Growth of GaAs nanowires on Si (111) for photovoltaic applications

Abstract

The goal of this thesis is to master the synthesis of GaAs nanowires ensembles on Si for their application in solar cells. Semiconductor nanowires present promising characteristics for photovoltaic applications: they benefit from their longitudinal high aspect ratio geometry to enhance light absorption, minimize material consumption and efficiently collect the carriers. To fully unleash their potential, the following properties have to be controlled: number density, diameter and orientation. The latter is of utmost importance to have uniform junctions and to avoid leakages/shortcuts, whereas number density and diameter allow to tune light absorption and minimize material utilization. Our nanowires have been grown by molecular beam epitaxy (MBE), a well-known technique for the high crystalline quality and atomically sharp interfaces in thin film applications. Moreover, to develop a scalable technique and to avoid any possible contamination we used a self-assembly and self-catalyzed approach, which involves only Ga and As, without any patterning of the surface. In a first place we studied the occurrence of GaAs nanowires growth for different types of silicon oxides, such as thermal oxide, native oxide and hydrogen silsesquioxane (HSQ). We determined the critical thicknesses to achieve nanowire growth and investigated the influence of surface roughness. This comparison study lead us to choose native oxide as oxide of choice for GaAs nanowires growth on Si. With this type of oxide, reproducibility and uniformity of results outpaced the others. Successively we developed a simple technique to control native oxide thickness and characterized the chemical composition and wetting. Once the behavior of the oxide properties as a function of oxide thickness was clarified we studied their influence over nanowire growth. We found that impacted the overall possibility of nanowires growth and to control their orientation with respect to the substrate. The root cause of the change in growth morphology was identified to be in the different thermal stability of the oxides with different compositions, and the wetting properties. The understanding of the influence of the surface properties over nanowires nucleation was of paramount importance to achieve reproducible, uniform and scalable growth of vertical nanowires. Once full control over the substrate was achieved, we investigated the tailoring of diameter and density by growth conditions using the self-assembly of Ga droplets. We demonstrated an approach to tailor diameter-density distribution that minimize nanowires-array reflectivity. These results give a clear pathway on how to obtain fully controlled nanowires growth in terms of diameter, density and orientation, paving the way to the development of GaAs nanowires based solar cells on Si.