Core-Shell GaAs Nanowire p-n Junctions from Growth to Photovoltaic and Optoelectronic Applications

Abstract

Semiconductor nanowires (NWs) are filamentary crystals with the diameter ranging from few tens up to few hundreds of nanometers. In the last 15 years, they have been intensively studied for the prospectives that their unique quasi-one dimensional shape offers to both fundamental and applied physics. More recently particular attention has been dedicated to the advantages that their geometry exhibits in the separation and extraction of photo-generated carriers with respect to standard thin film or bulk technologies. The aim of this thesis is the realization and characterization of single GaAs NW photovoltaic and optoelectronic devices. The work starts with the synthesis of radial NWp-n junctions. The core growth has been realized using the Vapor-Liquid-Solid (V LS) mechanism by Molecular Beam Epitaxy (MBE). Differently from the commonly used V LS method, which requires Au particles as a catalyst, we use a self-catalyzed process which avoids potential contaminations. The shell is then overgrown around, switching the growth parameters toward standard thin film conditions. The doping mechanism of such a structure has been deeply investigated, with the purpose of discriminating in which way the dopants get incorporated in the crystal structure. A reproducible technique has been developed to electrically contact the NWs and test their photovoltaic properties. They have been measured under different illumination conditions. Conversion efficiency of about 4.5% have been measured under diffuse illumination at 1.5AM and more information about the uniformity of the junction have been deduced by scanning photo-current measurements. Due to the size comparable with the wavelength of the visible light, NWs exhibit characteristic absorption peaks as a function of the photon energy, often referred as Mie resonances. In order to engineer these resonances we have deposited metal nanoparticles on their facets and studied how the plasmonic excitation of the latter could couple with the NW leaky modes. The results have been analyzed with Finite Difference Frequency Domain (FDFD) simulations. Since GaAs is a direct band-gap material, the same devices have also been tested in a reverse way as light emitting diodes and plasmon generators. Finally some preliminary studies about the effect of axial wurtzite/zincblende polytypism on the electronic properties have been performed.