral, Rapportrice & Examinatrice, Professeure, Ecole Polytechnique Fédérale de Lausanne (EPFL)

    Erwann Fourmond, Rapporteur & Examinateur, Maître de conférences, INSA Lyon

    José Alvarez, Examinateur,Chargé de recherche, Université Paris-Saclay, GEEPS

    Jean-François Guillemoles, Examinateur, Directeur de recherche, Insitut Polytechnique de Paris,  laboratoire IPVF

    Maria Tchernycheva, Directrice de thèse, Directrice de recherche, Université Paris-Saclay, C2N

    Martin Foldyna, Invité, Chargé de recherche, Insitut Polytechnique de Paris, LPICM

    Abstract :

    Among different materials for photovoltaic conversion, hydrogenated amorphous silicon (a-Si:H) displays excellent optical properties and can be used as a thin-film, requiring less material. However, the low mobilities of its charge carriers limit the efficiency in planar solar cells. Core-shell structures can solve this issue. This PhD work focuses on solar cells made of silicon/amorphous silicon (a-Si:H) core-shell nanowires (NWs). For these nanostructures the optimization of the top contact, which has to be both transparent and conductive is a major challenge. We developed a hybrid electrode, made of ITO and silver nanowires, to efficiently collect the charge carriers from every nanowire and over the whole cell (Fig. (a)). Changing the illumination level, we optimized the electrode transparency and conductivity directly on the device. The hybrid electrode increased the power conversion efficiency from 4.3 % to 6.6 % compared to the reference ITO electrode. In addition, we studied the change in performance of Si NW/a-Si:H solar cell under strong illumination (Fig. (b)). Amorphous silicon is generally believed to be suited only for low illumination levels, but there are no detailed studies on this topic, especially in a core-shell NW solar cell architecture. The comparison of I-V measurements of a Si NW/a-Si:H with a planar a-Si:H and a crystalline silicon solar cells evidenced different phenomena, both reversible and irreversible. In particular, it showed that the first limiting factors are not directly related to a-Si:H, but to the top-contact and the substrate.

    Link: https://cnrs.zoom.us/j/96292459096?pwd=Q0JxcGx1MW9BTk9Hcm1KQUkrbE52dz09 

    Passcode: HCqyW4

    Find a link in the the attached file below

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    Development of resonant nanostructures large area device for augmented reality

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Gil CARDOSO

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Cécile GOURGON, Directrice de Recherche, Université Grenoble Alpes, Rapporteur
    Aloyse DEGIRON, Chargé de Recherche, Université de Paris, Rapporteur
    Anne-Marie HAGHIRI-GOSNET, Directrice de recherche, Université Paris-Saclay, Examinatrice
    Daniel TUROVER, Ingénieur, SILSEF, Examinateur
    Serge RAVAINE, Professeur, Université de Bordeaux, Examinateur
    Rose-Marie SAUVAGE, Responsable Innovation, Agence Innovation Défense, Invitée
    Béatrice DAGENS, Directeur de recherche, Université Paris-Saclay, Directrice de thèse
    Frédéric HAMOUDA, Ingénieur de recherche, Université Paris-Saclay, Encadrant technologique

    Abstract :

    Recently, visualisation systems, and in particular augmented reality, have seen increased use in applications such as automotive head-up displays or head-mounted displays such as Google Glass. Due to the cost constraints of industrial applications, technological solutions are needed to improve the efficiency of these devices. In particular, these systems include a large surface blade that must be both transparent in the visible range and reflective at certain wavelengths. It can also perform the function of focusing or defocusing.
    In this context, wavelength-selective reflective metasurfaces are considered good candidates. This type of device has attracted a lot of attention in recent years due to the unique properties that can be obtained but nevertheless, significant improvements are still needed to make it a viable option.

    Link: https://us02web.zoom.us/j/86341050123?pwd=WmVDOHdHYXpoazRwNTNqSmFXVHBvUT09

    Find a link in the the attached file below

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    Non-linear physics associated to chiral symmetry in driven dissipative polariton lattices

    IOGS, Auditorium, Palaiseau

    Nicolas PERNET

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

     

    Mme Jacqueline BLOCH Université Paris-Saclay GS Physique, Directrice de thèse
    M. Christian SCHNEIDER Carl von Ossietzky Universität Oldenburg Institut für  Physik, Rapporteur
    Mme Anna MINGUZZI Laboratoire de Physique et Modélisation des Milieux Condensés, Rapporteure
    M. Iacopo CARUSOTTO INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, Examinateur
    Mme
    Emmanuelle DELEPORTE Ecole Normale Supérieure Paris-Saclay Examinatrice
    M. Sylvain NASCIMBENE Laboratoire Kastler Brossel, Collège de France Examinateur
    M. Sylvain RAVETS Invité

     

    Abstract :

    Microcavity polaritons originate from the strong coupling between cavity photons and electronic excitations of a semiconductor microcavity. These quasi-particles inherit properties from both constituents: the electronic component is responsible for giant Kerr non-linearities while the photonic part makes the system inherently dissipative and allows confining polaritons in micro-structures obtained via etching of the cavity. This thesis presents the study of the non-linear properties of micro-cavity arrays with chiral symmetry.

    The first part of the work is dedicated to one-dimensional topological lattices emulating the Su-Schrieffer-Heeger model. In the non-linear regime, driving the system coherently using a laser leads to the formation of gap solitons at the edges and in the bulk of the structure. We evidenced that such solitons present symmetry properties making them robust against certain types of defects. In addition, we unveiled that a careful engineering of the drive allows observing novel non-linear solutions which are specific to open systems. We analyzed the systems excitations spectrum in presence of such stationnary state and demonstrate the possibility to realize a topological phase transition induce by the interactions.

    In the second part, we explored the properties of the interaction between two gap solitons and showed that the interaction sign is strongly linked to the underlying structure of the lattice. This work showed the presence of a spontaneous symmetry breaking. The study of the systems non-linear response in the vicinity of such phase allows us to discover a novel effect of bistability allowing to induce chirality in the system depending on the drive protocol. We called this effect “helical bistability”. The helical bistability is a very general effect that can be observed in a simple set of two coupled Kerr resonators and is linked to the physics of a particle with spin one half. These results are supported both by numerical simulations and experiments. This thesis illustrates how symmetries and non-linearities enriches the physics of photonic systems in a driven-dissipative context.

    If you plan to attend the defense in person, please fill in the following document: https://docs.google.com/spreadsheets/d/1p3-8EWfpD3NS8eQNAhk4CfH4_3KzzVQ0819CWU8K6Ko/edit?usp=sharing

    Link: https://zoom.us/j/94169025971?pwd=VDJkUkxZZXBMUXl2aCtYN054blErQT09

    Caption: (a) Scanning electron microscopy image showing a chain of semiconductor cavities. The distance between the cavities is modulated, which gives rise to two different values of the coupling J and J’ between adjacent cavities (represented schematically by double white arrows). This modulation of the coupling provides the lattice with topological properties.  Two cavities of the lattice are excited by two lasers of amplitude F, angular frequency w and presenting a phase difference Dj. b) For Dj =1.13p, we have demonstrated a new family of solitons, which measured intensity profile is shown at the top of the figure. The intensity is very strong on one single site, thus leading to the spectral detuning of this site with respect to the rest of the chain. The chain is thus effectively broken, and an edge state appears in the excitation spectrum, as shown at the bottom of the figure.

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    Collage III-V//Si par couches transparentes conductrices pour cellules solaires tandem

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Phuong-linh NGUYEN

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    M. Stéphane COLLIN, Université Paris-Saclay GS Sciences de l’ingénierie et des systèmes,Directeur de thèse

    Mme Anne KAMINSKI-CACHOPO,Grenoble INP-Phelma ,Rapporteure

    M. Yvan CUMINAL, Université de Montpellier, Rapporteur

    Mme Anne TALNEAU, Université Paris-Saclay - C2N - CNRS, Examinatrice

    M. Romain CARIOU, CEA Liten, Examinateur

    M. Andrea CATTONI, Université Paris-Saclay - C2N - CNRS, Co-encadrant de thèse

    M. Philippe BARANEK, EDF R&D – Departement SYSTEME, Invité

    M. Oliver HöHN, Fraunhofer Institute for Solar Energy Systems ISE, Invité

    95% de la production photovoltaïque totale en 2020 est réalisée à partir de silicium. À l'échelle du laboratoire, le rendement record des cellules solaires silicium simple-jonction a atteint 27.6%, proche du rendement limite théorique de 29.4%. Une alternative pour mieux exploiter le spectre solaire consiste à combiner plusieurs semi-conducteurs dans des cellules solaires tandem, avec un rendement théorique de près de 45% pour une cellule solaire tandem optimale à double-jonction à base de Si. Les meilleures performances ont été obtenues par collage direct de III-V sur Si, mais la mise à l’échelle de cette technologie est difficile.

    Dans cette thèse, nous avons conçu et développé une nouvelle méthode de collage, potentiellement peu coûteuse, pour les cellules solaires tandem III-V sur silicium à deux terminaux, basée sur des couches transparentes conductrices (TCL). Cet empilement de TCL est composé de deux revêtements antireflets (ARCs) dérivés de sol-gels et d'une couche à faible indice de réfraction. Les ARCs assurent des contacts ohmiques avec les sous-cellules et la transmission des photons de faible énergie à la cellule de silicium. La couche à faible indice de réfraction assure à la fois l'interconnexion électrique entre les sous-cellules et un meilleur recyclage des photons dans la cellule supérieure (gain d'efficacité pouvant atteindre 0.9% en valeur absolue). L’empilement TCL a été optimisé à l'aide d'outils de simulation optique afin d'obtenir une concordance de courant avec une perte de courant minimale, similaire au cas de l'architecture à collage direct. Le processus de laminage est réalisé dans l'air, à basse température de polymérisation (120°C) à l'aide d'une simple presse hydraulique. Un processus de photolithographie reproductible et robuste à une basse température de 80°C a également été optimisé avec succès. Nous avons fabriqué une première génération de cellule tandem AlGaAs/TCLs/Si ayant une surface de 1 cm2, avec une cellule Si de type TOPCon et une colle à base de PEDOT:PSS, qui montre des résultats prometteurs. Une stratégie de piégeage de la lumière utilisant une couche nanostructurée insérée à l'arrière de la cellule supérieure a également été explorée par des simulations électromagnétiques, en tenant compte des contraintes pratiques. Cette stratégie permet de minimiser l'utilisation de matériaux III-V par un facteur de 2.6.

    Link: https://us02web.zoom.us/j/89732786691?pwd=MGNUbkNCS05vRUNhZEx2Z2dlTDJPUT09

    Find a link in the the attached file below

     

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    (in french) Plateforme microfluidique pour la détection électrochimique multiplexée de microARNs du cancer

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Claire POUJOULY

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Sophie GRIVEAU, Maîtresse de conférences, HDR, ENSCP, Université PSL, Rapporteure

    Gaelle LISSORGUES, Professeure, ESIEE Paris, Université Gustave Eiffel, Rapporteure

    Pierre-Yves JOUBERT, Professeur, C2N, Université Paris-Saclay, Examinateur

    Laurent THOUIN, Directeur de recherche, ENS Paris, CNRS, Examinateur

    Jean GAMBY, Chargé de recherche, C2N, CNRS, Directeur de thèse

    Abstract :

    Le diagnostic précoce de maladies, telles que les cancers, représente un enjeu sociétal majeur. Pour répondre à ce besoin, de nouveaux outils de diagnostic fiables, rapides et miniaturisés doivent être mis au point. Dans ce contexte, les microARNs ont été identifiés comme biomarqueurs clés pour le diagnostic précoce de cancers. Plus précisément, la détection de combinaisons précises de séquences de microARNs assure une fiabilité maximale du diagnostic.

    Le but de mon doctorat était de développer un dispositif microfluidique pour la détection électrochimique multiplexée de plusieurs séquences de microARNs. Ce dispositif composé de deux principaux modules, permet la détection de séquences spécifiques de microARNs, d'une vingtaine de paires de bases, en 30 minutes avec une limite de détection de 10-12 M, sans amplification préalable. Ce niveau de sensibilité et de spécificité est rendu possible grâce à l'intégration d'un capteur électrochimique à deux électrodes dans un dispositif microfluidique, et à l'utilisation d'un intercalant rédox, le bleu de méthylène, dans la solution électrolyte pour un transfert d'électrons longue distance. La géométrie de la puce microfluidique est conçue pour permettre la détection simultanée d'une combinaison de microARNs, composée de huit séquences différentes. En amont, un module microfluidique est adapté pour la dénaturation de doubles brins d'acides nucléiques, préalablement greffés sur des nanoparticules, par hyperthermie magnétique. Ce module permet la pré-concentration de séquences cibles de microARNs avant le module de détection.

    Figure 1 : Différentes étapes pour la détection spécifique d’une séquence d’acide nucléique cible. A. Capture des acides nucléiques cibles sur des nanoparticules magnétiques préalablement fonctionnalisées par des ADN sondes. B. Relargage des acides nucléiques cibles par hyperthermie magnétique en microfluidique. C. Détection électrochimique, intégrée en microfluidique, des acides nucléiques cibles.

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    New concepts for the development of polaritonic emitters in the mid-IR and THz range

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Paul GOULAIN

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Aloyse Degiron, Chargé de recherche, MPQ Paris : Rapporteur & Examinateur
    Karl Unterrainer, Professeur, TU Wien : Rapporteur & Examinateur
    Aristide Lemaitre, Directeur de recherche (HDR), C2N Palaiseau: Examinateur
    Eva Monroy, Docteure, NPSC Grenoble: Examinatrice
    Raffaele Colombelli, Directeur de recherche (HDR), C2N Palaiseau: Directeur de thèse
    Jean-Michel Manceau, Chargé de recherche, C2N Palaiseau: Coencadrant

    Abstract :

    The mid-infrared and terahetz bands of the electromagnetic spectrum have seen significant growth in applications in recent years, both in telecommunications and in the environmental and medical sciences.This interest is driving the demand for ever more compact and efficient sources and detectors. In this context, the development of coherent photon sources using the strong light-matter coupling regime is a promising avenue. Two axes of study of such sources will be explored in this thesis. First, a new approach to obtain mid-infrared photon emission through resonant optical pumping of intersubband polaritons was studied.
    This type of spontaneous emission is based on the use of non-dispersive cavities, allowing in principle to increase the phonon-polariton scattering rate and thus to reach more easily the threshold intensity of the coherent emission. The demonstration of photon emission in this configuration opens up the possibility of exploring this emission in a stimulated scattering regime by populating the final state of the system with a probe beam.
    In a second step, the focus will be on the THz domain. In order to overcome the thermal limitation imposed by the low energy of THz transitions, interdigitated parabolic quantum wells can be used to obtain resonant absorption up to 300K.To overcome the broadening introduced by multiple interfaces, alloy gradient wells were developed in collaboration with the University of Waterloo, Canada. This design resulted in very high quality THz transitions and an improvement in the operating temperature of the strong coupling regime by 170K. A particularity of intersubband polaritons is the possibility to obtain a high coupling constant through doping of the semiconductor.
    Consequently, these polaritons have been a platform of choice for demonstrating the ultra-strong coupling regime, in which the fundamental level of the system is populated by a non-negligible population of virtual photons. Theoretical studies have predicted that a non-adiabatic modulation of the ground state of the system, i.e. on a time scale smaller than the lifetime of the polaritons, this pool of light can be accessed. It is then possible to see these virtual photons being emitted as real photons. To explore this effect, three-dimensional LC cavities have been functionalised to achieve ultra-fast switching of their resonant frequency.
    Their development and characterisation by time-resolved THz spectroscopy will be presented.
    By using low temperature epitaxial GaAs, sub-picosecond modulation times have been achieved.
    Finally, the combination of these ultrafast switches with parabolic graded alloy quantum wells has allowed the achievement of a strong light-matter coupling regime between them.

    Figure 1: SEM image of a 3D LC resonator with parabolic quantum well active region in the patch. Figure 2: STEM image of a continuously graded parabolic quantum well. Overlay alloy profile extracted from the STEM signal amplitude.

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    Hybrid integration for on-chip optical emission and amplification in the near infrared

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre, Palaiseau

    Zhengrui TU

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Yannick DUMEIGE, Professeur des universités, FOTON (UMR 6082), Université de Rennes 1, Rapporteur & Examinateur

    Jean-Pierre VILCOT, Directeur de Recherche, IEMN (UMR 8520), Université de Lille, Rapporteur & Examinateur

    Lionel BASTARD, Maître de conférences, IMEP-LaHC (UMR 5130), INP - Phelma, Examinateur

    Philippe DELAYE, Directeur de Recherche, LCF (UMR 8501), IOGS, Examinateur

    Éric CASSAN, Professeur des universités, C2N (UMR 9001), Université Paris-Saclay, Directeur de thèse

     

    Abstract :

    One of the main challenges in the field of silicon photonics is to develop integrated optical sources and optical on-chip amplifiers. This thesis provides a contribution to the investigation of Erbium-based on-chip amplifiers and lasers. One main axis of the work carried out is to seek miniaturization of the active waveguides (<1mm, or even <<1 mm) and systematically consider a silicon integration perspective by favoring 1480 nm pump wavelength scheme.

    Active SiN waveguides coated with Erbium-doped Al2O3 layers at more than 1021 cm-3 and deposited by the ALD technique at Aalto university are the basis for the experimental demonstrations in this work. Modal gains of 10-20 dB/cm are demonstrated in these waveguides. Capitalizing on these results, theoretical and simulation work is further carried out in the purpose of investigating aggressive designs in terms of active structure lengths. Different types of single and double resonant cavities and resonators are studied and designed for the realization of integrated sources using the measured properties of experimentally studied active waveguides. As a whole, the carried out works contribute to the realization of compact light sources in silicon photonics exploiting recent advances in the growth of rare-earth doped active layers and the design of multiply resonant resonators.

    Link : https://teams.microsoft.com/l/meetup-join/19%3ameeting_NTNjMjVjYjMtZWRiMC00NjI4LThiMjYtNjYwMzgzYzExZTJm%40thread.v2/0?context=%7b%22Tid%22%3a%2268cdfebb-157b-4846-ba2f-d196a9124ac0%22%2c%22Oid%22%3a%2229dc529d-e78a-4197-ba7b-d7b7145626b5%22%7d

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    Switchable photovoltaic properties in ferroelectric PZT thin films

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre,

    Komalika RANI

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

     

    Houssny BOUYANFIF, Rapporteur, Maître de conférences-HDR, Université de Picardie (LPMC)

    Bohdan KUNDYS, Rapporteur, Chargé de recherche, HDR, Université de Strasbourg, CNRS

    Laurent DANIEL, Examinateur, Professeur des universités, Université Paris-Saclay

    Maryline GUILLOUX-VIRY, Examinatrice, Professeur des universités, Université de Rennes 1

    Hélène MAGNAN, Examinatrice, Ingénieure de recherche CEA

    Philippe LECOEUR, Directeur de thèse, Professeur, Université Paris-Saclay (C2N)

    Sylvia MATZEN, Encadrante, Maître de conférences, Université Paris-Saclay (C2N)

    Thomas MAROUTIAN, Invite, Chargé de recherche, Université Paris-Saclay-CNRS

    Abstract :

    Ferroelectric (FE) thin films are being explored for their possible use in photovoltaic (PV) applications. This is due to their high open-circuit voltage and switchable photovoltaic effect, which make them attractive for PV applications. Theoretically, 100% switching of the photocurrent can be achieved by varying the direction of the ferroelectric polarization through the ferroelectric layer. This is particularly intriguing for applications such as photo-ferroelectric memory. The presence of switchability in integrated ferroelectric films between electrodes, however, is not always achieved due to extrinsic parameters such as the nature of the electrode-ferroelectric interface (Schottky contact) or the presence of non-mobile charged defects in the ferroelectric film. In addition, the movement of charged defects, such as oxygen vacancies, under the influence of applied electric fields can have an effect on switchable photocurrent as well. It is not an easy process to disentangle all these contributions (polarization, interfaces, defects) to the photovoltaic properties of ferroelectric devices, and little is known about the quantitative link between photocurrent and ferroelectric polarization.

    In this work, a thorough investigation of the switchability of the PV properties of epitaxial lead zirconate titanate Pb(Zr,Ti)O3 (PZT) thin films has been carried out in order to study quantitatively the role of ferroelectric polarization. 100 nm thick PZT films were grown using pulsed laser deposition (PLD) and integrated into a capacitor geometry between bottom and top electrodes. The photoinduced current in the PZT devices was investigated under UV illumination (above the PZT band gap) and in different polarization states by poling the devices under increasing electric fields in order to achieve distinct electrical states while simultaneously monitoring their polarization value.

    A comparison study of different interfaces was also carried out, including Pt and ITO as top electrodes, SrRuO3 (SRO) and LaSrMnO3 as bottom electrodes, as well as the insertion of SrTiO3 dielectric layer at the PZT/electrode interface. This work has provided a quantitative determination of the switchable vs unswitchable parts of photocurrent. More precisely, the study of the dependence of the photocurrent as function of electrically controlled remanent polarization has shown that (1) the photocurrent depends linearly on the switchable part of the ferroelectric polarization and that (2) the analysis of this dependence allows extracting quantitatively the pinned polarization value in the FE layer. Such pinned polarization strongly affects the switchability of the PV properties in FEs and is otherwise rather difficult to probe by classical FE characterizations. In addition, the comparison study of different interfaces also revealed the contribution from the electrode-ferroelectric interface on the PV properties, which can induce really different switchability and amplitude of photocurrents.

    In conclusion, these results are thus particularly relevant for the optimization of FE thin films to achieve switchable PV properties which could have far-reaching implications for future photo-ferroelectric memory applications. In addition, the developed method of investigation of photocurrents switchability provides important insights on the ferroelectric behavior in all types of ferroelectric layers, in which pinned polarization could be significant but difficult to investigate otherwise.

    Link: soon

     

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    Nanostructured III-nitride light emitting diodes

    Centre de Nanosciences et de Nanotechnologies, Amphithéâtre, Palaiseau

    Nuno AMADOR-MENDEZ

    Centre Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Jury members :

    Jean-Christophe HARMAND, Directeur de Recherche, CNRS-C2N, France, Président

    Enrique CALLEJA, Professeur, ISOM-UPM, Espagne, Rapporteur

    Rachel GRANGE, Professeure, ETH, Suisse, Rapportrice

    Benjamin DAMILANO, Chargé de recherche, CNRS-CHREA, France, Examinateur

    Anna FONTCUBERTA i MORRAL, Professeure, EPFL, Suisse, Examinatrice

    Maria TCHERNYCHEVA,  Directrice de Recherche, CNRS-C2N, France, Directrice de thèse

     

    Abstract :

    Flexible light emitting diodes (LEDs) are today a topic of intense research driven by applications such as bendable displays, conformable light sources, bio-medical devices, etc. The conventional inorganic semiconductor devices are mechanically rigid; the fabrication of flexible devices from thin film structures is quite challenging and requires micro-structuring and lift-off of the active layer. Instead of two-dimensional films, in this thesis two types of III-nitride nanostructures are studied: (i) a bottom-up strategy using core shell nanowires, and (ii) a top-down strategy using a porous structure.

    Polymer-embedded nanowire membranes combine the high efficiency and the long lifetime of inorganic semiconductor materials with the high flexibility and transparency of polymers. I used MOCVD cores shell NWs for the fabrication of flexible blue and green NW LEDs, I also combined them with nanophosphors of different emission colors to produce a second generation of white LEDs with an improved color quality. For the fabrication of red flexible NW LEDs, I tested different strategies, namely an all-InGaN route based on In rich InGaN/GaN MQW NWs with a down-conversion of the blue light by a red phosphor and a red emission from GaAsP NWs.

    Selective area sublimation was demonstrated to be a promising approach to improve the luminous efficacy of defective GaN layers on Si. In my work, I analyzed the impact of porosification on InGaN/GaN single quantum wells and on p-i-n light emitting diode structures. The optical analyses were performed by cathodoluminescence demonstrating that the high temperature sublimation process does not degrade the QW emission while electron beam induced current microscopy showed that the p-i-n junction profile is also preserved after sublimation. I also describe the optimization of the technology for porous LED fabrication following several strategies. As a result, I demonstrated the first porous InGaN/GaN blue LED using parylene pore filling for electrical insulation.

    Link: soon