Dynamical theory and experiments on GaAs nanowire growth for photovoltaic applications

The geometry of nanowire solar cells provides many potential advantages compared to planar solar cells, such as reduced reflection, built-in light concentration due to absorption resonances, improved band gap tuning for multi-junction devices and an increased defect tolerance. Moreover, the use of nanowires reduces the quantity of material necessary to approach the limits of light to electric power conversion efficiency, allowing for substantial cost reductions if they are grown on a cheap substrate. However, it is far from straightforward to achieve optimum design of bottom up grown nanowire solar cells, as it requires control and an in-depth understanding of complex growth kinetics controlling the nanowire crystal formation and dopant incorporation. This thesis is concerned with the growth of self catalyzed GaAs based semiconductor nanowires on silicon substrates in a molecular beam epitaxy system, with the aim of growing nanowires for highly efficient photovoltaic applications. As it is crucial to control and understand the mechanisms behind the nanowire crystal formation, not only in terms of the overall nanowire morphology but also in terms of the crystal phase purity, this thesis begins with a formulation of a theoretical framework which can serve as a basis to model and understand the dynamics of III-V nanowire growth via the ‘vapor-liquid-solid’ method. The formalism is based on principles from transition state kinetics driven by a Gibbs free energy minimization process. The crystallization process is described in terms of a dynamic liquid-solid growth system which continuously seeks to lower the excess Gibbs free energy originating from the adatoms and gas states. Nucleation statistics and the nucleation limited growth at the topfacet which force the solid-liquid growth system far from equilibrium are discussed in detail. Self-catalyzed GaAs nanowire growth which will be the main focus for the photovoltaic applications is used as a model system, and examples of in depth dynamical simulations compared with experiments are shown. The work gradually involves more detailed growth experiments such as in-situ x-ray characterization of growing nanowires and growth of advanced photovoltaic structures and finally photovoltaic characterization of both lying and standing single nanowire devices are presented. All the different kind of single NW solar cell devices show an enormous potential as light absorbers. Especially the single vertical NW solar cells which are characterized as grown on the substrate shows an apparent solar cell efficiency of and a photo generated current which is much higher than previous reported on single NW solar cells.