Growth and Characterisation of GaAs/AlGaAs Core-shell Nanowires for Optoelectronic Device Applications
Date
2015
Authors
Jiang, Nian
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Abstract
III-V semiconductor nanowires have been investigated as key
components for future electronic and optoelectronic devices and
systems due to their direct band gap and high electron mobility.
Amongst the III-V semiconductors, the planar GaAs material system
has been extensively studied and used in industries. Accordingly,
GaAs nanowires are the prime candidates for nano-scale devices.
However, the electronic performance of GaAs nanowires has yet to
match that of state-of-the-art planar GaAs devices. The present
deficiency of GaAs nanowires is typically attributed to the large
surface-to- volume ratio and the tendency for non-radiative
recombination centres to form at the surface. The favoured
solution of this problem is by coating GaAs nanowires with AlGaAs
shells, which replaces the GaAs surface with GaAs/AlGaAs
interface.
This thesis presents a systematic study of GaAs/AlGaAs core-shell
nanowires grown by metal organic chemical vapour deposition
(MOCVD), including understanding the growth, and characterisation
of their structural and optical properties. The structures of the
nanowires were mainly studied by scanning electron microscopy and
transmis- sion electron microscopy (TEM). A procedure of
microtomy was developed to prepare the cross-sectional samples
for the TEM studies. The optical properties were charac- terised
by photoluminescence (PL) spectroscopy. Carrier lifetimes were
measured by time-resolved PL. The growth of AlGaAs shell was
optimised to obtain the best optical properties, e.g. the
strongest PL emission and the longest minority carrier
lifetimes.
The sidewalls of the vapour-liquid-solid (VLS) grown GaAs
nanowires were investi- gated. It was found that a Reuleaux
triangle with 3 {112}A curved surfaces is the actual shape of the
nanowire at the growth interface. This Reuleaux triangle changes
into well defined {112} facets as a result of the simultaneous
radial growth. A theoretical model was developed to explain the
orientations of nanowire sidewall facets.
The sidewalls of GaAs nanowires were found to transform to {110}
facets at high temperature as a result of surface atom migration.
The rate of the facet transformation was found to be controlled
by temperature and the difference in the surface energies, which
leads to different faceting behaviour along the length of the
nanowire. While the sidewalls of the top segment were fully
transformed into {110} facets, the sidewalls of the bottom of the
nanowires were a mixture of {110} and {112} facets. This
facet-change along the length of the nanowire directly affected
the subsequent growth of AlGaAs shell. This was relevant to the
non-uniform PL emission and the minority carrier lifetimes (tmc)
along the GaAs/AlGaAs core-shell nanowires. The strongest PL
emission and longest tmc was observed where the GaAs core had six
{110} facets. PL intensity and tmc decreased towards the bottom
of the nanowire where the sidewall facets of the GaAs core
consisted of both {110} and {112} facets.
The effect of AlGaAs shell growth parameters (including V/III
ratio, temperature and time) on the optical properties of
GaAs/AlGaAs core-shell nanowires was investigated on nanowires
catalysed by Au particles with a diameter of 50 nm. The V/III
ratio and shell growth temperature were found to profoundly
affect the optical properties. A high V/III ratio and/or a high
growth temperature dramatically increased tmc. Further increasing
the V/III ratio and/or growth temperature resulted in drop of
tmc. Interme- diate V/III ratio and shell growth temperature were
chosen as a compromise to achieve long tPL. The AlGaAs shell
growth time also showed a significant effect on tmc. tmc
increased with shell growth time to a maximum, followed by a
further drop with longer shell growth time. With the optimised
AlGaAs shell growth, an average carrier life- time of (1.02 ±
0.4) ns was achieved from single GaAs/AlGaAs core-shell nanowires
at room temperature. This is comparable to self-assisted
nanowires grown by molec- ular beam epitaxy and also proved that
Au catalyst is not detrimental to the optical properties in
VLS-grown GaAs nanowires. The long lifetimes are mainly
attributed to the improvement of the GaAs/AlGaAs interface
quality that is comparable with planar heterostructures.
The effect of AlGaAs shell growth time and shell thickness on tPL
were investigated. It was found that both the shell thickness and
shell growth time affected tmc. A certain shell thickness is
required to prevent the carriers generated in GaAs core from
tunnelling through the AlGaAs shell and recombining at the free
surface of GaAs cap layer. Beyond this thickness, the shell
growth time, which is related to the diffusion at the
heterointer- face, becomes the primary parameter controlling the
carrier lifetimes. Lifetimes as long as 1.9 ns were achieved by
reducing the effect of diffusion.
This work presents an in-depth understanding of the geometry of
GaAs nanowires, demonstrates GaAs/AlGaAs core-shell nanowires
with optical quality comparable with planar heterostructures and
reveals intriguing structural/optical behaviour of the nano-
wires. These findings will greatly assist the fabrication of
efficient nanowire devices and show a strong future for
nano-optoelectronic devices based on nanowires.
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Keywords
Metal Organic Chemical Vapour Deposition, Nanowires, semiconductor, electron microscopy, microtomy, spectroscopy, GaAs
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Thesis (PhD)
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DOI
10.25911/5d78d4a5b4052