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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.

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

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

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

We report on the dewetting process, in a high vacuum environment, of amorphous Ge thin films on SiO2/Si (001). A detailed insight of the dewetting is obtained by in situ reflection high-energy electron diffraction and ex situ scanning electron microscopy. These characterizations show that the amorphous Ge films dewet into Ge crystalline nano-islands with dynamics dominated by crystallization of the amorphous material into crystalline nano-seeds and material transport at Ge islands. Surface energy minimization determines the dewetting process of crystalline Ge and controls the final stages of the process. At very high temperatures, coarsening of the island size distribution is observed.

The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size distribution below 15%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling allow us to deconvolve the contribution of phonon broadening in the ensemble photoluminescence and study it in a wide temperature range., Comment: 9 pages, 4 figures

InGaN alloys are of raising interest for many applications. The possibility of tuning their functional properties with the composition sets the importance of finding methods to characterize these materials in a fast and non-destructive way. Raman spectroscopy is one of these techniques, being able to yield information about the composition, strain, and other relevant parameters. However, the method of measuring the composition with a calibration of the maximum A1(LO) band is today limited to regions which are either rich in In or in Ga. The middle composition range still needs a calibration. By measuring the Raman spectra of different In x Ga1-xN alloys grown epitaxially on Si and by comparing them with the results from X-ray diffraction, we investigated this missing region of compositions. Within the range of 30%-65%, we have found that the position of the maximum of A1(LO) scales with the In fraction x as ω x = 736-135x-24x 2 cm-1. With this calibration, it is possible to determine, by Raman spectroscopy, the composition of an unknown alloy with an uncertainty of 5%

The growth dynamics of Ga(In)N semiconductors by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) at low temperatures (T = 450 °C) is here investigated. The presence of droplets at the growth surface strongly affects the adatom incorporation dynamics, making the growth rate a decreasing function of the metal flux impinging on the surface. We explain this phenomenon via a model that considers droplet effects on the incorporation of metal adatoms into the crystal. A relevant role is played by the vapor-liquid-solid growth mode that takes place under the droplets due to nitrogen molecules directly impinging on the droplets. The role of droplets in the growth dynamics here observed and modeled in the case of Nitride semiconductors is general and it can be extended to describe the growth of the material class of binary compounds when droplets are present on the surface.

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.

The accurate determination of the geometrical features of quasi one-dimensional nanostructures is mandatory for reducing errors and improving repeatability in the estimation of a number of geometry-dependent properties in nanotechnology. In this paper a method for the reconstruction of length and spatial orientation of single nanowires (NWs) is presented. Those quantities are calculated from a sequence of scanning electron microscope (SEM) images taken at different tilt angles using a simple 3D geometric model. The proposed method is evaluated on a collection of SEM images of single GaAs NWs. It is validated through the reconstruction of known geometric features of a standard reference calibration pattern. An overall uncertainty of about 1% in the estimated length of the NWs is achieved.

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

Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dots—grown by Droplet Epitaxy on AlGaAs—with sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible. © 2017 Author(s).

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.

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

The structure, interface abruptness and strain relaxation in InAs/GaAs nanowires grown by molecular beam epitaxy in the Ga self-catalysed mode on (111) Si have been investigated by transmission electron microscopy. The nanowires had the zincblende phase. The InAs/GaAs interface was atomically and chemically sharp with a width around 1.5 nm, i.e. significantly smaller than previously reported values. This was achieved by the consumption of the Ga droplet and formation of a flat top facet of the GaAs followed by the growth of InAs by splitting the depositions of In and As. Both elastic and plastic strain relaxation took place simultaneously. Experimental TEM results about strain relaxation very well agree with linear elasticity theory calculations by the finite element methods. (C) 2016 Elsevier B.V. All rights reserved.

We present a detailed characterization of the growth dynamics of Ga(Al)As(111)A surfaces. We develop a theoretical growth model that well describes the observed behavior on the growth parameters and underlines the Ehrlich-Schw\"obel barrier as a leading factor that determines the growth dynamics. On such basis we analyze the factors that lead to the huge observed roughness on such surface orientations and we identify the growth conditions that drive the typical three-dimensional growth of Ga(Al)As(111)A towards an atomically flat surface. ${\mathrm{GaAs}/\mathrm{Al}}_{0.30}{\mathrm{Ga}}_{0.70}\mathrm{As}$ quantum wells realized on an optimized surface ($l0.2$ nm roughness) show a record low emission linewidth of 4.5 meV.

In this work we show the possibility to increase the optical quality of quantum dots grown by droplet epitaxy on Ge-on-Si substrates in terms of single dot emission linewidth, decay time and efficiency by operating on the As pressure during the crystallization step without increasing the thermal budget. © 2013 Elsevier B.V. All rights reserved.

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

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

Complex GaAs/AlGaAs nanostructures, constituted by a quantum dot sitting on a quantum disk, are fabricated. This was made possible by droplet epitaxy growth technique which allows for fine control of the crystallization kinetics, by means of As flux and substrate temperature, of nanometer-scale metallic Ga reservoirs deposited on the substrate. Such complex quantum nanostructures show good optical quality and the exciton recombination in both the dot and the disk could be detected by photoluminescence spectroscopy.

We report on the growth and optical characterization by macro and micro photoluminescence measurements of high optical quality GaAs quantum nanostructures grown by droplet epitaxy on Ge and Si substrates. The quantum nanostructures show optical performances comparable to those achievable with the most advanced realized on GaAs substrates. The adopted growth procedures show also the possibility to fabricate the active layer maintaining a low thermal budget compatible with back-end integration of the fabricated materials on integrated circuits. We demonstrate the possibility to embed GaAs nanostructured devices such as intersubband detectors and single quantum emitters on Si substrates. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interferometric correlation spectroscopy is performed to investigate the decoherence dynamics of localized excited states in a single GaAs/AlGaAs quantum ring. For increasing temperature and excitation, we observe that the decay of interferometric envelope and the Fourier-transformed spectrum remain exponential and Lorentzian, respectively. The linewidth broadening for temperature and excitation intensity can be attributed to enhanced acoustic phonon scattering and Auger scattering in the fast modulation limit through the barrier traps, where carriers are captured and escape randomly. Additionally, we suggest an internal scattering between the different vertical confinement states for the significantly short coherence time (

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 show that independent size and morphology engineering of epitaxial quantum dots can be obtained using a kinetically controlled quantum dot fabrication procedure, namely droplet epitaxy. Due to the far-from-equilibrium droplet epitaxy procedure, which is based on the crystallization, under As flux, of a nanometric droplet of Ga, independent and precise tuning of quantum dot size, aspect ratio, and faceting can be achieved. The dependence of the dot morphology on the growth conditions is interpreted and described quantitatively through a model that takes into account the crystallization kinetics of the Ga stored in the droplet under As flux.

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

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

We address the radiative emission of individual germanium extrinsic centers in ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ epilayers grown on germanium substrates. Microphotoluminescence experiments demonstrate the capability of high temperature emission (70 K) and complex exciton configurations (neutral exciton $X$ and biexciton $XX$, positive ${X}^{+}$ and negative ${X}^{\ensuremath{-}}$ charged excitons) of these quantum emitters. Finally, we investigate the renormalization of each energy level showing a large and systematic change of the binding energy of $XX$ and ${X}^{+}$ from positive to negative values (from $\ensuremath{\sim}+5$ meV up to $\ensuremath{\sim}\ensuremath{-}7$ meV covering $\ensuremath{\sim}70$ meV of the emission energy) with increasing quantum confinement. These light emitters, grown on a silicon substrate, may exhibit energy-degenerate $X$ and $XX$ energy levels. Furthermore, they emit at the highest detection efficiency window of Si-based single-photon detectors. These features render them a promising device platform for the generation of entangled photons in the time-reordering scheme.

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

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

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.

Some devices, like LEDs and multi-junction solar cells, make use of thick III-V layers. Due to the different thermal expansion coefficients such thick layers undergo cracking and bowing that can cause device failure. This issue is more dramatic when the III-V layers are grown on Si to get monolithic integration of III-V optoelectronic devices onto Si microelectronic CMOS platforms. It was shown recently that cracking and bowing of thick Ge layers on Si do not occur by growing on deeply patterned Si, i.e. on Si pillars [1]. This approach was applied here to grow crack-free GaAs microcrystals on Si pillars. The type and arrangement of extended defects, that are still expected due to the lattice mismatch and polarity of the III-V deposit, in such novel structures are discussed in this work. The GaAs was grown on (001) Si 6° off to [110] at 580 °C by MBE at a rate of 0.5 ML/s with V/III=50. Prior to growth the Si substrate was patterned by deep reactive ion etching (DRIE), based on the Borsch process, into square-based pillars, 8 µm high and 5 or 9 µm wide. Defect analysis was performed by TEM operated in the two beam diffraction, High Resolution and High Angle Annular Dark Field modes. The GaAs was thicker than 2 µm and its top was defined by {113} and {111} facets. Both 60° and edge misfit dislocations formed at the GaAs/Si interface, as expected from the 4.2% lattice mismatch. Threading dislocations generated from them propagate up the GaAs only for ~500 nm due to their mutual interactions that left behind tiny dislocation loops. They thus do not reach the top of the GaAs microcrystal that will serve as active region in devices. Threading dislocations reach the top surface only at the lateral edges of the GaAs, again far from the active area. Twins were also observed that originate at the interface very likely at the steps of the offcut substrate. Contrary to the threading dislocations, twins reach the topmost part of GaAs. However, as twins have no associated dangling bonds, they should not be electrically active. Rare antiphase boundaries exist at the interface, hence not harmful for device operation. [1] C. V. Falub, H. von Kaenel, F. Isa, R. Bergamaschini, A. Marzegalli, D. Chrastina, G. Isella, E. Mueller, P. Niedermann, L. Miglio, Science 335, 1330 (2012)