MOVPE self-assembly of III-V nanowires: advances in growth control and device fabrication

Free-standing nanowires (NWs) based on III-V semiconductors are considered ideal building blocks offering fascinating potential for future technological applications such as the realization of novel and efficient nanophotonic devices and photovoltaic cells. Self-assembly of III-V NWs by metalorganic vapor phase epitaxy (MOVPE) through the Au-catalyzed 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, we report on the self-assembly by MOVPE and the properties of GaAs NWs and core-shell GaAs-AlGaAs NWs, 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. 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 GaAs/Si hetero-substrates will be then discussed. The complex interplay between the NW size and height, and their array density in determining the radial (vapor-solid) growth rate of a shell material during the MOVPE process will be reported and assessed by validation of a shell vapour growth model. The fabrication of photodetectors based on Schottky-contacted single core-shell GaAs-AlGaAs NWs will be then presented. Noteworthy, as-fabricated detectors exhibit relatively strong polarization anisotropy of their spectral photocurrent, and record high external quantum efficiencies (about 10% at 600 nm). Also, core-shell devices exhibit significantly improved dc and high-speed performances over bare GaAs NWs, and comparable to planar MSM photodetectors. Picosecond temporal response coupled with picoampere dark currents, demonstrates the device potential for high-speed imaging arrays and on-chip optical interconnects. The dynamic control of hot electron transfer rates in nanoscale heterojunctions is relevant for novel photovoltaic devices: the hot photoexcited electron transfer across the coaxial interface of a single GaAs/AlGaAs core-shell nanowire device will be reported.