arXiv subjects

Sergio Bietti

Publications and source records attributed to Sergio Bietti.

13 recordsLinked to original sources

arXiv2024

Active nitrogen flux measurement during GaN growth based on the transmitted signal detected with a pyrometer

A novel approach for the measurement of the Nitrogen active species generated by a plasma source in the molecular beam epitaxy environment is here presented. The method is based on the analysis of the variations in the optical signal measured by a pyrometer during a two step, Gallium rich and Nitrogen controlled, growth modes. The method permits a precise, quantitative and direct measurement of the flux of active species as a function of the plasma generation parameters of the cell: nitrogen gas flux and RF-power.

physics.ins-det↗

arXiv2024

Conduction band resonant states absorption for quantum dot infrared detectors operating at room temperature

Long Wavelenght infrared devices, despite growing interest due to a wide range of applications in commercial, public, and academic sectors, are still struggling to achieve significant improvements over well-established technologies like HgCdTe detectors. Devices based on quantum nanostructures remain non competitive due to unresolved drawbacks, the most significant being the need to cool down to liquid nitrogen temperatures to improve the signal-to-noise ratio. In this work, we demonstrate an innovative solution to surpass the current generation of quantum dot-based detectors by exploiting the absorption from quantum dot localized states to resonant states in the continuum, that is states in the semiconductor conduction band with an enhanced probability density in the quantum dot region. This unprecedented approach takes advantage of the unique properties of such states to massively enhance carrier extraction, allowing to overcome one of the most crucial drawbacks of quantum dot-based infrared detectors. This innovative solution is discussed here from both theoretical and experimental perspectives. The measured room temperature operation with high detectivity demonstrates that exploiting resonant states absorption in quantum dots offers the long-sought solution for the next generation of infrared photodetectors.

physics.optics↗

arXiv2022

Fine structure splitting analysis of cavity-enhanced telecom-wavelength InAs quantum dots grown on a GaAs(111)A vicinal substrate

The effcient generation of entangled photons at telecom wavelength is crucial 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. Incorporating this kind of QDs into optical cavities is critical to achieve sufficient signal intensitiesfor 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 characterise 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 miscut steps influences the polarisation of the excitonic states and introduces a preferential orientation in the $C_{3v}$ symmetry of the surface.

quant-ph↗

arXiv2021

Optically controlled dual-band quantum dot infrared photodetector

We present the design for a novel type of dual-band photodetector in the thermal infrared spectral range, the Optically Controlled Dual-band quantum dot Infrared Photodetector (OCDIP). This concept is based on a quantum dot ensemble with a unimodal size distribution, whose absorption spectrum can be controlled by optically-injected carriers. An external pumping laser varies the electron density in the QDs, permitting to control the available electronic transitions and thus the absorption spectrum. We grew a test sample which we studied by AFM and photoluminescence. Based on the experimental data, we simulated the infrared absorption spectrum of the sample, which showed two absorption bands at 5.85 um and 8.98 um depending on the excitation power.

physics.app-ph↗

arXiv2020

Reentrant behavior of the density vs temperature of indium islands on GaAs(111)A

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 sizeable 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.

cond-mat.mtrl-sci↗

arXiv2019

High-temperature droplet epitaxy of symmetric GaAs/AlGaAs quantum dots

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 modelled.

cond-mat.mes-hall↗

arXiv2019

Metal Droplet Effects on the Composition of Ternary Nitrides

We investigate effects of metal droplets on the In incorporation in InGaN epilayers grown at low temperature (450 C) by plasma assisted molecular beam epitaxy. We find a strong reduction of the In incorporation when the surface is covered by metal droplets. The such reduction increases with the droplet density and the droplet surface coverage. We explain this phenomenonology via a model that considers droplet effects on the incorporation of In and Ga adatoms into the crystal by taking into account the combined effects of the higher mobility of In, with respect to Ga, and to the vapor-liquid-solid growth that takes place under the droplet by direct impingement of nitrogen. The proposed model is general and can be extended to describe the incorporation of adatoms during the growth of the material class of ternary compounds when droplets are present on the surface.

physics.app-ph↗

arXiv2019

Spectral broadening in self-assembled GaAs quantum dots with narrow size distribution

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.

cond-mat.mes-hall↗

arXiv2018

High-yield fabrication of entangled photon emitters for hybrid quantum networking using high-temperature droplet epitaxy

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.

cond-mat.mes-hall↗

arXiv2017

Droplet Controlled Growth Dynamics in Plasma-Assisted Molecular Beam Epitaxy of In(Ga)N Materials

We investigate the effect of the formation of metal droplets on the growth dynamics of InGaN by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) at low temperatures (T = 450°C). We find that the presence of droplets on the growth surface strongly affects the adatom incorporation dynamics, making the growth rate a decreasing function of the overall metal flux impinging on the surface as soon as the metal dose exceeds the critical amount required for the nucleation of droplets. We explain this phenomenon via a model that takes into account 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.

cond-mat.mtrl-sci↗

arXiv2017

Ehrlich-Schwoebel Effect on the Growth Dynamics of GaAs(111)A surfaces

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-Schwoebel barrier as 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 atomically flat surface. GaAs/AlGaAs quantum wells realized on optimized surface (<0.2 nm roughness) show a record low emission linewidth of 4.5 meV.

cond-mat.mtrl-sci↗

arXiv2015

Germanium-based quantum emitters for time-reordering entanglement scheme with degenerate exciton and biexciton states

We address the photoluminescence emission of individual germanium extrinsic centers in Al_0.3Ga0.7As epilayers grown on germanium substrates. Through a thorough analysis of micro-photoluminescence experiments we demonstrate the capability of high temperature emission (70 K) and multiexcitonic features (neutral exciton X, biexciton XX, positive X+ and negative X- charged exciton) 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 ~+5 meV up to ~-7 meV covering about ~ 70 meV of the emission energy) with increasing quantum confinement. These light emitters exhibiting energy-degenerate X and XX energy levels at ~1.855 eV (680 nm) are a promising resource for the generation of entangled photons in the time-reordering scheme on a silicon platform.

cond-mat.mes-hall↗

arXiv2013

A 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.

cond-mat.mtrl-sci↗