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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.

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.

rrl solar calls for Rapid Research Letter and Review@RRL contributions in the following areas: • Photovoltaic materials preparation, optimization, characterization and physics • Significant solar cell advances, modeling or characterization results and techniques • Important and verified advances in PV efficiency • Novel and urgent results in photovoltaics concepts, physics and technology • All materials and device systems Materials with perpendicular magnetic anisotropy for magnetic random access memory 3 2013

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.

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 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.

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.

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.

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 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.

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.

Semiconductor quantum dots (QDs) grown using self-assembly techniques in the Stranski- Krastanov (S-K) mode are expected to be useful for high-performance optical devices such as QD lasers. A significant amount of research has been carried out on the development of highperformance QD lasers because they offer the advantages of a low threshold current, temperature stability, high modulation bandwidth, and low chirp. To realize these high-performance devices, the surface QD density should be increased by fabricating a stacked structure. We have developed a growth method based on a strain-compensation technique that enables the fabrication of a high number of stacked InAs QD layers on an InP(311)B substrate. In this study, we employed the proposed method to fabricate QD laser diodes consisting of highly stacked QD layers and investigated the dependence of the diode parameters on the stacking layer number. We fabricated QD laser diodes with 5, 10, 15, and 20 QD layers in the active region. All of the laser diodes operated at around 1.55 μm at room temperature, and their threshold currents showed clear dependence on the stacking layer number. Laser diodes with more than 10 QD layers showed sufficient gain, i.e., the threshold currents decreased with a decrease in the cavity length. On the other hand, for laser diodes with less than 10 QD layers, the threshold currents increased with a decrease in the cavity length.

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.

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.

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.

The fabrication, by droplet epitaxy, of a class of quantum nanostructures characterized by a regular, nanometres high, flat disks with a diameter of hundreds of nanometres and a hole at the centre encircled by a ring a few nanometres high, is presented here. A detailed analysis of the growth kinetics performed via insitu and exsitu probes allows us to propose a working model for the formation of these structures. © 2010 IOP Publishing Ltd.

We presented an innovative fabrication technique for the self-assembly of GaAs islands on Si substrates by droplet epitaxy. The islands show highly tunable density (from 107 to some 109 islands/ cm 2) and size (from 75 to 250 nm), and small size dispersion (below 10%). The islands, made by single relaxed crystals with lattice parameters close to the GaAs bulk, show well defined shapes, with a high aspect ratio. The low thermal budget required for the island self-assembly, together with the high scalability of the process, make these islands good candidates for local artificial substrates or local strain sources with the required lattice parameters, band alignment, and crystalline quality as now required for the implementation of high quality devices on Si. © 2010 American Institute of Physics.

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 demonstrate the thermal tunability of the emission of polymer embedded in microcavities. The large thermo-optic coefficients of a conjugated polymer is combined with the possibility to tailor the emission properties by means of cavities acting as optical filters. Both the transmission and the photoluminescence spectra of the polymer in cavities can be finely tuned with slopes up to about -0.2 nm/degrees C. Such an effect could be exploited to realize thermally tunable organic light-emitting devices for optical communication or sensing applications. (c) 2008 American Institute of Physics.

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.

InAs segments were grown on top of GaAs islands, initially created by droplet epitaxy on silicon substrate. We systematically explored the growth-parameter space for the deposition of InAs, identifying the conditions for the selective growth on GaAs and for purely axial growth. The axial InAs segments were formed with their sidewalls rotated by 30° compared to the GaAs base islands underneath. Synchrotron X-ray diffraction experiments revealed that the InAs segments are grown relaxed on top of GaAs, with a predominantly zincblende crystal structure and stacking faults.

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.

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 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 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

We present the fabrication of a novel quantum nanostructure, constituted by three concentric quantum rings by droplet epitaxy. Atomic Force Microscopy and photoluminescence characterization of these nanostructures is reported.

We present here the fabrication, via droplet epitaxy, of GaAs/AlGaAs quantum dots with high optical efficiency on Si. The growth substrate lattice parameter was adapted to that of (Al)GaAs via Ge virtual substrates (GeVS). The samples clearly show the presence of quantum dot self-assembly, with the designed shape and density. Photoluminescence measurements, performed at low temperature, show an intense emission band from the quantum dots.

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 propose a self-assembling procedure for the fabrication of GaAs islands by Droplet Epitaxy on silicon substrate. Controlling substrate temperature and amount of supplied gallium is possible to tune the base size of the islands from 70 up to 250 nm and the density from 107 to 109 cm−2. The islands show a standard deviation of base size distribution below 10% and their shape evolves changing the aspect ratio from 0.3 to 0.5 as size increases. Due to their characteristics, these islands are suitable to be used as local artificial substrates for the integration of III–V quantum nanostructures directly on silicon substrate.

What makes three dimensional semiconductor quantum nanostructures so attractive is the possibility to tune their electronic properties by careful design of their size and composition. These parameters set the confinement potential of electrons and holes, thus determining the electronic and optical properties of the nanostructure. An often overlooked parameter, which has an even more relevant effect on the electronic properties of the nanostructure, is shape. Gaining a strong control over the electronic properties via shape tuning is the key to access subtle electronic design possibilities. The Pulsed Dropled Epitaxy is an innovative growth method for the fabrication of quantum nanostructures with highly designable shapes and complex morphologies. With Pulsed Dropled Epitaxy it is possible to combine different nanostructures, namely quantum dots, quantum rings and quantum disks, with tunable sizes and densities, into a single multi‐function nanostructure, thus allowing an unprecedented control over electronic properties.

We introduce a high–temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self–assemble with the desired emission wavelength and highly symmetric shape while maintaining a high optical quality. The role of the growth parameters during the droplet arsenization is discussed and modeled.