1,012 research outputs found
Growth and Characterisation of GaAs/AlGaAs Core-shell Nanowires for Optoelectronic Device Applications
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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Cyclin B1/CDK1-regulated mitochondrial bioenergetics in cell cycle progression and tumor resistance.
A mammalian cell houses two genomes located separately in the nucleus and mitochondria. During evolution, communications and adaptations between these two genomes occur extensively to achieve and sustain homeostasis for cellular functions and regeneration. Mitochondria provide the major cellular energy and contribute to gene regulation in the nucleus, whereas more than 98% of mitochondrial proteins are encoded by the nuclear genome. Such two-way signaling traffic presents an orchestrated dynamic between energy metabolism and consumption in cells. Recent reports have elucidated the way how mitochondrial bioenergetics synchronizes with the energy consumption for cell cycle progression mediated by cyclin B1/CDK1 as the communicator. This review is to recapitulate cyclin B1/CDK1 mediated mitochondrial activities in cell cycle progression and stress response as well as its potential link to reprogram energy metabolism in tumor adaptive resistance. Cyclin B1/CDK1-mediated mitochondrial bioenergetics is applied as an example to show how mitochondria could timely sense the cellular fuel demand and then coordinate ATP output. Such nucleus-mitochondria oscillation may play key roles in the flexible bioenergetics required for tumor cell survival and compromising the efficacy of anti-cancer therapy. Further deciphering the cyclin B1/CDK1-controlled mitochondrial metabolism may invent effect targets to treat resistant cancers
Continuous-variable controlled-Z gate using an atomic ensemble
The continuous-variable controlled-Z gate is a canonical two-mode gate for
universal continuous-variable quantum computation. It is considered as one of
the most fundamental continuous-variable quantum gates. Here we present a
scheme for realizing continuous-variable controlled-Z gate between two optical
beams using an atomic ensemble. The gate is performed by simply sending the two
beams propagating in two orthogonal directions twice through a spin-squeezed
atomic medium. Its fidelity can run up to one if the input atomic state is
infinitely squeezed. Considering the noise effects due to atomic decoherence
and light losses, we show that the observed fidelities of the scheme are still
quite high within presently available techniques.Comment: 7 pages, 3 figures, to appear in Physical Review
Dependency Stochastic Boolean Satisfiability: A Logical Formalism for NEXPTIME Decision Problems with Uncertainty
Stochastic Boolean Satisfiability (SSAT) is a logical formalism to model
decision problems with uncertainty, such as Partially Observable Markov
Decision Process (POMDP) for verification of probabilistic systems. SSAT,
however, is limited by its descriptive power within the PSPACE complexity
class. More complex problems, such as the NEXPTIME-complete Decentralized POMDP
(Dec-POMDP), cannot be succinctly encoded with SSAT. To provide a logical
formalism of such problems, we extend the Dependency Quantified Boolean Formula
(DQBF), a representative problem in the NEXPTIME-complete class, to its
stochastic variant, named Dependency SSAT (DSSAT), and show that DSSAT is also
NEXPTIME-complete. We demonstrate the potential applications of DSSAT to
circuit synthesis of probabilistic and approximate design. Furthermore, to
study the descriptive power of DSSAT, we establish a polynomial-time reduction
from Dec-POMDP to DSSAT. With the theoretical foundations paved in this work,
we hope to encourage the development of DSSAT solvers for potential broad
applications.Comment: 10 pages, 5 figures. A condensed version of this work is published in
the AAAI Conference on Artificial Intelligence (AAAI) 202
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