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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.

We present accurate measurements of Ga cation surface diffusion on GaAs surfaces. The measurement method relies on atomic force microscopy measurement of the morphology of nano-disks that evolve, under group V supply, from nanoscale group III droplets, earlier deposited on the substrate surface. The dependence of the radius of such nano-droplets on crystallization conditions gives direct access to Ga diffusion length. We found an activation energy for Ga on GaAs(001) diffusion EA=1.31±0.15 eV, a diffusivity prefactor of D0-=-0.53(×2.1±1) cm2s-1that we compare with the values present in literature. The obtained results permit to better understand the fundamental physics governing the motion of group III ad-atoms on III-V crystal surfaces and the fabrication of designable nanostructures.

Unlike in bulk materials, energy transport in low-dimensional and nanoscale systems may be governed by a coherent ballistic behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In order to study this coherent limit, both the spatial and temporal resolutions must be sufficient for the length-time scales involved. Here, we report observation of the lattice dynamics in nanoscale quantum dots of gallium arsenide using ultrafast electron diffraction. By varying the dot size from h = 11 to 46 nm, the length scale effect was examined, together with the temporal change. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes that travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. Because ultrafast diffraction provides the atomic-scale resolution and a sufficiently high time resolution, other nanostructured materials can be studied similarly to elucidate the nature of dynamical energy localization.

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 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 fabricate site-controlled, ordered arrays of embedded Ga nanoparticles on Si, using a combination of substrate patterning and molecular-beam epitaxial growth. The fabrication process consists of two steps. Ga droplets are initially nucleated in an ordered array of inverted pyramidal pits, and then partially crystallized by exposure to an As flux, which promotes the formation of a GaAs shell that seals the Ga nanoparticle within two semiconductor layers. The nanoparticle formation process has been investigated through a combination of extensive chemical and structural characterization and theoretical kinetic Monte Carlo simulations. © 2014 IOP Publishing Ltd.

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 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.

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.

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 contro

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

Metal nanoparticles (NPs) present unique optical properties, which are very different from those of bulk material. The localized surface plasmon resonance of these particles results in strong optical scattering and a strongly enhanced optical near-field around the particle [1]. Recently, metal nanoparticles have been investigated as a possible way to improve the performance of thin-film solar cells [2]. Metal NPs embedded in a semiconductor material act as antennas for the incident light and store energy in the localized surface plasmon resonance. The strong near-field absorption enhancement can be used to reduce the thickness of a thin film solar cell without a reduction of optical absorption. Here we show that, by the combination of substrate patterning and droplet epitaxy [3], it is possible to obtain the fabrication of and ordered and controlled array of embedded Ga nanoparticles in a semiconductor matrix. A Si(001) wafer patterned with regular arrays of half micron inverted pyramid pits was used as substrate for the subsequent fabrication of GaAs islands in the droplet epitaxy (DE) growth mode. DE separates Ga deposition, used for the formation of an ensemble of localized Ga reservoirs on the surface, from the As supply, necessary to crystallize the droplets into GaAs nanostructures. Figure 1(a) shows a SEM image taken before pre-growth cleaning on a typical array of {111} pits, etched into a Si(001) substrate surface. Figure 1(b) is taken on the same sample after DE growth, showing that a dot is present at the base of each pit. The capture of the Ga droplet by the inverted pits is caused by capillarity forces. The occupancy ratio of the pits by Ga droplets, obtained from several similar images, is 80%. The chemical and structural quality of the GaAs nanoislands at the bottom of pits was studied by means of set of complementary characterization techniques. In particular cross-sectional TEM was employed in order to extract the exact shapes and composition of the

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 de

We have recently shown that crystal defects and internal stresses commonly hampering Ge heteroepitaxy on Si can be greatly reduced by promoting the nucleation of micro-crystals at the top of Si pillars deeply patterned on the substrate [1]. Ge-on-Si epitaxy by low-energy plasma-enhanced CVD (LEPECVD), allows for controlling the microcrystal faceting, in turn affecting the evolution of threading dislocations and providing complete expulsion of threading arms to the microcrystal sidewalls [2]. In addition, the microcrystals are self-assembled in dense arrays, with a spacing ranging from tens to one hundred nanometers. The growth mechanism of the micro-crystals is an intriguing “nano” effect, which has been interpreted in terms of independent facet growth. This effect is not a unique feature of Ge deposition by LEPECVD, but a more general growth mode, which can be extended to other materials systems and deposition techniques. GaAs microcrystals deposited by MBE on patterned Si substrates show as well the complete tessellation of the surface by closely separated microcrystals. The complete elimination of thermal strain and reduction of threading dislocations is confirmed by the high photoluminescence yield. [1] C. V Falub et al. Science 2012, 335, 1330–4 ; [2] A. Marzegalli et al., Advanced materials 2013, 25, 4408

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 so

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

The integration of III-V nanostructures on silicon would allow to combine the high performance of III-V quantum photonic devices and of CMOS circuitry embedded on Si. In this work, we present the fabrication and characterization of single photon emission from high quality GaAs quantum dots (QDs) grown by droplet epitaxy (DE) [1] on Si through a thin Ge buffer layer deposited by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD) [2]. In figure 1 a sketch of the sample is reported. The deposition of a thin Ge layer by LEPECVD allows 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 Al0.30Ga0.70As barrier. To improve the quality of the GaAs QDs, two annealing procedure with reduced thermal budget are performed. The first one in chamber right after the quantum dots deposition to desorb As excess, the second one in a rapid thermal annealing system in order to improve 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 [3]. 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 (figure 2), 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 hig

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.

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.

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.

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.

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.

We present a novel approach for the growth of GaAs nanowires (NWs) with controllable number density and diameter, which consists of the combination between droplet epitaxy (DE) and self-assisted NW growth. 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. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and the same degree of control is transferred to the final GaAs NWs. We show how, by a suitable choice of V/III flux ratio, a single NW can be accommodated on top of each GaAs base island. By transmission electron microscopy, as well as cathodo-and photoluminescence spectroscopy, we confirmed the high structural and optical quality of GaAs NWs grown by our method. We believe that this combined approach can be more generally applied to the fabrication of different homo-or heteroepitaxial NWs, nucleated on the top of predefined islands obtained by DE. © 2013 American Chemical Society.