III-V core-shell Nanowires for Optoelectronic Devices

Core-shell nanowires (NWs) of III-V semiconductors possess unique superior characteristics over their planar counterparts for the realization of novel logic, photovoltaic, and light emitting devices. As building blocks, they offer 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, a most promising technology for the synthesis of NW-based devices, still requires demonstrating its entire potentials in terms of materials/device performances and industrial scalability. The growth of NW structures and the study of their physical properties are crucial in order to improve/optimize device performances. In this talk, we report on the optical/electronic and functional properties of GaAs NWs and core-shell GaAs-AlGaAs NWs, as a case study. The micro-structural properties (morphology, size, inner composition and crystal strain) of these free-standing NW nanostructures will be first presented. The characteristic photoluminescence (PL) core emission of GaAs-AlGaAs core-shell NWs will be then discussed as function of the NW relevant geometrical parameters, namely their hs/Rc=(shell thickness)/(core radius) ratio. The GaAs emission appears to redshifts with the hs/Rc ratio. Comparison between the NW excitonic energy position and the strain-shifted values of heavy- and light-hole excitons calculated upon assuming perfect coherence at the GaAs-AlGaAs hetero-interface and elastic energy equilibrium within the nanowire, allow identifying the GaAs core dominant PL emission as due to bound heavy-exciton recombination. Further, a tentative explanation in terms of exciton localization of observed spectral redshifts will be given. Understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in these NWs. In this respect, the application of photocurrent and photoluminescence spectroscopies to core-shell NW systems, allows to build up a band diagram of a single heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets. Finally, the fabrication of photodetectors based on Schottky-contacted single core-shell GaAs-AlGaAs NWs will be presented. Noteworthy, as-fabricated detectors exhibit relatively strong polarization anisotropy of their photocurrent, and record high external quantum efficiencies (about 10% at 600 nm). Also, core-shell NW devices exhibit significantly improved dc and high-speed performances over bare GaAs NWs, and comparable to planar MSM photodetectors. Picosecond temporal response coupled with pA dark currents demonstrate the device potential for high-speed imaging arrays and on-chip optical interconnects.