From Dopant to Source: The Use of Zinc as an Enabler in the Synthesis of Nanostructures by Metalorganic Vapour Phase Epitaxy
Date
2017
Authors
Burgess, Timothy
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Abstract
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.
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Nanotechnology, nanowires, epitaxy, III-V, II-V, zinc arsenide, zinc phosphide, MOCVD, MOVPE
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