Optical properties of semiconducting nanowires

Abstract

Semiconductor nanowires of high purity and crystallinity hold promise as building blocks for opto-electronical devices at the nanoscale.. They are commonly grown via a Vapor-Liquid-Solid (VLS) mechanism in which metal (nano) droplets collect the semiconductor precursors to form a solution which, when saturated, leads to the growth of a wire underneath the droplet. After a brief discussion of this general mechanism, the growth of InP and ZnO nanowires is detailed. The grown InP nanowires have an integrated alloy particle and have on average diameters of 50 nm and lengths of 10 μm. ZnO nanowires grown on silicon oxide covered substrates exhibit an integrated alloy particle, have diameters in the 50-100 nm range and lengths of up to 50 μm. In contrast, under the same growth conditions, ZnO nanowires grown epitaxially on Al2O3 substrates do not have an integrated gold particle and exhibit diameters mostly in the 100-300 nm range with lengths of up to 10 μm. Finally it is shown that using a method that is widely applicable for nanostructures, ZnO nanowires can be doped with cobalt ions which is an important step towards room-temperature ferromagnetic semiconducting nanowires for spintronic applications. The as-grown single crystal InP nanowires, covered with a In2O3 surface oxide, show a low photoluminescence efficiency that strongly varies from wire to wire. It is shown that the luminescence efficiency of single-crystal InP nanowires can be improved by photo-assisted wet chemical etching in a butanol solution containing HF and the indium-coordinating ligand tri-octyl-phosphineoxide (TOPO). Electron-hole photo generation, electron scavenging and oxidative dissolution combined with surface passivation by the indium-coordinating ligand are essential elements to improve the luminescence efficiency. Time-traces of the luminescence of surface-passivated wires show strong oscillations resembling the on-off blinking observed with single quantum dots. These results reflect the strong influence of a single or a few non-radiative recombination centre(s) on the luminescence properties of an entire wire. Using scanning-excitation single- wire emission spectroscopy, with a laser or electron beam as a spatially resolved excitation source, we observe standing-wave exciton-polaritons in ZnO nanowires at room temperature. The Rabi-splitting between the polariton branches is more than 100 meV indicative for huge light-matter interaction. Our results suggest that the remarkable sub-wavelength guiding in ZnO nanowires, reported before, is mediated by exciton-polaritons. The dispersion curve of "light" is substantially modified due to strong light-matter interaction; this will have to be taken into account in future nanophotonic circuitry. Finally in this thesis, the laser emission from individual ZnO nanowires is investigated. The energy spacing between sharp lasing modes scales with the reciprocal length of the nanowire; thus, laser emission peaks correspond to longitudinal Fabry-Pérot modes of the nanowire cavity. An interference pattern due to coherent laser emission from the wire end facets is observed. Comparison with numerical simulations shows that the laser light is emitted nearly spherically from the wire ends, with a zero or fixed phase difference.