Nitride nanowires are a promising material for the fabrication of efficient and compact piezogenerators. Their tremendous piezoelectric and mechanical properties give them the ability to convert efficiently mechanical energy into electrical energy. The piezoelectric material studied in this thesis is GaN, synthetised as nanowires by PA-Molecular Beam Epitaxy. Thanks to an adapted AFM résiscope, we show the great potential of nitride nanowires for piezogeneration and the correlation between the polarity of the nanostructure, its deformation and the establishment of the piezopotential. We also study the harvesting efficiency of the nanostructures’ polarization, through a nanometric Schottky contact. Due to scale effects, this Schottky nanocontact shows a reduced barrier height and resistance, which lead to an enhanced conduction and thus to a better harvesting of the piezoelectric energy generated by the GaN nanowires. Based on the understanding of those mechanisms, we have built a piezogenerator integrating a vertical array of p-type GaN nanowires, embedded in HSQ resist and with their top connected by a Pt metallic electrode, leading to a Schottky contact. This prototype delivered a power density of about 12,7 mW.cm-3, which is the state of the art for GaN nanowires based piezogenerator.

This work investigates III-V nanowires synthesized via the vapor-liquid-solid method, whereby a catalyst droplet promotes one-dimensional growth. By combining molecular beam epitaxy experiments, structural characterization and theoretical analyses, I study and clarify several critical issues. One of them is the control of the crystal phase, which is frequently found to be a mix of cubic and hexagonal segments. By performing a probabilistic analysis of the stacking sequence of InP nanowires, I show that phase selection is determined not only by growth conditions but also by interactions between layers. I highlight and discuss the role of the edge energy of the nucleus that mediates the formation of each monolayer. Another important problem is the formation of axial heterostructures, which interface sharpness is severely limited by material accumulation in the droplet (‘reservoir effect’). To this end, I study the formation of such heterostructures in Ga-catalyzed GaAs nanowires using either a second group V element (P) or a second group III element (Al). The composition profiles of the ternary insertions are analyzed with monolayer resolution. The interface widths are found to be larger [Ga(As,P)] or narrower [(Al,Ga)As] than expected, and the morphology of the growth front depends on supersaturation. In both cases, I demonstrate that the interface width can be reduced to a few monolayers and suggest further improvements. Attempts to achieve ultrathin GaAs and GaP nanowires that would permit lateral quantum confinement are presented. Finally, I consider the possibility of minimizing the stochastic character of nucleation ultimately to control the growth of single monolayers.

Bandwidth demand in optical communication systems is continually growing. Data rate values in the order of several hundreds of TBps are expected in the near future. In order to cope with those expectations silicon based technologies are believed to be the best suited. Its naturally compatibility with CMOS easily enables the electronics and photonics co-integration. In the short-term the way increase data rates in next generation optical communication systems goes through using advanced modulation format and increase symbol rates. In the long-term view, new multiplexing techniques will be required. In this sense, mode division multiplexing is nowa-days an attractive approach under consideration. In this Thesis work, the way to implement these new optical communication schemes in studied from the transmitter point of view. It includes, on the one hand, the modeling, design and charac-terization of silicon modulators. And in the other hand, it includes the proposition, design and characterization of novel mode handling devices for mode division multiplexing. A new way of modeling silicon modulators has been developed. This new model permits to re-duce the computation time of modulator analysis up to two orders of magnitude, while maintain-ing a good level of accuracy. Using the model, modulators based on lateral PN junctions and in-terdigitated PN junctions were designed to work in the O-Band of optical communications. Char-acterization work has been performed on these modulators with good results. Wide-open OOK eye diagrams were obtained at 10GBps. Furthermore, BPSK modulation was also demonstrated at 10GBps. New kind of mode converters and multiplexers, intended to work as mode division multiplexing subsystems have been proposed, designed, fabricated and characterized. Measured results show broad bandwidth operation high extinction ratio.

Nonlinearities in nanomechanical systems can arise from various sources such as spring and damping mechanisms and resistive, inductive, and capacitive circuit elements. Beyond fundamental interests for testing the dynamical response of discrete nonlinear systems with many degrees of freedom, non-linearities in nanomechanical devices, open new routes for motion transduction, nanomechanical sensing, and signal processing. We investigate the nonlinear response of a nanomechanical resonator consisting in a suspended photonic crystal membrane acting as a deformable mirror. Actuation of the membrane motion in the MHz frequency range is achieved via interdigitated electrodes placed underneath the membrane. The applied electrostatic force induces mechanical non-linearities, in particular bistability, superharmonic and stochastic resonances.

Membres du jury : Directeur de thèse : Laurent Vivien C2N (France) Rapporteurs : Prof. Lorenzo Pavesi University of Trento (Italy) Prof. Jeremy Witzens RWTH Aachen (Germany) Examinateurs : Valérie Véniard Ecole Polytechnique (France) Frédéric Boeuf STMicroelectronics (France)