Le photovoltaïque (PV) s’est imposé comme un acteur majeur de l’énergie. L’innovation dans ce domaine passera sans doute par le PV à haut rendement sur des couches minces flexibles et légères permettant son déploiement dans les applications mobiles. Cette thèse étudie le développement de cellules solaires III-V à structures quantiques visant des concepts PV hauts rendements tels les cellules solaires à bande intermédiaire (IBSC). Ces IBSC se sont montrés limités du fait de l’échappement thermique des porteurs à température ambiante ainsi que la faible absorption optique sous le gap. Nous avons évalué la topologie, le mécanisme d’échappement thermique, la structure quantique ainsi que l’absorption de boites quantiques en In(Ga)As dans un matériau hôte en Al0.2GaAs à grand gap. Nous avons aussi caractérisé de manière quantitative comment opère ce système et avons amélioré son design optique. Sous une forte irradiation, nous avons mis en évidence l’apparition d’une population de porteurs chauds dans les boites quantiques. Par ailleurs, l’effet d’absorption sequentielle à deux photons (S-TPA) a été démontré. Nous avons observé une augmentation de ce S-TPA d’un facteur x5-10 grâce à du management de la lumière réalisé notamment avec des cavités de Fabry-Pérot. Des nanostructures périodiques ont aussi été fabriquées dans le cas de cellules solaires à multi-puits quantiques par l’utilisation de lithographie en nanoimpression. Dans l’ensemble cette étude vise à discuter la possibilité de réaliser des cellules solaires à porteurs chauds assistés d’une bande intermédiaire et améliorées par un management optique afin d’ouvrir la voie pour des cellules à hauts rendements. / In the past decade, photovoltaics (PV) has become a key player for the future of worldwide energy generation. Innovation in PV is likely to rely on high efficiency PV with flexible and lightweight thin films to enable PV deployement for mobile applications. In the framework of the Japanese-French laboratory “NextPV”, this thesis investigates the development of III-V quantum structured solar cells to explore high-efficiency photovoltaic concepts especially intermediate band solar cells (IBSC). Quantum structured IBSC have proven to be limited by thermal escape at room temperature and by low subbandgap light absorption. Following a consistent approach, we evaluate the topology, thermal escape mechanism, quantum structure and optical absorption of In(Ga)As quantum dots in a wide gap Al0.2GaAs host material. We also characterize quantitatively the device operation and improve the optical design. For a high irradiation, we evidence a hot carrier population in the quantum dots. At the same time, sequential two-photon absorption (S-TPA) is demonstrated both optically and electrically. We also show that S-TPA for both subbandgap transitions can be enhanced by a factor x5-10 with light management techniques, for example by implementation of Fabry-Perot cavities with the different epitaxial transfer methods that we developed. More advanced periodical nanostructures were also fabricated in the case of multi-quantum well solar cells using nanoimprint lithography techniques. Overall we discuss the possibility of realizing intermediate-band-assisted hotcarrier solar cells with light management to open the path for high-efficiency quantum structured IBSC.

On présente dans ce travail un nouveau type de qubit de spin dont les performances reposent sur les propriétés d'un seul électron dans une double boîte quantique. Le fort moment dipolaire de la double boîte combiné à une large variation du champ magnétique entre les deux boîtes permettrait de réaliser des opérations logiques plus rapidement que dans une seule boîte quantique. Pour maximiser les variations du champ magnétique, on utilisera un micro-aimant placé le plus près possible d'une des deux boîtes. À cette fin, une hétérostructure de GaAs/A1GaAs sur laquelle sont déposées des grilles d'aluminium a été utilisée pour former une double boîte quantique latérale. L'occupation par un seul électron de la double boîte est confirmée par des mesures de transport électrique à basse température ainsi que par l'observation du blocage de spin. De plus, un procédé d'oxydation des grilles par plasma d'oxygène a été développé. Une étude des propriétés de l'oxyde formé par cette méthode montre qu'il est possible de placer un micro-aimant directement sur la surface de l'hétérostructure sans affecter l'isolation électrique entre les grilles. Cette nouvelle approche permet de produire des champs magnétiques encore plus intenses que dans les expériences antérieures, pour lesquelles le micro-aimant est placé beaucoup plus loin de la surface. L'ensemble du procédé de fabrication, de la photolithographie à l'électrolithographie, a été développé au cours de ce travail dans les salles blanches du département de génie électrique et dans les salles propres du département de physique de l'Université de Sherbrooke. Ce travail est une étape importante dans la réalisation de qubits de spin plus performants dans les boîtes quantiques latérales.

Actuellement, plusieurs types de photodétecteurs sont disponibles sur le marché. Leurs performances se caractérisent notamment par la réponse spectrale, le courant d'obscurité, le rapport signal sur bruit, le rendement quantique et le temps de réponse. L'émergence de nouvelles applications nécessite des photodétecteurs de plus en plus sensibles, afin de pouvoir détecter de très faibles niveaux de radiation, voire de pouvoir compter des photons un par un. Ce travail de thèse s'intéresse aux moyens de réalisation de nanopixels pour la détection de faibles niveaux de lumière visible, en utilisant l'absorption dans des nanocristaux de silicium. Après avoir discuté de l'influence de la réduction des dimensions sur les propriétés électroniques et optiques du silicium, ainsi que de l'utilisation du blocage de Coulomb pour la photodétection, nous présentons un procédé de fabrication et d'isolation de nanopiliers contenant des nanocristaux de silicium dans une matrice d'oxyde de silicium. Les caractéristiques électriques des nanopixels intégrant ces nanocristaux ont permis de mettre en évidence les phénomènes de piégeage de charges dans les îlots, ainsi que leur contribution aux mécanismes de transport. Nous présentons finalement une première étude des propriétés électro-optiques des nanopixels qui ont été caractérisés par des mesures de photocourant.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This report intends to show how able the Stellenbosch Electrical and Electronic Engineering Department’s micro fabrication laboratory is to manufacture nanomechanical switches and similar structures. Following the investigation, an attempt will be made to produce these devices. In attaining this goal, a literature study was performed focusing on mechanical switching and nanotechnology. Their origins, development and current application are investigated, as well as the requirements for developing these devices. Having completed the literature study, a series of current nanomechanical switches where investigated, selecting those most likely to be manufactured at the available facilities and having the required attributes for taking the place of silicon transistors in low speed, hostile environments. The most common method of manufacturing nanoswitches is simulated using several methods of predicting device failure. This will aid in the selection of manufacturing process guidelines, such as dimensions in the photoresist templates and layer thickness before etching, allowing for the repeated production of functional switches. The two manufacturable nanomechanical switches are investigated, using techniques and materials available at the micro fabrication laboratory to manufacture them. Subsequently, their electrical properties will be determined and used in simulating their failure characteristics. In conclusion, the result are discussed along with advice and improvements for the continued investigation and production of nanodevices at Stellenbosch University / AFRIKAANSE OPSOMMING: Die doel met hierdie tesis is om die vermoë van die Departement Elektriese en Elektroniese Ingenieurswese se mikro-elektronika laboratorium te ondersoek ten opsigte van die vervaardiging van nanomeganiese skakelaars en aanverwante komponente. Na afloop van die ondersoek sal ’n poging aangewend word om hierdie toestelle te vervaardig. In nastrewing van hierdie doel is ’n literatuurstudie uitgevoer wat fokus op meganiese skakeling en nanotegnologie. Die oorsprong, ontwikkeling en huidige aanwending van hierdie skakelaars word ondersoek, sowel as die vereistes vir die ontwikkeling daarvan. Na voltooiing van die literatuurstudie word ’n reeks van bestaande nanomeganiese skakelaars ondersoek. Skakelaars word dan geïdentifiseer op grond van hul waarskynlike vervaardigbaarheid met die beskikbare fasiliteite. Die vereiste eienskappe vir lae spoed toepassings in omgewings waar silikon-skakelaars nie kan werk nie, word ook in ag geneem. Die mees algemene metode vir die vervaardiging van nanoskakelaars word gesimuleer met behulp van verskeie tegnieke wat toestelfaling voorspel. Die simulasies sal help in die keuse van vervaardigingsriglyne van die proses, soos die dimensies van die fotolak template en die laagdiktes voor etsing, sodat goeie resultate verkry kan word wanneer die skakelaars vervaardig word, met goeie herhaalbaarheid. Die twee gekose vervaardigbare nanomeganiese skakelaars word dan ondersoek ten opsigte van die tegnieke en materiale wat in die mikro-elektronika laboratorium beskikbaar is. Daarna sal die elektriese eienskappe van die skakelaars bepaal word. Ter afsluiting word die resultate bespreek, saam met aanbevelings vir die voortsetting van die navorsing om nanomeganiese skakelaars by die Universiteit van Stellenbosch te vervaardig.

This thesis addresses nanostructure fabrication techniques based on electron beam lithography, which is the most widely employed nanofabrication techniques for R&D and for the prototyping or production of photo-mask or imprint mold. The focus is on the study of novel resist and development process, as well as pattern transfer procedure after lithography. Specifically, this thesis investigates the following topics that are related to either electron beam resists, their development, or pattern transfer process after electron beam lithography: (1) The dry thermal development (contrary to conventional solvent development) of negative electron beam resists polystyrene (PS) to achieve reasonably high contrast and resolution. (2) The solvent development for polycarbonate electron beam resist, which is more desirable than the usual hot aqueous solution of NaOH developer, to achieve a low contrast that is ideal for grayscale lithography. (3) The fabrication of metal nanostructure by electron beam lithography and dry liftoff (contrary to the conventional liftoff using a strong solvent or aqueous solution), to achieved down to ~50 nm resolution. (4) The study a novel electron beam resist poly(sodium 4-styrenesulfonate) (sodium PSS) that is water soluble and water developable, to fabricate the feature size down to ~ 40 nm. And finally, (5) The fabrication of gold nanostructure on a thin membrane, which will be used as an object for novel x-ray imaging, where we developed the fabrication process for silicon nitride membrane, electroplating of gold, and pattern transfer after electron beam lithography using single layer resist and tri-layer resist stack.

Au cours de ce travail de thèse, nous avons étudié la formation de nanogaps par électromigration dans des nanofils d'or. Cette technique consiste à provoquer la rupture d'un nanofil en lui appliquant de fortes densités de courant et peut être utilisée pour la caractérisation électrique de nano-objets. Néanmoins, les mécanismes de formation des nanogaps ne sont, à ce jour, pas encore totalement compris. L'étude en temps réel du processus d'électromigration par microscopie à force atomique a permis d'apporter un éclairage nouveau de la dynamique du processus. En effet, il apparaît que la structure globale du dispositif est définie dans les premiers temps de l'électromigration et nous avons montré que cette structure est directement liée à la microstructure du film métallique. Pour la première fois, des nanogaps ont été élaborés par électromigration dans des films monocristallins. Malgré l'absence de joints de grain, il est possible de former des nanogaps dans un matériau épitaxié. L'utilisation de ces matériaux permet d'obtenir des nanogaps avec une morphologie plus reproductible. Les propriétés de transports des nanogaps obtenus à partir de films polycristallins ont été caractérisées. Les caractéristiques obtenues présentent toutes des signatures particulières, attribuées à la présence d'agrégats d'or provenant de la procédure d'électromigration et/ou de polymères issus du procédé de nanofabrication. Ces résultats montrent la difficulté à réaliser des mesures à l'échelle de la molécule unique / We have studied the formation of nanogaps by électromigration of gold nanowires. Electromigration relies on large current densities to break a thin and narrow metallic wire and can be used for the electrical characterization of nanometer scale objects. Nevertheless, a complete description of the electromigration process is lacking. Real time atomic force microscopy during the electromigration process gave a new view the dynamic of the process. Indeed, this study reveals that the major structural changes appear at the early stage of the process and that the final global structure of the device is directly linked to the pre-existing microstructure. For the first time, we make nanogaps by électromigration of monocrystalline nanowires. We show that despite the lacking of grain boundaries, it is possible to form nanogaps in epitaxial materials. The morphology of those nanogaps is more reproducible. The electrical transport properties of the polycristalline nanogaps have been measured. The entire obtained characteristics show particular signature that can be attributed to the presence of gold cluster formed during the électromigration process and / or to polymer from the nanowire fabrication. These results show the difficulty to measure at the molecular scale

Neste trabalho apresentamos um estudo sistemático da transmissão da radiação eletromagnética em um conjunto de fendas metálicas individuais depositadas em substratos de vidro BK7 com larguras abaixo do comprimento de onda da luz incidente. As fendas foram obtidas através da evaporação térmica de prata e ouro sobre um substrato dielétrico (vidro BK7). A fabricação das fendas foi feita de forma direta através de um equipamento de feixe de íons de Gálio (FEI Quanta 3D). A transmissão óptica das fendas foi estudada com radiação laser para os comprimentos de onda de 488 nm e 632.8 nm. Observouse uma oscilação de intensidade na transmissão devido ao efeito de interferência dos surface plasmon polariton (SPP) gerados na superfície metálica como função da espessura do filme metálico. Os resultados experimentais obtidos foram confrontados com simulações computacionais, via método de elementos finitos, a partir das quais pode se compreender os mecanismos de propagação da radiação na estrutura em questão, e compará-los com detalhes da fabricação de fendas. Procedimento similar foi realizado ao variar a largura das mesmas. Também foi feita a análise da transmissão óptica em fendas fabricadas em filmes constituídos por camadas alternadas de prata e ouro, a fim de comparar como tal mudança na configuração da estrutura afeta os resultados anteriores. O trabalho serviu para mostrar que as dimensões dos filmes metálicos, assim como das fendas, apresentavam grande influência nas propriedades de transmissão e, além do que foi possível conferir diversos conceitos básicos da teoria eletromagnética num sistema relativamente simples. A compreensão dessas propriedades é fundamental para o desenvolvimento de futuros dispositivos que utilizam efeitos plasmônicos. / This work presents a systematic study of the transmission of electromagnetic radiation on a set of single metallic slits metallic with widths below the wavelength of incident light, deposited on BK7 glass substrates. The slits were obtained by thermal evaporation of silver and gold on a dielectric (BK7 glass). The fabrication of the slits was performed directly through a gallium ion beam equipment (FEI Quanta 3D). The optical transmission of the slits was studied by laser radiation for the wavelengths of 488 nm and 632.8 nm. An oscillation in the transmission intensity was observed due to the interference effect of the surface plasmon polariton (SPP) generated on the metallic surface as a function of the metal film thickness. The experimental results were compared with computational simulations, via finite element method, in order to comprehend the mechanisms of radiation propagation in the structure and compare them with details of slits fabrication. A similar procedure was performed by varying the width of the slits. An analagous analysis of the optical transmission in slits fabricated in films consisting of alternating layers of silver and gold was performed in order to compare how this change in structure configuration affects the previous results. The work served to show that the dimensions of the metal films, as well as of the slits, have a significant influence on the properties of transmission, and, moreover it was possible to confer different basic concepts of electromagnetic theory in a relatively simple system. Understanding these properties is fundamental for the development of future devices that make use of plasmonics effects.

"In the past decade nanomaterials have attracted the interest of scientists and engineers all over the world due to their unique properties. Through their devoted experimental efforts, limited advances have been made on the synthesis of nanomaterials, the integration of nanomaterials into the structures of larger scales, and the property study of nanomaterials to explore possible applications. Despite the huge amount of money, resources, and effort invested in nanomaterials, several challenges still remain as obstacles on the way towards the successful large scale use of nanomaterials to benefit human life and society. For example, the need for low-cost, robust, and highly productive manufacturing methods and the demand for efficient integration of nanomaterials with materials and devices of larger length scales are still left unmet. The objective of this work was to utilize cost-efficient nanofabrication methods such as template-assisted fabrication, electrodeposition, and chemical vapor deposition to fabricate nanomaterials, integrate nanomaterials with larger structures to form a hierarchical composite, and explore the application of unique nanostructured electrode in lithium-ion batteries. Thus the thesis consists of three main parts: (1) fabrication of one-dimensional inorganic nanomaterials such as metal nanowires, metal nanorods, and carbon nanotubes with good control over shape and dimension; (2) synthesis of hierarchical carbon nanofibers on carbon microfibers and/or glass microfibers; and (3) development of nanostructured anodes to improve high-rate capability of lithium-ion batteries by adapting nanorod arrays as miniature current collectors. "

The controlled organization of individual molecules and nanostructures with nanoscale accuracy is of great importance in the investigation of single-molecule events in biological and chemical assays, as well as for the fabrication of the next generation optoelectronic devices. In this regard, the precise patterning of individual molecules into hierarchical structures has attracted substantial research interest in recent years. DNA has been shown to be an ideal structural material for this purpose, due to the specificity of its programmability and outstanding chemical flexibility. DNA origami can display a high degree of positional and precise binding sites, allowing for complex arrangements and the assembly of different nanoscale architectures. In this project, we present a novel platform based on the use of DNA scaffolds for the organization of individual nanomoieties (with nanoscale spatial control), and their selective immobilisation on surfaces for single-molecule investigations. In particular, semiconductor quantum dots (QDs), fluorescence molecules, linear small peptides, and structural proteins were tethered with single-molecule accuracy on DNA origami; their subsequent organization in array configuration on nanopatterned surfaces allowed us to fabricate and test different platforms for single-molecule studies. In particular, we developed a Focused Ion Beam (FIB) nanofabrication strategy and demonstrated its general applicability for the assembly of functionalised DNA nanostructures in highly uniform nanoarrays, with single-molecule control. In addition, we further explored this nanofabricated platform for biological investigations at the single-molecule level, from protein-DNA interactions to cancer cell adhesion studies with single-molecule control. Investigations have been carried out via fluorescence microscopy, scanning electron microscopy (SEM), Focused Ion Beam (FIB) and atomic force microscopy (AFM). By and large, combining the programming ability of DNA as a scaffolding material with a one-step lithographic process, we have developed a platform of general applicability for the fabrication of nanoscale chips that can be employed in a variety of single-molecule investigations.

One dimensional carbon nanotubes (CNTs) and two-dimensional layered materials like graphene, MoS2, hexagonal boron nitride (hBN), etc. with different electrical and mechanical properties are great candidates for many applications in the future. In this study the synthesis and growth of carbon nanotubes on both conducting graphene and graphite substrates as well as insulating hBN substrate with precise crystallographic orientation is achieved. We show that the nanotubes have a clear preference to align to specific crystal directions of the underlying graphene or hBN substrate. On thicker flakes of graphite, the edges of these 2D materials can control the orientation of these carbon nanotubes. This integrated aligned growth of materials with similar lattices provides a promising route to achieving intricate nanoscale electrical circuits. Furthermore, short channel nanoscale devices consisting of the heterostructure of 1D and 2D materials are fabricated. In these nanoscale devices the nanogap is created due to etching of few layer graphene flake through hydrogenation and the channel is either carbon nanotubes or 2D materials like graphene and MoS2. Finally the transport properties of these nanoscale devices is studied.