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The long dreamed quantum internet would consist of a network of quantum nodes (solid-state or atomic systems) linked by flying qubits, naturally based on photons, travelling over long distances at the speed of light, with negligible decoherence. A key component is a light source, able to provide single or entangled photon pairs. Among the different platforms, semiconductor quantum dots are very attractive, as they can be integrated with other photonic and electronic components in miniaturized chips. In the early 1990s two approaches were developed to synthetize self-assembled epitaxial semiconductor quantum dots (QDs), or artificial atoms, namely the Stranski-Krastanov (SK) and the droplet epitaxy (DE) method. Because of its robustness and simplicity, the SK method became the workhorse to achieve several breakthroughs in both fundamental and technological areas. The need for specific emission wavelengths or structural and optical properties has nevertheless motivated further research on the DE method and its more recent development, the local-droplet-etching (LDE), as complementary routes to obtain high quality semiconductor nanostructures. The recent reports on the generation of highly entangled photon pairs, combined with good photon indistinguishability, suggest that DE and LDE QDs may complement (and sometime even outperform) conventional SK InGaAs QDs as quantum emitters. We present here a critical survey of the state of the art of DE and LDE, highlighting the advantages and weaknesses, the obtained achievements and the still open challenges, in view of applications in quantum communication and technology., 26 pages, 5 figures

GaN nanowires grown by molecular beam epitaxy generally suffer from dominant nonradiative recombination, which is believed to originate from point defects. To suppress the formation of these defects, we explore the synthesis of GaN nanowires at temperatures up to 915 °C enabled by the use of thermally stable TiNx/Al2O3 substrates. These samples exhibit indeed bound exciton decay times approaching those measured for state-of-the-art bulk GaN. However, the decay time is not correlated with the growth temperature, but rather with the nanowire diameter. The inverse dependence of the decay time on diameter suggests that the nonradiative process in GaN nanowires is not controlled by the defect density, but by the field ionization of excitons in the radial electric field caused by surface band bending. We propose a unified mechanism accounting for nonradiative recombination in GaN nanowires of arbitrary diameter.

Owing to their ability to generate non-classical light states, quantum dots (QDs) are very promising candidates for the large-scale implementation of quantum information technologies. However, the high photon collection efficiency demanded by these technologies may be impossible to reach for “standalone” semiconductor QDs, embedded in a high-refractive index medium. In this work a novel laser writing technique is presented, enabling the direct fabrication of a QD self-aligned—with a precision of ±30 nm—to a dielectric microsphere. The presence of the microsphere leads to an enhancement of the QD luminescence collection by a factor 7.3 ± 0.7 when an objective with 0.7 numerical aperture is employed. This technique exploits the possibility of breaking the N−H bonds in GaAs (Formula presented.) Nx:H by a laser light, obtaining a lower-bandgap material, GaAs (Formula presented.) Nx. The microsphere, deposited on top of a GaAs (Formula presented.) Nx:H/GaAs quantum well, is used to generate a photonic nanojet, which removes hydrogen exactly below the microsphere, creating a GaAs (Formula presented.) Nx QD at a predefined distance from the sample surface. Second-order autocorrelation measurements confirm the ability of the QDs obtained with this technique to emit singlephotons.

We demonstrate the feasibility of growing GaAs nanomembranes on a plastically-relaxed Ge layer deposited on Si (111) by exploiting selective area epitaxy in MBE. Our results are compared to the case of the GaAs homoepitaxy to highlight the criticalities arising by switching to heteroepitaxy. We found that the nanomembranes evolution strongly depends on the chosen growth parameters as well as mask pattern. The selectivity of III-V material with respect to the SiO2 mask can be obtained when the lifetime of Ga adatoms on SiO2 is reduced, so that the diffusion length of adsorbed Ga is high enough to drive the Ga adatoms towards the etched slits. The best condition for a heteroepitaxial selective area epitaxy is obtained using a growth rate equal to 0.3 ML/s of GaAs, with a As BEP pressure of about 2.5 &times, 10&minus, 6 torr and a temperature of 600 &deg, C.

In recent years, dielectric microspheres have been used in conjunction with optical microscopes to beat the diffraction limit and to obtain superresolution imaging. The use of microspheres on quantum dots (QDs) is investigated, for the first time, to enhance the light coupling efficiency. The enhancement of the QD luminescence collection in terms of extraction and directionality is demonstrated, as well as the enhancement of spatial resolution. In particular, it is found that a dielectric microsphere, placed on top of an epitaxial QD, increases the collected radiant energy by about a factor of 42, when a low numerical aperture objective is used. Moreover, if two or more QDs are present below the microsphere, the modification of the far field emission pattern allows selective collection of the luminescence from a single QD by simply changing the collection angle. Dielectric microspheres present a simple and efficient tool to improve the QD spectroscopy, and potentially QD-based devices.

The fabrication and characterization of an infrared photodetector based on GaAs droplet epitaxy quantum dots embedded in Al0.3Ga0.7As barrier is reported. The high control over dot electronic properties and the high achievable number density allowed by droplet epitaxy technique permitted us to realize a device using a single dot layer in the active region. Moreover, thanks to the independent control over dot height and width, we were able to obtain a very sharp absorption peak in the thermal infrared region (3-8 μm). Low temperature photocurrent spectrum was measured by Fourier spectroscopy, showing a narrow peak at 198 meV (∼6.3 μm) with a full width at half maximum of 25 meV. The observed absorption is in agreement with theoretical prediction based on effective mass approximation of the dot electronic transition.

We demonstrate the self-assembled formation of AlN nanowires by molecular beam epitaxy on sputtered TiN films on sapphire. This choice of substrate allows growth at an exceptionally high temperature of 1180 {\deg}C. In contrast to previous reports, the nanowires are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the nanowire sidewalls. The high crystalline quality of the nanowires is evidenced by the observation of near band edge emission in the cathodoluminescence spectrum. The key factor for the low nanowire coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices employing these nanowires as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range.

Silicon Deep Reactive Ion Etching (DRIE) process using a multi-layer mask containing a sputtered Aluminum thin film is studied. Aluminum is a candidate for very high aspect ratio DRIE, due to tis very low etch rate, but its adoption is hindered by the presence of micromasking. Effects of adding aluminum as the middle layer in a stack of silicon oxides are studied: heat dissipation and catalyst contribution to plasma chemistry are identified. The novel assumption that micromasking is generated only in the first part of the DRIE etching is demonstrated. Based on this, the effect of adding a protective coating on top of the mask is studied. The protective coating preserves aluminum in the first part of the etching, to reach a given depth before the aluminum mask is exposed to the etching plasma. A Safe aspect ratio value is obtained, beyond which micromasking does not take place. This aspect ratio is shown to be a function of the etching geometry and feature size.

We have successfully grown high quality InxGa1-xAs metamorphic layer on GaAs (111)A using molecular beam epitaxy. Inserting a thin 3.0-7.1 monolayer (ML) InAs interlayer between the In0.25Ga0.75As and GaAs allowed the formation of a nearly lattice-relaxed In0.25Ga0.75As with a very flat upper surface. However, when the thickness of the inserted InAs is thinner or thicker than these values, we observed degradation of crystal quality and/or surface morphology. We also revealed this technique to be applicable to the formation of a high quality metamorphic InxGa1-xAs layer with a range of In compositions (0.25 ≤ x ≤ 0.78) on GaAs (111)A. Cross-sectional scanning transmission electron microscope studies revealed that misfit dislocations formed only at the interface of InAs and GaAs, not at the interface of In0.25Ga0.75As and InAs. From the dislocation density analysis, it is suggested that the dislocation density was decreased by growing In0.25Ga0.75As on InAs, which effectively contribute the strain relaxation of In0.25Ga0.75As. The InGaAs/InAlAs quantum wells that were formed on the metamorphic layers exhibit clear photoluminescence emissions up to room temperature.

Mid-infrared sensors detect infrared radiation emitted from objects, and are actually widely used for monitoring gases and moisture as well as for imaging objects at or above room temperature. Infrared photodetectors offer fast detection, but many devices cannot provide high responsivity at room temperature. Here we demonstrate infrared sensing with high responsivity at room temperature. The central part of our device is an Al0.3Ga0.7As/GaAs heterostructure containing InAs quantum-dot (QD) layer with a 10-nm-thick GaAs spacer. In this device, the electrons that have been accumulated at the heterointerface are transferred to the conduction band of the Al0.3Ga0.7As barrier by absorbing infrared photons and the following drift due to the electric field at the interface. These intraband transitions at the heterointerface are sensitized by the QDs, suggesting that the presence of the QDs increases the strength of the intraband transition near the heterointerface. The room-temperature responsivity spectrum exhibits several peaks in the mid-infrared wavelength region, corresponding to transitions from the InAs QD and wetting layer states as well as the transition from the quantized state of the triangular potential well at the two-dimensional heterointerface. We find that the responsivity is almost independent of the temperature and the maximum value at 295 K is 0.8 A/W at ~ 6.6 µm for a bias of 1 V, where the specific detectivity is $$1.8\times {10}^{10}$$1.8×1010 cmHz1/2/W.

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.

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations., 14 pages, 3 figures

Semiconductor quantum dots (QDs), made of III-V semiconductors alloys, have attracted increasing interest in the last two decades, especially for their possible usage in quantum information technology. However, for such advanced applications, the requisite control of the QD's position cannot be achieved by the conventional growth techniques. Moreover, silicon (Si) dominates the microelectronic technology but its use is limited for optoelectronic applications due to its indirect bandgap. Therefore, the possibility to integrate QDs made of III-V alloys on a Si-based platform and circuitry is of the utmost importance. However, this is hindered by the very different lattice constants and thermal expansion coefficients of Si and GaAs, which generate strain and defects. In this paper we overcome the mismatch problems using the self-limited growth of germanium on micro-patterned (001) Si to obtain a relaxed GaAs(111) oriented epilayer for the subsequent heteroepitaxy of III-V nanostructures. In particular we optically characterize a stack of three GaAs/AlGaAs quantum wells (QWs) grown on top of the Si/Ge pillars. We provide clear evidence of the presence of naturally formed QDs, due to QW thickness fluctuations, and their position control with micrometer resolution, given by the pillar distance.

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. © 2013 Elsevier B.V. All rights reserved.

Three-dimensional, epitaxial GaAs crystals are fabricated on micro-pillars patterned into Si(001) substrates by exploiting kinetically controlled growth conditions in Molecular Beam Epitaxy. The evolution of crystal morphology during growth is assessed by considering samples with increasing GaAs deposit thickness. Experimental results are interpreted by a kinetic growth model, which takes into account the fundamental aspects of the growth and mutual deposition flux shielding between neighboring crystals. Different substrate pattern geometries with dissimilar lateral sizes and periodicities of the Si micro-pillars are considered and self-similar crystal structures are recognized. It is demonstrated that the top faceting of the GaAs crystals is tunable, which can pave the way to locally engineer compound semiconductor quantum structures on Si(001) substrates.

Epitaxial InN quantum dots (QDs) on In-rich InGaN, applied as an electrochemical electrode, activate Cl−-anion-selective surface attachment, bringing forth faradaic/pseudocapacitor-like behavior. In contrast to traditional pseudocapacitance, here, no chemical reaction of the electrode material occurs. The anion attachment is explained by the unique combination of the surface and quantum properties of the InN QDs. A high areal capacitance is obtained for this planar electrode together with rapid and reversible charge/discharge cycles. With the growth on cheap Si substrates, the InN/InGaN QD electrochemical electrode has great potential, opening up new application fields for III–nitride semiconductors.

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. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

We identified the growth conditions which lead to the formation of a flat top facet in GaAs nanowires, grown by the self-assisted method in molecular beam epitaxy. Low arsenic overpressure, intermediate temperature and relatively long annealing time are needed to transform the Ga droplets present at the top of the nanowires into a flat GaAs segment. This result opens the possibility to realize atomically sharp heterointerfaces in this type of semiconductor nanowires.

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (

We report the study on formation and thermal annealing of InAs quantum dots grown by droplet epitaxy on GaAs (111)A surface. By following the changes in RHEED pattern, we found that InAs quantum dots arsenized at low temperature are lattice matched with GaAs substrate, becoming almost fully relaxed when substrate temperature is increased. Morphological characterizations performed by atomic force microscopy show that annealing process is able to change density and aspect ratio of InAs quantum dots and also to narrow size distribution.

Effective monolithic integration strategies for high quality quantum III-V nanostructure fabrication on Si will be introduced. The methodologies presented range from ultrathin planar Ge virtual substrates for the integration of single photon emitters with high temperature stability to advanced three-dimensional Ge growths strategies.

We demonstrated the droplet epitaxial growth of InAs quantum dots on a GaAs(111)A substrate, which emitted at telecommunication wavelengths. A high-quality metamorphic In0.52Al0.48As layer was formed by inserting three monolayers of InAs between GaAs(111)A and InAlAs. InAs quantum dots were grown on the InAlAs surface by droplet epitaxy. They exhibited a laterally symmetrical shape owing to the C3v symmetry of the {111} surface. The photoluminescence signals of capped quantum dots indicated broadband spectra covering wavelengths from 1.3 to 1.55 µm. Thus, our dots are potentially useful for constructing entangled photon sources compatible with current telecommunication networks.

We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled lnAs/Al-y Ga1-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. (C) 2002 Elsevier Science B.V.. All rights reserved.

We demonstrate the formation of InAs quantum dots (QDs) on InAlAs/InP(111)A by means of droplet epitaxy. The C3v symmetry of the (111)A substrate enabled us to realize highly symmetric QDs that are free from lateral elongations. The QDs exhibit a disk-like truncated shape with an atomically flat top surface. Photoluminescence signals show broad-band spectra at telecommunication wavelengths of 1.3 and 1.5 μm. Strong luminescence signals are retained up to room temperature. Thus, our QDs are potentially useful for realizing an entangled photon-pair source that is compatible with current telecommunication fiber networks. © 2014 AIP Publishing LLC.

We present the growth of self-assisted GaAs nanowires (NWs) with selectable number density on bare Si(1 1 1), not covered by the silicon oxide. We determine the number density of the NWs by initially self-assembling GaAs islands on whose top a single NW is nucleated. The number density of the initial GaAs base islands can be tuned by droplet epitaxy and the same degree of control is then transferred to the NWs. This procedure is completely performed during a single growth in an ultra-high vacuum environment and requires neither an oxide layer covering the substrate, nor any pre-patterning technique.

We report on thermal strain relaxation in heteroepitaxial three dimensional crystals of GaAs grown on deeply patterned Si(001) substrates. The relaxation of the thermal strain induced by the three dimensionality and micrometric size of the GaAs crystals is investigated by comparing different pattern geometries. We exploit photoluminescence measurements to accurately evaluate the amount of residual strain. Our results confirm that deep substrate patterning is an effective way to relax the thermal strain of heteroepitaxial GaAs/Si.

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.

Liquid-phase epitaxy from an Si-In-Er solution at an average temperature of 950 degrees C has been used to grow 2- 4 mu m thick epilayers of erbium-doped silicon onto CZ and FZ silicon substrates in an oxygen-free hydrogen atmosphere. Most of the samples grown on CZ substrates presented detectable, but feeble photoluminescence at 2. K in the spectral range of emission of the Er3+ manifold at 0.8 eV. However, some of the samples presented intense photoluminescence characterized by two bands at 0.807 (at 10 K) and 0.873 eV, of which the first falls almost at the same energy of the Er3+ line, but whose intensity presents a quite remarkable persistence up to 250 Ii. From the energy position of the two bands, from their temperature dependence and from the levels found by deep level transient spectroscopy measurements, associated to TEM examinations, it was possible to attribute these bands, labelled D1 and D2, to dislocation luminescence. It will be shown in this paper that the presence of erbium enhances the D1 luminescence, possibly due to the fact that in these samples erbium is gettered at dislocations in an Er-O local configuration, as it results from EXAFS measurements. Apparently, also, a competition occurs with the Er-induced radiative recombination at dislocations, which is a fast process, and the indirect excitation of the Er manifold, which is the predominant process in dislocation-free materials. (C) 1999 Elsevier Science B.V. All rights reserved.

The multiexciton properties of extrinsic centers from AlGaAs layers on Ge and Si substrates are addressed. The two photon cascade is found both in steady state and in time resolved experiments. Polarization analysis of the photoluminescence provides clearcut attribution to neutral biexciton complexes. Our findings demonstrate the prospect of exploiting extrinsic centers for generating entangled photon pairs on a Si based device. © 2013 AIP Publishing LLC.

Quantum dot infrared detectors can be integrated on silicon using droplet epitaxy, enabling the simultaneous detection of multiple wavelengths in a format compatible with current semiconductor technology.

The structure of self-assembled quantum dots (QDs) grown by Droplet Epitaxy on Ge virtual substrates has been investigated by TEM. The QDs have a pyramidal shape with base and height of 50 nm. By (0 0 2) dark field TEM it was seen that the pyramid top is Ga poor and Al rich most likely because of the higher mobility of Ga along the pyramid sides down to the base. The investigated QDs contain defects identified as As precipitates by Moirè fringes. The smallest ones (3-5 nm) are coherent with the GaAs lattice suggesting that they could be a cubic phase of As precipitation. It seems to be a metastable phase since the hexagonal phase is recovered as the precipitate size increases above ∼5 nm. © 2012 Elsevier B.V. All rights reserved.

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.

We present a systematic study of the effects of in situ annealing of strain-free GaAs/AlGaAs quantum dots grown by droplet epitaxy, identifying the relation between the achievable shape anisotropy, aspect ratio and faceting and introducing a diffusion model able to describe the dot transformation during the annealing step.

The great interest in the implementation of GaAs quantum nanostructures (QNs) on silicon substrates is mainly due to the possibility of integrating specialized high efficiency optoelectronic and photonic devices on the existing complementary metal-oxide semiconductor technology developed on Si. This would allow the realization of specialized III–V devices such as nanoemitters and intersubband detectors directly embedded with a large number of existing Si devices. Of particular, technological interest is the possibility of carrying out the III–V device fabrication after the integrated circuit has been already realized, i.e., as a back-end process. In this case, the compatibility with the underlying integrated circuit is possible only imposing strict constraints on thermal budget for growth and processing of the epilayer.

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. © 2013 AIP Publishing LLC.

Dense arrays of micrometric crystals, with areal filling up to 93%, are obtained by depositing GaAs in a mask-less molecular beam epitaxy process onto Si substrates. The substrates are patterned into tall, micron sized pillars. Faceted high aspect ratio GaAs crystals are achieved by tuning the Ga adatom for short surface diffusion lengths. The crystals exhibit bulk-like optical quality due to defect termination at the sidewalls. Simultaneously, the thermal strain induced by different thermal expansion parameters of GaAs and Si is fully relieved. This opens the route to thick film applications without crack formation and wafer bowing. (C) 2013 AIP Publishing LLC.

We describe a method for the direct epitaxial growth of a single photon emitter, based on GaAs quantum dots fabricated by droplet epitaxy, working at liquid nitrogen temperatures on Si substrates. The achievement of quantum photon statistics up to T=80 K is directly proved by antibunching in the second order correlation function as measured with a H anbury Brown and Twiss interferometer. © 2013 AIP Publishing LLC.

Photoluminescence (PL) is one of the most widely diffused experimental techniques for the characterization of semiconductor nanostructures, in particular quantum wells (QWs), and for the study of their electronic properties. PL allows to study interesting intrinsic effects in QWs, like the exciton binding energy increase due to carrier localization, the homogeneous broadening of the exciton recombination line, the carrier–carrier interaction, and so on. In addition, the analysis of the radiative recombination spectra can help in the nanostructure characterization, providing information on the interface morphology and on the materials quality. The information that can be gained from PL spectra is critically summarized with the aim of providing a reference scheme for the characterization of nanostructures. The discussion is supported by a wide and detailed bibliography, in which basic textbooks, review articles, and research papers are included in order to provide the reader with up-to-date information on the application of PL to the study of optical and structural properties of semiconductor QWs. As a significant example, the PL characterization of Ge/SiGe QWs, an indirect band-gap model system whose optical properties are strongly influenced by transitions involving Γ Γ -type states, is presented in the last Section of the Chapter.

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]

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.

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 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.

A systematic optical study, including micro, ensemble and time resolved photoluminescence of GaAs/AlGaAs triple concentric quantum rings, self-assembled via droplet epitaxy, is presented. Clear emission from localized states belonging to the ring structures is reported. The triple rings show a fast decay dynamics, around 40 ps, which is expected to be useful for ultrafast optical switching applications.

The fabrication, by pure self-assembly, of GaAs/AlGaAs dot-ring quantum nanostructures is presented. The growth is performed via droplet epitaxy, which allows for the fine control, through As flux and substrate temperature, of the crystallization kinetics of nanometer scale metallic Ga reservoirs deposited on the surface. Such a procedure permits the combination of quantum dots and quantum rings into a single, multi-functional, complex quantum nanostructure.

The fabrication of submicrometer GaAs islands directly on Si substrates by droplet epitaxy is presented. Islands parameters, like density and size, are fully controlled through growth temperature and Ga coverage. The process is fully scalable and at low thermal budget, making these islands good candidates for local artificial substrates with lattice parameters, band alignment and crystalline quality as now required for the implementation of high quality III-As devices on Si.

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. © 2011 American Institute of Physics.

This review starts with a short introduction on the physical motivations for a new class of elementary particle detectors, known as bolometers, in which the detection is mediated by the production of phonons in a dielectric or superconductive material. Successively, the basic principles of these devices are exposed. In the next section the thermalization dynamics of the energy released in the absorber is analysed; the physics of the most common phonon sensors, doped semiconductors working in the variable range hopping conduction regime, is then discussed in depth. The last section is an outlook on the most important experimental results obtained with bolometers. Special attention is devoted to the materials (and to the related physical implications) chosen to realize the energy absorbers of the detectors.