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Elena DEGOLI

Professore Associato
Dipartimento di Scienze e Metodi dell'Ingegneria

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Pubblicazioni

2023 - A first-principles study of self-healing binders for next-generation Si-based lithium-ion batteries [Articolo su rivista]
Maji, R.; Salvador, M. A.; Ruini, A.; Magri, R.; Degoli, E.
abstract

Silicon anodes typically suffer from poor intrinsic conductivity and dramatic volume change during charge/discharge cycles, which hinders their commercialization in high energy density lithium-ion batteries (LiBs). This issue can be alleviated by embedding particles of the active material in an adhesive matrix, such as a polymer binder, that can accommodate large volume changes during lithiation and delithiation. Several research efforts have aimed at enhancing the adhesive, elastic, electrical, and ionic properties of binders for use in silicon anodes. Therefore, stable silicon/polymer interfaces are crucial for the performance of high capacity silicon-based LiBs. In this research, we focused on the definition of the mechanisms that determine the adhesion properties of a couple of recently proposed self-healing polymers, on Si-surfaces. The structural and electronic properties as well as the energetics of boronic acid-doped polyaniline and polyvinyl alcohol monomers absorbed on Si (110) and Si (111) surfaces have been investigated through first-principles calculations based on the density functional theory. We showed that the coabsorption of these two monomers increases the absorption energy and in general improves the adhesion properties of both polymers on both Si-surfaces, especially on the Si (111) facet.

2023 - Ab initio nonlinear optics in solids: linear electro-optic effect and electric-field induced second-harmonic generation [Articolo su rivista]
Prussel, L.; Maji, R.; Degoli, E.; Luppi, E.; Veniard, V.
abstract

Second-harmonic generation (SHG), linear electro-optic effect (LEO) and electric-field induced second-harmonic generation (EFISH) are nonlinear optical processes with important applications in optoelectronics and photovoltaics. SHG and LEO are second-order nonlinear optical processes described by second-order susceptibility. Instead, EFISH is a third-order nonlinear optical process described by third-order susceptibility. LEO and EFISH are only observed in the presence of a static electric field. These nonlinear processes are very sensitive to the symmetry of the systems. In particular, LEO is usually observed through a change in the dielectric properties of the material while EFISH can be used to generate a "second harmonic" response in centrosymmetric material. In this work, we present a first-principle formalism to calculate second- and third-order susceptibility for LEO and EFISH. LEO is studied for GaAs semiconductor and compared with the dielectric properties of this material. We also present how it is possible for LEO to include the ionic contribution to the second-order macroscopic susceptibility. Concerning EFISH we present for the first time the theory we developed in the framework of TDDFT to calculate this nonlinear optical process. Our approach permits to obtain an expression for EFISH which does not contain the mathematical divergences in the frequency-dependent second-order susceptibility that caused until now many difficulties for numerical calculations.

2023 - Measuring thermal conductivity of nanostructures with the 3ω method: the need for finite element modeling [Articolo su rivista]
Peri, L.; Prete, D.; Demontis, V.; Degoli, E.; Ruini, A.; Magri, R.; Rossella, F.
abstract

Conventional techniques of measuring thermal transport properties may be unreliable or unwieldy when applied to nanostructures. However, a simple, all-electrical technique is available for all samples featuring high-aspect-ratio: the 3? method. Nonetheless, its usual formulation relies on simple analytical results which may break down in real experimental conditions. In this work we clarify these limits and quantify them via adimensional numbers and present a more accurate, numerical solution to the 3? problem based on the Finite Element Method (FEM). Finally, we present a comparison of the two methods on experimental datasets from InAsSb nanostructures with different thermal transport properties, to stress the crucial need of a FEM counterpart to 3? measurements in nanostructures with low thermal conductivity.

2023 - Structural and Dynamic Characterization of Li-Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach [Articolo su rivista]
Salvador, Ma; Maji, R; Rossella, F; Degoli, E; Ruini, A; Magri, R
abstract

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li-PF6, Li-BF4, and Li-TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation-anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation-anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions.

2022 - Comparison of long-range corrected kernels and range-separated hybrids for excitons in solids [Articolo su rivista]
Maji, Rita; Degoli, Elena; Calatayud, Monica; V??niard, Val??rie; Luppi, Eleonora
abstract

At the moment, the most accurate theoretical method to describe excitons is the solution of the Bethe-Salpeter equation in the GW approximation (GW-BSE). However, because of its computation cost, time-dependent density functional theory (TDDFT) is becoming the alternative approach to GW-BSE to describe excitons in solids. Nowadays, the most efficient strategy to describe optical spectra of solids in TDDFT is to use long-range corrected exchange-correlation kernels on top of GW or scissor-corrected energies. In recent years, a different strategy based on range-separated hybrid functionals started to be developed in the framework of time-dependent generalized Kohn-Sham density functional theory. Here we compare the performance of long-range corrected kernels with range-separated hybrid functionals for the description of excitons in solids. This comparison has the purpose to weight the pros and cons of using range-separated hybrid functionals, giving new perspectives for theoretical developments of these functionals. We illustrate the comparison for the case of Si and LiF, representative of solid-state excitons.

2022 - Insight into the inclusion of heteroatom impurities in Silicon structures [Articolo su rivista]
Maji, Rita; Luppi, Eleonora; Degoli, Elena; Contreras-García, Julia
abstract

The bonding properties of tilt boundary in poly-silicon and the effect of interstitial impurities are investigated by first-principles. In order to obtain thorough information on the nature of chemical bondings in these solid systems, an accurate topological analysis is performed, through partitioning of the electron localization function. Although the mechanism of segregation of single light impurities, such as carbon, nitrogen, and oxygen in Si-based systems is known, it is only in the presence of multiple segregations that the distinctive structures of the various interstitial impurities emerge. The structural analysis of the modified Si systems and the comparison with the corresponding molecular structure within these solid phases provide an adequate description of interesting properties, for which bond charges provide more insight than bond length. It is shown that, in the presence of isovalent carbon, all systems try to preserve the tetrahedral coordination, on the contrary, trivalent nitrogen induces a strong local distortion to fit in the tetrahedral Si matrix while oxygen is the impurity that segregates more easily and more regularly. This work shows that impurities lead to local distortions and how the electron distribution rearranges to smooth it. Overall, it shows how the analysis of bonds and their correlation with energetics and electronic structure is of fundamental importance for the understanding of the defects induced properties and of the basic mechanisms that influence them.

2022 - Revealing the role of Σ3{112} Si grain boundary local structures in impurity segregation [Articolo su rivista]
Maji, Rita; Luppi, Eleonora; Degoli, Elena
abstract

The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical-chemical properties of the material. Therefore, GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different Σ3{112} Si-GB models. These models are (1×1) and (1×2) Σ3{112} Si-GBs which lead to different structural reconstruction. Starting from these two models, we have shown that geometry optimization has an important role on the structural reconstruction of the GB interface and therefore on its properties. For this reason, we discussed different methodologies to define an optimal relaxation protocol. The influence of the local structures in (1×1) and (1×2) models have also been investigated in the presence of vacancies where different light impurities of different valency (C, N, H, O) can segregate. We studied how local structures in (1×1) and (1×2) models are modified by the presence of vacancies and impurities. These structural modifications have been correlated with the changes of the energetics and electronic properties of the GBs. The behaviour of (1×1) and (1×2) models demonstrated to be significantly different. The interaction with vacancies and the segregation of C, N, H and O are significantly different depending on the type of local structures present in Σ3{112} Si-GB.

2021 - Ab initio study of oxygen segregation in silicon grain boundaries: The role of strain and vacancies [Articolo su rivista]
Maji, R.; Luppi, E.; Capron, N.; Degoli, E.
abstract

Multi-crystalline silicon is widely used for producing low-cost and high-efficiency solar cells. During crystal growth and device fabrication, silicon solar cells contain grain boundaries (GBs) which are preferential segregation sites for atomic impurities such as oxygen atoms. GBs can induce charge carriers recombination significantly reducing carrier lifetimes and therefore they can be detrimental for Si device performance. We studied the correlation between structural, energetic and electronic properties of Σ3{111} Si GB in the presence of vacancies, strain and multiple O segregation. The study of the structural and energetic properties of GBs in the presence of strain and vacancies gives an accurate description of the complex mechanisms that control the segregation of oxygen atoms. We analysed tensile and compressive strain and we obtained that local tensile strain around O impurities is very effective for segregation. We also studied the role of multiple O impurities in the presence of Si vacancies finding that the segregation is favorite for those structures which have restored tetrahedral covalent bonds. The presence of vacancies attract atomic impurities in order to restore the electronic stability: the interstitial impurity becomes substitutional. This analysis was the starting point to correlate the change of the electronic properties in Σ3{111} Si GBs with O impurities in the presence of strain and vacancies. For each structure we analysed the density of states and its projection on atoms and states, the band gaps, the segregation energy and their correlation in order to characterise the nature of new energy levels. Actually, knowing the origin of defined electronic states would allow the optimization of materials in order to reduce non radiative electron-hole recombination avoiding charge and energy losses and therefore improving solar cell efficiency.

2021 - Effect of Strain on Interactions of Σ3{111} Silicon Grain Boundary with Oxygen Impurities from First Principles [Articolo su rivista]
Maji, R.; Contreras-Garcia, J.; Luppi, E.; Degoli, E.
abstract

The interaction of grain boundaries (GBs) with inherent defects and/or impurity elements in multicrystalline silicon plays a decisive role in their electrical behavior. Strain, depending on the types of GBs and defects, plays an important role in these systems. Herein, the correlation between the structural and electronic properties of Σ3{111} Si-GB in the presence of interstitial oxygen impurities is studied from the first-principles framework, considering the global and local model of strain. It is observed that the distribution of strain along with the number of impurity atoms modifies the energetics of the material. However, the electronic properties of the considered Si-GBs are not particularly affected by the strain and by the oxygen impurities, unless a very high local distortion induces additional structural defects.

2021 - The role of Si vacancies in the segregation of O, C, and N at silicon grain boundaries: An ab initio study [Articolo su rivista]
Maji, R.; Contreras-Garcia, J.; Capron, N.; Degoli, E.; Luppi, E.
abstract

Grain boundaries (GBs) are defects originating in multi-crystalline silicon during crystal growth for device Si solar cell fabrication. The presence of GBs changes the coordination of Si, making it advantageous for charge carriers to recombine, which brings a significant reduction of carrier lifetimes. Therefore, GBs can be highly detrimental for device performances. Furthermore, GBs easily form vacancies with deep defect electronic states and are also preferential segregation sites for various impurity species, such as C, N, and O. We studied from first principles the correlation between structural, energetics, and electronic properties of the ς3{111} Si GB with and without vacancies, and the segregation of C, N, and O atoms. C and O atoms strongly increase their ability to segregate when vacancies are present. However, the electronic properties of the ς3{111} Si GB are not affected by the presence of O, while they can strongly change in the case of C. For N atoms, it is not possible to find a clear trend in the energetics and electronic properties both with and without vacancies in the GB. In fact, as N is not isovalent with Si, as C and O, it is more flexible in finding new chemical arrangements in the GB structure. This implies a stronger difficulty in controlling the properties of the material in the presence of N impurity atoms compared to C and O impurities.

2018 - First Principle Studies of B and P Doped Si Nanocrystals [Articolo su rivista]
Marri, Ivan; Degoli, Elena; Ossicini, Stefano
abstract

The properties of n- and p-doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free-standing and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total-energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co-doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co-doping, additional peaks are introduced in the absorption spectra, giving rise to a size-dependent red shift of the absorption spectra.

2017 - Ab Initio Study of Electronic Transport in Cubic-HfO2 Grain Boundaries [Articolo su rivista]
Degoli, Elena; Luppi, Eleonora; Capron, Nathalie
abstract

In polycrystalline materials the grain boundaries (GBs) are particularly important as they can act as a sink for atom defects and impurities, which may drive structural transformation of the materials and consequently modify their properties. Characterising the structure and properties of GBs is critical for understanding and controlling material property. Here, we investigated how GBs can modify the structural, electronic, and transport properties of the polycrystalline material . In general, grain boundaries are considered to be detrimental to the physical stability and electronic transport in . Anyway, studying by first principles the two most stable and common types of GBs, the tilt and the twist, we found substantial differences on the impact they have on the material properties. In fact, while tilt defects create channels of different sizes and shapes in hafnia along which the electronic transport is stronger in relation to leakage current through GBs, twist defects create a sort of amorphous structure that tends to resemble the bulk and which is independent of the number of rotated planes/atoms.

2017 - Doped and codoped silicon nanocrystals: The role of surfaces and interfaces [Articolo su rivista]
Marri, Ivan; Degoli, Elena; Ossicini, Stefano
abstract

Si nanocrystals have been extensively studied because of their novel properties and their potential applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. These new properties are achieved through the combination of the quantum confinement of carriers and the strong influence of surface chemistry. As in the case of bulk Si the tuning of the electronic, optical and transport properties is related to the possibility of doping, in a controlled way, the nanocrystals. This is a big challenge since several studies have revealed that doping in Si nanocrystals differs from the one of the bulk. Theory and experiments have underlined that doping and codoping are influenced by a large number of parameters such as size, shape, passivation and chemical environment of the silicon nanocrystals. However, the connection between these parameters and dopant localization as well as the occurrence of self-purification effects are still not clear. In this review we summarize the latest progress in this fascinating research field considering free-standing and matrix-embedded Si nanocrystals both from the theoretical and experimental point of view, with special attention given to the results obtained by ab-initio calculations and to size-, surface- and interface-induced effects.

2017 - Second Harmonic Generation in Silicon Based Heterostructures: The Role of Strain and Symmetry [Articolo su rivista]
Bertocchi, Matteo; Degoli, Elena; Véniard, V; Luppi, E; Ossicini, Stefano
abstract

Silicon is today the electronic material par excellence. Nevertheless the increasing demand for new, innovative and more efficient devices has driven scientists to explore new functionalities in Si-based materials. In silicon photonics the introduction of second-order nonlinearity by proper material engineering would be highly desirable. However a bulk second-order dipolar nonlinear optical susceptibility in Si is forbidden due to the bulk crystal centrosymmetry. Different approaches have been used to break this inversion symmetry: ...

2015 - Strain-designed strategy to induce and enhance second-harmonic generation in centrosymmetric and noncentrosymmetric materials [Articolo su rivista]
Luppi, Eleonora; Degoli, Elena; Bertocchi, Matteo; Ossicini, Stefano; Véniard, Valérie
abstract

Second-harmonic generation is described by the second-order nonlinear susceptibility χ(2) which, in the electric-dipole approximation, requires a noncentrosymmetric medium. It is very challenging and of high technological interest to search whether it is possible to find away to break inversion symmetry in centrosymmetric crystals in order to induce second-order nonlinearities. A new intriguing way to observe second-order nonlinear phenomena is strain. Here, we present a detailed analysis of the correlation between the strain and the χ(2) in both centrosymmetric and noncentrosymmetric materials. We considered Si and SiC as test materials and we studied different types of strain (tensile/compressive), in different directions (uniaxial/biaxial) and for different light-polarization directions.We found which is the type of strain necessary in order to induce, tune, and enhance second-harmonic generation in different energy regions for centrosymmetric and noncentrosymmetric materials.

2014 - Defects and strain enhancements of second-harmonic generation in Si/Ge superlattices [Articolo su rivista]
Bertocchi, Matteo; E., Luppi; Degoli, Elena; V., Véniard; Ossicini, Stefano
abstract

Starting from experimental findings and interface growth problems in Si/Ge superlattices, we have investigated through ab initio methods the concurrent and competitive behavior of strain and defects in the second-harmonic generation process. Interpreting the second-harmonic intensities as a function of the different nature and percentage of defects together with the strain induced at the interface between Si and Ge, we found a way to tune and enhance the second-harmonic generation response of these systems.

2014 - Determination of the Electronic Energy Levels of Colloidal Nanocrystals using Field-Effect Transistors and Ab-Initio Calculations [Articolo su rivista]
Satria Zulkarnaen, Bisri; Degoli, Elena; Nicola, Spallanzani; Gopi, Krishnan; Bart Jan, Kooi; Corneliu, Ghica; Maksym, Yarema; Wolfgang, Heiss; Olivia, Pulci; Ossicini, Stefano; Maria Antonietta, Loi
abstract

We investigate the absolute energy of the LUMO and HOMO energy states, and consequently the electronic bandgap values, for PbS NCs of different sizes, through a combined experimental and theoretical effort. The experimental method relies on the measurement of ambipolar field-effect transistors by using an ionic-liquid-based ion-gel gate. This represents the first successful application of this method to an array of fully quantum-confined systems. This method does not only allow the determination of the bandgap but also the higher energy levels of the NC array, which are characterized by slope variation (going true plateau) in the transfer characteristics. Our experimental results are directly compared with results obtained with advanced ab-initio calculations, performed with density functional theory (DFT) using a fully relativistic approach, which allow a direct determination of absolute energy levels. The agreement between measured and calculated energy values is excellent.

2013 - Ab Initio Electronic Gaps of Ge Nanodots: The Role of Self-Energy Effects [Articolo su rivista]
Margherita, Marsili; Silvana, Botti; Maurizia, Palummo; Degoli, Elena; Olivia, Pulci; Hans Christian, Weissker; Miguel A. L., Marques; Ossicini, Stefano; Rodolfo Del, Sole
abstract

Nanostructuring of a material leads to enormous effects on its excited state properties. This study, through the application of different state-of-the-art ab initio theoretical tools, investigates the effect of size on the electronic gap of germanium nanocrystals highlighting similarities and differences with respect to equivalent silicon nanostructures. We performed both GW and ΔSCF calculations for the determination of their electronic structure. While it is known that ΔSCF corrections to the Kohn−Sham gap vanish for extended systems, the two approaches were expected to be equivalent in the limit of small clusters. However, it has been recently found that for hydrogenated Si clusters the ΔSCF gaps are systematically smaller than the GW ones, while the opposite is true for Ag clusters. In this work we find that the GW gaps are larger than the ΔSCF ones for all the Ge dots, with the exception of the smallest one. Such crossing between the ΔSCF and the GW gap values was not expected and has never been observed before. Moreover, also for hydrogenated Si nanocrystals we found a similar behavior. The origin of this crossing might be found in the Rydberg character of the LUMO of the smallest clusters and can also explain the qualitative differences in the comparison between GW and ΔSCF found in the previous studies.

2012 - Large crystal local-field effects in second-harmonic generation of a Si/CaF2 interface:An ab initio study [Articolo su rivista]
Bertocchi, Matteo; E., Luppi; Degoli, Elena; V., Véniard; Ossicini, Stefano
abstract

In this work we present the ab initio study of crystal local-field effects in second-harmonic generationspectroscopy for an interface material such as Si/CaF2. Starting from an independent particle picture, wedemonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimationof the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF2 interface was doneby a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that themicroscopic fluctuations due to the inhomogeneities of the system cause a decrease of the intensities of the spectra.However, for second-harmonic generation the decrease is selective and completely inhomogeneous while for absorptionit is almost rigid.We also compare our theoretical study with experimental data showing unambiguouslythat only when crystal local fields are included, it is possible to correctly interpret experimental results.

2012 - Second-harmonic generation in silicon waveguides strained by silicon nitride [Articolo su rivista]
Cazzanelli, M.; Bianco, F.; Borga, E.; Pucker, G.; Ghulinyan, M.; Degoli, E.; Luppi, E.; Veniard, V.; Ossicini, S.; Modotto, D.; Wabnitz, S.; Pierobon, R.; Pavesi, L.
abstract

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks.All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are presentowing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper materialengineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively lowoptical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguideby using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principlecalculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40pm/V at2,300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated lightsources spanning the near- to mid-infrared spectrum from 1.2 to 10 micrometri.

2012 - Second-order nonlinear silicon photonics [Articolo su rivista]
M., Cazzanelli; F., Bianco; M., Ghulinyan; G., Pucker; D., Modotto; S., Wabnitz; F. L., Pigozzo; Ossicini, Stefano; Degoli, Elena; E., Luppi; V., Véniard; L., Pavesi
abstract

Breaking the internal crystalline symmetry of silicon by an external mechanical stress gives the material a novel nonlinear optical properties

2011 - Second-harmonic Generation Spectroscopy from Time-dependent Density-functional Theory [Relazione in Atti di Convegno]
E., Luppi; H., Huebener; Bertocchi, Matteo; Degoli, Elena; Ossicini, Stefano; V., Veniard
abstract

Nonlinear Optics is one of the most active fields for fundamental and applied research in physics. The interest for nonlinear optical phenomena is becoming extremely strong, because of their versatile and innovative properties and technological applications. Very recently we have developed a first-principles theory, based on the Time-Dependent Density-Functional Theory approach, for the calculation of the second-order susceptibility χ2. We find a general expression for χ2 valid for any fields, containing the ab initio relation between the microscopic and macroscopic formulation of the second-order responses. We consider the long wavelength limit and we develop our theory in the Time-Dependent Density-Functional Theory framework. This allows us to include straightforwardly many-body effects such as crystal local-field and excitonic effects. We apply this formalism to the calculation of the Second-Harmonic Generation spectra for different type of materials: cubic semiconductors, hexagonal SiC polytypes finding good agreement with experiments. We are also exploring with this formalism the effects of anysotrophies for more complex systems like CaF2/Si multi quantum-well and silicon surfaces.

2010 - Electronic and optical properties of Si and Ge nanocrystals: an ab-initio study [Articolo su rivista]
Olivia, Pulci; Degoli, Elena; Federico, Iori; Margherita, Marsili; Maurizia, Palummo; Rodolfo Del, Sole; Ossicini, Stefano
abstract

First-principles calculations within Density Functional Theory and Many-Bodyperturbation Theory have been carried out in order to investigate the structural,electronic and optical properties of undoped and doped silicon nanostructures.We consider Si nanoclusters co-doped with B and P. We ¯nd that the electronicband gap is reduced with respect to that of the undoped crystals suggesting thepossibility of an impurity based engineering of electronic and optical propertiesof Si nanocrystals. Finally, driven by recent suggestions concerning the chanceof exploiting Ge dots for photovoltaic nanodevices, we present calculations of theelectronic and optical properties of a Ge35H36 nanocrystal, and compare the re-sults with those of the corresponding Si35H36-nc and the co-doped Si33BPH36.

2010 - Local-fields and disorder effects in free-standing and embedded Si nanocrystallites [Articolo su rivista]
Guerra, Roberto; Degoli, Elena; M., Marsili; O., Pulci; Ossicini, Stefano
abstract

The case study of a 32-atoms Si nanocrystallite (NC) embeddedin a SiO2 matrix, both crystalline and amorphous, or freestandingwith different conditions of passivation and strain isanalyzed through ab-initio approaches. The Si32/SiO2 heterojunctionshows a type I band offset highlighting a separationbetween the NC plus the interface and the matrix around. Theconsequence of this separation is the possibility to correctlyreproduce the low energy electronic and optical properties ofthe composed system simply by studying the suspended NCplus interface oxygens with the appropriate strain. Moreover,through the definition of an optical absorption threshold wefound that, beside the quantum confinement trend, theamorphization introduces an additional redshift that increaseswith increasing NC size, i.e. the gap tends faster to the bulklimit. Finally, the important changes in the calculated DFTRPAoptical spectra upon inclusion of local fields point towardsthe need of a proper treatment of the optical response of theinterface region.

2010 - Size, oxydation, and strain of Si nanocrystallites: from large clusters to quantum dots. [Abstract in Rivista]
Guerra, Roberto; Degoli, Elena; Ossicini, Stefano
abstract

11th International Conference on Optics of Excitons in Confined Systems

2009 - Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures [Articolo su rivista]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; O., Pulci; Bisi, Olmes; Ossicini, Stefano
abstract

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.

2009 - Engineering quantum confined silicon nanostructures: ab-initio study of the structural, electronic and optical properties [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano
abstract

Chapter VI

2009 - Impurity screening in silicon nanocrystals [Articolo su rivista]
F., Trani; D., Ninno; G., Cantele; Degoli, Elena; Ossicini, Stefano
abstract

The impurity screening in silicon nanocrystals is analyzed using a first-principles approach based on density functional theory. The electron density induced by a positively charged impurity is evaluated as a function of the nanocrystal size. From our calculations we found that the impurity is responsible for anelectron density accumulation around the impurity site, fully compensated by a positive charge accumulation at the surface(electron depletion). The results are sound and shed new light on the most recent findings in this field. On the basis of the present first-principles results, we propose a Thomas–Fermi model of the impurity screening in silicon nanocrystals. The model gives reliableestimations of the screening function, that well compares to recent ab-initio calculations.

2009 - Optical properties of silicon nanocrystallites in SiO2 matrix:Crystalline vs. amorphous case [Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract

Within a first-principles framework we show the dependenceof the optical properties of silicon nanocrystallites embeddedin a silica matrix on the crystalline vs. amorphous order ofthe system. Moreover we calculate how many-body effectsmodify the electronic and optical properties of the embeddedsilicon nanodots. A discussion about the different roles playedby dimensionality, interface properties, disorder and excitoniceffects on the electronic and optical properties of the confinednanostructures is presented.

2009 - Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size [Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract

We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a SiO2 crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We findthat the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.

2009 - Size, oxidation, and strain in small Si/SiO2 nanocrystals [Articolo su rivista]
Guerra, Roberto; Degoli, Elena; Ossicini, Stefano
abstract

The structural, electronic, and optical properties of Si nanocrystals of different size and shape, passivated with hydrogens, OH groups, or embedded in a SiO2 matrix are studied. The comparison between the embedded and free, suspended nanocrystals (NCs) shows that the silica matrix produces a strain on the embedded NCs, which contributes to determine the band gap value. By including the strain on the hydroxided nanocrystals, we are able to reproduce the electronic and optical properties of the full Si/SiO2 systems. Moreover, we found that while the quantum confinement dominates in the hydrogenated nanocrystals of all sizes, the behavior of hydroxided and embedded nanocrystals strongly depends on the interface oxidation degree, in particular for diameters below 2 nm. Here, the proportion of NC atoms at the Si/SiO2 interface becomes relevant, producing surface-related states that may affect the quantum confinement appearing as inner band gap states and then drastically changing the optical response of the system.

2008 - First-Principles Study of Silicon Nanocrystals: Structural and Electronic Properties, Absorption, Emission, and Doping. [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Degoli, Elena; Marri, Ivan; F., Iori; E., Luppi; Magri, Rita; R., Poli; G., Cantele; D., Ninno; F., Trani; M., Marsili; O., Pulci; V., Olevano; M., Gatti; K., GAAL NAGY; A., Incze; G., Onida
abstract

Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural andoptical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced,favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility ofan impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

2008 - Novel optoelectronic properties of simultaneously n- and p-doped silicon nanostructures [Articolo su rivista]
F., Iori; Degoli, Elena; M., Palummo; Ossicini, Stefano
abstract

Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. For instance, it has been shownthat luminescence energies in silicon nanocrystals can be tuned by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. Starting from hydrogen-terminated silicon nanoclusters, we have previously calculated from first-principles that codoping results are always energetically favored with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The codoped Si nanoclusters present band-edge states localizedon the impurities which are responsible for the red-shift of the absorption thresholds with respect to that of pure undoped Si nanoclusters. Here we investigate how the properties of the codoped nanoclusters are influenced by adding one or two more impurities. Moreover we study also the effect of B- and P-codopingon the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality, 0-versus 1-dimensionality, of the systems.

2008 - Optical absorption spectra of doped and codoped Si nanocrystallites [Articolo su rivista]
L. E., Ramos; Degoli, Elena; G., Cantele; Ossicini, Stefano; D., Ninno; J., Furthmüller; F., Bechstedt
abstract

The effects of the incorporation of group-III (B and Al), group-IV (C and Ge), and group-V (N and P) impurities on the formation energies, electronic density of states, optical absorption spectra, and radiative lifetimes of Si nanocrystallites of different shape and with diameters up to 2 nm are studied by means of an abinitio pseudopotential method that takes into account spin polarization. The single doping with group-III or group-V impurities leads to significant changes on the onsets of the absorption spectra that are related to the minority-spin states. In contrast to the optical absorption spectra, the radiative lifetimes are sensitively influenced by the shape of the nanocrystallites, though this influence tends to disappear as the size of the nanocrystallites increase. Codoping is investigated for pairs of group-III and group-V impurities. We show that theimpurity formation energies decrease significantly when the nanocrystallites are codoped with B and P or withAl and P. Additional peaks are introduced in the absorption spectra due to codoping, giving rise to a redshift ofthe absorption onset with respect to the undoped nanocrystallites. Those additional peaks are more intense when codoping is performed with two different species either of the group III or of the group V. The values of radiative lifetimes for the codoped nanocrystallites are mostly in between the values for the nanocrystallites doped with the impurities separately.

2008 - THEORETICAL STUDIES OF ABSORPTION, EMISSION AND GAIN IN SILICON NANOSTRUCTURES [Capitolo/Saggio]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; Ossicini, Stefano
abstract

Density-functional and many body perturbation theory calculations have been carried out in order to study the structural, electronic, and opti- cal properties both in ground and excited state configuration, of silicon nanocrystals in different conditions of surface passivation and doping. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidences that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the phys- ical properties of these systems. In particular we show that only the presence of a surface Si-O-Si bridge bond induce an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible photoluminescence (PL) experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the electronic and optical properties are discussed in detail. The strong in- terplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. Finally, concerning doping, we will show that, thanks to electronic transitions between donor and acceptor states present at the band edges and considering also the effect of quantum confinement it is possible to engineer the absorp- tion and emission spectra of Si nanocrystals. For each considered system optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.

2007 - Ab-initio Electronic and Optical Properties of Low Dimensional Systems: from Single Particle to Many Body Approaches [Articolo su rivista]
M., Palummo; M., Bruno; O., Pulci; Luppi, Eleonora; Degoli, Elena; Ossicini, Stefano; R., DEL SOLE
abstract

Low dimensional systems, such as nanodots, nanotubes, nanowires, have attracted great interest in the last years, due to their possibleapplication in nanodevices. It is hence very important to describe accurately their electronic and optical properties within highly reliableand efficient ab-initio approaches. Density functional theory (DFT) has become in the last 20 years the standard technique for studyingthe ground-state properties, but this method often shows significant deviations from the experiment when electronic excited states areinvolved. The use of many-body Green’s functions theory, with DFT calculations taken as the zero order approximation, is todaythe state-of-the-art technique for obtaining quasi-particle excitation energies and optical spectra. In this paper we will present the currentstatus of this theoretical and computational approach, showing results for different kinds of low dimensional systems.

2007 - Codoping goes Nano: Structural and Optical Properties of Boron and Phosphorus Codoped Silicon Nanocrystals [Relazione in Atti di Convegno]
Magri, Rita; Iori, Federico; Degoli, Elena; O., Pulci; Ossicini, Stefano
abstract

Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. By using Density Functional Theory, Silicon Nanocrystals (Si-nc) of different size (diameter ranging from 1.1 nm to 1.8 nm) have been studied localizing impurities at different substitutional sites and calculating the impurity formation energies. Starting from hydrogen terminated silicon Si-nc, we found that codoping is always energetically favored with respect to a single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The formation energy depends on the distance between the two impurities. The codoped Si-nc present bandedge states localized on the impurities which are responsible for a red-shift of the absorption threshold with respect to that of pure undoped Si nanocrystals. Concerning the emission spectra, we find a Stokes shift of the photoluminescence to a lower energy with respect to the absorption edge due to the nanocrystal (nc) structural relaxation after the creation of the electron-hole pair. We have calculated the absorption and emission spectra going beyond a single-particle approach showing the important role played by the many-body effects. The presence of electronic quasi-direct optical transitions between the donor and acceptor states within the band-gap makes it possible to engineer the optical properties of Si-nc.

2007 - Doping in silicon nanocrystals [Articolo su rivista]
Ossicini, Stefano; Degoli, Elena; F., Iori; O., Pulci; G., Cantele; Magri, Rita; Bisi, Olmes; F., Trani; D., Ninno
abstract

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B–P co-doping results to be easier than simple B- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcome. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron–hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the selfenergycorrection and solving the Bethe–Salpeter equation scheme. Our procedure shows the important role played by the many-bodyeffects.

2007 - Doping in silicon nanostructures [Articolo su rivista]
F., Iori; Ossicini, Stefano; Degoli, Elena; E., Luppi; R., Poli; Magri, Rita; G., Cantele; F., Trani; AND D., Ninno
abstract

We report on an ab initio study of the structural, electronic and optical properties of boron and phosphorous doped silicon nanocrystals. The scaling with the Si-nanocrystal size is investigated for both the neutral formation energies (FE) and the impurity activation energies. Both these energies scale with the nanocrystal inverse radius. The optical properties reveal the existence of new absorption peaks in the low energy region related to the presence of the impurity. The effects of B and P co-doping show that the formation energies are always smaller than those of the corresponding single-doped cases due to both carriers compensation and minor structural distortion. Moreover in the case of co-doping the electronic and optical properties show a strong reduction of the band gap with respect to the pure silicon nanocrystals that makes possible to engineer the photoluminescence properties of silicon nanocrystals.

2007 - Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus [Articolo su rivista]
Iori, F.; Degoli, E.; Magri, R.; Marri, I.; Cantele, G.; Ninno, D.; Trani, F.; Pulci, O.; Ossicini, S.
abstract

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an ele...

2007 - Excitons in Silicon Nanocrystallites: the Nature of Luminescence [Articolo su rivista]
E., Luppi; F., Iori; Magri, Rita; O., Pulci; Degoli, Elena; Ossicini, Stefano; V., Olevano
abstract

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter have been calculated within a first-principles framework. Our calculations include geometry optimization and the many-body effects induced by the creation of an electron-hole pair. Starting from hydrogenated silicon clusters of different sizes, different Si/O bondings at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond causes significant excitonic luminescence features in the visible range that are in fair agreement with experiment

2007 - Role of surface passivation and doping in silicon nanocrystals [Articolo su rivista]
Magri, Rita; Degoli, Elena; Iori, Federico; Luppi, Eleonora; Pulci, O.; Ossicini, Stefano; Cantele, G; Trani, F; Ninno, D.
abstract

The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the atomic structure and on the optical spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, (i) different Si/O bonding at the cluster surface and (ii) different doping configurations have been considered. We have found that the presence of a Si-O-Si bridge bond at the nanocrystal surface gives rise to significant excitonic luminescence features in the near-visible range that are in fair agreement with photoluminescence (PL) measurements on oxidized and SiO_{2} embedded nanocrystals. The study of the structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have shown that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbors sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in agreement with the experiment.

2007 - Structural Features and Electronic Properties of Group-III, Group-IV and Group-V-doped Si Nanocrystallites [Articolo su rivista]
L. E., Ramos; Degoli, Elena; G., Cantele; Ossicini, Stefano; D., Ninno; J., Furthmüller; F., Bechstedt
abstract

We investigate the incorporation of group-III ( B and Al), group-IV ( C and Ge), and group-V ( N and P) impurities in Si nanocrystallites. The structural features and electronic properties of doped Si nanocrystallites, which are faceted or spherical-like, are studied by means of an ab initio pseudopotential method including spin polarization. Jahn-Teller distortions occur in the neighborhood of the impurity sites and the bond lengths show a dependence on size and shape of the nanocrystallites. We find that the acceptor ( group-III) and donor ( group-V) levels become deep as the nanocrystallites become small. The energy difference between the spin-up and spin-down levels of group-III and group-V impurities decreases as the size of the Si nanocrystallite increases and tends to the value calculated for Si bulk. Doping with carbon introduces an impurity-related level in the energy gap of the Si nanocrystallites.

2006 - Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties [Articolo su rivista]
Iori, Federico; Degoli, Elena; Luppi, Eleonora; Magri, Rita; Marri, Ivan; G., Cantele; D., Ninno; F., Trani; Ossicini, Stefano
abstract

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Screening in semiconductor nanocrystals: Ab initio results and Thomas-Fermi theory [Articolo su rivista]
F., Trani; D., Ninno; G., Cantele; G., Iadonisi; K., Hameeuw; Degoli, Elena; Ossicini, Stefano
abstract

A first-principles calculation of the impurity screening in Si and Ge nanocrystals is presented. We show that isocoric screening gives results in agreement with both the linear response and the point-charge approximations. Based on the present ab initio results, and by comparison with previous calculations, we propose a physical real-space interpretation of the several contributions to the screening. Combining the Thomas-Fermi theory and simple electrostatics, we show that it is possible to construct a model screening function that has the merit of being of simple physical interpretation. The main point upon which the model is based is that, up to distances of the order of a bond length from the perturbation, the charge response does not depend on the nanocrystal size. We show in a very clear way that the link between the screening at the nanoscale and in the bulk is given by the surface polarization. A detailed discussion is devoted to the importance of local field effects...

2006 - The structural, electronic and optical properties of Si nanoclusters: effects of size, doping and surface passivation [Capitolo/Saggio]
Degoli, Elena; E., Luppi; Magri, Rita; F., Iori; G., Cantele; Ossicini, Stefano
abstract

AB-initio routines

2006 - Thomas-Fermi model of electronic screening in semiconductor nanocrystals [Articolo su rivista]
D., Ninno; F., Trani; G., Cantele; Kj, Hameeuw; G., Iadonisi; Degoli, Elena; Ossicini, Stefano
abstract

Using first-principle density-functional theory in the GGA approximation we have studied the electronic screening in semiconductor nanocrystals. Combining simple electrostatics and the Thomas-Fermi theory it is shown that an analytical and general form of a model position-dependent screening function can be obtained. Taking as a case study silicon nanocrystals, the relative weights of the nanocrystal core and surface polarization contribution to the screening are thoroughly discussed. The connection between the screening at the nanoscale and in the bulk is clarified.

2006 - Understanding doping in silicon nanostructures [Articolo su rivista]
Ossicini, Stefano; F., Iori; Degoli, Elena; E., Luppi; Magri, Rita; R., Poli; G., Cantele; F., Trani; D., Ninno
abstract

The effects of both single doping and simultaneous codoping on the structural, electronic, and optical properties of Si nanocrystals are calculated by the first-principles method. We show that the amount of the nanocrystal relaxation around the impurity is directly related to the impurity valence. Moreover, both the neutral impurity formation energies and the impurity activation energies scale with the reciprocal radius. Interestingly, no significant variation of the activation energy on the impurity species is found, and the cluster relaxation gives a minor contribution to it. The role of the impurity position within the nanocrystal has also been elucidated showing that the subsurface positions are the most stable ones. We show that, if the carriers in the Si nanocrystals are perfectly compensated by simultaneous doping with the n- and p-type impurities, the nanocrystals undergo a minor structural distortion around the impurities. The formation energies are always smaller than that for the corresponding single-doped cases. Moreover, in the case of codoping, the bandgap is strongly reduced with respect to the gap of the pure crystals showing the possibility of an impurity-based engineering of the photoluminescence properties of the Si nanocrystals.

2005 - Ab-initio Calculations Of The Electronic Properties of Hydrogenated and Oxidized Silicon Nanocrystrals: Ground and Excited States [Relazione in Atti di Convegno]
Ossicini, Stefano; Bisi, Olmes; Cantele, G; Degoli, Elena; DEL SOLE, R; Gatti, M; Incze, A; Iori, Federico; Luppi, Eleonora; Magri, Rita; Ninno, D; Onida, G; Pulci, O.
abstract

The electronic and optical properties of hydrogenated silicon nanocrystals (H-Sinc) have been investigated through ab-initio techniques (Pseudopotential approach in ground and excited state electronic configurations, TDDFT, GW) as a function of size and symmetry. The presence of an electron-hole pair in the nanocrystals causes a strong deformation of the structures with respect to the ground-state configuration, and this is more evident for smaller systems and at the surface of the H-Si-nc. Also the nature of the distorsion changes: for small clusters it is strongly localized, while as the size increases the distortion is spread out over the entire structure. The structural modifications are immediately reflected into the electronic structure. Actually, we have found the expected decrease of the energy gap on increasing the nanocrystal dimension for the ground state and, for the excited state configuration, a reduction of the energy gap the more significant the smaller is the nanocrystal. For the excited nanoparticles the HOMO and LUMO become strongly localised in correspondence of the distortion, giving rise to defect-like states which reduce the gap; these effects are stronger in smaller clusters. Thus, we can deduce that the absorption of resonant radiation by the nanocrystal in its ground state configuration induces a transition between the HOMO and LUMO levels, which for all these nanocrystals is optically allowed. Such a transition is followed by a cluster relaxation in the excited state configuration giving rise to distorted geometries and to new LUMO and HOMO, whose energy difference is smaller than that in the ground-state geometry. It is between these two last states that emission occurs; the Stokes Shift between absorption and emission changes as a function of the dimension. Thus we have substituted some H with O both double bonded or in a bridge configuration with respect to Si. The substitution of H with O as passivating agent results in a different cluster geometry and in a reduction of the energy band gap depending on the type of O-Si bond. Moreover also the optical properties strongly depend on the different O-Si bond type. The results provide a consistent interpretation of the photoluminescence redshift observed in oxidized samples and of recent outcomes on Si single quantum dot photoluminescence bandwidth. Three fundamental aspects come out: first, there is a strong interplay between structural and electronic properties, mostly when excited configurations are concerned. Second, a consistent explanation of emission processes can be carried out only if such excited configurations are accounted for. Third, optical gaps cannot be calculated simply as the HOMO-LUMO energy separation without introducing errors as larger as smaller is the nanocrystal.

2005 - Ab-initio Calculations Of The Electronic Properties of Silicon Nanocrystals: Absorption, Emission, Stokes Shift [Relazione in Atti di Convegno]
Degoli, Elena; G., Cantele; E., Luppi; Magri, Rita; Ossicini, Stefano; D., Ninno; Bisi, Olmes; G., Onida; M., Gatti; A., Incze; O., Pulci; R., DEL SOLE
abstract

The structural, optical and electronic properties of silicon nanocrystals are investigated as a function of the dimension as well as the surface passivation. Both the ground- and an excited-state configuration are studied using ab-initio calculations. Atom relaxation under excitation is taken into account and related with the experimentally observed Stokes shift.

2005 - Electronic, structural and optical properties of hydrogenated silicon nanocrystals: the role of the excited states [Relazione in Atti di Convegno]
G., Cantele; Degoli, Elena; Luppi, Eleonora; Magri, Rita; D., Ninno; Bisi, Olmes; Ossicini, Stefano; G., Iadonisi
abstract

In this paper we report on a first-principle calculation of the electronic and structural properties of hydrogenated silicon nanocrystals both in the ground- and in an excited-state configuration. The presence of an electron-hole pair created under excitation is taken into account and its effects on both the electronic spectrum and the cluster geometry are pointed out. The interpretation of the results is done within a four-level model, which also allows the explanation of the experimentally observed Stokes shift. Size-related aspects are also analysed and discussed.

2005 - First-principles study of n- and p-doped silicon nanoclusters [Articolo su rivista]
G., Cantele; Degoli, Elena; Luppi, Eleonora; Magri, Rita; D., Ninno; G., Iadonisi; Ossicini, Stefano
abstract

We report on an ab initio study of the structural and electronic properties of B- and P-doped Si nanoclusters. The neutral impurities formation energies are calculated. We show that they are higher in smaller nanoclusters and that this is not related to the structural relaxation around the impurity. Their dependence on the impurity position within the nanocluster is also discussed. Finally, we have calculated the B and P activation energies showing the existence of a nearly linear scaling with the nanocluster inverse radius. Interestingly, no significant variation of the activation energy on the impurity species is found and the cluster relaxation gives a minor contribution to it.

2005 - Formation energies of silicon nanocrystals: role of dimension and passivation [Relazione in Atti di Convegno]
Degoli, Elena; Ossicini, Stefano; G., Cantele; Luppi, Eleonora; Magri, Rita; D., Ninno; Bisi, Olmes
abstract

The structural properties of small silicon nanoclusters as a function of dimension and surface passivation are studied from ab initio technique. The formation energies are calculated and the relative stability of the considered clusters is predicted and discussed.

2005 - P and B single- and co-doped silicon nanocrystals: Formation and activation energies, electronic and optical properties [Relazione in Atti di Convegno]
Ossicini, S.; Iori, F.; Degoli, E.; Luppi, E.; Magri, R.; Cantele, G.; Trani, F.; Ninno, D.
abstract

We report on an ab initio study of the structural and electronic properties of B and P doped Si nanocrystals (Si-nc). The formation energies (FE) scale with the radius, the activation energies with the inverse radius. The effects of B and P co-doping show that the FE are always smaller than that for the corresponding single-doped cases and that is possible to engineer the photoluminescence properties of Si-nc.

2005 - Simultaneously B- and P-doped silicon nanoclusters: Formation energies and electronic properties [Articolo su rivista]
Ossicini, Stefano; Degoli, Elena; Iori, F; Luppi, E; Magri, Rita; Cantele, G; Trani, F; Ninno, D.
abstract

The effects of B and P codoping on the impurity formation energies and electronic properties of Si nanocrystals (Si-nc) are calculated by a first-principles method. We show that, if carriers in the Si-nc are perfectly compensated by simultaneous doping with n- and p-type impurities, the Si-nc undergo a minor structural distortion around the impurities and that the formation energies are always smaller than those for the corresponding single-doped cases. The band gap of the codoped Si-nc is strongly reduced with respect to the gap of the pure ones showing the possibility of an impurity based engineering of the photoluminescence properties of Si-nc.

2005 - The electronic and optical properties of silicon nanoclusters: absorption and emission [Articolo su rivista]
Luppi, Eleonora; Degoli, Elena; G., Cantele; Ossicini, Stefano; Magri, Rita; D., Ninno; Bisi, Olmes; O., Pulci; G., Onida; M., Gatti; A., Incze; R., Del Sole
abstract

The electronic and optical properties of silicon nanocrystals passivated with hydrogen and oxygen have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron-hole pair leads to a strong interplay between the structural and optical properties of the system. The structural distortion of the nanocrystals induced by an electronic excitation is analysed together with the role of the symmetry constraint during the relaxation. The structural distortion can account for the experimentally observed Stokes Shift. Size-related aspects are also analysed and discussed.

2004 - Ab-initio structural and electronic properties of hydrogenated silicon nanoclusters in their ground and excited state [Articolo su rivista]
Degoli, Elena; G., Cantele; E., Luppi; Magri, Rita; D., Ninno; Bisi, Olmes; Ossicini, Stefano
abstract

Electronic and structural properties of small hydrogenated silicon nanoclusters as a function of dimension are calculated from ab initio technique. The effects induced by the creation of an electron-hole pair are discussed in detail, showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure after an electronic excitation of the cluster is analyzed together with the role of the symmetry constraint during the relaxation. We point out how the overall effect is that of significantly changing the electronic spectrum if no symmetry constraint is imposed to the system. Such distortion can account for the Stokes shift and provides a possible structural model to be linked to the four-level scheme invoked in the literature to explain recent results for the optical gain in silicon nanoclusters. Finally, formation energies for clusters with increasing dimension are calculated and their relative stability discussed.

2004 - Formation energies of silicon nanocrystals: role of dimension and passivation [Abstract in Atti di Convegno]
Ossicini, Stefano; Degoli, Elena; Cantele, G; Luppi, Eleonora; Magri, Rita; Ninno, D; Bisi, Olmes
abstract

The structural porperties of small silicon nanoclusters are investigated.

2004 - Structural, electronic and optical properties of silicon nanoclusters: the role of the size and surface passivation [Relazione in Atti di Convegno]
Cantele, G; Ossicini, Stefano; Degoli, Elena; Luppi, Eleonora; Magri, Rita; Ninno, D; Bisi, Olmes
abstract

Thes structural, optical and electronic properties of silicon nanocrystals are investigated.

2003 - Electronic and optical properties of silicon nanocrystals: structural effects [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano; M., Luppi; E., Luppi; Magri, Rita; G., Cantele; D., Ninno; N., Iadonisi
abstract

The aim of this work is to investigate the structural, electronic and optical properties of hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations. Recent results about ab-initio calculations of Stokes shift as a function of the cluster dimension and of optical gain will be presented here. A structural model that can be linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be suggested too.

2003 - Experimental and theoretical joint study on the electronic and structural properties of silicon nanocrystals embedded in SiO2: active role of the interface region [Relazione in Atti di Convegno]
N., Daldosso; M., Luppi; G., Dalba; L., Pavesi; F., Rocca; F., Priolo; G., Franzò; F., Iacona; Degoli, Elena; Magri, Rita; Ossicini, Stefano
abstract

The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energycalculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The comparisonbetween total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 nm. The optoelectronic properties show the appearance of localized states not only in the Si-nc core region but also in the modified SiO2 region

2003 - Gain theory and models in silicon nanostructures [Relazione in Atti di Convegno]
Ossicini, Stefano; C., Arcangeli; Bisi, Olmes; Degoli, Elena; M., Luppi; Magri, Rita; L., DAL NEGRO; L., Pavesi
abstract

The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon.Here we will review our activity in the field of the theoretical determination of the structural, electronic and optical properties of Si nanocrystals (Si-nc). The present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc.

2003 - Role of interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2 [Articolo su rivista]
N., Daldosso; M., Luppi; Ossicini, Stefano; Degoli, Elena; Magri, Rita; G., Dalba; P., Fornasini; R. GRISENTI, F. ROCCA; L., Pavesi; S. BONINELLI, F. PRIOLO; C., Spinella; F., Iacona
abstract

Light emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered TEM analysis and by ab-initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and of variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light emission process.

2003 - Surface and confinement effects on the optical and structural properties of silicon nanocrystals [Relazione in Atti di Convegno]
Ossicini, Stefano; Magri, Rita; Degoli, Elena; Luppi, Marcello; Luppi, Eleonora
abstract

In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory.In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth.

2003 - Will silicon be the photonics material of the third millennium? [Relazione in Atti di Convegno]
L., Pavesi; L., Dal Negro; N., Daldosso; Z., Gaburro; M., Cazzanelli; F., Iacona; G., Franzo; D., Pacifici; F., Priolo; Ossicini, Stefano; Luppi, Marcello; Degoli, Elena; Magri, Rita
abstract

CMOS circuitry dominates the current semiconductor market due to the astonishing power of silicon electronic integration technology. In contrast to the dominance of silicon in electronics, photonics utilises a diversity of materials for emitting, guiding, modulating and detecting light. In the last ten years a big research effort was aimed to render Si an optical active material so that it can be turned from an electronic material to a photonic material. For some the future of Si-based photonic lays in 'hybrid' solutions, for others the utilisation of more photonic functions by silicon itself. During the last two years many breakthroughs in the field have appeared. In this paper we will review what we believe the most important: optical gain in silicon nanostructures.

2002 - Density functional calculations of the structural, electronic and optical properties of semiconductor nanostructures [Capitolo/Saggio]
Ossicini, Stefano; Degoli, Elena; Luppi, M.; Magri, Rita
abstract

Series in Micro and Nanoengineering

2002 - Gain theory and models in silicon nanostructures [Monografia/Trattato scientifico]
Ossicini, Stefano; Arcangeli, C; Bisi, O.; Degoli, Elena; Luppi, M.; Magri, Rita; DAL NEGRO, L.; Pavesi, L.
abstract

The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon.Here we will review our activity in the field of the theoretical determination of the structural, electronic and optical properties of Si nanocrystals (Si-nc). The present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc.

2002 - Si nanostructures embedded in SiO[sub 2]: electronic and optical properties [Relazione in Atti di Convegno]
Ossicini, Stefano; Degoli, Elena; Luppi, Marcello; Magri, Rita
abstract

We present ab initio results for the structural, electronic and optical properties of silicon nanostructures confined by silicon dioxide. We investigate the role of the dimension, symmetry and bonding situations at the interfaces. In particular we consider Si/SiO2 superlattices and Si nanocrystals embedded in SiO2 matrix. In the case of Si/SiO2 superlattices the presence of oxygen defects at the interface and the dimensionality are the key points in order to explain the experimental outcomes concerning photoluminescence. For Si nanocrystals embedded in SiO2 we show, in agreement with experimental results, the close interplay between chemical and structural effects on the electronic and optical properties.

2001 - Optoelectronic interconnects for integrated circuits - Achievements 1998 – 2001 - Silicon based interconnects [Monografia/Trattato scientifico]
Degoli, Elena
abstract

Silicon is the leading semiconductor in the microelectronic industry.Not only silicon is dominating the past. Its technology is predicted to improve with the increasing demand for higher complexity integrated circuits and to maintain its role for a long time to go. At the same time the enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials - in which light can be generated, guided, modulated, amplified, and detected - need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. In particular, chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions.The use of optical interconnects among devices is indeed a solution to what is known as the "interconnects bottleneck". The total length of interconnecting conductors within a chip is continuously increasing towards values of several km in 1 cm2. This produces problems related to the Joule effect, heat dissipations and slow down of the system related to resistance and capacity of the system. The replacement of electrical interconnects with optical ones would solve the problem. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable, in its bulk form, for the implementation of fundamental optical functions such as light sources and modulators. Several approaches need hence to be investigated. The European Commission focussed a research program on the interconnects problem launching several projects within the Microelectronic Advanced Research Initiative (MEL-ARI).In the case of Si optoelectronics we are faced with a material problem to circumvent the physical inability of bulk silicon to emit light. Several strategies have been considered and explored. They can be divided in three different groups: (i) low dimensional systems, (ii) emitting impurities and (iii) semiconductor silicides. For low dimensional systems the cases of porous silicon, silicon nanocrystals, Si/CaF2 and Si/SiO2 quantum wells have been explored. In all of these cases nanometer sized silicon is embedded within an insulating host. The quantum confinement has several effects: it increases the radiative probability, it decreases the non-radiative recombination routes, it increases the energy of emission. Indeed intense room temperature photoluminescence can be achieved. The main problem here is the carrier injection to achieve electroluminescence. Several routes have been followed and several devices operating at room temperature were fabricated. The challenges here are to efficiently inject carriers in a semi-insulating material and to have sufficiently low operating voltages. The most promising case of light emitting impurities is that of Er in Si. Room temperature operating devices were fabricated. A great effort was spent in comprehending the basic physical mechanisms. The main advantage here is that standard technology based on single crystal silicon can be used introducing erbium as a dopant. Particularly interesting is the light emitting transistor in which erbium, introduced in the collector-base region, is excited through electrons injected from the emitter. The third approach of semiconducting silicides is based on the observation that these silicides can be grown on Si and should have a direct bandgap. Indeed, luminescent devices have been fabricated with beta-FeSi2 precipitates formed by ion implantation in a Si diode. The quantum efficiency obtained from these devices is however not yet sufficiently high. Though none of the approaches is at a stage ready for application, this European effort can be considered extremely successful. In fact, this booklet reports a picture

2001 - Role of defects in Si/SiO2 quantum wells [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano
abstract

Abstract The optical properties of Si/SiO2 superlattices (SLs) as a function of the Si layer thickness have been, for the first time, theoretically investigated. Through ab initio calculations we consider fully passivated structures, the presence of O vacancy at the Si/SiO2 interface or in the SiO2 matrix. We find that quantum confined states and O-related defect states play a key role in the experimentally observed visible luminescence in Si/SiO2 confined systems.

2001 - Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition [Articolo su rivista]
G. V., Prakash; N., Daldosso; Degoli, Elena; F., Iacona; M., Cazzanelli; Z., Gaburro; G., Pucker; P., Dalba; F., Rocca; E. C., Moreira; G., Franzo; D., Pacifici; F., Priolo; C., Arcangeli; A. B., Filonov; Ossicini, Stefano; L., Pavesi
abstract

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degreesC) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process, Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.

2000 - First principles optical properties of low dimensional silicon structures [Relazione in Atti di Convegno]
Ossicini, Stefano; Degoli, Elena
abstract

The book provides a snapshot of the state of the art and points out directions for future research in such different subjects as photonic band gap crystals, semiconductor quantum dot and wire lasers, silicon optoelectronics, carbon-based nanostructure physics, polymer based nano-composite and quantum wires, DNA nano-technology and silicon bio-compatibility, nano-scale optical characterisation, spray and cluster deposition, self assembly, imprint technology, quantum computing and quantum dot-based computation.

2000 - From undulating Si quantum wires to Si quantum dots: a model for porous silicon [Articolo su rivista]
Degoli, Elena; Luppi, Marcello; Ossicini, Stefano
abstract

ABSTRACT Freshly etched porous silicon shows the structure of a crystalline skeleton with a connected undulating-wire morphology, in aged porous silicon samples the presence of Si dots is predominant. In this paper we present, for the first time, ab-initio results of the electronic and optical properties of undulating Si quantum wires, moreover the transition from Si quantum wires to Si quantum dots is also discussed

2000 - Symmetry and passivation dependence of the optical properties of nanocrystalline silicon structures [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano; D., Barbato; M., Luppi; E., Pettenati
abstract

The electronic and optical properties of Si-based quantum wells (QW's) are studied ab initio by means of the linear-muffin-tin-orbital (LMTO) method in order to investigate their dependence on the symmetry of the lattice and on the passivating species that saturates the Si dangling bonds. We find that the symmetry of the lattice changes the nature of the gap that is indirect in the Si-H(111) saturated QW's and becomes direct in the Si-H(001) saturated QW's. The saturating species play instead an important role in the formation of interface states that can occupy or leave free the band gap so improving or making worse the optical properties of the material. Studying the Si-SiO2(001) superlattice we found that oxygen related defects play an important role in the determination of the optoelectronic properties of the material. (C) 2000 Elsevier Science S.A. All rights reserved.

2000 - The electronic and optical properties of Si/SiO2 superlattices: Role of confined and defect states [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano
abstract

The Si layer thickness dependence of the optical properties of silicon/silicon dioxide (Si/SiO2) superlattices has been,for the ®rst time, theoretically investigated. In our ®rst principle calculation we consider both fully passivated interfacesand the presence of oxygen vacancy at the interface. Our results show the key role played both by the quantum con®nedstates and interface states in the experimentally observed visible luminescence in Si/SiO2 con®ned systems.

1999 - First principle calculations of the electronic and optical properties of 2-,1-,and 0- dimensional confined Si structures. [Capitolo/Saggio]
Ossicini, Stefano; Degoli, Elena; Barbato, D; Luppi, M; Pettenati, E.
abstract

The optoelectronic structure of silicon shows remarkable changes if the material is reduced at the nanometre scale. Many efforts have been put on the study and the fabrication of light-emitting Si-based nanostructures. Here we present theoretical results concerning the eletronic and optical properties of Si confined quantum wells, wires and dots. The comparisons between our calculations and the experimental data help to elucidate the mechanisms involved in photoluminescence in confined Si. SINGAPORE: World Scientific.

1999 - Optical properties of Si/CaF2 superlattices [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano
abstract

We present a first-principle theoretical study of the dielectric functions of Si/CaF2 superlattices. In particular, we investigate how the optical response depends on the thickness of the Si layers. Our results show that for very thin Si slabs (well width less than ~20 Å) optical excitation peaks are present in the visible range. These peaks are related to strong transitions between localized states. Moreover, the static dielectric costant is considerably reduced. From the comparison made with recent experimental data on similar systems we conclude that the quantum confinement, a good surface passivation and the presence of localized states are the key ingredients in order to have photoluminescence in confined silicon based systems.

1998 - First-principles optical properties of Si/CaF2 multiple quantum wells [Articolo su rivista]
Degoli, Elena; Ossicini, Stefano
abstract

The optical properties of Si/CaF2 multiple quantum wells are studied ab initio by means of the linear-muffin-tin-orbital method. In particular, we investigate the dependence of the optoelectronic properties on the thickness of the Si wells. We find that below a well width of similar to 20 Angstrom, new transitions appear in the optical region with an evident polarization dependence. The oscillator strength of these transitions shows a dramatic increase as the width of the Si well decreases. A comparison is made with recent experimental work on similar systems. Our results show that quantum confinement and passivation are necessary in order to have photoluminescence in confined silicon-based materials.