Alberto Casadei
Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 20 years, they have been intensively studied for the prospects that their unique quasi-one dimensional shape offers to both fundamental and applied science. More recently particular attention has been dedicated to use NWs as building blocks for nano-electronic devices. In this thesis we investigate the electro-optical properties of NWs in order to put some light on the mechanisms governing the electrical transport and the light coupling between NWs and metal nanostructures. We investigate Be and C doping in GaAs NWs synthesized by Molecular Beam Epitaxy (MBE). We obtain a doping control over a large range of densities and we identify a new in situ incorporation path. Since strong surface impurity scattering in III-V materials degrade the electronic performances, we grew NWs passivated with an AlGaAs layer and we investigate their properties. The NW passivation allows for the increase of the electron mean free path by a factor of almost 10. In addition, we designed AlGaAs/GaAs modulation doping NWs. The modulation doping structure allows for the to enhancement of the NW electron mobility revealing excellent properties for the realization of nano-electronic devices. We calculate the electron distribution in the modulation doped NWs and we observe a six-fold symmetry with six 1D electron channels when the carrier concentration is high, while for low concentrations, electrons are delocalized in the GaAs NW core. Thanks to their special interaction with light, semiconductor NWs have opened new avenues in photonics, quantum optics and solar energy harvesting. Here, we design a new system composed of a NW and an array of nanoantennas. Initially, we successfully demonstrated the plasmonic coupling between NWs and nanoantennas, observing an electric field enhancement in the NW as a function of the nanoantenna's gap distance. This finding represented an initial step toward the development of coupled nano-structures for the realization of a new generation of solar cells, detectors and non linear optical devices. Near field coupling was also used between a NW and Yagi-Uda antennas to obtain directional emission. In particular the precise tuning of the Yagi-Uda dimensions and positions leads a strong variation of the NW emission, being able to change this from backward to forward. One of the major challenges for NWs full technological deployment has been their strong polarization dependence in light absorption and emission. Here, we demonstrate that a hybrid structure formed by GaAs NWs with a highly dense array of bow-tie antennas is able to modify the polarization response of a NW. As a result, the increase in light absorption for transverse polarized light changes the NW polarization response, including the inversion of the polarization response. We calculated that the absorption of transverse polarized light can be enhanced up to 15 times. We also fabricate several electrical devices proving our calculated predictions.
EPFL2016
