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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).

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).

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 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 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 Droplet Epitaxy allows for the design and the realization of extremely complex semiconductor quantum ring structures, which range from single ring, multiple concentric quantum rings and coupled ring/disk structures. The fabrication and the characterization of such quantum ring complexes is here reviewed. Electronic structure, carrier dynamics, magnetic properties and single photon emission from droplet epitaxy ring structures will be discussed.

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.

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 experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs/AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3003578]

We demonstrate the self-assembled formation of concentric quantum double rings with high uniformity and excellent rotational symmetry using the droplet epitaxy technique. Varying the growth process conditions can control each ring's size. Photoluminescence spectra emitted from an individual quantum ring complex show peculiar quantized levels that are specified by the carriers' orbital trajectories.

Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (approximate to1x10(9)cm(-2)) self-assembled GaAs/AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion. (C) 2004 American Institute of Physics.

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures—grown by modified droplet epitaxy—where the dimensionality of the barrier states has been varied from 2D to 3D. This is realized by the insertion of a variable thickness quantum well at the quantum dot base. We find a fast carrier relaxation, which is made faster by the increase of the photogenerated carrier injection in all the samples due to Auger-like mechanisms. Hence the dimensionality of the electronic states in the barrier is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in the literature. Our findings suggest that the Auger-like processes only involve the zero-dimensional levels of the quantum dots.

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 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.