GaAs Self-assembled Quantum Dots Research Articles

Sequential Photon Emissions from Quad-excitons in Single GaAs Quantum Dots

We studied photoluminescence (PL) spectra of a single self-assembled GaAs quantum dot (QD) grown by modified droplet epitaxy. The single QD exhibits several sharp peaks in the PL spectrum indicating the formation of multi-excitonic complexes. Most of these peaks are originated from the recombinations of electron-hole pairs in the lowest level of the QD. We observed several emission peaks arising from recombinations from the second level of the QD at the condition of strong excitation. The difference of the recombination energy between the lowest and second levels is about 30 meV. We identified the emissions of tri-excitons and quad-excitons in the PL spectrum by single photon correlation spectroscopy. Quad-exciton, which is the four-exciton complex, emits correlated four photons in the cascade recombination. We observed bunching of photons emitted from the cascade by cross-correlation measurements.

Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots

We report on the implementation of a scanning Fabry-Pérot interferometer for photoluminescence spectroscopy investigation. We choose a conveniently small reflectivity of the two planar semitransparent mirrors which, in spite of a moderate cavity finesse, ensures a good mechanical stability over a long time. We also exploit the large tuneability of the cavity length (i.e., of the free spectral range) for changing the spectral resolution over two order of magnitude (from ~300 μeV to ~4 μeV in full width at half maximum). Such a characteristic easily allows to scan both sharp and broad luminescence bands. We test our Fabry-Pérot interferometer on sharp photoluminescence lines resulting from excitonic recombination in self-assembled GaAs quantum dots. We demonstrate the ability of our system to resolve linewidth as small as 4 μeV.

Facetting of the self-assembled droplet epitaxial GaAs quantum dot

In this work, droplet epitaxially grown GaAs quantum dots on AlGaAs surface are studied. The quantum dots are investigated in situ with RHEED and ex situ with TEM method. The TEM picture shows that the quantum dot is perfectly crystalline and fits very well to the crystal structure of the substrate. Furthermore, the side of the quantum dot shows stepped facet shape. Here, we show, how the stepped side shape forms from the droplet during crystallization. The RHEED picture shows broadened chevron-tail, which can be explained by the shape of the quantum dot.

Electron delocalization of tensily strained GaAs quantum dots in GaSb matrix

The magneto-optical response of type-II tensily strained GaAs self-assembled quantum dots in GaSb was investigated in magnetic fields up to 14 T. By depositing different GaAs amount, the dot sizes and the corresponding emission energies were varied. We analyzed the carrier wave function extent of different dots using the diamagnetic shift results. It was found that, with the increase in the energy (the reduction in the dot size), the diamagnetic coefficient first rises quickly and then saturates at around 21 μeV/T2. Based on a simple calculation model, this unusual tendency is attributed to the electrons gradually spilling out of the quantum dot to the wetting layer as the dots get smaller. This delocalization effect is enhanced in this material system due to the tensile strain relaxation within the dots, which raises the conduction band edge over that in the wetting layer.

Fine structure splitting reduction in droplet epitaxy GaAs quantum dots grown on (111)A surface

In this contribution we compare the geometrical symmetry of self-assembled GaAs quantum dots (QDs) grown by a refined droplet epitaxy (DE) method on (100) and (111)A AlGaAs substrate. By atomic force microscopy (AFM) we study the morphology of the QDs and by micro–photoluminescence spectroscopy (μPL) we compare the dependence of the fine structure splitting on QD size and shape. We show that: i) the refined DE enhances the elongation of QDs grown on (100) AlGaAs substrate increasing the fine structure splitting; ii) by growing the QDs on the (111)A AlGaAs surface it is possible to recover a good geometrical circular symmetry obtaining a small value of the fine structure splitting .

High quality relaxed GaAs quantum dots in GaP matrix

A system of GaAs self-assembled quantum dots (QDs) embedded in GaP matrix was studied by means of transmission electron microscopy, steady-state, and transient photoluminescence. Unusually, the QDs are fully unstrained but they have no nonradiative centers introduced by dislocations at GaAs/GaP heterostructure. The band alignment in the QDs is shown to be of type I.

Magneto-optical properties of excitonic complexes in GaAs self-assembled quantum dots

We report a detailed experimental investigation of the magneto-optical properties of different excitonic complexes (neutral exciton, neutral biexciton, and charged exciton) confined in self-assembled GaAs/AlGaAs strain-free quantum dots grown by droplet epitaxy, where all piezoelectric effects are lacking. We measured the Land\`e $g$ factor and the diamagnetic coefficient $\ensuremath{\gamma}$ for several quantum dots spanning an interval of $\ensuremath{\sim}200\text{ }\text{meV}$ of the emission energy. The dependence of $g$ and $\ensuremath{\gamma}$ on quantum dot size and shape is discussed together with a comparison with Stranski-Krastanov and fluctuation-induced GaAs quantum dots, as well as with excitons confined in quantum wells.

Ultra-narrow emission from single GaAs self-assembled quantum dots grown by dropletepitaxy

We realized ultra-narrow excitonic emission from single GaAs/AlGaAs quantum dots (QDs) grownby a refined droplet epitaxy technique. We found that uncapped quantum dots can be annealed at400 °C without major changes in their morphology, thus enabling an AlGaAs capping layer to begrown at that temperature. Consequently, we demonstrate a fourfold reduction of thelinewidth of the emission together with an increased recombination lifetime, compared tothe conventional droplet epitaxial QDs. The averaged linewidth of neutral excitonsmeasured by micro-photoluminescence on single quantum dots was around 35 µeV.

Correlated photons from multi-carrier complexes in GaAs quantum dots

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.

The RHEED tracking of the droplet epitaxial grown quantum dot and ring structures

Self-assembled strain-free GaAs quantum dot and quantum ring structures grown by droplet epitaxy were investigated by the RHEED technique. The temporal evolution of these nanometer-scale objects were tracked in situ manner during the growth process. The initial stage of the surfaces was the same for both kinds of quantum objects. We have compared step by step the RHEED pattern in [ 1 1 ¯ 0 ] direction in the case of both quantum objects. After the Ga deposition the pattern becomes diffuse. Almost at the same time with the offering of arsenic the RHEED pattern changed from diffuse to spotty and later changed slowly to spots with chevrons in case of the quantum dot. In case of the quantum ring the situation is quite different. The pattern, which consists of streaks and spots, evaluate slowly and continuously. The different patterns and their different temporal evaluations can be explained with the shape and the kinetic of development of different quantum objects.

Bunching visibility for correlated photons from single GaAs quantum dots

We study photon bunching phenomena associated with biexciton-exciton cascade in single GaAs self-assembled quantum dots. Experiments carried out with a pulsed excitation source show that significant bunching is only detectable at very low excitation, where the typical intensity of photon streams is less than the half of their saturation value. Our findings are qualitatively understood with a model which accounts for Poissonian statistics in the number of excitons, predicting the height of a bunching peak being determined by the inverse of probability of finding more than one exciton.

Further insight into the temperature quenching of photoluminescence from InAs∕GaAs self-assembled quantum dots

The possibility of controlling the photoluminescence (PL) intensity and its temperature dependence by means of in-growth and postgrowth technological procedures has been demonstrated for InAs∕GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). The improvement of the QD emission at room temperature (RT), achieved due to a treatment with tetrachloromethane used during the growth, is explained by the reduction of the point defect concentration in the capping layer. It is shown that the PL quenching at RT appears again if the samples are irradiated with protons, above a certain dose. These findings are accounted for by the variations in the quasi-Fermi level position of the minority carriers, which are related to the concentration of trapping centers in the GaAs matrix and have been calculated using a photocarrier statistical model including both radiative and nonradiative recombination channels. By taking into consideration the temperature dependent distribution of the majority and minority carriers between the QDs, embedding QW and GaAs barriers, our calculated results for the PL intensity reproduce very well the experimentally observed trends.

Determination of spin polarization in InAs∕GaAs self-assembled quantum dots

The spin polarization of electrons trapped in InAs self-assembled quantum dot ensembles is investigated. A statistical approach for the population of the spin levels allows one to infer the spin polarization from the measured values of the addition energies. From the magnetocapacitance spectroscopy data, the authors found a fully polarized ensemble of electronic spins above 10T when B∥[001] and at 2.8K. Finally, by including the g-tensor anisotropy, the angular dependence of the spin polarization with the magnetic field B orientation and strength could be determined.

Multi-excitonic structures in GaAs quantum dots proved by photon correlation spectroscopy

We studied micro-photoluminescence (μ-PL) spectra of a single GaAs self-assembled quantum dot (QD) using a pulsed excitation light source. At low excitation, we observed single photon emission from a neutral exciton in an isolated QD. For higher excitation, multiple peaks appeared in the spectra, reflecting the formation of exciton complexes. Cross-correlation functions between these lines showed either bunching or antibunching behavior, depending on whether the relevant emission was from a biexcitonic cascade or a charged exciton recombination.

Optical investigation of type II GaSb∕GaAs self-assembled quantum dots

We have studied the emission and absorption properties of type II GaSb∕GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ∼500meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system.

POLARIZED PHOTOLUMINESCENCE OF EXCITONS IN n-, p- AND UNDOPED InAs/GaAs QUANTUM DOTS

Polarized photoluminescence from InAs / GaAs self-assembled quantum dots has been investigated under continuous wave excitation by circularly polarized light. Depolarization of the photoluminescence in a magnetic field perpendicular to the wavevector of the exciting light (Hanle effect) was studied. For n-doped, undoped and p-doped quantum dots the observed Hanle curves exhibit qualitatively different shapes: single-, bi-, and tri-Lorentzian, respectively. In the two latter cases the complicated shape of the Hanle curve is attributed to the different processes of radiative recombination which take place simultaneously. Recombination of uncharged excitons and recombination of positively charged exciton complexes are considered. Corresponding spin lifetimes are determined.

Enhancement of the room temperature luminescence of InAs quantum dots by GaSb capping

The authors have studied the use of antimony for the optimization of the InAs∕GaAs(001) self-assembled quantum dot (QD) luminescence characteristics in the 1.3μm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at 1.3μm emission wavelength.

Lasing characteristics of GaSb∕GaAs self-assembled quantum dots embedded in an InGaAs quantum well

The authors report the optical characteristics of GaSb∕GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). Variations in the In composition of the QW can significantly alter the emission wavelength up to 1.3μm and emission efficiency. Lasing operation at room temperature is obtained from a 2-mm-long device containing five stacked GaSb QDs in In0.13Ga0.87As QWs at 1.026μm with a threshold current density of 860A∕cm2. The probable lasing transition involves electrons and holes confined in the QW and QDs, respectively, resulting in a large peak modal gain of 45cm−1. A significant blueshift of the electroluminescence peak is observed with increased injection current and suggests a type-II band structure.

Ultrafast Electronic Dynamics in Unipolar n-Doped InGaAs–GaAs Self-Assembled Quantum Dots

The dynamics of electron capture and relaxation in an n-doped quantum-dot (QD) infrared detector structure are studied directly in the time domain using ultrafast intraband-pump-interband-probe differential transmission spectroscopy. Femtosecond midinfrared pulses are used to excite electrons from the doped QDs into the conduction band continuum, and the complete electron distribution functions are monitored as a function of time using an interband probe. Because only electrons are excited and no holes are present, the electron-hole scattering which dominates the relaxation in bipolar systems is not present, and the measurement yields the electron dynamics exclusively. Excitation-dependent electron capture times were measured from 40 to <10 ps with increasing pump intensity. Intradot inter-level relaxation times were observed to be ~100 ps, driven by Auger-type electron-electron scattering. Nanosecond-scale dynamics in the n=1 state were also observed and attributed to transport effects. Our results indicate that the phonon bottleneck in the QDs is circumvented by Auger scattering; nevertheless, the electron dynamics in the unipolar device are found to be slower than those observed in bipolar systems, which confirms the significance of the holes in the carrier relaxation in bipolar devices. The results also support the improved operation of QD infrared photodetectors relative to quantum-well-based devices

Observation of the Purcell effect in high-index-contrast micropillars

The authors have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and aluminum oxide. Compared to the more widely studied GaAs∕AlAs micropillars, these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. The authors have studied a number of samples containing a low density of InGaAs∕GaAs self-assembled quantum dots in a cavity and here report observation of a threefold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect.