Free-standing Quantum Dots Research Articles

The features of moiré pattern on electron-microscope images of free-standing quantum dots containing dislocations

Simulation of moiré pattern arising on plan-view transmission electron-microscope image of nanoisland (quantum dot) containing dislocations was performed on the base of finite-element method. The presence of dislocations with a screw component of Burgers vector in the nanoisland is shown to be energetically unfavourable. On the contrary, edge dislocations allow relieving mismatch stress most effectively. The simulation results provide a means to determine the type of dislocation inside quantum dot by the specific features of the moiré pattern on transmission electron microscope image.

Morphology of self-assembled InSb/GaAs quantum dots on Ge substrate

In this work, we report on the growth of self-assembled InSb/GaAs quantum dots (QDs) on (001) Ge substrate by molecular beam epitaxy. Due to the polar/non-polar nature of GaAs grown on Ge, antiphase domains are formed. Effects of the domain and QD growth temperature on the morphology of realized QDs are presented. InSb QDs are mostly formed at the antiphase-domain boundaries (APBs). The QD size, shape and density are varied with the QD growth temperature. These free-standing QDs have irregular lens and stripe-shapes with {10n} side facets according to the analysis of atomic force microscopy images. InSb QDs is formed at the APBs, where two orthogonal GaAs surfaces are met.

Highly Directional Room-Temperature Single Photon Device.

One of the most important challenges in modern quantum optical applications is the demonstration of efficient, scalable, on-chip single photon sources, which can operate at room temperature. In this paper we demonstrate a room-temperature single photon source based on a single colloidal nanocrystal quantum dot positioned inside a circular bulls-eye shaped hybrid metal-dielectric nanoantenna. Experimental results show that 20% of the photons are emitted into a very low numerical aperture (NA < 0.25), a 20-fold improvement over a free-standing quantum dot, and with a probability of more than 70% for a single photon emission. With an NA = 0.65 more than 35% of the single photon emission is collected. The single photon purity is limited only by emission from the metal, an obstacle that can be bypassed with careful design and fabrication. The concept presented here can be extended to many other types of quantum emitters. Such a device paves a promising route for a high purity, high efficiency, on-chip single photon source operating at room temperature.

Excitation transfer in stacked quantum dot chains

Stacked InAs quantum dot chains (QDCs) on InGaAs/GaAs cross-hatch pattern (CHP) templates yield a rich emission spectrum with an unusual carrier transfer characteristic compared to conventional quantum dot (QD) stacks. The photoluminescent spectra of the controlled, single QDC layer comprise multiple peaks from the orthogonal QDCs, the free-standing QDs, the CHP, the wetting layers and the GaAs substrate. When the QDC layers are stacked, employing a 10 nm GaAs spacer between adjacent QDC layers, the PL spectra are dominated by the top-most stack, indicating that the QDC layers are nominally uncoupled. Under high excitation power densities when the high-energy peaks of the top stack are saturated, however, low-energy PL peaks from the bottom stacks emerge as a result of carrier transfers across the GaAs spacers. These unique PL signatures contrast with the state-filling effects in conventional, coupled QD stacks and serve as a means to quickly assess the presence of electronic coupling in stacks of dissimilar-sized nanostructures.

Fabrication of wafer-scale free-standing quantum dot/polymer nanohybrid films for white-light-emitting diodes using an electrospray method

Wafer-scale free-standing quantum dot (QD)/polymer nanohybrid (NHB) based down-conversion (DC) phosphor films for remote-type white-light-emitting diodes (W-LEDs) are developed.

Optically pumped rolled-up InGaAs/GaAs quantum dot microtube lasers

The authors report on the achievement of lasing in rolled-up semiconductor microtubes at room temperature, wherein self-organized InGaAs/GaAs quantum dots are incorporated as the gain medium. The free-standing quantum dot microtubes, with a diameter of approximately 5-6 microm and wall thickness of approximately 100 nm, are formed when the coherently strained InGaAs/GaAs quantum dot heterostructure is selectively released from the GaAs substrate. The devices are characterized by an ultralow threshold (approximately 4 microW) and a minimum intrinsic linewidth of approximately 0.2 - 0.3 nm at room temperature. The multiple lasing modes are analyzed using both the finite-difference time domain method and also a planar dielectric waveguide model.

Finite element analysis on stress distribution in buried quantum dots

The stacked, self-assembled and vertically aligned quantum dot superlattices are fabricated by alternating growth of substrate and epitaxial materials, the stress/strain fields in the buried quantum dots can influence their optical and piezoelectric properties and mechanical stability. The distributions of stresses, strains, hydrostatic strains and biaxial strains in buried strain self-assembled Ge/Si semiconductor quantum dot are investigated based on the theory of anisotropy elasticity and also compared with those of free-standing quantum dot. The sameness and difference of the stresses/strains between the buried and the free-standing quantum dots, and the influence of cap layer on the stress/strain fields in quantum dots are given.

Single electron transport in a free-standing quantum dot

We have fabricated a single quantum dot in the two-dimensional electron gas of a free-standing GaAs/AlGaAs heterostructure. Standard processing techniques are used to define the dot on a narrow mesa and selective etching undercuts the device to form a 120nm thick free-standing beam connected to the substrate by source and drain regions. Work by Weig et al. [Phys. Rev. Lett. 92(4) (2004) 046804] on similar devices has provided evidence for phonon blockade which suppresses conduction at low temperatures and bias. Further studies are presented here that can only partially be explained by this theory. A similar gap in conductance has been seen, but evolution of the structure in a magnetic field suggests an alternative explanation of multiple dots in series, may better explain the results.

Acoustic modes in free and embedded quantum dots

Acoustic phonon spectra were calculated for different quantum dots for the free-standing case and for the case in which the quantum dot is embedded in a selection of different matrix materials including semiconductors, plastic, and water. The case of water as a matrix embedding a quantum dot is of special interest in biology where quantum dots are being used as biological tags. The acoustic phonon modes can be normalized in the free-standing case and the Hamiltonians for the deformation potential interactions derived. The results demonstrate that the matrix can have a large effect on the acoustic phonon spectrum and therefore it should be included when calculating the acoustic modes of the quantum-dot heterostructures. Simple analytic results are used to specify completely the lowest-order spherical breathing mode for free-standing quantum dots.

THE INFLUENCE OF ENVIRONMENTAL EFFECTS ON THE ACOUSTIC PHONON SPECTRA IN QUANTUM-DOT HETEROSTRUCTURES

We present calculations of the acoustic phonon spectra for a variety of quantum dots and consider the cases where the quantum dots are both free-standing and embedded in a selection of different matrix materials — including semiconductors, plastic, and water. These results go beyond previous calculations for free-standing quantum dots and demonstrate that the matrix material can have a large effect on the acoustic phonon spectrum and consequently on a variety of phonon-assisted transitions in quantum-dot heterostructures.

Interface phonons in GaAs/AlAs quantum dots

Electron scattering by interface phonon modes in GaAs/AlAs quantum dots is investigated. The emission rates are calculated using an electron-phonon interaction Hamiltonian derived in the framework of the dielectric continuum model. The effect of the embedding material AlAs on the electron relaxation by interface phonons is analyzed by comparing the emission rates with those obtained for GaAs free standing quantum dots in vacuum. This effect reduces the emission rates yielding relaxation times slower than the radiative recombination lifetime of electrons and holes in their ground levels. In GaAs/AlAs quantum dots, it appears that the contribution of the electron relaxation via interface phonon emission to the radiative decay processes is less significant than the relaxation mechanisms via longitudinal optical and longitudinal acoustic phonon. Faster relaxation times are obtained if the interface phonon mode involved in the scattering processes is antisymmetric.

Photoluminescence studies of GaAs-AlGaAs quantum dots

Free standing quantum dots with diameters between 40 and 400nm have been fabricated by electron beam lithography and reactive ion etching. The extent of the damage and the importance of surface recombination have been assessed optically using low temperature photoluminescence and photoluminescence excitation spectroscopy. It was found that the luminescence efficiency of the quantum dots scaled approximately with the volume of well material excited.

Photoluminescence of overgrown GaAs-GaAlAs quantum dots

Buried GaAs-GaAlAs quantum dots have been grown by MOCVD for the first time. Free-standing quantum dots were formed in GaAs-GaAlAs single quantum well material by a combination of electron beam lithography and dry etching, then the dots were buried by regrowing GaAlAs. Low temperature (60K) photoluminescence spectra shows an emission which is increased in intensity and shifted to higher energies from the dots after they are buried. The efficiency of the emission scales approximately with the volume of material remaining after patterning.