Search

Dense arrays of self-assembled nanoholes are fabricated in GaAs(100) surfaces by As-free oxide cleaning thermal process. The formation of pit-like structures above 500 °C occurs by congruent evaporation of GaAs in the areas where the oxides have already desorbed. Thermally etched nanoholes exhibit inverted symmetric conical shape with an average depth/base diameter ratio of 0.21. Shallow and deep nanoholes are formed with this method, their depth ranging from 2 nm to 15 nm. Metallic Indium deposited onto the etched surfaces accumulates at the bottom of the nanoholes, thus making the etched pits a convenient template for the fabrication of high density quantum dot structures.

We demonstrated nanopit formation by Ga-assisted local droplet etching technique in InGaAs metamorphic layers grown on vicinal GaAs(111)A substrates. We studied nanopit formation depending on the substrate temperature, Ga flux and Ga amount. The etched pits show a highly symmetrical pyramidal shape with an equilateral triangular base and the edges of the triangle are along <11 ̄0> directions. The observed behavior, in terms of nanopit density, depth and, aspect ratio is well described by a model taking into account the dynamics of the droplet etching process.

The development of quantum nanostructures is of primary importance for the advancement of optoelectronic technologies, and particularly the successful implementation of quantum dots (QDs) has the potential to result in a generational leap in the field. Despite the promises, the performance of QD-based optoelectronic devices is still lacking compared to the state-of-theart technology. One approach to overcome their limitations is to fully exploit the engineering possibilities allowed by QDs, by controlling not only their quantum states but also the carrier dynamics in the nanostructures. This approach allows control both wavefunction shaping and interlevel coupling, permitting to selectively enhance or suppress recombination channels. Our group is facing this challenge by designing a hybrid nanostructure driven by the concept of quantum confinement modulation. This approach exploits the ability of Droplet Epitaxy to grow homoepitaxial QDs on a quantum well, combining the advantages of 1D and 3D confinement while overcoming their individual drawbacks. In this talk I will discuss the impact of this approach on optoelectronic devices, focusing on the application in long-wave infrared detectors and intermediate band solar cells.

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.

We investigated the nucleation of Ga droplets on singular GaAs(111)A substrates in the view of their use as the seeds for the self-assembled droplet epitaxial quantum dots. A small critical cluster size of 1–2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around 350∘C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich–Schwöbel barrier at the terrace edges.

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.

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.

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.

We have successfully grown, through the detailed control of the growth kinetics, flat InAlAs metamorphic buffer layers on 2 degrees -off GaAs(111)A substrates using molecular beam epitaxy. Almost full plastic relaxation is obtained for a layer thickness > 40 nm. The control of an adatom diffusion length and a step ejection probability from the bunches permits a reduction of the InAlAs epilayer root-mean-square surface roughness to 0.55 nm.

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.

We investigated the composition uniformity of InGaN epilayers in presence of metal droplets on the surface. We used Plasma Assisted MBE to grow an InGaN sample partially covered by metal droplets and performed structural and compositional analysis. The results showed a marked difference in indium incorporation between the region under the droplets and between them. Based on this observation we proposed a theoretical model able to explain the results by taking into account the vapour liquid solid growth that takes place under the droplet by direct impingement of nitrogen adatoms.

We demonstrate the growth of low density anti-phase boundaries, crack-free GaAs epilayers, by Molecular Beam Epitaxy on silicon (001) substrates. The method relies on the deposition of thick GaAs on a suspended Ge buffer realized on top of deeply patterned Si substrates by means of a three-temperature procedure for the growth. This approach allows to suppress, at the same time, both threading dislocations and thermal strain in the epilayer and to remove anti-phase boundaries even in absence of substrate tilt. Photoluminescence measurements show the good uniformity and the high optical quality of AlGaAs/GaAs quantum well structures realized on top of such GaAs layer.

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 (-1-12). At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

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

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 x 10(-6) torr and a temperature of 600 degrees C.

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 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 dispersion below 10%, 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.

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.

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 toward 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 with 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.

The search for a suitable entangled photon source is an active research direction because such sources play an important role in key quantum communication protocols and different quantum computation approaches. We present the fabrication and the characterization of an entangled photon source based on GaAs/AlGaAs QDs grown on a ( l l l )A substrate by a novel approach based on droplet epitaxy. DE is still quite far from meeting the fundamental requirements for the practical realization of a hybrid semiconductor−atomic quantum network. The average fine structure splitting value is still too high and gives rise to a minor fraction of only approximately 5% of entanglement-ready emitters. Additionally, we want to match the QDs emission with Rb—based quantum memories (around 780 nm). Finally, a long-standing drawback is the low substrate temperature during the formation of the nanostructures and the surrounding barrier, which places a limit on the crystalline and optical quality of the material, issue that can be only partially overcome with an annealing process. Thanks to the specific choice of substrate orientation, the use of low Al concentration in the barrier and finely tuned growth conditions, we overcome this limitations. 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. Fidelity measurements under two-photon resonant excitation yielded a value of 0.8. which already reaches the state-of the art for the more studied InGaAs/GaAs QDs in absence of postselection or external FSS tilting, thus confirming the intriguing potentiality of this technique.

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 (<25 ps) compared to that observed in quantum dots.

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.

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).

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-Schwöbel 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. GaAs/Al0.30Ga0.70As quantum wells realized on an optimized surface (<0.2 nm roughness) show a record low emission linewidth of 4.5 meV.

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.

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 investigate the effect of surface Ga accumulation on the growth of In0.5Ga0.5N by PAMBE at low temperatures (T= 440 °C). We find that the control of the surface metal condition is very important because the crystallization process strongly depends on the metal flux impinging on the surface. The growth rate rapidly decreases when the supply of metal flux exceeds certain amount. Such phenomenon can be explained by the model taking into account droplet effects on the incorporation of metals adatom into the crystal.

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.

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 growt

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.