Stefano Sanguinetti

Fabrication of Multiple Concentric Nanoring Structures

We present the fabrication of GaAs/AlGaAs Multiple (from three to five) concentric nanoring structures by an innovative growth method based on droplet epitaxy and characterized by short time As supply to the Ga droplets at different substrate temperatures. The formation mechanism has been interpreted on the basis of a detailed ex situ and in situ characterization of nanostructure morphology and surface reconstruction. We introduce design criteria which will allow to obtain concentric quantum ring structures of the desired complexity.

Self-assembled GaAs/AlGaAs coupled quantum ring-disk structures by droplet epitaxy

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 in situ and ex situ probes allows us to propose a working model for the formation of these structures.

Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory

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

Control over the number density and diameter of GaAs nanowires on Si(111) mediated by droplet epitaxy.

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.

Hallow diamonds: stability and elastic properties

Abstract We conjecture about the possibility of periodic sp 3 -bonded carbon lattices formed by regular arrays of fullerenic cavities (hollow diamonds (HDs)) as a generalization of clathrates. We describe three infinite series of HDs having Nn ( n + 1)(2 n +1)/3 atoms per unit cell, for any natural number n and N = 17 (face center cubic), 20 (hexagonal) or 23 (simple cubic lattices). Each of the series tends to diamond for n → ∞. Using the semi-empirical many-body Tersoff potential for the carbon-carbon interaction, we have calculated with a simulated annealing procedure the ground-state structure, the total energy and the elastic constants of the smallest HDs (fcc-C 34 , hex-C 40 and sc-S 46 ), comparing the results to diamond.

High temperature single photon emitter monolithically integrated on silicon

We report on triggered single photon emission from GaAs quantum dots, grown on Si substrates and obtained by means of fabrication protocols compatible with the monolithic integration on Si based microelectronics. Very bright and sharp individual exciton lines are resolved in the spectra and can be followed up to 150 K. The nature of quantum emitters of single photon pulses can be measured up to liquid nitrogen temperature by Hanbury Brown and Twiss interferometric correlations.

Shape control via surface reconstruction kinetics of droplet epitaxy nanostructures

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.

Fabrication of high efficiency III-V quantum nanostructures at low thermal budget on Si

We fabricate high efficiency GaAs∕AlGaAs quantum nanostructure active layer for intersubband detectors and light emitting devices on a silicon substrate. The whole process of formation of the GaAs∕AlGaAs active layer was realized via droplet epitaxy and migration enhanced epitaxy maintaining the growth temperature ⩽350°C, thus resulting in a low thermal budget procedure compatible with back-end integration of the fabricated materials on integrated circuits. The realized quantum nanostructures show optical efficiencies comparable to those achievable with state of the art quantum dot materials grown on GaAs substrates.

Dynamics of mass transport during nanohole drilling by local droplet etching

Local droplet etching (LDE) utilizes metal droplets during molecular beam epitaxy for the self-assembled drilling of nanoholes into III/V semiconductor surfaces. An essential process during LDE is the removal of the deposited droplet material from its initial position during post-growth annealing. This paper studies the droplet material removal experimentally and discusses the results in terms of a simple model. The first set of experiments demonstrates that the droplet material is removed by detachment of atoms and spreading over the substrate surface. Further experiments establish that droplet etching requires a small arsenic background pressure to inhibit re-attachment of the detached atoms. Surfaces processed under completely minimized As pressure show no hole formation but instead a conservation of the initial droplets. Under consideration of these results, a simple kinetic scaling model of the etching process is proposed that quantitatively reproduces experimental data on the hole depth as a function of the process temperature and deposited amount of droplet material. Furthermore, the depth dependence of the hole side-facet angle is analyzed.

Droplet epitaxy of nanostructures

The droplet epitaxy is an innovative growth method, performed in the molecular beam epitaxy environment, for the fabrication of quantum nanostructures with highly designable shapes and complex morphologies. Droplet epitaxy is based on the split of the deposition of III- and V-column elements at controlled temperatures and fluxes. The first step of droplet epitaxy is the formation of nanoscale metal atoms reservoirs on the growth surface in forms of nanometre-size droplets with small size dispersion. The metallic droplets on the surface will constitute the group III localised sources from which the nanostructures will evolve. Second, and more relevant for the nanostructure shape control, is the supply of group V elements at different temperatures and fluxes. This allows for the possibility to finely control, through flux and temperature, the transformation kinetics of the metal droplets into III–V nanocrystals, thus leading to the formation of nanostructures with complex and controlled shapes. In this chapter, droplet epitaxy founding concepts are introduced. A detailed review of the droplet epitaxy fabrication procedure of III–V nanostructures, as well of the main droplet epitaxy achievements, is presented.