Advances in 1-dimensional semiconductor nanostructures: from self-assembly to functional nanodevices

Recently, free-standing nanowires (NWs) based on III-V compound semiconductors have attracted much interest due to their unique physical properties and huge potential for future realization of nanodevices with innovative and/or enhanced functionalities: these systems are indeed, being considered as ideal building blocks for the fabrication of novel and efficient nanophotonic and nano-electronic devices, and III-generation photovoltaic cells. High-quality III-V NW-based heterostructures, with sharp interfaces and good control in size, shape and material composition, can nowadays be grown using various techniques, including metalorganic vapor phase epitaxy (MOVPE. Self-assembly of III-V NWs by MOVPE through the Au-catalyst assisted (so-called Vapour-Liquid-Solid, VLS) mechanism is a most promising technology for the synthesis of NW-based devices, but it still requires demonstrating its entire potentials in terms of materials/device performances and industrial scalability. The growth of device-quality NW structures and the study of their physical properties is crucial in order to improve/optimize the performances of NW-based devices. In this lecture, the self-assembly by MOVPE and the properties of GaAs NWs and core-shell GaAs-AlGaAs NWs will be reported as a case study. The controlled growth of vertically well-aligned GaAs, AlGaAs and GaAs-AlGaAs core-shell NWs will be presented, focusing on both NW properties (morphology, size, growth rate, inner composition, defects content, and luminescence) and on their dependence on MOVPE self-assembly conditions. Moreover, the fabrication of novel single-NW photo-detectors based on MOVPE-grown GaAs and GaAs-AlGaAs core-shell nanostructures will be reported, along with their spectral photocurrent properties. Core-shell structures are advantageous in this respect, as they reduce substantially carrier recombination at the NW surface, thus achieving higher quantum efficiency. Moreover, the GaAs/AlGaAs heterostructure offers other benefits such as an almost strain-free NW system due to the close lattice matching between core and shell materials, and possibility of band bending at the hetero-interface which confines the electrons and facilitates their transport. The potentials of self-assembly MOVPE technology for photovoltaic applications through the growth of large and dense arrays of well-aligned GaAs and GaAs-AlGaAs NWs on Si substrates will be finally presented and discussed.