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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.

A TEM study of GaAs nanoislands grown on (001) Si substrate by the Droplet Epitaxy technique is presented. The nanoislands turn out to be monocrystalline in perfect epitaxial relationship with Si. By X-ray microanalysis in the TEM it is also seen that the islands are stoichiometric. TEM images of the moiré fringes revealed the presence of dislocations at the nanoislands suggesting strain relaxation.

We report the effect of the quantum dot aspect ratio on the sub-gap absorption properties of GaAs/AlGaAs quantum dot intermediate band solar cells. We have grown AlGaAs solar cells containing GaAs quantum dots made by droplet epitaxy. This technique allows the realization of strain-free nanostructures with lattice matched materials, enabling the possibility to tune the size, shape, and aspect ratio to engineer the optical and electrical properties of devices. Intermediate band solar cells have been grown with different dot aspect ratio, thus tuning the energy levels of the intermediate band. Here, we show how it is possible to tune the sub-gap absorption spectrum and the extraction of charge carriers from the intermediate band states by simply changing the aspect ratio of the dots. The tradeoff between thermal and optical extraction is in fact fundamental for the correct functioning of the intermediate band solar cells. The combination of the two effects makes the photonic extraction mechanism from the quantum dots increasingly dominant at room temperature, allowing for a reduction of the open circuit voltage of only 14 mV, compared to the reference cell. publishedVersion

We investigate in detail the role of strain relaxation and capping overgrowth in the self-assembly of InAs quantum dots by droplet epitaxy. InAs quantum dots were realized on an In (Formula presented.) Al (Formula presented.) As metamorphic buffer layer grown on a GaAs(111)A misoriented substrate. The comparison between the quantum electronic calculations of the optical transitions and the emission properties of the quantum dots highlights the presence of a strong quenching of the emission from larger quantum dots. Detailed analysis of the surface morphology during the capping procedure show the presence of a critical size over which the quantum dots are plastically relaxed.

The efficient generation of entangled photons at telecom wavelength is essential for the success of many quantum communication protocols and the development of fiber-based quantum networks. Entangled light can be generated by solid-state quantum emitters with naturally low fine-structure splitting, such as highly symmetric InAs quantum dots (QDs) grown on (111)-oriented surfaces. The incorporation of these QDs into optical cavities is crucial to achieve sufficient signal intensities for applications but has so far shown major complications. In this work, we present droplet epitaxy of telecom-wavelength InAs QDs within an optical cavity on a vicinal (2° miscut) GaAs(111)A substrate. We show a remarkable enhancement of the photon extraction efficiency compared to previous reports together with a reduction of the density that facilitates the isolation of single spectral lines. Moreover, we characterize the exciton-fine-structure splitting and employ numerical simulations under the framework of the empirical-pseudopotential and configuration-interaction methods to study the impact of the miscut on the optical properties of the QDs. We demonstrate that the presence of the miscut steps influences the polarization of the neutral excitons and introduces a preferential orientation in the C3v symmetry of the surface.

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards $$(\bar{1}\bar{1}2)$$ ( 1 ¯ 1 ¯ 2 ) . At low temperature (

We show that the density of indium islands on GaAs(111)A substrates have a non-monotonic, reentrant behavior as a function of the indium deposition temperature. The expected increase in the density with decreasing temperature, indeed, is observed only down to 160 ∘C, where the indium islands undertake the expected liquid-to-solid phase transition. Further decreasing the temperature causes a sizable reduction of the island density. An additional reentrant increasing behavior is observed below 80 ∘C. We attribute the above complex behavior to the liquid&ndash, solid phase transition and to the complex island&ndash, island interaction which takes place between crystalline islands in the presence of strain. Indium solid islands grown at temperatures below 160 ∘C have a face-centered cubic crystal structure.

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

In As atmosphere, we analyzed the crystallization dynamics during post-growth annealing of Ga droplets residing at the top of self-assisted GaAs nanowires grown by molecular beam epitaxy. The final crystallization steps, fundamental to determining the top facet nanowire morphology, proceeded via a balance of Ga crystallization via vapor-liquid-solid and layer-by-layer growth around the droplet, promoted by Ga diffusion out of the droplet perimeter, As desorption, and diffusion dynamics. By controlling As flux and substrate temperature the transformation of Ga droplets into nanowire segments with a top surface flat and parallel to the substrate was achieved, thus opening the possibility to realize atomically sharp vertical heterostructures in III-As self-assisted nanowires through group III exchange.

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.

Entanglement plays a crucial role in many protocols for quantum cryptography and in various approaches for quantum computation. Semiconductor quantum dots (QDs) have been proposed as a source of polarization-entangled photons which can be integrated in an electrically driven solid state device [1]. The generation process relies on the biexciton-exciton radiative cascade. However, e-h exchange interaction often induces a fine structure splitting (FSS) between the bright exciton states and destroys quantum correlation. In the case of QDs grown on commonly used (100) substrates, this degeneracy lifting is caused by the C2v symmetry stemming from asymmetric interfaces, strain anisotropy, piezoelectric fields and shape elongation [2,3]. A viable alternative relies on growing QDs on the higher symmetry (111) substrate. Theoretical investigations have shown that QDs with C3v symmetry should exhibit zero FSS [4,5]. The most studied III-V materials do not grow in the Stranski-Krastanov mode on a (111) surface, however different techniques such as droplet epitaxy [6] or the use of patterned substrates [7] have demonstrated to be able to overcome this limitation. In our work we focus on GaAs/AlGaAs (111) QDs grown by droplet epitaxy. Polarization resolved single dot photoluminescence measurements on hexagonal QDs are presented. Charged and bi-excitonic complexes are consistently identified by means of power and polarization dependence analyses. A broad FSS energy distribution is observed, with an average value smaller than the one reported for QDs grown on (100) substrates using the same technique and emitting at similar wavelengths. The phase distribution of the polarization axis evidences the absence of systematic anisotropies. These results are in agreement with previous studies on similar samples [8]. Recent advances in fabrication have proven the ability to obtain atomically flat substrates and to gradually tune the shape from hexagonal to triangular by changing the growth parameters. This lays the groundwork for a systematic investigation of the impact of geometry on the excitonic fine structure, with the goal of finding the best conditions for vanishing FSS. [1] O. Benson, C. Santori, M. Pelton, Y. Yamamoto, in: Physical Review Letters 84, 2513 (2000). [2] G. Bester, S. Nair, A. Zunger, in: Physical Review B 67, 161306 (2003). [3] R. Seguin, A. Schliwa, S. Rodt, K. Poetschke, U. W. Pohl, D. Bimberg, in: Physical Review Letters 95, 257402 (2005). [4] R. Singh, G. Bester, in: Physical Review Letters 103, 063601 (2009). [5] A. Schliwa, M. Winkelnkemper, A. Lochmann, E. Stock, D. Bimberg, in: Physical Review B 80, 161307 (2009). [6] E. Stock, T. Warming, I. Ostapenko, S. Rodt, A. Schliwa, J. A. Toefflinger, A. Lochmann, A. Toropov, S. Moshchenko, D. Dmitriev, V. Haisler, D. Bimberg, in: Applied Physics Letters 96, 093112 (2010). [7] Y. Sugiyama, Y. Sakuma, S. Muto, N. Yokoyama, in: Applied Physics Letters 67, 256 (1995). [8] T. Mano, M. Abbarchi, T. Kuroda, B. McSkimming,, A. Ohtake, K. Mitsuishi, K. Sakoda, in: Applied Physics Express 3, 065203 (2010).

We present a fully self‐assembly technique, based on Droplet Epitaxy, to fabricate semiconductor surfaces functionalized with asymmetric planar quantum dot molecules –metal nanoparticles hybrid systems for efficient THz generation. The role of growth parameters on molecule fabrication and metal nanoparticle alignment for the matching of the grown system with the design requirements is discussed.

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.

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 semiconductor quantum dot (QD) intermediate band solar cells (IBSC) design introduce an extension of the absorption coefficient of the host semiconductor to lower energies without voltage loss [1]. The IBSC working mechanisms have been demonstrated via Stranski-Krastavov self-assembly of InAs QDs in GaAs, but many issues such as strain defect nucleation, increased carrier escape, induced also by wetting layer states, are still unsolved [2]. A different approach is to self-assemble the QDs by droplet epitaxy (DE) [3], a molecular beam epitaxy technique. This technique allows for the growth of QDs without the introduction of strain, without a wetting layer, and with the independent control of number density, size and shape. DE also allows to tune the electronic states, controlling the size and aspect ratio of the QDs. Thus DE is a perfect candidate for the realization of IBSC. In this presentation the production of sub-gap two-photon photocurrent in solar cell containing lattice matched GaAs/Al0.3Ga0.7As QDs grown by DE will be shown [4]. By using DE for QD fabrication it is also possible to tune the size and the aspect ratio of the QD, to tailor proper electronic levels in order to reduce the temperature activated quenching of the sub-gap absorption spectrum of the IBSC (see Fig. 1). Moreover, the lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the QDs [5], leaving photon-induced transitions the dominant ones, as requested by IBSC theory [1], and avoiding the quasi-Fermi energy pinning to QD states which causes the open circuit voltage reduction in QD based IBSCs. References [1] A. Luque, A. Martì, Physical Review Letters, 78, 5014 (1997). [2] A. Luque and A. Martì, Prog. Photovolt: Res. Appl. 9, 73-86 (2001) [3] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics 32, 2052-2058 (1993). [4] S. Sanguinetti, K. Watanabe, T. Tateno, et. al., Applied Physics Letters 81, 613 (2002) [5] A. Scaccabarozzi, S. Adorno, S. Bietti, M. Acciarri and S. Sanguinetti, Phys. Satus Solidi RRL 7, 174-176 (2013)

Droplet epitaxy (DE) is a flexible growth technique based on molecular beam epitaxy which allows for the fabrication of density, size and shape controlled nanostructures. Being based on control of crystallization kinetic of nanoscale reservoirs of group III metal by group V irradiation, it allows for the growth 3D nanostructures on a large variety of substrates, including Si(111) and GaAs(111)A. On patterned Si substrates, we show that it is possible to obtain the fabrication of ordered and controlled array of embedded Ga nanoparticles (NPs) in a semiconductor matrix. We demonstrate that Ga droplets can be successfully trapped at the bottom of the pits due to the combined effects of capillarity condensation and nucleation kinetics, as also shown by kinetic Monte Carlo simulations [1]. DE allows for the fabrication of highly symmetric GaAs QDs on GaAs(111)A, which can be used as entangled photon emitters. We developed a procedure to control surface flatness of AlGaAs on a GaAs(111)A substrate and to control the shape of GaAs QDs to obtain highly symmetric QDs. In addition it is possible, by DE, to create a self-assembled nanopatterning of the Si(111) substrate for the subsequent growth of density and size controlled GaAs nanowires (NWs). In our method, GaAs islands are initially formed on Si(111) by DE and, subsequently, GaAs NWs are selectively grown on their top facet, which acts as a nucleation site on Si substrates with or without an oxide layer [2,3]. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and successively transfer the same degree of control to the final GaAs NWs. [1]M. Bollani, S. Bietti, C. Frigeri, D. Chrastina, K. Reyes, P. Smereka, J.M. Millunchick, G.M. Vanacore, M. Burghammer, a Tagliaferri, and S. Sanguinetti, Nanotechnology 25, 205301 (2014). [2]C. Somaschini, S. Bietti, A. Trampert, U. Jahn, C. Hauswald, H. Riechert, S. Sanguinetti, and L. Geelhaar, Nano Letters 13, 3607 (2013). [3]S Bietti , C Somaschini, C Frigeri, A Fedorov, L Esposito, L Geelhaar and S Sanguinetti, Journal of Physics D (accepted for publication)

The authors present a procedure to obtain uniform, ordered arrays of Ga droplets on GaAs(001) substrates. The growth process relies on an interplay between the substrate patterning, in form of a two dimensional array of nanoholes periodically modulated obtained via local anodic oxidation, and self-assembly of Ga droplets in a molecular beam epitaxy environment. The formation of site controlled Ga droplets, characterized by atomic force microscopy, is the outcome of the combined effects of capillary condensation and nucleation kinetics. (C) 2014 American Vacuum Society.

Droplet epitaxy (DE) is a growth technique proposed in 1991 by Dr. N. Koguchi [1] based on molecular beam epitaxy. In DE, first a beam of Ga atoms impinges on the sample surface in absence of arsenic, leading to the formation of nanometric size gallium droplets. Then, an As flux is supplied in order to crystallize the droplets into GaAs nanocrystals. The separation of the group III and group V atom irradiation on the substrate, makes possible to obtain a fine tuning of the quantum dot (QD) shape and density by simply changing the parameters that control the group III atom diffusion on the surface and allows for the fabrication of III-V QDs by self-assembly without wetting layer, lattice-matched with the barrier layer and strain-free. In this presentation we will show how it is possible to control the size dispersion of the Ga droplets, and then of the final GaAs QDs, by simply tuning during the Ga deposition step the substrate temperature and the Ga flux. This allow for the fabrication of nanostructures with narrow size dispersion (few percent) using optimal substrate temperatures and Ga fluxes. In our experiments performed in an MBE chamber on GaAs (001) surface, we changed the substrate temperature between 200 °C and 450 °C and the Ga flux between 0.01 and 1 ML/s, comparing the island size distribution (ISD) and the capture zone distribution (CZD). The CZD is estimated with Voronoi polygons, whose boundaries are defined as the locus of points equidistant from the two nearest island centers. This tessellation of the plane was carried out by using AFM images of each sample. The standard nucleation theory predicts that the minimum variance of ISD (σISD) is achieved when it matches the CZD (σCZD). In usual conditions, σISD > σCZD. For a substrate temperature of 200 °C during the Ga droplet deposition, the ISD variance for each sample is well above the σCZD. The fabricated QD thus show a rather broad emission due to the large size dispersion. If the substrate temperature is increased up to 300°C during Ga droplet deposition, it is possible to observe a range of Ga fluxes where σISD becomes comparable to σCZD . In these conditions an extremely narrow emission from the QDs can be achieved. As shown in figure 2, photoluminescence of two samples grown in different conditions (ISD variance much higher than CZD, black line, ISD variance comparable CZD, red line) shows a different full widht half maximum, reaching the value of 20 meV comparable with the best results present in scientific literature for Stranski Krastanow QDs. [1] N.Koguchi, S.Takahashi, T.Chikyow, Journal of Crystal Growth 111, 688 (1991) [2] Venables, Spiller, Hanbuenchen, Nucleation and growth of thin films, Rep. Prog. Phys. 47, 399 (1984)

Droplet epitaxy (DE) is a non-conventional growth technique based on molecular beam epitaxy. This method allows for the fabrication of lattice-matched and strain-free self-assembled III-V nanostructures, reducing the size dispersion between 5 and 20%. Thanks to the versatility of the DE, a fine tuning of the dot shape and density is possible by changing the parameters that control the Ga atom diffusion on the surface. The density can span between 10 7 and 1011 cm-2. The control on the shape allows to select the aspect ratio of the dots, thus permitting to change the quantum confinement and to tailor the optical and electronic property of the dots to fit the needs of different applications. In this talk the main aspects of the growth and characterization of DE nanostructures will be discussed, together with different applications recently developed with DE. In particular, I will show the results of the insertion of a layer of quantum dots grown by DE in a solar cell. By tuning the size of the QDs it is possible to change the position of the intermediate band, and by tuning their aspect ratio the high energy states of the QDs can also be tuned in order to have a small electron-phonon coupling with the barrier. Moreover, the lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the IB, leaving photon-induced transitions the dominant ones, as requested by intermediate band theory [1]. I will also show the fabrication of GaAs single photon emitters integrated on Si substrates demonstrating how DE makes also possible the growth of bright III-V quantum emitters on silicon substrates thus paving the route to the integration of optically efficient III-V nanostructures on CMOS technology [2]. The last application I will describe is the growth of ultra high density QDs for the fabrication of quantum dot infrared photodetectors (QDIP) active in the range between 2 and 8 μm, as a first step toward the the fabrication of a III-V based QDIP integrated on Si [3]. [1] Scaccabarozzi, Adorno, Bietti, Acciarri, Sanguinetti, Physica Status Solidi (RRL) - Rapid Research Letters 3, 173, (2013) [2] Cavigli, Bietti, Accanto et al., Applied Physics Letters, 100, 231112, 2012. [3] Frigerio, Isella, Bietti, Sanguinetti, SPIE Newsroom, 2013

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.

Quantum dot intermediate band (IB) solar cells have been proposed in order to increase the efficiency of traditional solar cells and immediately attracted a lot of attention and research since the time they were first proposed [1]. The IB introduces an extension of the absorption coefficient of the semiconductor to lower energies, allowing a more thorough collection of the solar spectrum, via a two-step absorption of low-energy photons. Quantum dot (QD) structures are good candidates for IB solar cells, because their confined energy levels can overlap and form a miniband in dense arrays and the position of these levels and bands can be tuned varying the size and spacing of the QDs. The IB working mechanisms have been demonstrated for InAs QDs in GaAs [2], and a lot of research is devoted to reduce the problems due to strain, and defect nucleation increasing carrier escape [3]. Droplet epitaxy (DE) [4] is a molecular beam epitaxy technique that allows for the growth of quantum dots of materials lattice matched to the barrier and the removal of the wetting layer. DE makes possible to indipendently control density, size and shape of the nanostructures. Densities as high as some 1011 cm-2 per layer have been reported [5] and potentially a large number of layers can be stacked because the system is strain-free, leading to a much higher density of states in the IB than the more conventional Stranski-Krastanov techniques. Since it is a strain-free technique, there are virtually no defects in DE-grown materials, and this is fundamental to have high performance devices. Moreover, DE nanostructures can be grown without the presence of a wetting layer [6], that would introduce unwanted quantum-well-like states in the system. By tuning the size of the QDs it is obviously possible to change the position of the IB, and by tuning their aspect ratio the high energy states of the QDs can also be tuned in order to have a small electron-phonon coupling with the barrier. The lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the IB, leaving photon-induced transitions the dominant ones, as requested by IB theory [1]. For these reasons DE is a good candidate for the realization of QD-IB solar cells. We demonstrate the key working principle of IB materials, that is the production of sub-gap two-photon photocurrent, with Al0.3Ga0.7As solar cells containing GaAs QDs grown by DE (figure 1a). The devices were illuminated at 15K by continuous monochromatic light and chopped broadband IR light (1.5 – 20 m), and the signal was demodulated by a lock-in amplifier. The continuous light pumps electrons into the QDs and the chopped IR promotes them to the CB where they can be collected generating an electric signal. As a control, a reference sample without QDs was grown and measured in the same conditions, but no two-photon signal was detected (see figure 2b). Measurements show that the shape of the two-photon photocurrent signal is different from the VB-CB photoresponse, indicating clearly that a two-photon process involving the QD levels is taking place. The two-photon signal response is in good agreement with the PL spectrum of the QD samples (figure 1b), that lets us easily understand the position of the energy levels of the system. [1] A. Luque, A. Martì, Physical Review Letters, 78, 5014 (1997). [2] A. Martì, E. Antolìn, C. R. Stanley, C. D. Farmer, N. Lòpez, P. Dìaz, E. Cànovas, P. G. Linares and A. Luque, Physical Review Letters 97, 247701 (2006). [3] A. Luque and A. Martì, Prog. Photovolt: Res. Appl. 9, 73-86 (2001) [4] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics 32, 2052–2058 (1993). [5] M. Jo, T. Mano, Y. Sakuma and K. Sakoda, Applied Physics Letters 100, 212113 (2012) [6] S. Sanguinetti, K. Watanabe, T. Tateno, M. Wakaki, N. Koguchi, T. Kuroda, F. Minami, and M. Gurioli, Applied Physics Letters 81, 613 (2002)

Droplet epitaxy (DE) is a non-conventional growth technique based on molecular beam epitaxy. This method, differently from strain-induced 3-dimensional nano-structures, enables the growth of lattice-matched and strain-free self-assembled III-V nano-emitters. Thanks to the versatility of the DE, different kinds of nano-structures can be implemented: quantum dots, coupled quantum dots, multiple concentric quantum rings, quantum disks, as well as combinations of these different shapes can be obtained by playing with the growth conditions. Moreover, DE enables the growth on different substrates orientations (such as the (100), (311)A, (111)A) enabling ultra-low or ultra-high nano-structure density. Most importantly, DE makes possible the growth of bright III-V quantum emitters on substrates made of Silicon and Germanium. In this talk I will introduce some features of growth and photoluminescence spectroscopy of single GaAs/AlGaAs DE nano-structures. In particular I will concentrate on quantum dots and rings addressing their electronic structure, fine structure and line broadening: I will show how the composition, shape, geometrical anisotropy and disorder rule the optical properties and how, thanks to recent advances in the DE method, bright and sharp photoluminescence lines can be obtained in circular symmetric quantum dots. Finally, I will show recent advances in the growth of III-V nano-structures on IV-IV substrates demonstrating single photon emission at high temperature fully compatible with CMOS silicon devices.

Quantum dot intermediate band (IB) solar cells have been proposed in order to increase the efficiency of traditional solar cells and immediately attracted a lot of attention and research since the time they were first proposed. The IB introduces an extension of the absorption coefficient of the semiconductor to lower energies, allowing a more thorough collection of the solar spectrum, via a two-step absorption of low-energy photons. Quantum dot (QD) structures are good candidates for IB solar cells, because their confined energy levels can overlap and form a miniband in dense arrays and the position of these levels and bands can be tuned varying the size and spacing of the QDs. The IB working mechanisms have been demonstrated for InAs QDs in GaAs, and many efforts are devoted to reduce the problems due to strain, and defect nucleation increasing carrier escape. Droplet epitaxy (DE) is a molecular beam epitaxy technique that allows for the growth of quantum dots of materials lattice matched to the barrier and the removal of the wetting layer. DE makes possible to indipendently control density, size and shape of the nanostructures. Densities as high as some 1011 cm-2 per layer have been reported and potentially a large number of layers can be stacked because the system is strain-free. Being a strain-free technique, there are virtually no defects in DE-grown materials. By tuning the size of the QDs it is obviously possible to change the position of the IB, and by tuning their aspect ratio the high energy states of the QDs can also be tuned in order to have a small electron-phonon coupling with the barrier. The lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the IB, leaving photon-induced transitions the dominant ones, as requested by IB theory. For these reasons DE is a good candidate for the realization of QD-IB solar cells. We demonstrate the key working principle of IB materials, that is the production of sub-gap two-photon photocurrent, with Al0.3Ga0.7As solar cells containing GaAs QDs grown by DE. Measurements show that the shape of the two-photon photocurrent signal is different from the VB-CB photoresponse, indicating clearly that a two-photon process involving the QD levels is taking place. The two-photon signal response is in good agreement with the PL spectrum of the QD samples, that lets us easily understand the position of the energy levels of the system.

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.

We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with experimental results that can describe and predict the mechanisms for the formation of various types of nanostructures observed during droplet epitaxy. The crucial features of the model include the explicit and independent representation of atoms with different species and the ability to treat solid and liquid phases independently. Using this model, we examine nanostructural evolution in droplet epitaxy. The model faithfully captures several of the experimentally observed structures, including compact islands and nanorings. Moreover, simulations show the presence of Ga/GaAs core-shell structures that we validate experimentally. A fully analytical model of droplet epitaxy that explains the relationship between growth conditions and the resulting nanostructures is presented, yielding key insight into the mechanisms of droplet epitaxy. © 2013 American Physical Society.

Self-assembled quantum dots (QDs) are object of extensive research and application in optoelectronics because of the possibility to tune their optical and electronic performances by a proper design of their morphological properties, like size and shape.Morphological control is therefore of primary importance to obtain fine-tuning of the electron states and of the emission of the QDs. Size and shape determine the confinement potential of electrons and holes, the transitions between different energy levels and the consequent optical emission. The available design degrees of freedom remain limited due to energetic driven evolution of the case of Stranski-Krasyanov (SK) QD self-assembling, thus reducing the possibilities of a real on demand design of their electronic properties. To overcome the SK growth limitations, a kinetic limited growth procedure, the Droplet Epitaxy (DE) was introduced [1-2]. Unlike the SK self-assembly technique, DE does not rely on strain for the formation of three-dimensional (3D) crystals. DE is based on the subsequent deposition of III and V column elements at controlled temperatures and fluxes. In this work, we show that it is possible to control the QD aspect ratio (height to base ratio) and QD exposed facets by choosing proper As pressure and substrate temperature during the crystallization step of the growth (figure 1). The DE-QD aspect ratio can be changed between 0.5 and 0.1 while the exposed facets continuously change from planes compatible with {111} to {511} orientation. In order to explain the observed phenomenology we developed a model, considering the relevant phenomena taking place during the cristallization, that is As impinging flux. As diffusion in the droplet, Ga diffusion etc. The results of the numerical simulation of QD profiles at different As fluxed and substrate temperatures are shown in figure 2. The model predictions are good agreement with the experimental results. In conclusion, we show that it is possible to control faceting and aspect ratio in DE-QDs thus allowing for a complete fine-tuning of electron states and electron-phonon interaction in self-assembled DE QDs, of great importance for many optoelectronic applications. [1] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics32, 2052–2058 (1993). [2] C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti, Nano Letters9, 3419–24 (2009).

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 (DE) [1] is an emerging and powerful MBE growth technique alternative to the commonly used Stranski-Krastanov approach for the self-assembly of quantum dots (QDs). Strain-free GaAs quantum dots can be grown by DE crystallizing group III metallic atoms, stored in nanometer scale droplets, with a group V atomic flux. This allows for an independent control over size and density of the fabricated QDs. However, despite the high level of design control achieved by DE, the details of the growth kinetics of the DE-QDs is still unclear. Two fundamental aspects of the QD growth kinetics by DE will be here presented: 1) The crystallization kinetics of a nanometer size Ga droplet in to a QD under As flux 2) The faceting of a DE-QD and its dependence on the growth parameters. The crystallization kinetics of the metallic Ga contained in the droplet into GaAs nanocrystals under the As flux is followed step-by-step, investigating the amount and the morphology of the crystallized GaAs in the droplet at different As doses by means of a combination of selective wet chemical etching and Atomic Force Microscopy (AFM), in a nano-tomography approach. The crystallization of the Ga in the droplet starts from a ring of GaAs which is formed just after the Ga deposition at the perimeter of the droplet. This ring acts as nucleation seed for the subsequent QD growth. During the As supply, the ring increases its size at the expenses of the metallic droplet. The QD growth the proceeds at the contact surface between the liquid Ga and the ring until the complete depletion of the Ga contained in the droplet. The control of the faceting of DE-QD is a fundamental aspect for the fabrication of QDs with on-demand density of states and a reduced electron-phonon interaction. We will show that it is possible to determine, by the control of the crystallization kinetic, the shape of DE-QDs. The QD shape depends on the Ga diffusion length in the crystallization condition, thus showing a marked dependence on As pressure and temperature. The facet orientation evolves, by reducing As pressure or increasing substrate temperature, from {111} facets, exposed at low temperatures and high As presure, towards {311} facets. A model, based on the effect of the diffusion of the metallic Ga from the droplet during the crystallization step, is able to reproduce the observed faceting evolution on temperature and As pressure. [1] N. Koguchi, S. Takahashi, T. Chikyow, J. Crystal Growth 111 (1991) 688

One of the fundamental ingredients for quantum cryptography and quantum information applications is the fabrication of single and entangled photon emitters. Single photon emitter should produce exactly a single photon on demand, should work not only at cryogenic temperature and should be easy to produce and to integrate on existing electronic devices. In this work, we present the fabrication and characterization of single photon emission from high quality GaAs quantum dots (QDs) by droplet epitaxy (DE) and from impurity centers in AlGaAs layers grown on Si through a thin Ge buffer layer deposited by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD). The deposition of a thin Ge layer by LEPECVD and subsequent annealing cycles allow for the reduction of threading dislocation density down to few 107 cm-2. DE is an intrinsically low thermal budget technique, being performed at temperatures between 200 and 350 °C. This makes DE perfectly suited for the implementation of growth procedures compliant with back-end integration of III-V nanostructures on CMOS. GaAs QDs with a density of few 108 cm-2 and a mean height of 8 nm are fabricated by DE inside a Al0.3Ga0.7As barrier. Bright and sharp emission lines are observed in a micro-photoluminescence experiment around 700 nm, with pure radiative excitonic lifetime and clear evidence of exciton-biexciton cascade. The achievement of quantum photon statistics is directly proved by antibunching in the second order correlation function as measured with a Hanbury Brown and Twiss interferometer up to T=80 K, thus making the single photon emitter working at liquid nitrogen temperature and compatible with present CMOS technology. The optical quality of the GaAs quantum dots grown on Si substrate is almost comparable with quantum dots directly grown on GaAs substrates. We also show that the epitaxial growth of thin layers of Al0.3Ga0.7As on GaAs buffer layers grown on Si and Ge substrates allows to obtain a single photon source by exploiting the strewn and unintentional contamination with defects of the Al0.3Ga0.7As. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in QDs, but their realization is by far much easier, since it does not require 3D nucleation or spatially selective doping. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements. In both cases (GaAs QDs by DE and impuritues in AlGaAs layer) it is clearly demonstrated a new procedure for the integration of high efficient light emitters, based on III-V semiconductors, directly on Si substrates, and opening the route to wide applications to optoelectronics, photonics and quantum information technology.

The integration of III-V nanostructures on silicon would allow to combine the high performance of quantum photonic devices and of CMOS circuitry on Si. In this work, we present the first demonstration of single photon emission from high quality GaAs quantum dots (QDs) grown by droplet epitaxy on Si substrates using a thin Ge buffer layer deposited by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD). Droplet epitaxy allows for the separate control of the QD size and density, and provides the possibility to fabricate different classes of quantum nanostructures with tailored wavefunctions and electronic levels. Droplet epitaxy is also an intrinsically low thermal budget growth, being performed at temperatures between 200 and 350 C. This makes droplet epitaxy perfectly suited for the realization of growth procedures compatible with back-end integration of III-V nanostructures on CMOS. The deposition of a thin Ge layer by LEPECVD allows for the reduction of threading dislocation density down to few 10^-7 cm . GaAs QDs with a density of few 10 cm and a mean height of 8 nm are fabricated by droplet epitaxy inside a Al0.30Ga0.70 As barrier. Two annealing procedure are performed, the first one in chamber right after the quantum dots growth to desorb As excess while the second one (performed in a rapid thermal annealing system) improves crystal quality. Bright and sharp emission lines are observed in a micro-photoluminescence experiment around 700 nm, with pure radiative excitonic lifetime and clear evidence of exciton-biexciton cascade. The achievement of quantum photon statistics is directly proved by antibunching in the second order correlation function as measured with a Hanbury Brown and Twiss interferometer up to T=80 K, thus making the single photon emitter working at liquid nitrogen temperature and compatible with present CMOS technology. The optical quality of the GaAs quantum dots grown on Si substrate is almost comparable with quantum dots directly grown on GaAs substrates, clearly demonstrating a new procedure for the integration of high efficient light emitters, based on III-V semiconductors, directly on Si substrates, and opening the route to wide applications to optoelectronics, photonics and quantum information technology.

The integration of III-V nanostructures on silicon would open the possibility to pursue integration between high performance quantum photonic devices and quantum information technology devices based on CMOS circuitry on Si. In this work, we present the growth and optical characterization of high quality GaAs quantum dots (as single photon emitters) grown by droplet epitaxy on Si substrates through the deposition of a thin Ge layer. Droplet epitaxy [1] is intrinsically a low thermal budget growth, being performed at temperatures between 200 and 350 °C. This makes droplet epitaxy perfectly suited for the realization of growth procedures compatible with back-end integration of III-V nanostructures on CMOS [2]. The control of the growth kinetics allows the fabrication of quantum dot samples with an areal density down to few 108 cm-2. Bright and sharp emission lines are observed in a micro-photoluminescence experiment around 700 nm, with pure radiative excitonic lifetime and clear evidence of exciton-biexciton cascade. The achievement of quantum photon statistics is directly proved by antibunching in the second order correlation function as measured with a Hanbury Brown and Twiss interferometer up to T=80 K, thus making the single photon emitter working at liquid nitrogen temperature and compatible with present CMOS technology. Optical quality of the GaAs quantum dots grown on Si substrate is almost comparable with quantum dots directly grown on GaAs substrates, clearly demonstrating a new procedure for the integration of high efficient light emitters, based on III-V semiconductors, directly on Si substrates, and opening the route to wide applications to optoelectronics, photonics and quantum information technology. [1] N. Koguchi, S. Takahashi, T. Chikyow, J. Crystal Growth 111 (1991) 688. [2] S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, G. Isella, and D. Chrastina, Applied Physics Letters 95, 241102 (2009)

The integration of III-V devices on Si substrates would allow the fabrication of specialized devices for optoelectronics and photonics directly on the highly refined silicon infrastructure, based on CMOS technology. In this work of thesis, Droplet Epitaxy technique is used for the low thermal budget fabrication of GaAs quantum nanostructures on silicon substrates through a Ge layer and for the fabrication of GaAs local artificial substrates directly on Si substrate. Quantum nanostructures grown on Si substrate through a Ge layer showed an intense photoluminescence emission, detectable up to room temperature and with a ratio between number of photon emitted and photogenerated carriers similar to the one obtained for GaAs quantum nanostructures grown by droplet epitaxy on GaAs substrate. GaAs local artificial substrates fabricated on Si showed high tunability in size and density, a size dispersion below 10%, a good crystalline quality and well defined shapes with a high aspect ratio.

What makes three dimensional semiconductor quantum nanostructures (QN) 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 QN. An often overlooked parameter, which has a even more relevant effect on the electronic properties of the QN, is shape. Gaining a strong control over the electronic properties of semiconductor nanostructure via shape tuning is the key to access electronic fine design possibilities. We present an innovative growth method, the Dropled Epitaxy (DE) [1,2], a variant of molecular beam epitaxy, for the fabrication of semiconductor III-V QNs with highly designable shapes and complex morphologies. In short, the DE growth procedure consists of first irradiating the substrate with a group III molecular beam flux, leading to the formation of numerous, nanometer-sized, metallic droplets on the surface which are subsequently crystallized into nanostructures by a group V molecular beam. With DE is possible to combine multiple single QNs, namely quantum dots, quantum rings and quantum disks, with tunable sizes and densities, into a single multi-functional QN thus allowing an unprecedented control over the electronic properties of the QNs [2,3] (see Figure 1). In addition, DE is intrinsically a low thermal budget growth of III-V materials, being fully performed at 200- 350°C. This makes DE perfectly suited for the realization of growth procedures compatible with back- end integration of III-V materials on Si [4,5]. [1] N. Koguchi, et al., J. Cryst. Growth (1991), 111, 688 [2] C. Somaschini, et al., Nano Letters 2009, 9, 3419 [3] C. Somaschini, et al., Nanotechnology 2010, 21, 125601 [4] S. Bietti, et al., Appl. Phys. Lett. 2009, 95, 241102 [5] C. Somaschini, et al. Appl. Phys. Lett. 2010, 97, 053101

Physical properties of nanostructures are strongly influenced by their dimensions and shapes, so that only a precise control on the nanocrystals morphology can allow for the fine tuning of their electronic properties. The droplet epitaxy (DE) [1] is a flexible growth method, based on the molecular beam epitaxy, which allows for the fabrication of a large variety of three-dimensional nanostructures with different geometries. During the growth of GaAs by DE the substrate is first irradiated by a Ga molecular beam flux, leading to the formation of numerous, nanometer-size, Ga droplets on the surface with uniform size, which are subsequently crystallized into GaAs nanostructures by an As molecular beam supply. The intrinsic design flexibility of the DE variant of MBE is permitted mainly by such splitting in time of the III-column and V-column element supply. This allows an independent choice for each of the two elements of specific growth conditions. In this presentation a set of samples which showed a large morphological tunability, ranging from quantum dots (QDs) to quantum dots molecules (QDMs), quantum rings (QRs), concentric double quantum rings (CQDRs), concentric multiple quantum rings (CMQRs)[2], coupled rings/disks (CRDs) [3], Dot/Ring, Dot/Disk, Ring/Ring and Ring/Disk complexes, was successfully fabricated. The wide growth parameter space (defined by the substrate temperature and the As flux used for the crystallization) has been explored, studying the influence of the growth conditions on the nanocrystals configuration. We investigated the growth mechanism by means of Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and selective chemical etching. We introduce a model for the growth mechanism of GaAs nanostructures, which accounts for the fabrication of different types of nanostructure on the GaAs/AlGaAs system, based on the interplay between the As adsorption on the Ga-rich (4×6) surface and the Ga migration on the As-stabilized (2×4) activated by the substrate temperature and limited by the As impingement rate on the surface. [1] N. Koguchi, S. Takahashi, T. Chikyow, J. Crystal Growth 111 (1991) 688. [2] C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti, Nano Letters 9, 3419-24 (2009). [3] C. Somaschini, S. Bietti, S. Sanguinetti, N. Koguchi, and A. Fedorov, Nanotechnology 21, 125601 (2010).

The droplet epitaxy (DE) growth method for the fabrication of III-V material quantum nanostructures [1], is an intrinsically Low Thermal Budget technique, being fully performed at temperature between 200 and 350 °C. This makes DE perfectly suited for the realization of growth procedures compatible with back-end integration. In short, the DE growth procedure consists in the deposition at different times for the group III and group V elements. Group III elements create a regular pattern of liquid droplet on a substrate, group V elements are incorporated inside group III element crystalling the droplets into a quantum nanostructure. We can distinguish two main areas where fabrication of III-V quantum nanostructures on Si substrate could play a fundamental role. The first is the fabrication of nanostructured active layers at LTB with designed DOS for optimum device performance [2]. The second area concerns the realization of local artificial substrates for heterogeneous integration of quantum nanostructures [3]. The nucleation of quantum dots atop an island is an attractive approach to address radiative recombination issues and dot uniformity as the island both separates the dot from the interface with the substrate and provides a nucleation platform of sufficiently small dimension to realize quantum size effects. For this purpose we fabricated self-assembly of GaAs islands by DE which show highly tunable density (from 107 to 109 cm-2) and size (from 75 nm to 250 nm) and size dispersion below 10%. Changing the substrate temperature during the Ga deposition and the amount of irradiated Ga is possible to independently control the density and the size of the nanostructures (figure 1a). The islands, made by single relaxed crystals, show well defined shapes, with a high aspect ratio (Figure 1b). 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 on Si. [1] N. Koguchi and K. Ishige, Japanese Journal Of Applied Physics 32, 2052-2058 (1993). [2] S.Bietti, C.Somaschini, S. Sanguinetti, N. Koguchi, G. Isella, and D. Chrastina, Applied Physics Letters 95, 241102 (2009). [3] C. Somaschini, S. Bietti, N. Koguchi, F. Montalenti, C. Frigeri, and S. Sanguinetti, Applied Physics Letters 97, 053101 (2010).

We present the molecular beam epitaxy (MBE) fabrication of GaAs ring/disk nanostructures. In this system, a central quantum ring is surrounded by a flat outer disk-like region, which is developed following a layer-by-layer growth mode. We studied the influence of the growth temperature on the morphology of these nanostructures and found out a pronounced dependence only for the outer region diameter, which is interpreted in terms of larger Ga atoms surface diffusion length at higher temperatures. Our experimental data provide a fundamental parameter to control the final shape of GaAs coupled ring/disk nanostructures. © 2010 Elsevier B.V. All rights reserved.

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.

Implementation of III-V quantum nanodevices on Si based-circuitry is an important goal to pursue the integration between “classical” and “quantum” electronics on a single technological Si based platform [1-2]. In this contribution we explore the results obtained by droplet epitaxy for the fabrication of III-V material quantum nanostructures. Droplet epitaxy is an intrinsically low thermal budget technique, being fully performed at temperature between 200 and 350°C, perfectly suited for the realization of growth procedures compatible with CMOS back-end integration. We can distinguish two main areas where fabrication of III-V quantum nanostructures on Si substrate could play a fundamental role. The first area is the realization of local artificial substrates for heterogeneous integration of quantum nanostructures. The second concerns the fabrication of nanostructured active layers with designed density of states for optimum device performance. For the first approach we present the fabrication of GaAs islands on Si as local artificial substrates. Nucleation of quantum dots atop an island is an attractive approach to address radiative recombination issues and dot uniformity as the island both separates the dot from the interface with the substrate and provides a nucleation platform of sufficiently small dimension to realize quantum size effects. For this purpose we fabricated self-assembled GaAs islands highly tunable in density (from 10 7 to 109 cm-2) and size (from 75 nm to 250 nm, see fig. 1) and with a size dispersion below 10%. The islands, made by single relaxed crystals, show well defined shapes with a high aspect ratio [2]. For the second approach, we present the growth and optical characterization of high quality GaAs quantum nanostructures grown by droplet epitaxy on Ge substrates and on Si through a Ge virtual substrates [3]. Single quantum dot spectroscopic characterization has been performed by means of a micro-photoluminescence apparatus, without the need of further sample processing. Optical quality of the GaAs quantum dots is almost comparable with quantum dots directly grown on GaAs substrates, clearly demonstrating a new procedure for the integration of high efficient light emitters, based on III-V semiconductors, directly on IV-column semiconductor substrates, and opening the route to wide applications to optoelectronics, photonics and quantum information technology. [1] S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, G. Isella, and D. Chrastina, Applied Physics Letters 95, 241102 (2009). [2] C. Somaschini, S. Bietti, N. Koguchi, F. Montalenti, C. Frigeri, and S. Sanguinetti, Applied Physics Letters 97, 053101 (2010). [3] L. Cavigli, M. Abbarchi, S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, A. Vinattieri and M. Gurioli, Applied Physics Letters, 98(10), 103104 (2011).

The integration of III-V nanostructured active layers on silicon could open interesting perspectives for implementation of high performance devices for optoelectronics, photonics and quantum information technology within CMOS circuits [1]. Nanostructures based on III-V semiconductor materials have better optical properties compared to Si, and their use has largely improved optoelectronic devices performances, such as semiconductor laser. Even more striking is the possibility to exploit quantum dots with three-dimensional quantum confinement and deltalike density of states for quantum devices such as single photon [2] or entangled photon pairs emitters [3]. In this presentation, we explore the growth and optical characterization of high quality and low density GaAs quantum dots grown by droplet epitaxy [4] on Ge substrates and the possibility to integrate the same nanostructures on Si through Ge virtual substrates [5]. The adoption of droplet epitaxy method is of utter importance for our target. Droplet epitaxy is intrinsically a low thermal budget growth, being performed at temperatures between 200 and 350 °C. This makes droplet epitaxy perfectly suited for the realization of growth procedures compatible with back-end integration of III-V nanostructures on CMOS emitters. The control of the growth kinetics permitted the fabrication of quantum dot samples with extremely low areal densities (down to few 108cm -2). Single quantum dot spectroscopic characterization has been performed, by means of a micro-photoluminescence apparatus, without the need of further sample processing. Optical quality of the GaAs quantum dots is almost comparable with quantum dots directly grown on GaAs substrates, clearly demonstrating with such achievement, a new procedure for the integration of high efficient light emitters, based on III- V semiconductors, directly on IV semiconductor substrates, and opening the route to wide applications to optoelectronics, photonics and quantum information technology. [1] S. Bietti, C. Somaschini, S. Sanguinetti, N. Koguchi, G. Isella, and D. Chrastina, Applied Physics Letters 95, 241102 (2009). [2] T. Kuroda, M. Abbarchi, T. Mano, K. Watanabe, M. Yamagiwa, K. Kuroda, K. Sakoda, G. Kido, N. Koguchi, C. Mastrandrea, L. Cavigli, M. Gurioli, Y. Ogawa, and F. Minami, , Applied Physics Express 1 042001(2008) [3] A. Dousse, J. Suffczynski, Alexios Beveratos, Olivier Krebs, Aristide Lemaıtre, Isabelle Sagnes, Jacqueline Bloch, Paul Voisin, and Pascale Senellart, Nature (London) 466, 217 (2010) [4] N. Koguchi, S. Takahashi, T. Chikyow, J. Crystal Growth 111 (1991) 688. [5] J. Osmond, G. Isella, D. Chrastina, R. Kaufmann, M. Acciarri, and H. von Kaenel, Applied Physics Letters 94, 201106 (2009).