108,581 research outputs found
Excitonic properties of strained wurtzite and zinc-blende GaN/Al(x)Ga(1-x)N quantum dots
We investigate exciton states theoretically in strained GaN/AlN quantum dots
with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained
WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies
the conduction and valence band edges of GaN quantum dots. The piezoelectric
field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while
it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN
quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN
quantum dots, with heights larger than 3 nm, exhibits a red shift with respect
to the bulk WZ GaN energy gap. The radiative decay time of the red-shifted
transitions is large and increases almost exponentially from 6.6 ns for quantum
dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ
GaN/AlGaN quantum dots, both the radiative decay time and its increase with
quantum dot height are smaller than those in WZ GaN/AlN quantum dots. On the
other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order
of 0.3 ns, and is almost independent of the quantum dot height. Our results are
in good agreement with available experimental data and can be used to optimize
GaN quantum dot parameters for proposed optoelectronic applications.Comment: 18 pages, accepted for publication in the Journal of Applied Physic
Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy
We investigate the optical emission and decay dynamics of excitons confined
in large strain-free GaAs quantum dots grown by droplet epitaxy. From
time-resolved measurements combined with a theoretical model we show that
droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an
oscillator strength between 8 and 10. The quantum dots are found to be fully
described by a model for strongly-confined excitons, in contrast to the
theoretical prediction that excitons in large quantum dots exhibit the
so-called giant oscillator strength. We attribute these findings to localized
ground-state excitons in potential minima created by material intermixing
during growth. We provide further evidence for the strong-confinement regime of
excitons by extracting the size of electron and hole wavefunctions from the
phonon-broadened photoluminescence spectra. Furthermore, we explore the
temperature dependence of the decay dynamics and, for some quantum dots,
observe a pronounced reduction in the effective transition strength with
temperature. We quantify and explain these effects as being an intrinsic
property of large quantum dots owing to thermal excitation of the ground-state
exciton. Our results provide a detailed understanding of the optical properties
of large quantum dots in general, and of quantum dots grown by droplet epitaxy
in particular.Comment: 13 pages, 7 figure
Tunable few electron quantum dots in InAs nanowires
Quantum dots realized in InAs are versatile systems to study the effect of
spin-orbit interaction on the spin coherence, as well as the possibility to
manipulate single spins using an electric field. We present transport
measurements on quantum dots realized in InAs nanowires. Lithographically
defined top-gates are used to locally deplete the nanowire and to form
tunneling barriers. By using three gates, we can form either single quantum
dots, or two quantum dots in series along the nanowire. Measurements of the
stability diagrams for both cases show that this method is suitable for
producing high quality quantum dots in InAs.Comment: 8 pages, 4 figure
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