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Silicene has recently been revisited as a 2D material with potential thermoelectric applications. Here, the thermal properties of supported silicene are determined by optothermal Raman spectroscopy. Single and multilayer silicene is grown either directly on silver (Ag) substrates or on an intermediate tin (Sn) monolayer, introduced to reduce the influence of the substrate on the physical properties of silicene. Experimental values of an effective cross-plane thermal conductivity of 0.5 W/mK are obtained for silicene on Ag and 0.3 W/mK for silicene on stanene-Ag. The values of the interfacial thermal conductance, on the other hand, are 0.3 and 0.5 GW/m2K, respectively. Heterostack engineering is confirmed as a versatile strategy for extracting relevant physical parameters in silicene and for modulating the thermal response in the 2D limit.

Dall'isolamento del grafene, la nuova classe dei materiali bidimensionali (2D) si è continuamente ampliata, aprendo la strada a molte opportunità nell'evoluzione delle nanotecnologie. In questo gruppo di materiali, gli Xeni sono una classe emergente di materiali simili al grafene, sintetizzati mediante epitassia da fasci molecolari (MBE), che mostrano proprietà eccezionali grazie alla loro natura 2D. Tra questi, il silicene gioca un ruolo cruciale, grazie alla potenziale facilità di integrazione nell'attuale tecnologia al silicio, mentre lo stanene presenta interessanti proprietà, soprattutto legate all'accoppiamento spin-orbita (SOC) e alla sua possibile integrazione come isolante topologico 2D. In questo quadro, lo scopo di questo lavoro di tesi è legato allo studio e all'ottimizzazione della reale manipolazione di Xeni, come il silicene e lo stanene, dal punto di vista del processo, al fine di discutere e risolvere le principali problematiche versola loro implementazione in strutture di dispositivi. In particolare, i campioni considerati sono di tre tipi: silicene cresciuto su Ag(111) su mica, eterostrutture di silicene-stanene cresciute su Ag(111) su mica e campioni di stanene cresciuti su substrato InSb. A differenza del grafene, l'ibridazione mista sp2-sp3 degli Xeni, e in particolare del silicene, comporta un'elevata reattività ambientale che porta alla degradazione del materiale. Lo sviluppo di schemi di incapsulamento affidabili è fondamentale per la scoperta di nuove possibili applicazioni; pertanto, un ruolo chiave in questo lavoro è legato alle eterostrutture di silicene-stanene, in cui il silicene è inserito tra uno strato Al2O3 di protezione sulla superficie superiore e lo stanene su Ag(111) sulla faccia inferiore. L'introduzione di uno strato di stanene ha permesso di sviluppare uno schema di incapsulamento completo per proteggere il silicene dalla degradazione in condizioni ambientali quando viene disassemblato dal substrato di crescita. Co, Since the isolation of graphene, the new class of two-dimensional (2D) materials has been continuously expanded, paving the way for many opportunities in the evolution of nanotechnology. In this materials group, Xenes are an emerging class of graphene-like materials, synthesized by molecular beam epitaxy (MBE), that show exceptional properties due to their 2D nature. Among these, silicene plays a crucial role, as a possible future integration in the current silicon technology, and stanene presents interesting properties, especially related to the spin-orbit coupling (SOC) and its possible integration as a 2D topological insulator. In this framework, the purpose of this work of thesis is related to the study and optimization of the actual manipulation of Xenes, such as silicene and stanene, from a processing point of view, in order to discuss and solve the main issues toward the implementation into device structures. In particular, the considered samples are of three types: silicene grown on Ag(111)-on-mica substrate, heterostructures of silicene-stanene grown on Ag(111)-on-mica and samples of few-layer stanene grown on InSb substrate. Unlike graphene, the mixed sp2-sp3 hybridization of Xenes, and in particular of silicene, results in a high environmental reactivity leading to material degradation. The development of reliable encapsulation schemes is fundamental for the discovery of new possible applications, hence, a key role in this work is related to the vertically stacked silicene-stanene heterostructures, where silicene is sandwiched in between an Al2O3 capping layer on the top surface and the stanene on Ag(111) crystal at the bottom face. The introduction of a stanene buffer layer enabled to develop an all-around encapsulation scheme to protect silicene against degradation under environmental conditions when disassembled from the native substrate. By combining different chemical and structural techniques, such as X-ray photoelectron spectroscopy (XPS) and Raman

Due to their superior mechanical properties, 2D materials have gained interest as active layers in flexible devices co-integrating electronic, photonic, and straintronic functions altogether. To this end, 2D bendable membranes compatible with the technological process standards and endowed with large-scale uniformity are highly desired. Here, it is reported on the realization of bendable membranes based on silicene layers (the 2D form of silicon) by means of a process in which the layers are fully detached from the native substrate and transferred onto arbitrary flexible substrates. The application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. It is also shown that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene wrinkles. Finally, as compelling evidence of the technological potential of the silicene membranes, it is demonstrated that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures, a piezoresistor, and thus paving the way to a viable advance in a fully silicon-compatible technology framework.

La tesi si pone l'obiettivo di studiare le proprietà ottiche degli Xeni. Gli Xeni, reticoli monoelementali artificiali simili al grafene, hanno mostrato sin dalla loro scoperta grandi potenzialità per applicazioni negli ambiti dell'elettronica e della fotonica. Purtroppo, il loro utilizzo è ancora limitato dall'elevata reattività chimica e dai forti effetti di ibridazione con i substrati nativi. Conoscere la risposta ottica o termo-ottica degli Xeni, escludendo o controllando eventuali effetti del substrato, è quindi funzionale allo sviluppo di nuove tecnologie. La ricerca è stata condotta nell'ambito del progetto X-Fab (ERC COG Grant n. 772261), presso i laboratori CNR-IMM di Agrate Brianza. In questo studio vengono esaminate diverse configurazioni a base di Xene, combinando tecniche di analisi in situ (XPS e LEED) e di spettroscopia ottica (FTIR, Vis-UV e Raman), con lo scopo di determinarne la risposta ottica e valutare l'integrazione degli Xeni in un dispositivo che ne sfrutti le potenzialità. La prima configurazione analizzata prevede l'introduzione di un substrato isolante e otticamente trasparente, come lo zaffiro, a sostituzione del più diffuso argento. Lo zaffiro, nella sua orientazione (0001) presenta un reticolo a simmetria esagonale commensurato a quello di silicene e stanene. La conducibilità ottica di nano-film di stagno cresciuti tramite MBE è stata ricavata da misure di trasmittanza e riflettanza, acquisite tra 0.01 e 6.5 eV. L'analisi spettrale e il successivo confronto con dati teorici già presenti in letteratura hanno permesso di identificare delle caratteristiche non banali, in parte riconducibili allo stanene. L'analisi chimica eseguita in situ durante la crescita ha però evidenziato anche la presenza di una componente dello stagno ossidata dovuta ad effetti del substrato. Il problema emerso è stato affrontato introducendo uno strato di grafene tra i film di stagno e l'Al2O3. L'introduzione dello strato di grafene ha permesso di evi, The aim of this thesis is to investigate the optical properties of Xenes. Since their discovery, Xenes, artificial graphene-like single-element lattices, have shown great potential for applications in electronics and photonics. Unfortunately, their use is still limited by their high chemical reactivity and strong hybridisation effects with native substrates. Understanding the optical or thermo-optical response of Xenes, and thus excluding or controlling possible substrate effects, is therefore crucial for the development of new technologies. The research was carried out as part of the X-Fab (ERC COG Grant n. 772261) project at the CNR-IMM laboratories in Agrate Brianza. In this study, several Xenes-based configurations are investigated using a combination of in situ analysis (XPS and LEED) and optical spectroscopy (FTIR, Vis-UV and Raman) techniques to determine their optical response and evaluate the integration of Xenes into a device that exploits their potential. The first configuration analyzed involved the introduction of an insulating and optically transparent substrate, such as sapphire (Al2O3), to replace the more common silver. Sapphire, in its (0001) orientation, has a hexagonal symmetry lattice commensurate to that of silicene and stanene. The optical conductivity of tin nano-sheets grown by MBE was derived from transmittance and reflectance measurements. The measurements were carried out covering a range from 0.01 to 6.5 eV. Spectral analysis and subsequent comparison with theoretical data from the literature made it possible to identify non-trivial features, some of which can be attributed to stanene. However, in situ chemical analysis during growth also revealed the presence of an oxidised tin component due to substrate effects. This issue was solved by introducing a graphene layer between the tin films and Al2O3. The introduction of the graphene layer prevented oxidation of the tin, as confirmed by XPS analysis. The results of the optical analysis, on

With black phosphorus being a promising two-dimensional layered semiconductor for application to electronics and optoelectronics, an issue remains as to how heat diffusion is managed when black phosphorus is interfaced with metals, namely in a typical device heterojunction. We use Raman spectroscopy to investigate how the laser-induced heat affects the phonon modes at the interface by comparing the experimental data with a finite element simulation based on a localized heat diffusion. The best convergence is found taking into account an effective interface thermal conductance, thus indicating that heat dissipation at the Au-supported black phosphorus nanosheets is limited by interface effect.

Stabilization of silicene and preservation of its structural and electronic properties are essential for its processing and future integration into devices. The stacking of silicene on stanene, creating a Xene-based heterostructure, proves to be a viable new route in this respect. Here we demonstrate the effectiveness of a stanene layer in breaking the strong interaction between silicene and the Ag(111) substrate. The role of stanene as a ‘buffer’ layer is investigated by analyzing the optical response of epitaxial silicene through both power-dependent Raman spectroscopy and reflectivity measurements in the near infrared (NIR)-ultraviolet (UV) spectral range. Finally, we point out a Xene-induced shift of the silver plasma edge that paves the way for the development of a new approach to engineering the metal plasmonic response.

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.

Nel campo dei semiconduttori, lo studio delle proprietà spin dipendenti forniscono informazioni fondamentali per la realizzazione di dispositivi che uniscano spin, fotonica ed elettronica. In questi dispositivi l’informazione è codificata nel grado di libertà (DOF) dello spin, sfruttando l’accoppiamento spin-orbita (SOC) tra il momento angolare del fotone e lo spin del portatore. Ho concentrato la mia ricerca sullo studio del SOC con spettroscopia ottica in Si, Ge, Sn e loro leghe. Questi materiali possiedono proprietà promettenti per applicazioni di spintronica, tra cui lunghi tempi di vita e lunghezze di diffusione dello spin. I processi di fabbricazione aprono la strada all’ingegnerizzazione del bandgap e dello strain come DOF addizionali per sintonizzare i fenomeni spin-dipendenti. La spettroscopia ottica permette di superare i problemi delle misure elettriche, come la qualità dei contatti, che impediscono una stima corretta dei parametri cinetici. I pozzi quantici (QW), sono valide piattaforme per unire i DOF sopracitati e permettere la manipolazione dello spin con campi elettrici. Nei sistemi a QW che mancano della simmetria di inversione di punto o di quella strutturale (BIA/SIA), la degenerazione di spin è rimossa dai campi Dresselhaus o Rashba. In quanto campi magnetici, possono agire agiscono sullo spin cambiandone l’orientazione. La SIA può sorgere da un drogaggio asimmetrico. In questo caso, il dispositivo possiede anche un campo elettrico che può essere sfruttato. Infatti, con un campo elettrico esterno si può modulare il campo Rashba, manipolando lo spin. Questa possibilità è significativa per la spintronica, si pensi allo spin-FET, dove la tensione di gate seleziona l’orientazione dello spin e quindi lo stato on/off. Ho eseguito misure di fotoluminescenza (PL) su campioni costituiti da uno stack di 50 QW di Ge/Si0.15Ge0.85 cresciuto nella zona intrinseca di un diodo p-i-n. Il drogaggio asimmetrico introduce la SIA, permettendo di manipolare, In the field of semiconductors, the study of spin-dependent properties provides fundamental information needed for the realization of devices that merge spin, photonic and electronic functionalities. In these devices the information is encoded in the spin degree of freedom (DOF), exploiting the interaction between the angular momentum of the photon and the carrier spin via the spin-orbit coupling (SOC). I focused on the study of SOC in Si, Ge, Sn and their alloys using optical spectroscopy. These materials possess promising properties for spintronics applications such as long spin lifetime, diffusion length and decoherence time. Notably, the advanced manufacture also opens the way to bandgap and strain engineering as further DOF to tune spin-dependent phenomena, whereas the application of optical spectroscopy allows to overcome typical problems of electrical measurements, e.g., the quality of contacts, that hamper the estimation of carrier kinetics parameters Quantum well (QW) systems are valid platforms to merge all the aforementioned DOF and to also introduce a way to manipulate the spin via electric fields. Indeed, in QW systems that possess bulk or structure inversion asymmetry (BIA/SIA), the spin degeneracy is removed due to the Dresselhaus or Rashba fields. As effective magnetic fields, they can act on the spin of a carrier, ultimately changing its orientation. SIA can arise from an asymmetric doping of the device. In this case, the device also possesses an intrinsic electric field, which can be of practical use for applications. Indeed, an external field can be applied to tune the Rashba field, achieving spin manipulation. This opportunity has a strong impact in spintronics devices, such as the spin-FET, where the gate voltage selects the orientation of the spin and switch between on/off states I carried out photoluminescence (PL) investigations on a stack of 50 Ge/Si0.15Ge0.85 QWs grown within the intrinsic region of a p-i-n diode. The asymmetric doping intr

Stabilization of silicene and preservation of its structural and electronic properties are essential for its processing and future integration into devices. The stacking of silicene on stanene, creating a Xene-based heterostructure, proves to be a viable new route in this respect. Here we demonstrate the effectiveness of a stanene layer in breaking the strong interaction between silicene and the Ag(111) substrate. The role of stanene as a ‘buffer’ layer is investigated by analyzing the optical response of epitaxial silicene through both power-dependent Raman spectroscopy and reflectivity measurements in the near infrared (NIR)-ultraviolet (UV) spectral range. Finally, we point out a Xene-induced shift of the silver plasma edge that paves the way for the development of a new approach to engineering the metal plasmonic response.

With black phosphorus being a promising two-dimensional layered semiconductor for application to electronics and optoelectronics, an issue remains as to how heat diffusion is managed when black phosphorus is interfaced with metals, namely in a typical device heterojunction. We use Raman spectroscopy to investigate how the laser-induced heat affects the phonon modes at the interface by comparing the experimental data with a finite element simulation based on a localized heat diffusion. The best convergence is found taking into account an effective interface thermal conductance, thus indicating that heat dissipation at the Au-supported black phosphorus nanosheets is limited by interface effect.

In the realm of two-dimensional material frameworks, single-element graphene-like lattices, known as Xenes, pose several issues concerning their environmental stability, with implications for their use in technology transfer to a device layout. In this Discussion, we scrutinize the chemical reactivity of epitaxial silicene, taken as a case in point, in oxygen-rich environments. The oxidation of silicene is detailed by means of a photoemission spectroscopy study upon carefully dosing molecular oxygen under vacuum and subsequent exposure to ambient conditions, showing different chemical reactivity. We therefore propose a sequential Al2O3 encapsulation of silicene as a solution to face degradation, proving its effectiveness by virtue of the interaction between silicene and a silver substrate. Based on this method, we generalize our encapsulation scheme to a large number of metal-supported Xenes by taking into account the case of epitaxial phosphorene-on-gold.

Here, two novel approaches for disassembling epitaxial silicene from the native substrate and transferring onto arbitrary target substrates are presented. From the processing perspective, the two methodologies open up a new route for handling silicene, and in general any epitaxial Xene, in view of establishing reliable process flows for the development of a Xene-based nanotechnology. Integration of silicene in a back-gated controlled device architecture is demonstrated and the built up of unique multi-stack heterostructures between silicene, but potentially every Xene, and other technological relevant materials, like transparent conductive oxides and transition metal dichalcogenides is shown.

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.

Here, two novel approaches for disassembling epitaxial silicene from the native substrate and transferring onto arbitrary target substrates are presented. From the processing perspective, the two methodologies open up a new route for handling silicene, and in general any epitaxial Xene, in view of establishing reliable process flows for the development of a Xene-based nanotechnology. Integration of silicene in a back-gated controlled device architecture is demonstrated and the built up of unique multi-stack heterostructures between silicene, but potentially every Xene, and other technological relevant materials, like transparent conductive oxides and transition metal dichalcogenides is shown.

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.

The large foreseeable use of two-dimensional materials in nanotechnology consequently demands precise methods for their thickness measurements. Usually, having a quick and easy methodology is a key requisite for the inspection of the large number of flakes produced by exfoliation methods. An effective option in this respect relies on the measurement of the intensity of Raman spectra, which can be used even when the flakes are encapsulated by a transparent protective layer. However, when using this methodology, special attention should be paid to the crystalline anisotropy of the examined material. Specifically, for the case of black phosphorus flakes, the absolute experimental determination of the thickness is rather difficult because the material is characterized by a low symmetry and also because the Raman tensors are complex quantities. In this work, we exploited Raman spectroscopy to measure the thickness of black phosphorous flakes using silicon as reference material for intensity calibrations. We found out that we can determine the thickness of a flake above 5 nm with an accuracy of about 20%. We tested the reproducibility of the method on two different setups, finding similar results. The method can be applied also to other van der Waals materials with a Raman band characterized by the same Raman tensor.

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.

I punti quantici (quantum dots, QDs) epitassiali possono generare fotoni in uno stato di polarizzazione entangled tramite la cascata radiativa bieccitone-eccitone. Le potenzialità di funzionamento deterministico e di scalabilità dei dispositivi sono vantaggi unici per le applicazioni in reti quantistiche. Tuttavia, una alta simmetria strutturale e una scelta ponderata dei materiali sono cruciali per affrontare le principali cause di degradazione dell'entanglement, ossia la separazione in energia (fine structure splitting, FSS) tra i due stati eccitonici ed i campi magnetici nucleari oscillanti legati all'interazione iperfine. Questa tesi si concentra su QDs di GaAs/AlGaAs cresciuti su un substrato di GaAs (111)A con un nuovo approccio alla epitassia da goccia, in cui la fase fondamentale di cristallizzazione è eseguita ad una temperatura significativamente più alta rispetto ai precedenti tentativi. La scelta specifica di orientazione del substrato, denotata da un basso coefficiente di adsorbimento per l'As, favorisce l'incorporazione di As nella goccia piuttosto che con adatomi di Ga sulla superficie. Diversamente dall'epitassia da goccia convenzionale, limitata a temperature del substrato sotto i 250°C, la formazione di nanostrutture è osservata fino a 520°C. Ciò porta ad una maggiore qualità cristallina dei QDs e della barriera circostante e ad un ridotto impatto dell'interdiffusione, evidenziato dal confronto tra macro-fotoluminescenza e simulazioni dei livelli energetici basate sulle geometrie misurate tramite microscopia a forza atomica su campioni senza copertura. Il controllo sulla dinamica di crescita porta alla fabbricazione di QDs con diversi rapporti di forma e, quindi, alla progettazione riproducibile della lunghezza d'onda d'emissione. Così è dimostrato il funzionamento attorno a 780 nm, che permette l'integrazione con memorie ottiche al Rb, un importante obiettivo per la realizzazione di ripetitori quantistici. La forma di piramide troncata co, Epitaxial semiconductor quantum dots (QDs) can generate polarization-entangled photons through the biexciton-exciton radiative cascade. The potential for on demand operation and device scalability are unique assets for future applications in quantum networking. However, a high QD structural symmetry and a proper choice of materials are crucial to tackle the main sources of entanglement degradation, namely the presence of a fine structure energy splitting (FSS) between the two bright exciton states and fluctuating nuclear magnetic fields due to the hyperfine interaction. This thesis focuses on GaAs/AlGaAs QDs grown on a (111)A GaAs substrate by a novel approach based on droplet epitaxy, where the fundamental crystallization step is performed at a temperature which is significantly higher than in previous reports. The specific choice of substrate orientation, characterized by a very low As sticking coefficient, favors As incorporation inside the droplet rather than with Ga adatoms on the surface. In contrast to standard droplet epitaxy, which is restricted to substrate temperatures below 250°C, quantum dot formation is observed up to 520°C. The increased growth temperature improves the crystalline quality of the QDs and their surrounding barrier and strongly reduces the impact of interdiffusion. This is confirmed by comparing ensemble photoluminescence with energy level simulations based on the geometrical features probed by atomic force microscopy on uncapped samples. The control over the growth dynamics leads to the fabrication of QDs with different aspect ratios and, therefore, to the reproducible design of the emission wavelength. Thus, operation in the 780 nm range is demonstrated, which allows the frequency-matching of these QDs with Rb-based optical quantum memories, an important target for the realization of quantum repeaters. At the same time, a truncated pyramid shape with regular hexagonal base is achieved, also fulfilling the requirements on high in-plane

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.

L'integrazione monolitica di funzionalità fotoniche nella micro-tecnologia basata su silicio è un obiettivo ampiamente perseguito dalla comunità scientifica, con applicazioni nella comunicazione di dati e nella rilevazione nel vicino e medio infrarosso. L’integrazione di componenti attivi e passivi con processi di fabbricazione standard è già avanzata. Tuttavia, ancora non esiste una soluzione per una sorgente di radiazione efficace attivata elettricamente. Un grande sforzo di ricerca si concentra sulle eterostrutture di germanio (Ge) e silicio (Si). Tuttavia, il Ge è un semiconduttore a gap indiretto con efficienza quantica scarsa rispetto ai materiali a gap diretta. Un’alta deformazione e un pesante drogaggio sono stati proposti come metodi per migliorare la densità di portatori nella valle della banda di conduzione diretta del Ge e sono generalmente accettati come una strategia valida per raggiungere un guadagno ottico positivo negli strati di Ge. In questa tesi proponiamo un approccio top-down per la realizzazione di stressori nanostrutturati di SiGe, basati sulla differenza di passo reticolare tra SiGe e Ge. Per mezzo di spettroscopia micro-Raman dimostriamo che il Ge localmente viene deformato uniassialmente fino al 4%. Per limitare la diffusione degli elettroni eccitati dalle regioni altamente deformate, viene studiato sistematicamente l'inserimento di una barriera di SiGe sotto uno strato superficiale di Ge. I risultati dimostrano che durante la termalizzazione gli elettroni superano la barriera di diffusione. Per aumentare il volume deformato e utilizzare il vuoto come barriera, gli stressori di SiGe sono stati trasferiti su un sottile ponte di Ge. Confrontando lo strain ottenuto per il ponte e il bulk, i valori assoluti di strain sono nell'ordine di 0,7%. Tuttavia, la deformazione relativa indotta da nanostrutture nel micro-ponte è 1,3% a causa della alta condivisione di energia elastica tra nanostrutture e ponte. Quindi, la realizzazione di nanost, The monolithic integration of photonic functionality into silicon micro-technology is a widely-sought goal with projected applications in data-communication and sensing for the near- and mid-infrared spectral range. The realization of integrated passive and active components with means of standard silicon manufacturing processes is already widely advanced. Yet there is no final solution for an effective, electrically pumped light source. A lot of research effort focuses on germanium (Ge) on silicon (Si) hetero-structures. However, Ge is an indirect gap semiconductor with scarce quantum efficiency as compared to intrinsic direct gap materials. High tensile strain and heavy n-type doping have been proposed to enhance the carrier density in the Ge direct conduction band valley and are accepted as a route to achieve positive optical gain in Ge layers. We propose a top-down fabrication for external stressors made of SiGe nanostructures that is based on the lattice mismatch of SiGe and Ge. By means of micro-Raman spectroscopy we demonstrate that Ge is locally deformed and a uniaxial tensile strain of up to 4 % is reached. However, the photo-luminescence from the strained volume is scarce and the emission spectra are bulk like. To constrain the excited electrons to the highly strained regions, the insertion of a SiGe barrier below a shallow layer of Ge is investigated systematically showing that during thermalization carriers overcome the barrier by diffusion. To enlarge the strained volume and to have vacuum as the barrier, we transfer the SiGe stressors to a thin Ge micro-bridge and compare the obtained strain to the case of an attached bulk-like Ge layer. Absolute strain values are of the order of ~ 0.7 % for both micro-bridge and bulk. However, the relative strain induced by the nanostructures in the micro-bridge is 1.3 % due to the high sharing of elastic energy between nanostructures and bridge. Hence, the suspension doubles the strain value with respect to a bulk li

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.

Epitaxial growth of Ge films on Si introduces an in-plane biaxial tensile strain, opening up Ge applications in photonic, as recently demonstrated by the room temperature lasing action in Ge-on-Si heterostructure. The optical access to the direct gap transitions remarkably provides the possibility of injecting spin-polarized carriers via absorption of circular-polarized light. Nevertheless the effects of tensile-strain on the spin properties of Ge-on-Si have still to be addressed. In this work we applied polarization-resolved photoluminescence (PL) to spectrally resolve the radiative recombination involving strain-split light and heavy hole bands. We found that at low temperature, the fundamental transition has a record-high circular polarization degree of 85%, despite an off-resonance excitation of 300 meV. We show that this very high value is due to the characteristic energy dependence of the optically induced electron spin population and their concomitant dynamics. Finally, our observation of a direct gap PL doublet clarifies that the light hole contribution to the optical spectrum can dominate the room temperature PL. Our results provide a step forward the investigation of the dynamics of non-equilibrium spin populations and of the evaluation of optical gain in group IV materials. Above all, our findings confirm Ge as a prominent candidate for the development of next-generation CMOS-compatible devices featuring spintronics and photonics functionalities.

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

Our work, focused on the heteroepitaxial Ge/Si (001) system shows that patterned Si substrates in appropriate pillar arrays, featuring micrometric dimensions within the common deep-etching capabilities and array size suitable for several applications (up to few hundreds microns), do provide exceptional compliance to the thermal strain of a continuous Ge film deposited on top. Micro-Raman and XRD measurements on the Ge suspended layer, as obtained by Low Energy Plasma Enhanced Chemical Vapor Deposition, confirm the theoretical predictions. Actually, the sharp onset with pillar aspect ratio of such an effect is demonstrated to be triggered by the free rotation of the pillars, as induced by the thermal contraction of the film. The strain relaxation is deeply analyzed both by FEM simulations and experimental XRD results, showing a very good quantitative agreement. Our findings have far reaching consequences beyond the crystalline heteroepitaxial systems, whenever a film of any microstructure, or composition, is deposited in suspended patches on suitable pillar arrays.

We present a new concept applicable to the epitaxial growth of dislocation-free semiconductor structures on a mismatched substrate with a thickness far exceeding the conventional critical thickness for plastic strain relaxation. This innovative concept is based on the out-of-equilibrium growth of compositionally graded alloys on deeply patterned substrates. We obtain space-filling arrays of individual crystals several micrometers wide in which the mechanism of strain relaxation is fundamentally changed from plastic to elastic. The complete absence of dislocations at and near the heterointerface may pave the way to realize CMOS integrated SiGe X-ray detectors.

The monolithic integration of photonic functionality into silicon microtechnology is widely advanced. Yet, there is no final solution for the realization of a light source compatible with the prevailing complementary metal-oxide-semiconductor technology. A lot of research effort focuses on germanium (Ge) on silicon (Si) heterostructures and tensile strain application to Ge is accepted as one feasible route to make Ge an efficient light emitter. Prior work has documented the special suitability of Ge membranes to reach the high tensile strain. We present a top-down approach for the creation of SiGe stressors on Ge micro-bridges and compare the obtained strain to the case of an attached bulk-like Ge layer. We could show that the Ge influenced by a SiGe stressor is under tensile strain; absolute strain values are of the order of 0.7% for both micro-bridge and bulk. The relative strain induced by the nanostructures in the micro-bridge is 1.3% due to the high sharing of elastic energy between nanostructures and bridges.

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 (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

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 (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

We show that a relatively simple top-down fabrication can be used to locally deform germanium in order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically predicted to transform germanium from an indirect to a direct gap semiconductor. These values of strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers.

The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.

The polarization of the photoluminescence (PL) from the indirect gap of bulk Ge is studied after optical orientation by absorption of circularly polarized light through the direct gap. The spin relaxation time ts is then estimated by combining the polarization degree and the decay time of the indirect gap emission. Ge is a promising material for spin-optoelectronics due to its feasible integration on Si, its noticeable spin properties and the possibility of achieving room temperature lasing action. In order to merge the spin degree of freedom and the photonics functionalities, the knowledge of properties such as the electron spin lifetime is of practical and fundamental importance. In this work we address this issue reporting the first evidence that low temperature PL from the indirect gap of bulk Ge is circularly polarized. This is the smoking gun proof that spin-polarized electrons preserve their orientation during the long dwell time in the L valley. We experimentally verified the selection rules for phonon mediated transitions showing an excellent agreement with the theory. Finally, to estimate ts we applied time-resolved PL, yielding in addition accurate data for the carrier lifetime. Hundreds of ns long ts is found providing the longest estimation reported at T<30K. Our results demonstrate the prominent and viable role of Ge for merging photonics and spintronics on the same materials platform.

Germanium is known to become a direct band gap material when subject to a biaxial tensile strain of 2% or to a uniaxial tensile strain of 4%. In this work we demonstrate that the latter condition can be obtained by exploiting suitable SiGe nanostructures lithographycally defined on top of a Ge film. Such unprecedented strain value is caused by the perimetral elastic forces exerted on the film by the nanostructures, in their attempt to recover their bulk lattice parameter. These findings might lead to the realization of strained-Ge integrated emitters with unprecedented properties, while being compatible with current silicon technology.

Germanium is known to become a direct band gap material when subject to a biaxial tensile strain of 2% (Vogl et al 1993 Phys. Scr. T49B 476) or uniaxial tensile strain of 4% (Aldaghri et al 2012 J. Appl. Phys. 111 053106). This makes it appealing for the integration of optoelectronics into current CMOS technology. It is known that the induced strain is highly dependent on the geometry and composition of the whole system (stressors and substrate), leaving a large number of variables to the experimenters willing to realize this transition and just a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a finite element approach to systematically study the elastic strain induced by different configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on their composition and geometries. We numerically investigate the role played by the Ge substrate by comparing the strain induced on a bulk or on a suspended membrane. These results and their interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported. © 2014 IOP Publishing Ltd.

In this paper we experimentally study the growth of self-assembled SiGe islands formed on Si(001) by exploiting the thermally activated surface diffusion of Ge atoms from a local Ge source stripe in the temperature range 600-700 °C. This new growth strategy allows us to vary continuously the Ge coverage from 8 to 0 monolayers as the distance from the source increases, and thus enables the investigation of the island growth over a wide range of dynamical regimes at the same time, providing a unique birds eye view of the factors governing the growth process and the dominant mechanism for the mass collection by a critical nucleus. Our results give experimental evidence that the nucleation process evolves within a diffusion limited regime. At a given annealing temperature, we find that the nucleation density depends only on the kinetics of the Ge surface diffusion resulting in a universal scaling distribution depending only on the Ge coverage. An analytical model is able to reproduce quantitatively the trend of the island density. Following the nucleation, the growth process appears to be driven mainly by short-range interactions between an island and the atoms diffusing within its vicinities. The islands volume distribution is, in fact, well described in the whole range of parameters by the Mulheran's capture zone model. The complex growth mechanism leads to a strong intermixing of Si and Ge within the island volume. Our growth strategy allows us to directly investigate the correlation between the Si incorporation and the Ge coverage in the same experimental conditions: higher intermixing is found for lower Ge coverage. This confirms that, besides the Ge gathering from the surface, also the Si incorporation from the substrate is driven by the diffusion kinetics, thus imposing a strict constraint on the initial Ge coverage, its diffusion properties and the final island volume © 2014 IOP Publishing Ltd.

Dense arrays of micrometric crystals, with areal filling up to 93%, are obtained by depositing GaAs in a mask-less molecular beam epitaxy process onto Si substrates. The substrates are patterned into tall, micron sized pillars. Faceted high aspect ratio GaAs crystals are achieved by tuning the Ga adatom for short surface diffusion lengths. The crystals exhibit bulk-like optical quality due to defect termination at the sidewalls. Simultaneously, the thermal strain induced by different thermal expansion parameters of GaAs and Si is fully relieved. This opens the route to thick film applications without crack formation and wafer bowing. (C) 2013 AIP Publishing LLC.

We studied the plastic deformation of an ultrathin silicon-on-insulator with epitaxial Si1−xGex by transmission electron microscopy, Raman spectroscopy, and finite-element method. We analyzed a top Si layer of 10 nm (testing also a 2 nm layer) with epitaxial Si0.64Ge0.36 stressors of 50 and 100 nm. SiGe plastically deforms the top Si layer, and this strain remains even when Si1−xGex is removed. For low dislocation densities, dislocations are gettered close to the Si/SiO2 interface, while the SiGe/Si interface is coherent. Beyond a threshold dislocation density, interactions between dislocations force additional dislocations to position at the Si1−xGex/Si interface.

Edge forces exerted by epitaxial nanostructures are shown to induce high levels of strain in the substrate. These very high localized forces appear at the perimeter and the resulting strain can be exploited to engineer the functional properties of the substrate. High levels of strain in a Si substrate are induced by SiGe nanostructures, starting from both top-down and bottom-up approaches. Compressive uniaxial strains of up to −0.7% are demonstrated.

In the last two decades, attention has raised towards SiGe nanostructures for applications in microelectronics and optoelectronics. Many efforts have been payed in order to control the growth of the nanostructures and characterize their structural properties: for this purpose, a wide series of different experimental techniques have been developed. Raman spectroscopy of SiGe is commonly used to obtain information about the average values of composition and strain inside the nanostructures. Actually, due to the lack of data about the Raman efficiency of SiGe as a function of the alloy composition and the excitation light frequency, it is not possible to extract information about the composition profiles which are known to be present inside the structures and deeply affect their optoelectronic properties. In this work, the Raman efficiency of SiGe is measured across the whole compositional range for six different excitation light energies in the UV and VIS range. Strong resonances in the Raman efficiency of SiGe are observed, related to the direct electronic transitions in the material: their origin is explained by means of the theory and confirmed by semiempirical calculations. Then, a numerical tool is presented, which is able to analyze the Raman spectrum of an inhomogeneous SiGe structure and extract information about its internal composition profile by taking into account the effects of the resonance on the SiGe Raman efficiency. The methodology is validated by means of a calibrated SiGe sample, and applied to the study of SiGe islands. The internal composition profile of the islands obtained with this methodology is in good agreement with the results of independent techniques. Finally, the powers and limitations of the methodology are investigated.

In the quest for a Ge x-ray detector mono-lithically integrated onto a Si-CMOS chip we developed a novel method for combining dissimilar materials that may provide a solution to the main problems of heteroepitaxy, e.g. high threading dislocation densities, wafer bowing and cracks. It consists of replacing the conventional continuous layers by space-filling arrays of strain- and defect-free Ge crystals, the width, height and shape of which are controlled by tuning epitaxial growth onto micrometer-sized features deeply etched into Si-substrates. Heterojunctions formed between the Ge-crystals and the Si-substrate exhibit the required rectifying diode behavior with low dark currents (<1 mA/cm2).

This paper collects the results of some experiments aimed at investigating the physical mechanisms of defect generation in devices. It is shown that suitable limits for the mechanical stress can be defined to prevent defect generation. In addition, a high temperature stress release annealing can be beneficial for stress reduction and defect prevention. The annealing of implanted layers may result in crystal defect formation even with no contribution from mechanical stress. In this case, the silicon surface plays a relevant role in reducing the point defect concentration and hence the nucleation of extended defects. In the annealing process of high energy implantations, the most damaged region is far from the wafer surface. Under these conditions, impurities in the silicon substrate play a relevant role in determining the resulting defect morphology. The presence of interstitial oxygen forces defects to grow along <110> directions parallel to the silicon surface, thus preventing them to reach the device region.