From Dopant to Source: The Use of Zinc as an Enabler in the Synthesis of Nanostructures by Metalorganic Vapour Phase Epitaxy
Creators: name: Burgess, Timothy
Publisher: name: The Australian National University
Year: 2017
Subjects: subject: Nanotechnology
subject: FOS: Nanotechnology
subject: nanowires
subject: epitaxy
subject: III-V
subject: II-V
subject: zinc arsenide
subject: zinc phosphide
subject: MOCVD
subject: MOVPE

description: As conventional methods of semiconductor fabrication approach fundamental physical limits, new paradigms are required for progress. One concept with the potential to deliver such a paradigm shift is the bottom-up synthesis of semiconductor nanostructures. Beyond further scaling, bottom-up methods promise novel geometries and heterostructures unavailable by conventional top-down methods. This is particularly true in the case of nanostructure growth by the vapour-liquid-solid (VLS) method. Commonly realised using existing vapour phase epitaxy techniques, a range of high-performance VLS devices have now been demonstrated including photovoltaic cells, lasers and high-frequency-transistors. In this dissertation, selected applications of diethylzinc (DEZn) are used to step through a range of opportunities and challenges arising from the VLS synthesis of semiconductor nanostructures by metal-organic vapour phase epitaxy (MOVPE). These applications are broadly grouped into four chapters focusing on the use of zinc firstly as a dopant and then morphological agent, internal quantum efficiency (IQE) enhancer and finally, source. In the context of doping, relatively high DEZn flows are shown to alter the morphology of GaAs nanowires by introducing planar defects, kinking and seed-splitting. Growth studies are used to establish the threshold for these effects and thus the range of DEZn flows suitable for doping. Successful incorporation of up to 5 x1020 Zn/cm3 is demonstrated through atom probe tomography (APT) and electrical characterisation. Building on these results, DEZn is then used to generate periodic twinning in GaAs nanowires. The morphology and overgrowth of these twinning superlattice (TSL) nanowires is studied. Unlike for other III-V materials, twin spacing is found to be a linear function of nanowire diameter. By analysing the probability of twin formation, this result is related to the relatively high twin plane and solid-liquid interface energies of GaAs. Values for the wetting angle and supersaturation of the seed particle during growth are also extracted. In addition to acting as a dopant, zinc is also shown to produce an orders of magnitude increase in the IQE of GaAs nanowires. Performance gains are quantified by measuringthe absolute efficiency of individual nanowires. This increase in IQE with doping enables room-temperature lasing from unpassivated GaAs nanowires. The performance of doped nanolasers, including the transition to lasing, is fully characterised. In addition to increasing radiative efficiency, Zn doping also increases differential gain while reducing the transparency carrier density. The threshold pump power of a Zn doped nanowire is thus shown to be less than that of an equivalent AlGaAs passivated structure. In the final chapter, DEZn is used as a source for the growth of ZnAs, ZnP and ZnSb nanostructures by MOVPE. A range of growth conditions, substrates and seed materials are investigated. Individual nanostructures of both ZnAs and ZnP are shown to exhibit excellent optoelectronic performance with emission from individual nanostructures at 1.0 and 1.5 eV respectively. Overall, this thesis underlines the vast range of possibilities offered by VLS growth and opens to the door to both a variety of new techniques and new family of semiconductor nanomaterials.