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Molecular dynamics (MD) simulations are applied to elucidate the anisotropic characteristics in the material responses for crystallographic nickel substrates with (100), (110) and (111) surface orientations during nanoindentation. The strain energy of the substrate exerted by the tip is stored by the formation of the homogeneous nucleation, and is dissipated by the dislocation sliding of the {111} plane. The steep variations of the indentation curve from the local peak to the local minimums are affected by the numbers of slip angle of {111} sliding plane. The pile-up patterns of the three nickel substrates prove that the crystalline nickel materials demonstrate the pile-up phenomenon from nanoindentation on the nanoscale. The three crystallographic nickel substrates exhibit differing amounts of pile-up dislocation spreading at different crystallographic orientations. The effects of surface orientation in material properties of F.C.C. nickel material on the nanoscale are observable through the slip angle numbers of {111} sliding planes which influence hardness values, as well as the cohesive energy of different crystallographic surfaces that indicate Young¡¦s modulus. Furthermore, the multiscale simulations are performed on the (100) monocrystal nickel substrate by using nanoindentation, compensating for MD limitation of a large specimen simulation without significant increase in the size of the problem. This study examines the accuracy of the coupling method for the multiscale model by means of the indentation curve and the deformation profile. Nanoindentation-induced mechanical deformation in GaN thin films prepared by metal-organic chemical-vapor deposition (MOCVD) was investigated using the Berkovich diamond tip in combining with the cross-sectional transmission electron microscopy (XTEM). By using the focused ion beam (FIB) milling to accurately position the cross-section of the indented region, the XTEM results demonstrate that the major plastic deformation was taking place through the propagation of dislocations. The present observations are in support of attributing the pop-ins appeared in the load-displacement curves to the massive dislocation activities occurring underneath the indenter during loading cycle. The absence of indentation-induced new phases might have been due to the stress relaxation via substrate and is also consistent with the fact that no discontinuity was found upon unloading.

La microélectronique cherche à produire des composants toujours plus performants. Un axe d'amélioration est l'intégration de plus de fonctionnalités dans un volume toujours plus compact. L'approche planaire classiquement utilisée jusqu'à présent atteint ses limites. Une solution à ce défi technologique est l'intégration 3D permettant d'empiler verticalement plusieurs circuits. Les étapes d'assemblage sont cruciales dans ces schémas d'intégration. Parmi les différentes techniques d'assemblage, le collage direct de couches minces métalliques est une alternative très intéressante. En effet, elle offre simultanément un lien mécanique et électrique vertical entre les couches actives de composants.Les propriétés microstructurales, physiques et chimiques des couches minces métalliques déposées ont été largement rapportées dans l'état de l'art antérieur. Cependant, elles n'ont jamais été étudiées dans l'environnement particulier du collage. Le but de notre étude est d'évaluer l'impact de cet environnement sur les couches minces métalliques assemblées pendant et après le procédé d'assemblage.Le collage direct consiste en la mise en contact de surfaces lisses à température ambiante et sous atmosphère ambiant afin de créer une adhérence entre elles. Puisque le collage n'est pas réalisé sous vide, des espèces adsorbées sont piégées à l'interface et une couche d'oxyde natif limite l'obtention du contact métal-métal. L'environnement de collage nous pousse donc à considérer ces différentes espèces qui interfèrent avec le procédé de collage et l'établissement du contact électrique.Dans cette étude, nous avons assemblé différents métaux dans différentes configurations de couches minces. Ainsi, les couches d'oxyde surfaciques ont été désignées comme influentes sur le comportement en adhésion des assemblages. Dans le cas précis du collage direct Cu-Cu, la réaction de l'eau interfaciale est primordiale au renforcement de la tenue mécanique dès la température ambiante. À plus haute température, la dissolution de l'oxyde piégé et la croissance de grain verticale sont des moteurs du scellement dépendant de phénomènes diffusifs. Il est apparu que les joints de grains sont des chemins de diffusion privilégiés dont le rôle dans la microstructure est majeur. Il a également mis en évidence que les couches de métaux réfractaires ne pouvaient pas être assemblées en utilisant les mêmes forces motrices que les métaux de transition dans la gamme de température considérée. La compréhension des différents mécanismes apporte un éclairage nouveau dans l'utilisation du collage direct dans les schémas d'intégration des composants de demain. / The semiconductor industry is driven by an increasing need of computation speed and functionalities. In the development of next generation devices the integration of more functionalities in an ever smaller volume becomes paramount. So far, classical planar integration was privileged but it is currently reaching its limits. One solution to this technological challenge is to consider the 3D dimension as pathway of integration. To ensure the vertical stacking of circuits, the development and control of assembly processes becomes crucial. Among the different techniques under development, direct bonding of metal thin films is a promising solution. It is a straightforward option that offers both a mechanical and an electrical link between the active strata.Microstructural, physical and chemical properties of deposited metal thin films were widely reported in previous state of art. However, they have not yet been studied in the specific bonding environment. The main goal of our study is to pinpoint the impact of this environment during and after the process of assembly.Direct bonding process consists in putting into contact smooth surfaces at room temperature and ambient air which in appropriate conditions leads to the establishment of attractive forces. Since bonding is not operated under vacuum, adsorbed species are trapped at the interface and the metal bonding suffers from the formation of native oxide. The encapsulation of these species as well as the native metal oxide interfere with the bonding process and the establishment of an electrical contact.In this study, various bonded structures have been realized using an extended set of metals in different thin film configurations. Metal oxide layers impact is clearly highlighted via the monitoring of adhesion properties of the assemblies. In the Cu-Cu direct bonding case, the interfacial water reaction is primordial in the strengthening of bonding toughness at room temperature. At higher temperature, oxide dissolution and vertical grain growth are driving forces in the sealing of bonding interface. The microstructure play a role in all these phenomena since grain boundaries are favorite diffusion pathway in thin films. Considering the temperature limitation imposed by the integration, we also highlight that refractory metal thin films needs another bonding approach compared to the transient metals. The understanding of bonding mechanisms throws new light on the use of direct bonding process in the realization of future electrical components.

In linear accelerator driven 4th generation Free Electron Lasers (FELs), the final beam quality is set by the linac and ultimately by its photoinjector and photocathode. Therefore, to deliver cutting-edge beam characteristics, there are stringent requirements for the photocathode used in the photoinjector. Understanding how surface properties of materials influence photocathode properties such as quantum efficiency (QE) and intrinsic emittance is critical for such sources. Metal photocathode research at Daresbury Laboratory (DL) is driven by our on-site accelerators VELA (Versatile Electron Linear Accelerator) and CLARA (Compact Linear Accelerator for Research and Applications), a free electron laser test facility. Metals offer the advantage of a fast response time which enable the generation of short electron pulses. Additionally, they are robust to conditions within the gun cavity. The main challenge with metal photocathodes is to maximise their (relatively) low electron yield. In this PhD thesis, the goal has been to carry out an experimental investigation on alternative metals to copper, correlating surface properties with photoemissive properties. A range of surface analysis techniques have been employed: surface composition was investigated using X-ray Photoelectron Spectroscopy and Medium Energy Ion Scattering, Kelvin Probe apparatus and Ultra-violet Photoelectron Spectroscopy were used to measure work function, and Atomic Force Microscopy and Interferometric microscope provided images characterising surface morphology. The photocathode properties studied include: QE measured using a 265 nm UV LED source that was later upgraded to a 266 nm UV LASER, and Mean Transverse Energy measured using the Transverse Energy Spread Spectrometer. As a result of this work, Mg, Nb, Pb, Ti and Zr have all been identified as photocathode candidate materials, each exhibiting a QE greater than Cu. Additionally, surface preparation procedures for optimising QE from a selection of metals has been explored; the findings of these experiments would suggest that ex-situ Ar plasma treatment followed by in-situ heat treatment is well suited to remove surface contaminants without altering the surface morphology of the cathode. As part of this work, metallic thin films produced by magnetron sputtering have been produced; ultimately the chosen cathode metal will be deposited onto a cathode plug which will be inserted into the electron gun that will drive CLARA. Thus the preparation of metal thin films has been investigated and the effect of different substrate materials on the film properties has been explored. Preliminary experiments studying the effects of surface roughness on photoelectron energy distribution have been conducted; the findings have not been conclusive, thus further systematic studies are required.

Organic solar cells offer attractive properties for novel applications and continuous advances in material and concept development have led to significant improvements in device performance. To exploit their full potential (roll-to-roll production of flexible and top-illuminated devices, using e.g. opaque metal foil or textile as substrate), highly transparent, conductive, mechanically flexible, and cost-efficient top electrodes are of great importance. The current standard material indium tin oxide (ITO) is rigid, expensive and requires a high energy / high temperature deposition process, limiting ITO (and other transparent conductive oxides) to bottom electrode applications. This work presents fundamental investigations to understand and control the properties of transparent conductors and documents four different approaches to prepare transparent electrodes on top of efficient small molecule organic solar cells, with the aim to replace ITO. Fullerene C60 layers are investigated as completely carbon-based electrodes. For an optimized doping concentration, sheet resistance and transmittance are improved and efficient solar cells are realized. Since the lateral charge transport is still limited, a combination with a microstructured conductor is suggested. Pulsed laser deposition allows for the first time a damage-free preparation of gallium doped zinc oxide (ZnO:Ga) layers on top of organic devices by careful optimization of the deposition atmosphere. ZnO:Ga electrodes with a transmittance of Tvis = 82.7 % and sheet resistance Rs = 83 Ohm/sq are obtained. The formation of local shunts due to ZnO:Ga droplets is identified and then prevented by a shadow mask between the target and the sample, enabling solar cells with similar efficiency (2.9 %) compared to a reference device using a state-of-the-art metal top contact. Another very promising alternative are intrinsically flexible, ultra-thin silver layers. By introducing an oxide interlayer, the adverse interpenetration of silver and organic materials is prevented and the charge extraction from the solar cells is improved. With a second oxide layer on top, the silver electrode is significantly stabilized, leading to an increased solar cell lifetime of 4500 h (factor of 107). Scanning electron micrographs of Ag thin films reveal a poor wetting on organic and oxide substrates, which strongly limits the electrode performance. However, it is significantly improved by a 1 nm thin seed layer. An optimized Au/Ag film reaches Tvis = 78.1 % and Rs = 19 Ohm/sq, superior to ITO. Finally, silver electrodes blended with calcium show a unique microstructure which enables unusually high transmittance (84.3 % at 27.3 Ohm/sq) even above the expectations from bulk material properties and thin film optics. Such values have not been reached for transparent electrodes on top of organic material so far. Solar cells with a Ca:Ag top electrode achieve an efficiency of 7.2 %, which exceeds the 6.9 % of bottom-illuminated reference cells with conventional ITO electrodes and defines a new world record for top-illuminated organic solar cells. With these electrodes, semi-transparent and large-area devices, as well as devices on opaque and flexible substrates are successfully prepared. In summary, it is shown that ZnO:Ga and thin metal electrodes can replace ITO and fill the lack of high performance top electrodes. Moreover, the introduced concepts are not restricted to specific solar cell architectures or organic compounds but are widely applicable for a variety of organic devices.

Organic solar cells offer attractive properties for novel applications and continuous advances in material and concept development have led to significant improvements in device performance. To exploit their full potential (roll-to-roll production of flexible and top-illuminated devices, using e.g. opaque metal foil or textile as substrate), highly transparent, conductive, mechanically flexible, and cost-efficient top electrodes are of great importance. The current standard material indium tin oxide (ITO) is rigid, expensive and requires a high energy / high temperature deposition process, limiting ITO (and other transparent conductive oxides) to bottom electrode applications. This work presents fundamental investigations to understand and control the properties of transparent conductors and documents four different approaches to prepare transparent electrodes on top of efficient small molecule organic solar cells, with the aim to replace ITO. Fullerene C60 layers are investigated as completely carbon-based electrodes. For an optimized doping concentration, sheet resistance and transmittance are improved and efficient solar cells are realized. Since the lateral charge transport is still limited, a combination with a microstructured conductor is suggested. Pulsed laser deposition allows for the first time a damage-free preparation of gallium doped zinc oxide (ZnO:Ga) layers on top of organic devices by careful optimization of the deposition atmosphere. ZnO:Ga electrodes with a transmittance of Tvis = 82.7 % and sheet resistance Rs = 83 Ohm/sq are obtained. The formation of local shunts due to ZnO:Ga droplets is identified and then prevented by a shadow mask between the target and the sample, enabling solar cells with similar efficiency (2.9 %) compared to a reference device using a state-of-the-art metal top contact. Another very promising alternative are intrinsically flexible, ultra-thin silver layers. By introducing an oxide interlayer, the adverse interpenetration of silver and organic materials is prevented and the charge extraction from the solar cells is improved. With a second oxide layer on top, the silver electrode is significantly stabilized, leading to an increased solar cell lifetime of 4500 h (factor of 107). Scanning electron micrographs of Ag thin films reveal a poor wetting on organic and oxide substrates, which strongly limits the electrode performance. However, it is significantly improved by a 1 nm thin seed layer. An optimized Au/Ag film reaches Tvis = 78.1 % and Rs = 19 Ohm/sq, superior to ITO. Finally, silver electrodes blended with calcium show a unique microstructure which enables unusually high transmittance (84.3 % at 27.3 Ohm/sq) even above the expectations from bulk material properties and thin film optics. Such values have not been reached for transparent electrodes on top of organic material so far. Solar cells with a Ca:Ag top electrode achieve an efficiency of 7.2 %, which exceeds the 6.9 % of bottom-illuminated reference cells with conventional ITO electrodes and defines a new world record for top-illuminated organic solar cells. With these electrodes, semi-transparent and large-area devices, as well as devices on opaque and flexible substrates are successfully prepared. In summary, it is shown that ZnO:Ga and thin metal electrodes can replace ITO and fill the lack of high performance top electrodes. Moreover, the introduced concepts are not restricted to specific solar cell architectures or organic compounds but are widely applicable for a variety of organic devices.