GaAs-AlGaAs core-shell nanowires: unfolding their nano-scale structure and the role of hetero-interface on optical and functional properties

GaAs nanowires have shown unique potentials for the realization of fast and efficient opto-electronic and photovoltaic nano-devices. The growth of an AlGaAs alloy shell around GaAs nanowires ensures long-term stable passivation of GaAs surface states: enhanced luminescence intensity, improved minority carrier diffusion lengths and recombination lifetimes have been indeed, demonstrated for GaAs-AlGaAs core-shell nanowires. Self-assembly of these core-shell nanostructures has been reported by a variety of growth methods, including MBE and MOVPE. However, the occurrence of defects, elastic strain fields, and growth-dependent inhomogeneity may affect their electronic/optical and functional properties. Understanding the nano-scale properties of these core-shell nanowires is necessary for any future device application. In this talk we present a detailed study of the structural, optical and functional properties of GaAs-AlGaAs core-shell nanowires grown on GaAs and Si substrates by Au-catalyst assisted MOVPE. The morphology (size, shape) and structure of bare GaAs nanowires and of GaAs-AlGaAs core-shell nanowires is investigated, showing that nano-faceting occurs at the shell/core interface as a result of twin defect nucleation. Also, built-in elastic strain fields along the core-shell nanowire axis are experimentally determined by high-resolution XRD and low temperature photoluminescence (PL) measurements, and compared to values calculated based on a uniaxial elastic energy equilibrium model. Application of a vapour mass transport model for AlGaAs shell growth in the case of well-aligned dense nanowire arrays demonstrates the dependence of shell growth rate on nanowire density; this allow us to use a 'combinatorial' approach to explore the PL properties of core-shell nanowires as function of their relevant geometrical parameter, i.e. the shell-thickness to core-radius ratio. Besides the expected strain-dependent effects on the excitonic emission of the GaAs nanowire cores, we demonstrate an additional shell-dependent localization (up to 13 meV), whose possible interface-related origin will be discussed. Spatially indirect electronic transitions in both emission (PL) and absorption (photocurrent) spectra were also recorded for GaAs-AlGaAs core-shell nanowires, further pointing towards a role of the core-shell hetero-interface in determining the nanostructure optical properties. This idea will be discussed also in relation to the previously reported observation of negative differential resistance in core-shell nanowire devices.