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A hybrid metal-semiconductor nanosystem for the generation of THz radiation, based on the fabrication of GaAs quantum molecules-Ga metal nanoparticles complexes through a self assembly approach, is proposed. The role of the growth parameters, the substrate temperature, the Ga and As flux during the quantum dot molecule (QDM) fabrication and the metal nanoparticle alignment are discussed. The tuning of the relative positioning of QDMs and metal nanoparticles is obtained through the careful control of Ga droplet nucleation sites via Ga surface diffusion. The electronic structure of a typical QDM was evaluated on the base of the morphological characterizations performed by atomic force microscopy and cross sectional scanning electron microscopy, and the predicted results confirmed by micro-photoluminescence experiments, showing that the Ga metal nanoparticle-GaAs quantum molecule complexes are suitable for terahertz generation from intraband transition.

Droplet Epitaxy is a variant of molecular beam epitaxy for the fabrication of quantum nanostructures with highly designable shapes and complex morphologies. With Droplet Epitaxy it is possible to combine different quantum structures, namely quantum dots, quantum rings and quantum disks into a single unit, thus allowing an unprecedented control over electronic properties of the fabricated quantum nanostructures.

We report on the growth and optical characterization by macro and micro photoluminescence measurements of high optical quality GaAs quantum nanostructures grown by droplet epitaxy on Ge and Si substrates. The quantum nanostructures show optical performances comparable to those achievable with the most advanced realized on GaAs substrates. The adopted growth procedures show also the possibility to fabricate the active layer maintaining a low thermal budget compatible with back-end integration of the fabricated materials on integrated circuits. We demonstrate the possibility to embed GaAs nanostructured devices such as intersubband detectors and single quantum emitters on Si substrates.

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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4726189]

Complex GaAs/AlGaAs nanostructures, constituted by a quantum dot sitting on a quantum disk, are fabricated. This was made possible by droplet epitaxy growth technique which allows for fine control of the crystallization kinetics, by means of As flux and substrate temperature, of nanometer-scale metallic Ga reservoirs deposited on the substrate. Such complex quantum nanostructures show good optical quality and the exciton recombination in both the dot and the disk could be detected by photoluminescence spectroscopy.

The emission dynamics in GaAs/AlGaAs coupled ring-disk (CRD) quantum structures fabricated on silicon substrates is presented. The CRD structures are self-assembled via droplet epitaxy, a growth technique which, due to its low thermal budget, is compatible with the monolithic integration of III-V devices on Si based electronic circuits. Continuous wave, time resolved photoluminescence and theoretical calculations in the effective mass approximations are presented for the assessment of the electronic and carrier properties of the CRDs. The CRDs show a fast carrier dynamics which is expected to be suitable for ultrafast optical switching applications integrated on silicon.

We present of a detailed photoluminescence characterization of high efficiency GaAs/AlGaAs quantum nanostructures grown on silicon substrates. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature a parts per thousand currency sign350A degrees C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits.

We present the Molecular Beam Epitaxy fabrication of complex GaAs/AlGaAs nanostructures by Droplet Epitaxy, characterized by the presence of concentric multiple rings. We propose an innovative experimental procedure that allows the fabrication of individual portions of the structure, controlling their diameter by only changing the substrate temperature. The obtained nanocrystals show a significant anisotropy between [110] and [1-10] crystallographic directions, which can be ascribed to different activation energies for the Ga atoms migration processes. © 2010 The Author(s).

We show the possibility to integrate high quality III-V quantum nanostructures tunable in shape and emission energy on Si-Ge Virtual Substrate. Strong photoemission is observed, also at room temperature, from two different kind of GaAs quantum nanostructures fabricated on Silicon substrate. Due to the low thermal budget of the procedure used for the fabrication of the active layer, Droplet Epitaxy is to be considered an excellent candidate for implementation of optoelectronic devices on CMOS circuits.

We present the detailed fabrication method of two different GaAs nanostructures with cylindrical symmetry by the Droplet Epitaxy technique. Concentric Multiple Rings or Coupled Rings/Disks are successfully obtained, exploiting the lateral growth around the Ga droplets, through a fine control of the crystallization dynamics. © 2010 IOP Publishing Ltd.

We present the fabrication and discuss the growth dynamics of two classes of GaAs quantum nanostructures fabricated by droplet epitaxy, namely, double rings and coupled ring-disks. Their morphological differences has been investigated and found to be originated by the kinetic of the changes in the surface reconstruction around the initially formed Ga droplets during the arsenization step. The control of surface reconstruction dynamics thus permits a fine tuning of the actual nanostructure shape at the nanoscale, based on pure self-assembling techniques. © 2010 American Institute of Physics.

We present the molecular beam epitaxy fabrication and optical properties of complex GaAs nanostructures by droplet epitaxy: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

Making use of a novel droplet epitaxy technique. we realize GaAs ring shaped quantum dots of high circular symmetry. Here we report on photoluminescence (PL) study of single isolated quantum rings in magnetic field up to T. Polarized PL signals show a significant reduction in their intensity at >6 T. Possible origins accounting for this observation are discussed. (c) 2009 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim

We address the question whether a quantum ring nanostructure can be considered as a single-photon emitter by means of intensity time-correlation measurement based on a Hanbury-Brown and Twiss interferometer. The second-order correlation function g((2)) well characterizes the quantum nature of the emitter by the photon antibunching dip at zero delay. In concentric quantum rings we find evidence that the inner ring satisfies the requirement of single-photon source, while in the outer ring this requirement is relaxed.

We present the fabrication of GaAs/AlGaAs Multiple (from three to five) concentric nanoring structures by an innovative growth method based on droplet epitaxy and characterized by short time As supply to the Ga droplets at different substrate temperatures. The formation mechanismhas been Interpreted on the basis of a detailed ex situ and in situ characterization of nanostructure morphology and surface reconstruction, We introduce design criteria which will allow to obtain concentric quantum ring structures of the desired complexity.

We realized ultra-narrow excitonic emission from single GaAs/AlGaAs quantum dots (QDs) grown by a refined droplet epitaxy technique. We found that uncapped quantum dots can be annealed at 400 degrees C without major changes in their morphology, thus enabling an AlGaAs capping layer to be grown at that temperature. Consequently, we demonstrate a fourfold reduction of the linewidth of the emission together with an increased recombination lifetime, compared to the conventional droplet epitaxial QDs. The averaged linewidth of neutral excitons measured by micro-photoluminescence on single quantum dots was around 35 mu eV.

We fabricate high efficiency GaAs/AlGaAs quantum nanostructure active layer for intersubband detectors and light emitting devices on a silicon substrate. The whole process of formation of the GaAs/AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature <= 350 degrees C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits. The realized quantum nanostructures show optical efficiencies comparable to those achievable with state of the art quantum dot materials grown on GaAs substrates

We report polarization-resolved high spectral resolution photoluminescence measurements in self-assembled strain-free GaAs/Al0.3Ga0.7As quantum dots designed and realized in order to reduce as much as possible strain and segregation, which affected previous fine-structure splitting (FSS) experiments. Photoluminescence from isolated quantum dots exhibits a linearly polarized FSS. FSS clearly shows a quantum size effect monotonically decreasing from 90 to 20 mu eV by decreasing the quantum dot size (increasing emission energy). While this finding is similar to that observed in strained In (Ga) As/GaAs quantum dots, clearly it requires a different explanation, being our quantum dots not affected by strain-induced piezoelectricity. We ascribed the observed FSS to a size dependent reduction in dot shape anisotropy as evidenced by structural data analysis. Moreover the linear polarization in dots with shape close to cylindrical symmetry is not along the {[110] crystallographic axis but it turns out randomly distributed, highlighting the role of extrinsic effects.}

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 reducin the perturbation of the fluctuating environment.

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. (C) 2008 American Institute of Physics.

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 Lande g 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. (c) 2007 Elsevier B.V.

Low temperature photoluminescence spectroscopy is used to analyze the effects of the postgrowth thermal annealing on the electronic structure ad carrier dynamics of GaAs/AlGaAs quantum dot and quantum ring structures grown by droplet epitaxy. All the samples show a large increase in the photoluminescence efficiencies after the thermal treatment due to sizeable reduction in the material defectivity. Modifications of the photoluminescence band, which depend on thermal annealing temperature, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion. (c) 2008 American Institute of Physics.

We present the temperature dependence of the photoluminescence emission of single concentric quantum rings. The two rings which form the nanostructure show a decoupled recombination kinetics in the whole temperature range. The emission is characterized by a doublet. The emission linewidth, which is already larger than 1 meV at low temperatures and increases as the temperature raises, is dominated by the thermal activation of carriers to higher excited states

Quantized energy levels of nanoscopic concentric double rings are identified by means of micro photoluminescence spectroscopy. High-yield emissions from a single quantum structures are observed. The spectra show several discrete lines reflecting the carrier confinement in the rings. Effective mass calculation is performed to characterize the quantized motion of carriers with rotational and orbital degrees of freedoms. Excellent agreement between the observed spectra and the numerical ones is found. (c) 2006 Elsevier B.V. All rights reserved.

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.

We analyze the photoluminescence temperature behavior of InGaAs/GaAs quantum dots grown by heterogeneous droplet epitaxy. Morphologically, these dots are nanocrystal InGaAs inclusions in the GaAs matrix, with a concave disk shape and, more important, no wetting layer is connecting the dots. The photoluminescence of the dots does not show any of the typical of the Stranski-Krastanov dots temperature properties, such as sigmoidal peak energy position and linewidth narrowing. We demonstrate that such behavior stems from the lacking of the thermally activated dot-dot coupling channel provided by the wetting layer thus preventing the establishment of a common quasiequilibrium in the whole dot ensemble.

Photoluminescence spectroscopy is used to analyze the effects of post-growth thermal annealing on the electronic properties and capture processes of self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Post-growth annealing induces deep changes in the electronic structure of the quantum dots material, modifying both capture processes and photoluminescence quenching channels. The optical data, together with theoretical models, are used to quantify such structural and electronic modifications in the annealed materials.