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We present a detailed analysis of the emission of individual GaAs/AlGaAs complex nano-systems composed of two concentric and topologically distinct quantum nanostructures, namely, a quantum dot and a quantum ring. Time resolved, temperature and excitation power density dependence of the photoluminescence from single and ensemble dot/ring structures have been used in order to determine the carrier dynamics. Despite the small spatial separation between the dot and the ring, the exciton dynamics in the two nanostructures is completely decoupled at low temperatures. At higher temperatures, we observe a clear change in the carrier dynamics, which shows the onset of the coupling between the two nanostructures. We attribute such change in carrier dynamics to the breaking of topology induced selection rules which allows the transfer of the carriers between the dot and the ring via an electronic quantum state, common to the two nanostructures. [ABSTRACT FROM AUTHOR]

We present self-assembly of InAs/InAlAs quantum dots by the droplet epitaxy technique on vicinal GaAs(111)A substrates. The small miscut angle, while maintaining the symmetries imposed on the quantum dot from the surface, allows a fast growth rate thanks to the presence of preferential nucleation sites at the step edges. A 100 nm InAlAs metamorphic layer with In content ≥ 50% directly deposited on the GaAs substrate is already almost fully relaxed with a very flat surface. The quantum dots emit at the 1.3 μm telecom O-band with fine structure splitting as low as 16 μeV, thus making them suitable as photon sources in quantum communication networks using entangled photons. [ABSTRACT FROM AUTHOR]

We show that the epitaxial growth of height-controlled GaAs quantum dots, leading to the reduction of the inhomogeneous emission bandwidth, produces individual nanostructures of peculiar morphology. Besides the height controlled quantum dots, we observe nanodisks formation. Exploiting time resolved and spatially resolved photoluminescence we establish the decoupling between quantum dots and nanodisks and demonstrate the high optical properties of the individual quantum dots, despite the processing steps needed for height control. [ABSTRACT FROM AUTHOR]

We report a detailed experimental investigation of the power dependence of excitonic complexes (neutral exciton, neutral biexciton, and charged exciton) confined in single self-assembled GaAs/AlGaAs strain-free quantum dots grown by droplet epitaxy. By using the random population theory we show that, under stationary excitation, the power dependence of the excitonic complexes precisely follows the Poissonian statistics. This result allows us to determine with great accuracy the state filling condition of the quantum dots (QDs) and therefore to estimate the capture volume of the QDs. [ABSTRACT FROM AUTHOR]

We report an experimental study on the recombination kinetics of single strain-free GaAs quantum dots (QDs) grown by modified droplet epitaxy. The different bands composing the single quantum dot emission line at high temperature show identical dynamics, proving the common origin of all contributions. Our results thus agree with the interpretation of the broad pedestal band appearing when increasing the temperature as originated from the phonon replica. Finally, the relative weight of the phonon replicas depends both on temperature and on the QD size, in agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

The effects of hydrogen incorporation on carrier relaxation and recombination efficiencies in a large series of InAs self-assembled quantum dot structures deposited on InGaAs lower confining layers with different thicknesses and compositions have been addressed. With increasing H dose we observe an improvement in the radiative efficiency. By comparing steady state and time resolved photoluminescence measurements, it is established that the H passivation does not enhance the relaxation and capture efficiencies, but instead directly improves the emission yield from carriers in the dots. We therefore conclude that the H-passivated defects are located nearby, or even inside, the dots. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate that a very simple, noninvasive, and common technique, i.e., standard nonresonant photoluminescence of large ensemble of quantum dots (QDs), can be used to determine the temperature dependence of homogeneous broadening. The method can be applied to a single quantum dot layer independently of the QD density and it allows to follow the increase of the homogeneous broadening up to high T. The experimental results show that different pictures apply depending on the QD size. [ABSTRACT FROM AUTHOR]

Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (≈1×109cm-2) self-assembled GaAs/AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion. [ABSTRACT FROM AUTHOR]

The temperature dependence of carrier confinement in states of self-assembled In[sub 0.5]Ga[sub 0.5]As quantum dots (QDs) embedded in Al[sub y]Ga[sub 1-y]As barriers has been investigated by means of photoluminescence (PL) measurements. We show that photoexcited carriers above the AlGaAs barriers have two recombination channels that contribute to the temperature quenching of the PL from QDs: (a) carrier losses in the AlGaAs layers during the relaxation process and (b) thermal evaporation of captured carriers out of QDs. The interplay between these two mechanisms determines the behavior of the nonresonantly excited photoluminescence as a function of temperature. Eliminating the first contribution by using resonant excitation of the QD PL, we demonstrate a definite enhancement of the carrier confinement at room temperature in InGaAs/AlGaAs QDs by increasing the Al content. We show that this effect is related to the increase in the energy separation between the electronic states in the QD and the wetting layer. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study the multiexciton properties of GaAs quantum dots self aggregated on Si substrates. Sequential emission of two photons radiative cascade is observed both in continuous wave and in time resolved measurements. Polarization resolved measurements, with high spectral resolution, allow us to attribute the observed photon cascade to positively charged biexciton. Our results highlight the possibility of obtaining systems showing quantum correlations on a Si based device. [ABSTRACT FROM AUTHOR]

We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements. [ABSTRACT FROM AUTHOR]

We report on the nucleation of low density and defect-free GaAs quantum dots (QDs) on Ge substrates. The growth of III-V nanostructures was realized via droplet epitaxy technique. A detailed micro- and macro-photoluminescence analysis shows that the optical quality of the GaAs QDs is almost comparable with state-of-the-art QDs directly grown on GaAs substrates. Bright and sharp exciton and biexciton lines of individual QDs have been observed. This achievement opens the route to the realization of quantum optoelectronic devices on IV semiconductor substrates. [ABSTRACT FROM AUTHOR]

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs/AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment. [ABSTRACT FROM AUTHOR]

We present a study of the emission of GaN/AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence. [ABSTRACT FROM AUTHOR]

We have investigated in detail the dependence of the photoluminescence decay times in stacked quantum dot (QD) structures, showing that they strongly depend on the emission energy. We propose that the dispersion of the lifetimes has to be related to the presence of vertical disorder in the quantum dot column. Due to the disorder the energy distribution of the carrier extended states shrinks and localized states are created in the tails. This idea is supported by a theoretical model, which takes into account the presence of a vertical disorder along the QD columns. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled In0.5Ga0.5As/AlxGa1−xAs quantum dot (QD) structures, grown by atomic layer molecular beam epitaxy. A blue shift of the energy transition of both the QD and the wetting layer (WL) is found when increasing the Al content in the barrier. Nevertheless, the energy separation between 0D and 2D levels increases with x and, therefore, the coupling between the fundamental QD state and the WL electronic level can be tuned by increasing the barrier band gap. Since the WL state acts as the intermediate level on which escape and thermalization occur, we found that the larger separation between QD and WL states reduces the QD thermal quenching and enhances the QD radiative efficiency at high temperature. [Copyright &y& Elsevier]

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures—grown by modified droplet epitaxy—where no wetting layer is connecting the dots. We find a fast carrier relaxation time (30 ps) to the dot ground state, which becomes even faster for increasing the photogenerated carrier injection. This shows that the two-dimensional character of the wetting layer is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in literature. We discuss the role of the barrier states as well as the possibility of Auger processes involving the zero-dimensional levels of the quantum dots. [ABSTRACT FROM AUTHOR]

We present a detailed investigation of the carrier dynamics in a set of InAs/GaAs (N11) quantum dots (QD) by means of time-resolved photoluminesce (PL) techniques. A dynamical red shift of the PL bands when increasing the delay time after the pulse excitation is observed. We attribute this intrinsic optical nonlinearity to the photoinduced screening of internal built-in electric field. The value of the redshift of the QD emission band decays with the carrier population demonstrating the intrinsic nature of the built-in field. Its dependence on the substrate orientation and termination agrees with the expected piezoelectric induced quantum confined Stark effects of the QD optical transitions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present time resolved photoluminescence measurements of GaAs/Al0.3Ga0.7 As quantum dot structures where no wetting layer is connecting the dots. We show that the two-dimensional character of the wetting layer is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in the literature. [Copyright &y& Elsevier]

We report on triggered single photon emission from GaAs quantum dots, grown on Si substrates and obtained by means of fabrication protocols compatible with the monolithic integration on Si based microelectronics. Very bright and sharp individual exciton lines are resolved in the spectra and can be followed up to 150 K. The nature of quantum emitters of single photon pulses can be measured up to liquid nitrogen temperature by Hanbury Brown and Twiss interferometric correlations. [ABSTRACT FROM AUTHOR]

We investigate the optical properties of InAs self-assembled quantum dots grown on (N11)A/B GaAs substrates, by means of cw photoluminescence under different excitation power densities. We observe a sizeable blue-shift of photoluminescence band induced by increasing the photogenerated carrier density. The shift depends on the substrate orientation and exhibits a strong asymmetric dependence on the substrate termination. We attribute the photoluminescence blue-shift to a reverse quantum confined Stark shift of ground state transition energies in the quantum dots. This effect arises from the photogenerated charge screening of the built-in piezoelectric field present in such strained structures grown on high index planes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed study of the carrier transfer and photoluminescence (PL) quenching in stacked InAs/GaAs quantum dots (QDs) is presented. Vertically aligned QD structures, grown by atomic layer molecular beam epitaxy, with different numbers N of dot planes and different spacer thicknesses (d) were prepared and studied. The dependencies of carrier transfer from the GaAs barriers to the InAs QDs and of the PL quenching channels on the design parameters N and d have been identified by performing continuous wave and time-resolved PL measurements. We have found that both the radiative recombination and capture efficiency into the QDs are reduced by increasing N and by decreasing d, as a consequence of the deterioration of the interdot GaAs spacers induced by stacking. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled InAs/AlyGa1−yAs/GaAs quantum dot (QD) structures, grown by Atomic Layer Molecular Beam Epitaxy. We found a blue shift of the fundamental QD energy transition when increasing the Al content in the barrier. The comparison of the experimental data with previous findings and with a simple effective mass model suggests that the emission blue shift cannot be completely attributed to the increase of the confining barriers band gap. At the same time we show that the use of ALMBE allows a better control of the QD size distribution with respect to standard MBE growth. The increase of the barrier height enhances the QD radiative efficiency at high temperature, allowing to observe the QD emission up to T=430 K. Important pieces of information on the thermal activation of non radiative channels are obtained by comparing the QD PL temperature dependence with non-resonant and resonant excitation of the QD levels. [Copyright &y& Elsevier]

We report on a novel lithographic approach for the fabrication of integrated quantum dot (QD)-photonic crystal (PhC) nanocavity systems. We exploit unique hydrogen's ability to tailor the band gap energy of dilute nitride semiconductors to fabricate isolated site-controlled QDs via a spatially selective hydrogenation at the nanometer-scale. A deterministic integration of the realized site-controlled QDs with PhC nanocavities is provided by the inherent realignment precision (~ 20 nm) of the electron beam lithography system used for the fabrication of both QDs and PhC cavities. A detailed description of the fabrication steps leading to the realization of integrated QD-PhC cavity systems is provided, together with the experimental evidence of a weak coupling effect between the single-photon emitter and the PhC cavity. [ABSTRACT FROM AUTHOR]

Single and two-layer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical conditions, leading to the selective growth of self-assembled InAs Quantum Dot chains over mounded GaAs surfaces. Changing the thickness of the spacer layer and the InAs deposition made it possible to tune the nucleation of 2-fold or single chains in the second layer. Finite Element Method simulations evidenced the major role of the strain field in favoring the formation of single stacked chains. On the other hand, tuning properly the As 4 /In flux ratio contributed to improving the QD ordering along the chains. Microphotoluminescence experiments demonstrated single photon emission properties of the observed QDs. Our growth approach did not degrade the optical quality of the InAs QDs, allowing a significant spatial correlation between the QDs aligned along the chain. [ABSTRACT FROM AUTHOR]

Abstract: In this work we show the possibility to increase the optical quality of quantum dots grown by droplet epitaxy on Ge-on-Si substrates in terms of single dot emission linewidth, decay time and efficiency by operating on the As pressure during the crystallization step without increasing the thermal budget. [Copyright &y& Elsevier]

We present a detailed analysis of the Ga coverage and of the post-growth annealing effects on the optical properties of very-low-density self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Through theoretical calculation of the QD electronic states, including thermally activated Al–Ga interdiffusion processes, we were able to relate our spectroscopic observations to QD structural properties. [Copyright &y& Elsevier]

We show that the use of modified droplet epitaxy allows to tune the wetting layer thickness in GaAs quantum dot structures. Morphological observations demonstrate that the wetting layer at the base of the dots can be controlled or even removed by changing the surface stoichiometry of substrates before droplet formations. Spectroscopical measurements show that the variation of the wetting layer thickness strongly influences the optical properties of the dots. The experimental transition energies of the dots well agree with a theoretical model based on effective mass approximation. [Copyright &y& Elsevier]

Abstract: We report on the power dependence of the exciton photoluminescence of single self-assembled GaAs/AlGaAs quantum dots. Interpreting the experimental data with the random population theory we demonstrate that the exciton states occupation probability can be described by Poissonian statistics, allowing to study the state filling condition and evaluate the QDs capture volume. In addition we study the effect of temperature on the QDs capture efficiency. [Copyright &y& Elsevier]

Abstract: We present the results of high resolution photoluminescence spectroscopy of single GaAs/AlGaAs quantum dots grown by droplet epitaxy technique. By measuring the line broadening and the linear polarization of the excitonic photoluminescence we get new insights on the role of extrinsic effects on the quantum dot symmetry. Taking advantage of the lack of strain in the GaAs/AlGaAs system we study the relationship between the geometrical elongation of the confining potential and the exciton fine structure splitting. [Copyright &y& Elsevier]

Abstract: Modified droplet epitaxy GaAs/AlGaAs single quantum dots have been studied by low-temperature magneto-photoluminescence spectroscopy. We observe Zeeman splitting and diamagnetic shift of the neutral exciton line. The Landég factor of different quantum dots has been measured in a wide range of emission energies (i.e. quantum dot size). The measured g factors and diamagnetic shift are compared with conventional quantum dots grown by the Stranski–Krastanov method, as well as GaAs/AlGaAs quantum wells and fluctuation-induced dots. [Copyright &y& Elsevier]

Abstract: We directly investigate, by means of near-field spectroscopy, the spatial distribution of the optical cavity modes of 2D photonic crystal microcavities. Numerical simulations confirm that the photoluminescence maps of quantum dots embedded in the photonic structure qualitatively match the spatial modulation of the electric field intensity. [Copyright &y& Elsevier]

We investigated a large series of self-assembled InAs and InxGa1−xAs quantum dot (QD) structures by means of photoluminescence (PL) and PL excitation (PLE) techniques. A pronounced dip of the QD PLE spectra just below the GaAs absorption edge has been observed in all the investigated samples, denoting a resonant quenching of the QD PL intensity. PL spectra with excitation in such spectral region show, beside the QD PL band, a new extrinsic band at 1.356 eV with strong GaAs–LO phonon replicas. The PLE spectrum of this extrinsic band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and the defects associated with the 1.356 eV emission. The chemical etching of the QD and of the surrounding barrier layers does not eliminate the extrinsic PL band, which is, therefore, attributed to a native defect of the GaAs buffer. The spectral position and shape of the 1.356 eV band allows us to associate it to a complex of Ga vacancy with deep acceptor states in the GaAs layers. We conclude that the resonant PL quenching of QDs arises from the competition in the carrier capture between the QDs and deep defects in the GaAs layers. [Copyright &y& Elsevier]