Quantum dots (QD) are an exciting new area of research; they have valuable properties that could further the future of opto-electronic devices. Such properties can be probed by performing photo-luminescence (PL) experiments, which is essentially excitation of the dots by optical means and detecting...
2. Introduction to quantum dots
2.1 Self assembly of quantum dots
2.2 Quantum dot ensembles
2.3 Optical properties of quantum dots and single dot spectroscopy
2.4 Electronic properties
2.5 Quantisation in quantum dots
3. Experiment
3.1 The sample
3.2 Experimental setup
4. Results and discussion
4.1 Analysis of dots
4.11 Quantum dot sizes
4.12 Energy state separation
4.13 State filling
4.2 Temperature dependence experiment
4.21 Photoluminescence line width
4.22 Temperature excitation intensity dependence
4.23 Photoluminescence red shift
4.3 Excitation intensity dependence of the quantum dot photoluminescence
, 1. Abstract
Quantum dots (QD) are an exciting new area of research; they
have valuable properties that could further the future of
optoelectronic devices. Such properties can be probed by
performing photoluminescence (PL) experiments, which is
essentially excitation of the dots by optical means and detecting
the photoluminescence emitted. Using this technique, the
temperature and excitation intensity dependence of the PL from
four different quantum dots have been investigated. The results
show the emission linewidth of the dots is strongly affected by
altering the lattice temperature. Deviation from a lorentzian
profile at low temperatures and linewidth broadening due to
acoustic phonon scattering is observed. Temperature dependent
experiments have also shown a systematic red shift and weakening
of the emission peak as temperature is increased, the former
attributed to band-gap reduction and the latter due to thermal
excitation of excitons from quantum dot confinement into the
quantum well (QW). Intensity variations show a red shift, state
filling and furthermore saturation occurs at low temperatures,
while at higher temperatures a quadratic dependence followed by
a linear dependence of excitation intensity is observed.
2. Introduction to quantum dots
Quantum dots are nano-sized structures that confine excitons in all three
directions. They represent the limit in carrier confinement due to their
quasi zero dimensionality, which gives rise to discrete quantised levels.
Due to the small size of quantum dots, coulomb interactions between
charged carriers are enhanced.
The form of the energy states within the quantum dots is quite different
to that of higher dimensional structures such as bulk (3D), quantum wells
(2D), and quantum wires (1D). The density of states is delta function like,
the wave functions are localised inside the dot, but can extend over many
periods of the lattice. The zero-dimensional properties of the quantum
dots can be applied to many systems such as efficient lasers.
2.1 Self assembly of quantum dots
There are many ways of developing the dots; one of the most studied is
the self assembled approach (Stranski-Krastanow (SK) method.)
The SK growth method can be used to form spontaneous quantum dots.
Self assembly of strained dots will only occur if the deposited
semiconductor material has a lager lattice constant than the
semiconductor substrate. For InAs on GaAs there is a lattice mismatch of
approximately 7%. Initially an InAs layer is grown on top of GaAs, and at
a certain thickness known as the critical thickness of about
1.5monolayers, dots form spontaneously. The formation of the dots
occurs on an InAs layer known as the wetting layer.1
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