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STEFANO OSSICINI

Professore emerito
Dipartimento di Scienze e Metodi dell'Ingegneria

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Pubblicazioni

2023 - Interplay of Quantum Confinement and Strain Effects in Type I to Type II Transition in GeSi Core-Shell Nanocrystals [Articolo su rivista]
Marri, Ivan; Grillo, Simone; Amato, Michele; Ossicini, Stefano; Pulci, Olivia
abstract

The electronic properties of hydrogenated, spherical SiGe and GeSi core-shell nanocrystals, with a diameter ranging from 1.8 to 4.0 nm, are studied within density functional theory. Effects induced by quantum confinement and strain on the near band-edge state localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that SiGe core-shell nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) GeSi core- shell nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms, i.e., in the core region. In larger GeSi core-shell nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The factors which determine the band-offset character at the Ge/Si interface are discussed in detail.

2023 - L'aureola della gloria.Huygens, Newton e la natura della luce. [Monografia/Trattato scientifico]
Ossicini, Stefano
abstract

Testo Commedia in un atto

2021 - Electronic and Optical Properties of Si, Ge and Sige Low Dimensional Systems: Ab-Initio Results [Abstract in Rivista]
Marri, Ivan; Ossicini, Stefano
abstract

In recent years,considerable efforts have been done to study silicon, germanium, and silicon/germanium slabs, nanocrystals, and nanowires for optoelectronics and photovoltaic solar energy applications. These zero- one- and two-dimensional systems, with sizes ranging from few to some tenths of nanometers, show unique electronic, optical, and transport properties that are intrinsically associated with their low dimensionality and to the quantum confinement effect. The possibility of understanding the microscopic properties of these systems and modulating their characteristics by doping and passivation can open new perspectives in the development of new, advanced, photovoltaics and optoelectronics devices. In this talk, we will discuss ab-initio theoretical results obtained by our group in the study of electronic, optical, and transport properties of silicon, germanium, and silicon/germanium low dimensional systems. The role played by size, passivation, and doping will be discussed. Moreover, we will show how the interaction between different nanostructures is a promising route to foster the establishment of third-generation photovoltaics.

2021 - Il cristallo e la balena [Monografia/Trattato scientifico]
Ossicini, Stefano; Ossicini, Charlotte
abstract

Eunan Maxwell è un giovane archeologo del mare esperto in relitti navali. Dopo essere stato coinvolto in un progetto di ricerca, l’Università Libera di Berlino lo ha incaricato di analizzare i reperti rinvenuti negli alloggi degli ufficiali di una baleniera basca del quindicesimo secolo, affondata di fronte all’Irlanda, nella baia di Galway. Lo scozzese, spronato dall’approssimarsi della scadenza del suo contratto e costretto a fare i conti con la scarsità dei risultati finora conseguiti, decide di “prendere in prestito” alcuni di questi reperti e di coinvolgere nella sua indagine i suoi coinquilini. Quattro giovani con cui Eunan non solo condivide l’appartamento ma anche interessi, speranze, delusioni, amori e passioni, precarietà: una basca appassionata di filosofia, una dottoranda in fisica italiana, un giapponese studioso di moda e di balene, un bavarese con il talento di saper aggiustare qualunque cosa. Il giovane archeologo rivela che, in particolare, è un reperto a lasciarlo perplesso: una sorta di “mattoncino sghembo” di color bianco grigiastro, che somiglia a un insignificante blocco di sale duro, grezzo, sporco e tutto rigato. Di cosa potrebbe trattarsi? Cosa un ufficiale a bordo di una baleniera potrebbe aver custodito con tanta cura nei propri alloggi? Risolvere l’enigma, in compagnia degli amici, finirà per rappresentare per ognuno di loro un viaggio nel tempo e nello spazio, dal Medioevo a oggi, da una parte all’altra dell’Europa, attraverso scienza, filosofia, natura, esoterismo e società segrete: occasione di un percorso di crescita e presa di consapevolezza.

2021 - Multiple exciton generation in isolated and interacting silicon nanocrystals [Articolo su rivista]
Marri, I.; Ossicini, S.
abstract

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

2020 - Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase [Articolo su rivista]
Ossicini, S.; Marri, I.; Amato, M.; Palummo, M.; Canadell, E.; Rurali, R.
abstract

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results from ab initio calculations for undoped and doped Si nanocrystals and nanowires, showing how theory can aid and improve comprehension of the structural, electronic and optical properties of these systems.

2020 - Breve viaggio multilingue nel nanomondo [Monografia/Trattato scientifico]
Ossicini, Stefano
abstract

Otto storie su nanoscienza e nanotecnologi, con annesso glossario di nanoscienze e nanotecnologie in sei diverse lingue

2020 - Doping of III-V Arsenide and Phosphide Wurtzite Semiconductors [Articolo su rivista]
Giorgi, G.; Amato, M.; Ossicini, S.; Cartoixa, X.; Canadell, E.; Rurali, R.
abstract

The formation energies of n- and p-type dopants in III-V arsenide and phosphide semiconductors (GaAs, GaP, and InP) are calculated within a first-principles total energy approach. Our findings indicate that - for all the considered systems - both the solubility and the shallowness of the dopant level depend on the crystal phase of the host material (wurtzite or zincblende) and are the result of a complex equilibrium between local structural distortion and electronic charge reorganization. In particular, in the case of acceptors, we demonstrate that impurities are always more stable in the wurtzite lattice with an associated transition energy smaller with respect to the zincblende case. Roughly speaking, this means that it is easier to p-type dope a wurtzite crystal and the charge carrier concentration at a given temperature and doping dose is larger in the wurtzite as well. As for donors, we show that neutral chalcogen impurities have no clear preference for a specific crystal phase, while charged chalcogen impurities favor substitution in the zincblende structure with a transition energy that is smaller when compared to the wurtzite case (thus, charge carriers are more easily thermally excited to the conduction band in the zincblende phase).

2020 - Surface chemistry effects on work function, ionization potential and electronic affinity of Si(100), Ge(100) surfaces and SiGe heterostructures [Articolo su rivista]
Marri, Ivan; Amato, Michele; Bertocchi, Matteo; Ferretti, Andrea; Varsano, Daniele; Ossicini, Stefano
abstract

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (–I, –Br, –Cl, –F) and other chemical functionalizations (–H, –OH, –CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar –OH and –CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

2019 - L'imganno di Mesmer e la commissione Franklin-Lvoisier [Monografia/Trattato scientifico]
Ossicini, S
abstract

Nel 1784 re Luigi XVI di Francia istituisce una commissione di inchiesta per indagare sulle teorie e sull’operato di Franz Mesmer, medico e scienziato tedesco, che a Parigi sta vivendo un successo eclatante. Le sue sedute terapeutiche sono diventate un vero e proprio fenomeno di massa. Secondo Mesmer, un fluido fisico, detto magnetismo animale, riempie l’universo. Le malattie nascono dalla carenza di tale fluido nel corpo umano, ma con l’aiuto di diverse tecniche, come ad esempio l’uso di occhi, mani, sbarre, acqua e alberi “magnetizzati”, questo fluido può essere utilmente incanalato nei pazienti provocando “crisi” salutari. Fra i membri della commissione due grandi scienziati, Accademici di Francia, Antoine Lavoisier, il fondatore della chimica moderna e Benjamin Franklin, l’inventore del parafulmine, che metteranno in atto diversi sagaci esperimenti per studiare il fenomeno del mesmerismo. La commissione conclude che il fluido magnetico semplicemente non esiste e che gli effetti delle pratiche mesmeriche sono dovuti al potere della immaginazione, e nel far questo costruisce una metodologia di indagine, che risulta utile anche oggi per non rivivere simili errori.

2019 - Preferential Positioning, Stability, and Segregation of Dopants in Hexagonal Si Nanowires [Articolo su rivista]
Amato, M.; Ossicini, S.; Canadell, E.; Rurali, R.
abstract

We studied the physics of common p- and ntype dopants in hexagonal-diamond Si, a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions, by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed that (i) as observed in recent experiments, at larger diameters (beyond the quantum confinement regime) p-type dopants prefer the hexagonaldiamond phase with respect to the cubic one as a consequence of the stronger degree of three-fold coordination of the former, while n-type dopants are at afirst approximation indifferent to the polytype of the host lattice; (ii) in ultrathin nanowires, because of the lower symmetry with respect to bulk systems and the greater freedom of structural relaxation, the order is reversed and both types of dopant slightly favor substitution at cubic lattice sites; (iii) the difference in formation energies leads, particularly in thicker nanowires, to larger concentration differences in different polytypes, which can be relevant for cubic-hexagonal homojunctions; (iv) ultrasmall diameters exhibit, regardless of the crystal phase, a pronounced surface segregation tendency for p-type dopants. Overall these findings shed light on the role of crystal phase in the doping mechanism at the nanoscale and could have a great potential in view of the recent experimental works on group IV nanowires polytypes.

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.

2018 - First Principles Modeling of Si/Ge Nanostructures for Photovoltaic and Optoelectronic Applications [Articolo su rivista]
Marri, I.; Amato, M.; Guerra, R.; Ossicini, S.
abstract

We discuss results of ab initio calculations for Si, Ge, and Si/Ge nanowires and nanocrystals showing that theory can improve the comprehension of the properties of these systems. First, we consider doped and undoped freestanding hydrogenated nanowires and we explore their properties as a function of the size, geometry, and composition. Secondly, we focus the discussion on the electronic properties of matrix embedded Si, Ge, and Si/Ge nanocrystals by pointing out the role played by composition, quantum confinement, and strain. The discussed results show that, for Si/Ge nanowires, the interface between Si and Ge region plays an important role determining, in some case, the formation of a type II band offset, which is essential for photovoltaic applications. Moreover, for Si/Ge core–shell nanowires, it is shown that: i) selective doping results in the formation of hole or electron accumulation, with interesting consequences for the use of these materials in thermoelectrics and ii) through compensated doping, it is possible to tune the optical properties of these systems. For the embedded nanocrystals, the outcomes suggest that Ge nanocrystals can be suitable as optical absorption centers and that Si/Ge nanocrystals, owing to the localization of the band edge states, are interesting for photovoltaic cells.

2018 - First-principle investigations of carrier multiplication in Si nanocrystals: A short review [Relazione in Atti di Convegno]
Marri, Ivan; Ossicini, Stefano
abstract

Carrier Multiplication (CM) is a Coulomb-driven non-radiative recombination mechanism which leads to the generation of multiple electron-hole pairs after absorption of a single high-energy photon. Recently a new CM process, termed space separated quantum cutting, was introduced to explain a set of new experiments conducted in dense arrays of silicon nanocrystals. The occurrence of this effect was hypothesized to generate the formation of Auger unaffected multiexciton configurations constituted by single electron-hole pairs distributed on different interacting naocrystals. In this work we discuss ab-initio results obtained by our group in the study of CM effects in systems of strongly interacting silicon nanocrystals. By solving a set of rate equations, we simulate the time evolution of the number of electron-hole pairs generated in dense arrays of silicon nanocrystals after absorption of high energy photons, by describing the circumstances under which CM dynamics can lead to the generation of Auger unaffected multiexciton configurations.

2018 - I Pinocchi della scienza: Cattive condotte e la struttura della ricerca scientifica THE PINOCCHIOS OF SCIENCE: Misconduct and the Structure of Scientific Research [Articolo su rivista]
Ossicini, Stefano
abstract

Steven Weinberg, Nobel Prize in Physics, in his 1990’s book said “As far as I know there has never been a case of open falsification of data in physics”. The numerous scandals exploded in this first part of the millennium joined forces immediately to prove him wrong. In recent years, in fact, false discoveries, plagiarism, manipulation, exaggerated results are so increased that many observers have concluded that not only strategies must be implemented to curb the phenomenon, but that we must also question the actual evolution in the structure of scientific research.

2017 - Carrier Multiplication in Silicon Nanocrystals: Theoretical Methodologies and Role of the Passivation [Articolo su rivista]
Marri, I.; Govoni, M.; Ossicini, S.
abstract

Carrier multiplication is a non-radiative recombination mechanism that leads to the generation of two or more electron–hole pairs after absorption of a single photon. By reducing the occurrence of dissipative effects, this process can be exploited to increase solar cell performance. In this work, we introduce two different theoretical fully ab initio tools that can be adopted to study carrier multiplication in nanocrystals. The tools are described in detail and compared. Subsequently, we calculate carrier multiplication lifetimes in H- and OH-terminated silicon nanocrystals, pointed out the role played by the passivation on the carrier multiplication processes.

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 - Multiple excitation generation in silicon nanocrystals [Relazione in Atti di Convegno]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

We present density functional theory calculations of carrier multiplication processes in a system of strongly coupled silicon nanocrystals

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: ...

2017 - TEACHING SILICON NEW POSSIBILITIES [Articolo su rivista]
Ossicini, S.; Marri, I.
abstract

Although silicon is the most widely used material in microelectronics, photonics and photovoltaics, in recent years concerns about the evolu-tion of these sectors have been raised which seem related to funda-mental materials and processing aspects. Example are the limitations of the operating speed of microelectronic devices due to the intercon-nect, the lack of optical second-order nonlinearities in bulk silicon, the necessity to improve the efficiency in solar cells. Silicon based systems with new functionalities able to solve these and other prob-lems are highly desirable. In the last years, our group has developed new theoretical tools to study structural, electronic and optical proper-ties of silicon based systems, in particular silicon nanostructures. Our analyses have permitted to predict new properties of silicon, thus opening new possibilities for applications of this material in different technological fields. Here we review our main results.

2017 - Tuning the Work Function of Si(100) Surface by Halogen Absorption: A DFT Study [Articolo su rivista]
Bertocchi, M.; Amato, M.; Marri, I.; Ossicini, S.
abstract

First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well as to the charge redistribution at the interface. All these results are examined with respect to previous theoretical works and experimental data obtained for the (100) as well as other Si surface orientations. Based on this analysis, we argue that the changes in the electronic properties caused by variations of the interfacial chemistry strongly depend on the chemisorbed species and much less on the surface crystal orientation.

2016 - First-principles calculations of electronic coupling effects in silicon nanocrystals: Influence on near band-edge states and on carrier multiplication processes [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

Arrays of closely packed nanocrystals show interesting properties that can be exploited to induce new features in nanostructured optoelectronic devices. In this work we study, by first principles calculations, effects induced on near band-edge states and on carrier multiplication by nanocrystals interplay. By considering both hydrogenated and oxygenated structures, we prove that interaction between silicon nanocrystals can alter both the energy gap of the system and dynamics of excited states with a relevance that depends on the nanocrystal-nanocrystal separation, on nanocrystals orientation and on nanocrystals surface properties.

2016 - Silicon Nanocrystals for Photonics and Photovoltaics: Ab-initio Results [Capitolo/Saggio]
Ossicini, Stefano; Govoni, Marco; Guerra, Roberto; Marri, Ivan
abstract

Review Article on electronic and optical properties of silcon nanocrystals for photonic and photovoltaic applications

2016 - Work function bowing in Si1−xGex heterostructures: Ab initio results [Articolo su rivista]
Amato, Michele; Bertocchi, Matteo; Ossicini, Stefano
abstract

Abstract A systematic theoretical study of the work function behavior for Si1−xGexheterostructures over the whole composition range, from Si (x = 0) to Ge (x = 1), is presented. Our results, obtained through Density Functional Theory calculations and in good agreement with experimental evidences, show that increasing the Ge content lowers the work function value. We find that in order to exactly reproduce this behaviour in relation to the work function of pure Ge and Si systems and their concentrations, a deviation from the linear Vegard's rule is necessary. However, the calculated bowing parameter is very small, thus making the simple linear interpolation a valid approximation to obtain the work function of complex SiGe alloys.

2015 - Ab initio calculations of the electronic and optical properties of group IV semiconductor nanostructures embedded in different matrices [Capitolo/Saggio]
Guerra, Roberto; Ossicini, Stefano
abstract

n/a

2015 - Ab initio calculations of the electronic and optical properties of silicon quantum dots embedded in different matrices [Capitolo/Saggio]
Guerra, R.; Ossicini, S.
abstract

2015 - Ab initio energy loss spectra of Si and Ge nanowires [Articolo su rivista]
Palummo, Maurizia; Hogan, Conor; Ossicini, Stefano
abstract

We report an ab initio investigation of fast electron energy-loss probability in silicon and germanium nanowires. Computed energy loss spectra are characterized by a strong enhancement of the direct interband transition peak at low energy, in good agreement with experimental data. Our calculations predict an important diameter dependence of the bulk volume plasmon peak for very thin wires which is consistent with the blue shift observed experimentally in thicker wires.

2015 - Carrier Multiplication in Isolated and Interacting Silicon Nanocrystals [Capitolo/Saggio]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

n/a

2015 - Carrier multiplication in silicon nanocrystals: ab initio results [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

One of the most important goals in the field of renewable energy is the development of original solar cell schemes employing new materials to overcome the performance limitations of traditional solar cell devices. Among such innovative materials, nanostructures have emerged as an important class of materials that can be used to realize efficient photovoltaic devices. When these systems are implemented into solar cells, new effects can be exploited to maximize the harvest of solar radiation and to minimize the loss factors. In this context, carrier multiplication seems one promising way to minimize the effects induced by thermalization loss processes thereby significantly increasing the solar cell power conversion. In this work we analyze and quantify different types of carrier multiplication decay dynamics by analyzing systems of isolated and coupled silicon nanocrystals. The effects on carrier multiplication dynamics by energy and charge transfer processes are also discussed.

2015 - Energetics and carrier transport in doped Si/SiO2 quantum dots [Articolo su rivista]
Garcia Castello, Nuria; Illera, Sergio; Prades, Joan Daniel; Ossicini, Stefano; Cirera, Albert; Guerra, Roberto
abstract

In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5 nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime. At higher voltages, our findings indicate a significant increase of transport only for P-doped Si-QDs, while the electrical response of B-doped ones does not stray from the undoped case. These findings are of support for the employment of doped Si-QDs in a wide range of applications, such as Si-based photonics or photovoltaic solar cells.

2015 - Marie Curie, Hertha Ayrton e le altre. Donne e Scienziate [Articolo su rivista]
Ossicini, Stefano
abstract

Il ruolo di Marie Curie e Hertha Ayrton come pioniere della scienza.

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.

2015 - Work Function Measurement of Silicon Germanium Heterostructures Combining Kelvin Force Microscopy and X-ray Photoelectron Emission Microscopy [Articolo su rivista]
Pouch, Sylvain; Amato, Michele; Bertocchi, Matteo; Ossicini, Stefano; Chevalier, Nicolas; Melin, Thierry; Hartmann, Jean Michel; Renault, Olivier; Delaye, Vincent; Mariolle, Denis; Borowik, Lukasz
abstract

Work function in Si1-xGex heterostructures with Ge content in the 6% to 49% range was studied with high energy resolution by combining Kelvin force microscopy and X-ray photoelectron emission microscopy. Although the two methods are based on distinct physical mechanisms, we show that both techniques give the same work function differences between each Si1-xGex layer, as small as 20 meV. To detect such small work function differences, we put in evidence the necessity of preparing the Si1-xGex sample surface with polishing, HF etching and Ar+ sputtering. Such surface preparation allows, in principle, to reduce the deleterious influence of surface states, coming for instance from carbon atoms or native oxide, on quantitative work function extraction. We show in this paper that even after such a sample surface preparation, a strong band bending can be present, which causes a contrast inversion on the surface of the material and yields an artificially lower surface work function with respect to theoretical values. By using density functional theory simulations, we demonstrate that such inversion is likely due to residual carbon present on the surface.

2014 - A chi la vera Gloria? Le verità della Scienza sulla Luce [Esposizione]
Brunetti, Rossella; Bisi, Olmes; Ossicini, Stefano
abstract

Il progetto ha presentato esperimenti illustranti le teorie corpuscolare e ondulatoria della luce commentati e drammatizzati per un pubblico di non esperti.

2014 - Conductance fluctuations in Si nanowires studied from first-principles [Articolo su rivista]
Iori, Federico; Ossicini, Stefano; Riccardo, Rurali
abstract

We study how the variability of the conductance associated with single-dopant configurations affects the overall conductivity of long, realistic ultrathin Si nanowires (NW). We calculate the resistance associated with each single-dopant configuration from density-functional theory (DFT) calculations and we sum them up classically to obtain the resistance of the long wire. This allows to identify limiting factors for the performance of Si NWs based devices.

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.

2014 - Preferential Positionings of Dopants and Co-dopants in Embedded and Freestanding Silicon Nanocrystals [Articolo su rivista]
Guerra, Roberto; Ossicini, Stefano
abstract

In this work we aim at understanding the effect of n- and p-type substitutional doping in the case of matrix embedded and freestanding Si nanocrystals. By means of ab initio calculations we identify the preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case. Subsequently, we consider the case of phosphorus and boron co-doped nanocrystals showing that, against the single-doping situation, the energetics strongly favors the binding of the impurities at the nanocrystal surface. Indeed we demonstrate that the polar B−P bond forms a stable permanent electric dipole that radially points inward in the nanocrystal. Such a noteworthy characteristic and its physical consequences are discussed alongside new potential applications.

2014 - Red-shifted carrier multiplication energy threshold and exciton recycling mechanisms in strongly interacting silicon nanocrystals. [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

We present density functional theory calculations of carrier multiplication properties in a system of strongly coupled silicon nanocrystals. Our results suggest that nanocrystal-nanocrystal interaction can lead to a reduction of the carrier multiplication energy threshold without altering the carrier multiplication efficiency at high energies, in agreement with experiments. The time evolution of the number of electron-hole pairs generated in a system of strongly interacting nanocrystals upon absorption of high-energy photons is analyzed by solving a system of coupled rate equations, where exciton recycling mechanisms are implemented. We reconsider the role played by Auger recombination which is here accounted also as an active, nondetrimental process.

2014 - SiGe nanowires for thermoelectric applications [Capitolo/Saggio]
M., Amato; M., Palummo; R., Rurali; Ossicini, Stefano
abstract

The possibility to reduce the thermal conductivity leaving essentially unaltered the electron transport makes semiconducting nanowires ideal materials for the engineering of high-efficiency thermoelectric devices. A simple and appealing route to achieve these goals is bringing together Si and Ge, giving rise to Si1 x Gex alloy nanowires with tunable Ge concentration, core–shell structures and multiple axial junctions, i.e. superlattices. In this chapter we review the most recent pro- gresses in this field.

2014 - Silicon nanocrystals in carbide matrix [Articolo su rivista]
C., Summonte; M., Allegrezza; M., Bellettato; F., Liscio; M., Canino; A., Desalvo; J., Lopez Vidrier; S., Hernandez; L., Lopez Conesa; S., Estrade; F., Peiro; B., Garrido; P., Loeper; M., Schnabel; S., Janz; Guerra, Roberto; Ossicini, Stefano
abstract

Ordered silicon nanocrystals in silicon carbide are produced by Plasma Enhanced Chemical Vapor Deposition by means of the multilayer approach followed by annealing at 1100 C. The crystallization is verified by Raman scattering, X-ray diffraction, Transmission Electron Microscopy, and UV–vis spectroscopy. The conditions for the periodic structure to survive the high temperature annealing and for the SiC barrier to confine the Si crystal growth are examined by energy-filtered transmission electron microscopy and X-ray reflection. The final layout appears to be strongly influenced by the structural features of the as-deposited multilayer. Threshold values of Si-rich carbide sublayer thickness and Si-to-C ratio are identified in order to preserve the ordered structure. The crystallized fraction is observed to be correlated with the total silicon volume fraction. The constraints are examined through the use of ab-initio calculations of matrix-embedded silicon nanocrystals, as well as in terms of existing models for nanocrystal formation, in order to establish the role played by the interface energy on nanocrystal outgrowth, residual amorphous fraction, and continuous crystallization. A parameter space of formation of ordered Si nanocrystals is proposed. The diffusivity of carbon in the crystallized material is evaluated, and estimated to be around 10–16 cm2/s at 1100 C.

2014 - Silicon−Germanium Nanowires: Chemistry and Physics in Play, from Basic Principles to Advanced Applications [Articolo su rivista]
M., Amato; M., Palummo; R., Rurali; Ossicini, Stefano
abstract

CONTENTS 1. Introduction 1371 2. Growth Techniques, Morphology, and Structural Properties 1373 2.1. Alloyed Nanowires 1373 2.2. Axial Heterostructures 1375 2.3. Radial Heterostructures 1377 3. Chemical and Physical Properties 1379 3.1. Electronic Properties 1379 3.1.1. Modulation of the Electronic Properties by Composition Control 1379 3.1.2. Interfaces at Work: Strain, Band-Offset, and Carrier Gases 1381 3.1.3. Doped Nanowires 1384 3.2. Thermal and Thermoelectric Properties 1385 4. Theoretical Modeling 1389 4.1. Electronic Structure 1390 4.1.1. Quantum Confinement Effect and Band Offset 1391 4.1.2. Size Effects 1393 4.1.3. Alloying and Interface Effects 1394 4.1.4. Strain Effects 1395 4.1.5. Addition of Impurities 1395 4.1.6. Electronic Transport 1396 4.1.7. Optical Properties 1397 4.2. Phonons and Thermal Conductivity 1398 4.2.1. Breakdown of Fourier’s Law at Nanoscale 1398 4.2.2. Numerical Simulations of Thermal Properties 1398 5. Devices and Applications 1402 5.1. High-Performance Nanoelectronic Components 1403 5.1.1. Si1−xGex Alloy Nanowire Transistor 1403 5.1.2. Si-Shell Ge-Core Nanowire Transistor 1404 5.2. From Quantum Transport to Superconductivity: SiGe Nanowires As Platforms for Fundamental Physics Studies 1405 6. Conclusions and Perspectives 1405 Author Information 1406 Corresponding Authors 1406 Notes 1406 Biographies 1407 Acknowledgments 1408 References 1408

2014 - Structural and electronic properties of Si1-xGex alloy nanowires [Articolo su rivista]
Ossicini, Stefano; Iori, Federico; Rurali, R.
abstract

We present first-principles density-functional calculations of Si1-xGex alloy nanowires. We show that given the composition of the alloy, the structural properties of the nanowires can be predicted with great accuracy by means of Vegard’s law, linearly interpolating the values of a pure Si and a pure Ge nanowire of the same diameter. The same holds, to some extent, also for electronic properties such as the band-gap. We also assess to what extend the band-gap varies as a function of disorder, i.e., how it changes for different random realization of a given concentration. These results make possible to tailor the desired properties of SiGe alloy nanowires starting directly from the data relative to the pristine wires.

2014 - Understanding doping at the nanoscale: the case of codoped Si and Ge nanowires [Articolo su rivista]
Michele, Amato; Riccardo, Rurali; Maurizia, Palummo; Ossicini, Stefano
abstract

Results of first-principles DFT calculations of the structural and electronic properties of B-P codoped Si and Ge NWs are presented and discussed. We find that, according to experiments, for both Si and Ge NWs, impurities tend to get closer together and to occupy edge positions, as a result of minor structural relaxation and hence lower formation energy. The study of the electronic structure shows that the simultaneous addition of B and P only slightly modifies the energy band gap value with respect to the pure wire, and is strongly dependent on the particular codoping configuration considered.

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.

2013 - Non ho nulla da rimproverarmi. [Monografia/Trattato scientifico]
Ossicini, Stefano
abstract

“In lei c’era un femminismo intransigente, una rivolta contro lo stato sociale presente”. Una frase che non ci viene da accostare a Marie Curie, unico scienziato premiato con due Nobel in diverse discipline, fisica e chimica. Marie considerata una sorta di santa della scienza, vestale della radioattività, genio ossessivo, guaritrice dell’umanità, solo per stare ai titoli di libri a lei dedicati. Eppure quella frase è di Irène Curie, la figlia maggiore di Marie, e si riferisce proprio a sua madre. E allora? Allora vale la pena ripartire da quella frase per guardare a Marie non solo come a una grande scienziata, ma come a una donna capace di mettere in piedi una rete di affetti e amicizie importanti e decisive per la vita e la carriera, una donna pronta a utilizzare l’autorità duramente conquistata per rendere possibile ad altre donne la conquista dei propri sogni. Simile in questo alla sua amica, la scienziata inglese Hertha Ayrton, quell’amica che sarà fondamentale nel farle superare il suo periodo più oscuro.

2013 - Optical absorption and emission of silicon nanocrystals: From single to collective response [Articolo su rivista]
Guerra, Roberto; Francesco, Cigarini; Ossicini, Stefano
abstract

We report on the possibility of describing the absorption and emission characteristics of an ensemble of silicon nanocrystals (NCs) with realistic distributions in the NC size, by the sum of the responses of the single NCs. The individual NC responses are evaluated by means of ab initio theoretical calculations and the summation is performed by taking into account the trend of the optical properties as a function of NC size and oxidation degree. The comparison with experimental results shows a nice matching of the spectra, also without any tuning of the parameters. Finally, the possibility of adapting the model in order to reproduce the experimental data is explored and discussed.

2013 - Role of strain in interacting silicon nanoclusters [Articolo su rivista]
Guerra, Roberto; Ossicini, Stefano
abstract

The possibility of controlling the optical transition probability between neighboring silicon nanoclusters (Si-NCs) constitutes nowadays an attractive prospect in nanophotonics and photovoltaics. In this work, by means of theoretical ab initio calculations, we investigate the effect of strain on the optoelectronic properties of Si-NCs pairs. We consider two sources of strain: the strain induced by an embedding SiO2 matrix and the strain generated by mutual NC-NC forces occurring at small distances. Independently on its source, we observe a fundamental impact of the strain on the orbital localization and, as a consequence, on the transition probability between energy states belonging or not to the same NC. The resulting picture allots to the structural strain a fundamental role in the NC-NC interaction mechanisms, suggesting the possibility of enabling a strain-controlled response in Si-NC ensembles.

2013 - Silicon Nanostructures Embedded in SiO2 Matrices: Ab-Initio Results [Capitolo/Saggio]
Ossicini, Stefano; Guerra, Roberto
abstract

Silicon has become the most studied material in the past decades owing to its unique characteristics: Si is the second most abundant element (after oxygen) in the Earth’s crust, making up 25.7% of its mass; it can be produced with impurity levels of less than 10−9; it remains a semiconductor at higher temperatures than germanium; its native oxide is easily grown in a furnace and forms a better semiconductor/insulator interface than any other material. These properties have made Si the widest used material for electronic devices, such as photovoltaic (PV) cells, light emitters, lasers, environmental probes, and so on. Nevertheless, the use of Si in photonic applications remains highly limited because the indirect gap of the Si band structure—radiative interband transitions from the conduction-band minimum (Δ-point) to the top of the valence band (Γ-point)—requires electron–phonon coupling in order to satisfy the momentum conservation rule.

2013 - Silicon nanocrystals embedded in SiO2matrices: Ab initio results [Capitolo/Saggio]
Guerra, R.; Ossicini, S.
abstract

2013 - Silicon nanocrystals from high-temperature annealing: Characterization on device level [Articolo su rivista]
Philipp, Löper; Mariaconcetta, Canino; Julian López, Vidrier; Manuel, Schnabel; Florian, Schindler; Friedemann, Heinz; Anke, Witzky; Michele, Bellettato; Marco, Allegrezza; Daniel, Hiller; Andreas, Hartel; Sebastian, Gutsch; Sergi, Hernández; Guerra, Roberto; Ossicini, Stefano; Blas, Garrido; Stefan, Janz; Margit, Zacharias
abstract

Silicon nanocrystals (Si NCs) embedded in Si-based dielectrics provide a Si-based high band gap material (1.7 eV) and enable the construction of all-crystalline Si tandem solar cells. However, Si nanocrystal formation involves high-temperature annealing which deteriorates the properties of any previously established selective contacts. The inter-diffusion of dopants during high-temperature annealing alters Si NC formation and limits the built-in voltage. Furthermore, most devices presented so far also involve electrically active bulk Si and therefore do not allow a clear separation of the observed photovoltaic effect of the nanocrystal layer from that of the bulk Si substrate. A membrane route is presented for nanocrystal based p–i–n solar cells to overcome these limitations. In this approach, the formation of both selective contacts is carried out after hightemperature annealing and therefore not affected by the latter. p–i–n Solar cells are investigated with Si NCs embedded in silicon carbide in the intrinsic region. Device failure due to damaged insulation layers is analyzed by electron- and light beam induced current measurements. Open-circuit voltages of 176mV are shown for the NC layer. An optical model of the device is presented for improving the cell current. Comparison of the optical limit and the measured short circuit current demonstrates that the device is governed by recombination within the absorber layer. Vertical p–i–n solar cell with a Si nanocrystal thin film in the intrinsic region and selective electron and hole contacts by doped amorphous silicon carbide (a-SixC1x:H).

2013 - Silicon quantum dots embedded in a SiO2 matrix: From structural study to carrier transport properties [Articolo su rivista]
N., Garcia Castello; S., Illera; Guerra, Roberto; J. D., Prades; Ossicini, Stefano; A., Cirera
abstract

We study the details of electronic transport related to the atomistic structure of silicon quantum dots embedded in a silicon dioxide matrix using ab initio calculations of the density of states. Several structural and composition features of quantum dots (QDs), such as diameter and amorphization level, are studied and correlated with transport under transfer Hamiltonian formalism. The current is strongly dependent on the QD density of states and on the conduction gap, both dependent on the dot diameter. In particular, as size increases, the available states inside the QD increase, while the QD band gap decreases due to relaxation of quantum confinement. Both effects contribute to increasing the current with the dot size. Besides, valence band offset between the band edges of the QD and the silica, and conduction band offset in a minor grade, increases with the QD diameter up to the theoretical value corresponding to planar heterostructures, thus decreasing the tunneling transmission probability and hence the total current.We discuss the influence of these parameters on electron and hole transport, evidencing a correlation between the electron (hole) barrier value and the electron (hole) current, and obtaining a general enhancement of the electron (hole) transport for larger (smaller) QD. Finally, we show that crystalline and amorphous structures exhibit enhanced probability of hole and electron current, respectively.

2013 - Silicon-Based Light Sources [Capitolo/Saggio]
Aleksei, Anopchenko; Alexei, Prokofiev; Irina N., Yassievich; Ossicini, Stefano; Leonid, Tsybeskov; David J., Lockwood; Saba, Saeed; Tom, Gregorkiewicz; Maciek, Wojdak; Jifeng, Liu; Al, Meldrum
abstract

The search for active silicon-based light sources is an active research area.

2012 - Band structure analysis in SiGe nanowires [Articolo su rivista]
M., Amato; M., Palummo; Ossicini, Stefano
abstract

One of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modificationof the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological applications.

2012 - Band-offset driven efficiency of the doping of SiGe core-shell. nanowires [Relazione in Atti di Convegno]
R., Ruralli; Amato, Michele; Ossicini, Stefano
abstract

Impurity doping of semiconducting nanowires is expected to become increasingly inefficient as the wire diameter shrinks down, because impurity states get deeper due to quantum and dielectric confinement. Here we show that efficient n- and p-type doping can be achieved in strongly confined SiGe core-shell nanowires, taking advantage of the type-II band offset at the Si/Ge interface. A one-dimensional electron (hole) gas is created at the band-edge and the carrier density is uniquely controlled by the impurity concentration with no need of thermal activation. Additionally, SiGe core-shell nanowires provide naturally the separation between the different types of carriers, electron and holes, and are ideally suited for photovoltaic applications.

2012 - Carrier multiplication between interacting nanocrystals for fostering silicon-based photovoltaics [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

The conversion of solar radiation into electric current with high efficiency is one of the most important topics of modern scientific research, as it holds great potential as a source of clean and renewable energy. Exploitation of interaction between nanocrystals seems to be a promising route to the establishment of third-generation photovoltaics. Here, we adopt a fully ab initio scheme to estimate the role of nanoparticle interplay in the carrier multiplication dynamics of interacting silicon nanocrystals. Energy and charge transfer-based carrier multiplication events are studied as a function of nanocrystal separation, demonstrating the benefits induced by the wavefunction sharing regime. We prove the relevance of these recombinative mechanisms for photovoltaic applications in the case of silicon nanocrystals arranged in dense arrays, quantifying at an atomic scale which conditions maximize the outcome.

2012 - Doping of SiGe core-shell nanowires [Articolo su rivista]
M., Amato; R., Rurali; Ossicini, Stefano
abstract

Dopant deactivation in pure Si and pure Ge nanowires (NWs) can compromise the efficiency of the doping process at nanoscale. Quantum confinement, surface segregation and dielectric mismatch, in different ways, strongly reduce the carrier generation induced by intentional addition of dopants. This issue seems to be critical for the fabrication of high-quality electrical devices for various future applications, such as photovoltaics and nanoelectronics. By means of Density Functional Theory simulations, we show how this limit can be rode out in core-shell silicon/germanium NWs (SiGe NWs), playing on the particular energy band alignment that comes out at the Si/Ge interface. We demonstrate how, by choosing the appropriate doping configurations, it is possible to obtain a 1-D electron or hole gas, which has not to be thermally activated and which can furnish carriers also at very low temperatures. Our findings suggest core-shell NWs as possible building blocks for highspeed electronic device and new generation solar cells.

2012 - Electron Transport in SiGe Alloy nanowires in the Ballistic Regime from First Principles [Articolo su rivista]
Amato, Michele; Ossicini, Stefano; R., Rurali
abstract

Silicon−germanium alloying is emerging as one of the most promising strategies to engineer heat transport at the nanoscale. Here, we perform first-principles electron transport calculations to assess at what extent such approach can be followed without worsening the electrical conduction properties of the system, providing then a path toward high-efficiency thermoelectric materials.

2012 - L'universo è fatto di storie non solo di atomi [Monografia/Trattato scientifico]
Ossicini, Stefano
abstract

Steven Weinberg, premio Nobel per la fisica, recentemente affermava: «Per quanto ne sappia non vi è mai stato una caso di aperta falsificazione dei dati in fisica». Gli scandali esplosi in questo primo scorcio di secolo si sono immediatamente incaricati di smentirlo. Negli ultimi anni, infatti, false scoperte, errori, risultati esagerati sono così aumentati che molti osservatori hanno concluso che non solo i truffatori debbano essere scovati e puniti, ma che la stessa struttura della ricerca scientifica sia da porre sotto indagine. Questo libro narra un secolo di storie controverse, frodi, errori, illusioni prese dal fronte della scienza: dai raggi N alle nanotecnologie, dalla natura della luce ai nuovi elementi chimici, dalle forme dell'acqua alle energie inesauribili.Assistiamo in questi anni all'emergere di una nuova tipologia di fare scienza, una sorta di arrangiamento ibrido tra università, centri di ricerca, ruolo del pubblico, governi nazionali e sovranazionali, finanziatori e imprese private, dove il numero di ricercatori, esperimenti, analisi e pubblicazioni diventa sempre più grande mentre i normali meccanismi di controllo stentano a tenere il passo. La competizione diventa più spietata, la ricerca più frammentata, sponsorizzata, sempre meno guidata dalla curiosità e dalla ricerca della verità. I conflitti di interesse, finanziari ed etici, aumentano e i ricercatori a progetto, tipica forma postmoderna, che non possono avere una conoscenza d'insieme dello studio intrapreso, dei suoi scopi e metodi , non solo sono chiamati a svolgere spesso lavori ripetitivi ma in più sono costretti a produrre comunque risultati per sperare in una prosecuzione del progetto, per aspirare al mantenimento del posto. Ed ecco che nascono, inevitabili, tentazioni devianti.Certo è che i meccanismi di autocorrezione continuano ad essere un tratto distintivo della scienza, ma tutti dobbiamo conoscere come la scienza realmente opera per poter più correttamente giudicare.Una volta Peter Medawar, premio Nobel per la medicina, descrisse così i ricercatori: «Gli scienziati sono persone di temperamento molto dissimile, che fanno cose differenti, in modi fortemente diversi. Tra gli scienziati ci sono collezionisti, classificatori e compulsivi inseguitori delle nuove mode. Molti sono detective e molti sono esploratori. Ci sono scienziati-poeti, scienziati-filosofi e persino qualche mistico». Qualche tempo dopo scrisse, dispiaciuto, di aver dimenticato di aggiungere «... e anche qualche imbroglione».

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 - Optical absorption modulation by selective codoping of SiGe core-shell nanowires [Articolo su rivista]
M., Amato; M., Palummo; R., Rurali; Ossicini, Stefano
abstract

First-principles calculations on the structural, electronic, and optical properties of B-P codoped SiGe core-shell nanowires are discussed. We show that the simultaneous addition of B and P impurities into the wire can be energetically favored with respect to the single-doping. We demonstrate that impurities energetic levels in the band gap are dependent by the Si/Ge band offset, as well as by their location in the wire (i.e., core or shell region). This electronic tunability results in a significant optical modulation, as demonstrated by the red-shift of the first optical peak when B and P locations are switched in the wire.

2012 - Photonic Properties of Silicon-Based Materials [Articolo su rivista]
L., Khriachtchev; Ossicini, Stefano; F., Iacona; F., Gourbilleau
abstract

Photonics is a key technology of this century. The fast development of optical communications is a consequence of the advantages of photonics with respect to electrical signaling. As a bright demonstration, Charles K. Kao was awarded by the 2009 Nobel Prize in physics for groundbreaking achievements concerning the transmission of light in fibers for optical communication. The combination of photonics and Si technology is a great challenge because of the potentiality of coupling electronics and optical functions on a single chip, which is a dream for optical computing. Si-based light emitting device with high efficiency is a missing part in the design of complete optoelectronic circuits based on Si technology.

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

2012 - Silicon Nanoscale Materials: From Theoretical Simulations to Photonic Applications [Articolo su rivista]
L., Khriachtchev; Ossicini, Stefano; F., Iacona; F., Gourbilleau
abstract

The combination of photonics and silicon technology is a great challenge because of the potentiality of coupling electronics and optical functions on a single chip. Silicon nanocrystals are promising in various areas of photonics especially for light-emitting functionality and for photovoltaic cells. This review describes the recent achievements and remaining challenges of Si photonics with emphasis on the perspectives of Si nanoscale materials. Many of the results and properties can be simulated and understood based on theoretical studies. However, some of the key questions like the light-emitting mechanism are subjects of intense debates despite a remarkable progress in the recent years. Even more complex and important is to move the known experimental observations towards practical applications. The demonstrated devices and approaches are often too complex and/or have too low efficiency. However, the challenge to combine optical and electrical functions on a chip is very strong, and we expect more research activity in the field of Si nanophotonics in the future

2012 - Silicon quantum dots in photovoltaic devices: device fabrication, characterization and comparison of materials [Relazione in Atti di Convegno]
Loper, P.; Canino, M.; Lopez Vidrier, J.; Schnabel, M.; Witzky, A.; Belletato, M.; Allegrezza, M.; Hiller, D.; Hartel, A.; Gutsch, S.; Hernandez, S.; Guerra, Roberto; Ossicini, Stefano; Garrido, B.; Janz, S.; Zacharias, M.
abstract

The realization of crystalline silicon tandem solar cells relies on silicon nanocrystal (Si NC) quantum dots as the absorber of the top solar cell. Quantum confinement of charge carriers within the nanocrystals permits to achieve a band gap up to 2 eV which can be adjusted according to the size of the nanocrystals. This enables the construction of all-crystalline Si tandem solar cells, and increases the theoretical efficiency limit from 33 % to 42.5 % due to the addition of a second band gap. In this work, the electrical and photovoltaic properties of Si NC films are assessed and the most prominent material systems (SiO2, Si3N4, SiC) are compared. P-i-n solar cells are presented which feature Si NC as the intrinsic absorber layer and permit to characterize quantum confinement electrically on device level. P-i-n solar cells with Si NC in SiC in the i-region have yielded open circuit voltages of up to 370mV.

2012 - The influence of silicon nanoclusters on the optical properties of a-SiNx samples: A theoretical study [Articolo su rivista]
Guerra, Roberto; M., Ippolito; S., Meloni; Ossicini, Stefano
abstract

By means of ab-initio calculations, we investigate the optical properties of pure a-SiNx samples, with x [0.4;1.8], and samples embedding silicon nanoclusters (NCs) of diameter0:5=<d<=1:0 nm. In the pure samples, the optical absorption gap and the radiative recombination rate vary according to the concentration of Si-N bonds. In the presence of NCs, the radiative rate of the samples is barely affected, indicating that the intense photoluminescence of experimental samples is mostly due to the matrix itself rather than to the NCs. Besides, we evidence animportant role of Si-N-Si bonds at the NC/matrix interface in the observed photoluminescence trend.

2012 - What is What in the NanoworldA Handbook on Nanoscience and Nanotechnology [Monografia/Trattato scientifico]
V. E., Borisenko; Ossicini, Stefano
abstract

This introductory reference with more than 2,200 entries summarizes the terms and definitions, most important phenomena, and regulations found in the physics, chemistry, technology, materials science and application of nanostructures.

2011 - Auger recombination in Si and GaAs semiconductors : Ab initio results [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

A detailed description, at the atomistic scale, of the dynamics of excess electrons and holes is fundamentalin order to improve the performance of many optoelectronic devices. Among all recombination processes,nonradiative decay paths play a fundamental role in most semiconductor devices, such as optoelectronic devicesand solar cells, limiting their efficiency. In this work, a precise ab initio analysis of the direct Auger recombinationprocesses in both n- and p-type Si and GaAs crystals is presented. Our simulations of minority carrier Augerlifetimes rely on an accurate electronic band structure, calculated using density functional theory with theinclusion of quasiparticle corrections. The results obtained are in good agreement with experimental data forboth n-Si and p-GaAs, proving the importance of the direct Auger recombination mechanism in such systems. Onthe contrary, we show that different nonradiative recombination paths are necessary to explain the experimentalresults for both p-Si and n-GaAs.

2011 - Band-offset driven efficiency of the doping of SiGe core-shell nanowires [Articolo su rivista]
Amato, Michele; Ossicini, Stefano; R., Rurali
abstract

Impurity doping of semiconducting nanowireshas been predicted to become increasingly inefficient as thewire diameter is reduced, because impurity states get deeperdue to quantum and dielectric confinement. We show thatefficient n- and p-type doping can be achieved in SiGe coreshellnanowires as thin as 2 nm, taking advantage of the bandoffset at the Si/Ge interface. A one-dimensional electron(hole) gas is created at the band-edge and the carrier densityis uniquely controlled by the impurity concentration with noneed of thermal activation. Additionally, SiGe core-shellnanowires provide naturally the separation between thedifferent types of carriers, electron and holes, and are ideallysuited for photovoltaic applications.

2011 - Local-fields effects in silicon nanoclusters [Articolo su rivista]
Guerra, Roberto; M., Marsili; O., Pulci; Ossicini, Stefano
abstract

The effect of the local fields on the absorption spectra of silicon nanoclusters (NCs), freestanding or embeddedin SiO2, is investigated in the DFT-RPA framework for different size and amorphization of the samples.We showthat local field effects have a great influence on the optical absorption of the NCs. Their effect can be described bytwo separate contributions, both arising from polarization effects at the NC interface. First, local fields producea reduction of the absorption that is stronger in the low energy limit. This contribution is a direct consequence ofthe screening induced by polarization effects on the incoming field. Secondly, local fields cause a blue shift onthe main absorption peak that has been explained in terms of perturbation of the absorption resonance conditions.Both contributions do not depend either on the NC diameter nor on its amorphization degree, while showing ahigh sensitivity to the environment enclosing the NCs.

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.

2011 - Selective doping of SiGe core-shell nanowires: a possible route to photovoltaics [Abstract in Atti di Convegno]
Ossicini, Stefano
abstract

Ab-initio results

2011 - The NASCEnT project [Relazione in Atti di Convegno]
Janz, S.; Loper, P.; Schnabel, M.; Zacharias, M.; Hiller, D.; Gutsch, S.; Hartel, A. M.; Summonte, C.; Xanino M., Allegrezza; Ossicini, Stefano; Guerra, Roberto; Marri, Ivan; Garrido, B.; Hernandez, S.; Lopez Vidrier, J.; Valenta, J.; Kubera, T.; Foti, M.; Gerardi, C.
abstract

NAscent Project

2010 - Ab initio optoelectronic properties of SiGe nanowires: Role of many-body effects [Articolo su rivista]
M., Palummo; Amato, Michele; Ossicini, Stefano
abstract

The self-energy and electron-hole interaction corrections to the one-particle approximation for SiGe nanowireshave been calculated for different geometries and diameters. We show that, at fixed nanowire diameterand orientation, the self-energy corrections for the SiGe nanowires can be obtained as a weighted average, onthe relative composition of one type of atom with respect to the total numbers of atoms in the unit cell, of thecorrections for the pure Si and Ge nanowires, thus circumventing cumbersome computations and allowing adirect and practical determination of the electronic band gap. Moreover we show that particular geometricalconfigurations are at the origin of an enhancement of the optical oscillator strength that should be important foroptoelectronic applications.

2010 - Electronic and Optical Properties of Silicon Nanocrystals [Capitolo/Saggio]
C., Bulutay; Ossicini, Stefano
abstract

Several strategies have been researched over the last few years for light generation and amplification in silicon.

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 - Giant excitonic exchange splitting in Si nanowires: First-principle calculations [Articolo su rivista]
M., Palummo; F., Iori; R., Del Sole; Ossicini, Stefano
abstract

The size and doping dependence of the electron-hole exchange interaction in Si nanowires is investigated from first principles. In pure Si nanowires we found excitonic exchange splittings in very good agreement with the experimental results for porous silicon. For n-doped Si nanowires a giant singlet-triplet splitting, three order of magnitude bigger than in bulk silicon, is predicted as due to the dramatic enhancement of the electron and the hole probability of being in the same place at the same time.

2010 - High Luminescence in small Si/SiO2 Nanocrystals: a theoretical study [Articolo su rivista]
Guerra, Roberto; Ossicini, Stefano
abstract

In recent years, many experiments have demonstrated the possibility to achieve efficient photoluminescencefrom SiSiO2 nanocrystals. While it is widely known that only a minor portion of the nanocrystals in thesamples contributes to the observed photoluminescence, the high complexity of the SiSiO2 interface and thedramatic sensitivity to the fabrication conditions make the identification of the most active structures at theexperimental level not a trivial task. Focusing on this aspect, we have addressed the problem theoretically, bycalculating the radiative recombination rates for different classes of Si nanocrystals in the diameter range of0.2–1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination ratesof hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is theprincipal quantity in determining the system response. Interestingly, a completely different behavior emergesfrom the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seemsintimately connected to the recombination rates, resulting the most important quantity for the characterizationof the optical yield in such systems. Besides, additional conditions for the achievement of high rates areconstituted by a high crystallinity of the nanocrystals and by high confinement energies and mall diameters

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 - Many-body effects on the electronic and optical properties of Si nanowires from ab-initio approaches [Articolo su rivista]
M., Palummo; Ossicini, Stefano; R., Del Sole
abstract

The study of semiconducting nanowires is one of the mostrapidly growing research areas in materials science andnanotechnology, not only from the point of view of the possibleapplications, but also regarding the use of the latest developmentsin the theory. In this paper, we review the general abinitio many-body theory and methods and resume some of ourvery recent results regarding the structural, electronic, andoptical properties of Silicon nanowires (Si-NWs), outliningboth the reached achievements and some of the technicalaspects necessary to obtain them.

2010 - Segregation, quantum confinement effect and band offset for [110] SiGe NWs [Articolo su rivista]
Amato, Michele; M., Palummo; Ossicini, Stefano
abstract

Results of first-principles DFT simulations provide strongevidence that, at zero temperature, for [110] oriented SiGenanowires (NWs), the segregated structure is favoured withrespect to the mixed ones; for this observation two differentschemes of calculations are presented and discussed. Moreoverthe segregation strongly influences the NWs electronic properties,inducing a reduced quantum confined effect (RQCE). Weshow here that it depends on the effect of strain in the planenormal to the direction of growth and not on the choice of latticeparameter in the direction of growth.Aqualitative evaluation ofthe band offset between Si and Ge for SiGe NWs is alsopresented.

2010 - Self-Energy and Excitonic Effects in the Electronic and Optical Properties of TiO2 crystalline phases [Articolo su rivista]
L., Chiodo; J. M., Garcia Lastra; A., Iacomino; Ossicini, Stefano; J., Zhao; H., Petek; A., Rubio
abstract

We present a unified ab initio study of electronic and optical properties of TiO2 rutile and anatase phaseswith a combination of density-functional theory and many-body perturbation-theory techniques. The consistenttreatment of exchange and correlation, with the inclusion of many-body one-particle and two-particles effectsin self-energy and electron-hole interaction, produces a high-quality description of electronic and opticalproperties, giving, for some quantities, the first available estimation for this compound. In particular, we givea quantitative estimate of the electronic and direct optical gaps, clarifying their role with respect to previousmeasurements obtained by various experimental techniques. We obtain a description for both electronic gapand optical spectra that is consistent with experiments by analyzing the role of different contributions to theexperimental optical gap and relating them to the level of theory used in our calculations. We also show thespatial properties of excitons in the two crystalline phases, highlighting the localization character of differentoptical transitions. This paper aims at understanding and firmly establishing electro-optical bulk properties, yetto be clarified, of this material of fundamental and technological interest for green energy applications.

2010 - Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results [Articolo su rivista]
Ossicini, Stefano; Amato, Michele; Guerra, Roberto; M., Palummo; O., Pulci
abstract

Actually, most of the electric energy is beingproduced by fossil fuels and great is the search for viablealternatives. The most appealing and promising technologyis photovoltaics. It will become truly mainstream when itscost will be comparable to other energy sources. One way isto significantly enhance device efficiencies, for example byincreasing the number of band gaps in multijunction solarcells or by favoring charge separation in the devices. Thiscan be done by using cells based on nanostructured semiconductors.In this paper, we will present ab-initio results ofthe structural, electronic and optical properties of (1) siliconand germanium nanoparticles embedded in wide band gapmaterials and (2) mixed silicon-germanium nanowires. Weshow that theory can help in understanding the microscopicprocesses important for devices performances. In particular,we calculated for embedded Si and Ge nanoparticles thedependence of the absorption threshold on size and oxidation,the role of crystallinity and, in some cases, the recombinationrates, and we demonstrated that in the case of mixednanowires, those with a clear interface between Si and Geshow not only a reduced quantum confinement effect butdisplay also a natural geometrical separation between electronand hole.

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 - Effects of simultaneous doping with boron and phosphorous on the structural, electronic and optical properties of silicon nanostructures [Articolo su rivista]
F., Iori; Ossicini, Stefano
abstract

We show, by means of ab-initio calculations, that by properly controlling the doping a significant modification of both the absorption and the emission of light of silicon nanocrystals can be achieved. We have considered impurities, boron and phosphorous (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupynearest neighbour 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 electron–hole pair due tolight absorption determines a geometry distortion that in turn results in a Stokes shift between absorption and emission spectra. In order to give a deeper insight in this effect, in one case, we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. Moreover, we also investigate how the properties of the codoped nanoclusters are influenced by the insertion of more impurities (multidoping). Finally, we have studied the effect of B and P codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanostructures.

2009 - Electronic properties and dielectric response of surfacesand nanowires of silicon from ab-initio approaches [Articolo su rivista]
M., Palummo; F., Iori; R., Del Sole; Ossicini, Stefano
abstract

We present here an ab-initio study, within the Density FunctionalTheory (DFT), of the formation energy of doped Silicon Nanowires(Si-NWs). While this theoretical approach is appropriate tocalculate the ground-state properties of materials, other methods,like Many-Body Perturbation Theory (MPBT) or Time DependentDensity Functional Theory (TDDFT), formally provide a correctdescription of the electronic excited states. Then, in the secondpart of this paper, we show how the many-body effects, introduced using the MBPT, modify the optical properties of the Si(100) surface.

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 - Reduced quantum confinement effect and electron-hole separation in SiGe nanowires [Articolo su rivista]
Amato, Michele; M., Palummo; Ossicini, Stefano
abstract

Using first-principles methods, we investigate the structural and electronic properties of SiGe nanowiresbasedheterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a different manner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only form the most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with the inclusion of many-body effects, prove that these nanowires—under optical excitation—display a clear electron-holeseparation property which can have relevant technological applications.

2009 - SiGe nanowires: structural stability, quantum confinement and electronic properties [Articolo su rivista]
Amato, Michele; M., Palummo; Ossicini, Stefano
abstract

We report first-principles calculations of 110 SiGe NWs; we discuss the effect of geometry and composition on their thermodynamic stability, on their electronic properties, and on the nature of the quantum confinement effect. The analysis of formation enthalpy reveals that Gecore/Sishell NWs represent the most stablestructure at any diameter, as a confirmation of the results of many experimental works. The study of the dependence of the energy band gap on the composition and geometry shows how abrupt NWs (wires with a clear flat interface between Si and Ge) present strongly reduced quantum confinement effect and offer a very easy way to predict and to engine energy band gap, which can have a strong relevance from a technological point of view. A careful analysis of the influence of composition on the wave-function localization and quantum confinement effect is also presented, in particular, for core-shell NWs.

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.

2009 - The Role of the Surface Coverage on the Structural and the Electronic Properties of TiO2 Nanocrystals [Articolo su rivista]
A., Iacomino; G., Cantele; F., Trani; D., Ninno; Marri, Ivan; Ossicini, Stefano
abstract

We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio density functional theory (DFT) calculations. We analyze the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates modeling the basical interactions of TiO2 nanosystems with the hydration sphere. We thus address the electronic reorganization and the surface role in determining the overall properties of the nanostructures. The structural reconstruction is found to depend on the surface coverage and the experimental evidences on the structural variations can be explained by a topological analysis of the Ti-O bonds. Q-size effects are observed through the bandgap widening, but the surface competes to determine the energy distribution of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the conduction bands, which can enhance the conductive properties of the nanowires. In the hydrogenated cluster such levels present a localized charge distribution with strong similarities (orbital character, energy position) to the defect states arising after oxygens desorption. From the analysis of the electronic density of states we found that Ti-H bonds induce in-gap states above the valence bands, whereas hydration leads to occupied states that shift the valence bands to lower binding energies.

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 - Oxygen vacancy effects on the Schottky barrier height at the Au/TiO2(100) interface: a first principle study [Articolo su rivista]
Marri, Ivan; Ossicini, Stefano
abstract

Motivated by the pioneering work of McFarland and Tang on multilayer photovoltaic devices, we discuss here structural and electronic properties of the Au/TiO2(110) interface for a coverage of 1 monolayer (1 ML) of gold, both for a stoichiometric and a reduced (Ti-rich) rutile surface. A detailed analysis of theSchottky barrier height for such systems is presented and the effects generated on this barrier by the presence of an oxygen vacancy (localized on the rutile support) are discussed.

2008 - Structural, Electronic and Surface Properties of Anatase TiO2 Nanocrystals from First Principles [Articolo su rivista]
A., Iacomino; G., Cantele; D., Ninno; Marri, Ivan; Ossicini, Stefano
abstract

The structural and electronic properties of anatase TiO2 nanocrystals (NCs) are investigated through firstprinciplescalculations. The dependence of the structural properties,e.g., NC volume variations on the surface chemistry is discussed by considering two different surface coverages (dissociated water and hydrogens). Both prevent a pronounced reconstruction of the surface, thus ensuring a better crystalline organization of the atoms with respect to the bare NC. In particular, the results for the hydrated NC do show the largest overlap with theexperimental findings. The band-gap blueshift with respect to the bulk shows up for both the bare and the hydrated NC, whereas hydrogen coverage or oxygen desorption from the bare NCs induce occupied electronic states below the conduction levels thus hindering the gap opening due to quantum confinement. These states are spatially localized in a restricted region and can be progressively annihilated by oxygen adsorption onundercoordinated surface titanium atoms. Formation energy calculations reveal that surface hydration leads to the most stable NC, in agreement with the experimental finding that the truncated bipyramidal morphology is typical of the moderate acidic environment. Oxygen desorption from the bare NC is unfavorable, thus highlighting the stabilizing role of surface oxygen stoichiometry for TiO2. Available experimental data on theelectronic and structural properties of TiO2 NCs are summarized and compared with our results.

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.

2008 - What is What in the Nanoworld A Handbook on Nanoscience and Nanotechnology Second, Completely Revied and Enlarged Edition [Monografia/Trattato scientifico]
V. E., Borisenko; Ossicini, Stefano
abstract

This is the second, enlarged and updated edition of our book. Starting from the initial more than 1400 entries of the first edition we arrived now to about 2000 entries. Moreover a large number of the old entries have been extended.The gallery of illustrations is enriched by new figures and new tables are added throughout the book.The presented terms, phenomena, regulations, experimental and theoretical tools are very easy to consult since they are arranged in alphabetical order, with a chapter for each letter. The great majorities of the terms have additional information in the form of notes such as ”First described in: ...”, ”Recognition: ...”, ”More details in: ..”, thus giving a historical retrospectiveof the subject with reference to further sources of extended information, which can be articles, books, review articles, websites. In particular in this second edition we have try, for the overwhelming majority of the items, to find out who was the initiator and when and where the term is born, or has been defined or discussed. We think that all these additional notes are quite useful, since they give the possibility to all the readers to start independently their personal research regarding an adequate argument. Only four years separate this second edition form the first one, nevertheless we have seen a true explosion of researches in nanoscience and developments in nanotechnologies. One measure of the emergence of these fieldsis the growth of the literature dedicated to the new disciplines. Nanoscience and nanotechnology have, in the last years, witnessed not only an explosive growth in the number of relevant and important ”classical” scientific journals, which have devoted, more and more, a consistent part of their published papers to ”nano”-related researches, but also in the number of new journals, which contain the stem ”nano” in their title.

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 - First-principles optical properties of silicon and germanium nanowires [Articolo su rivista]
M., Bruno; M., Palummo; Ossicini, Stefano; R., DEL SOLE
abstract

In this work we study the optical properties of hydrogen-passivated, free-standing silicon and germanium nanowires, oriented along the [1 0 0], [1 1 0], [1 1 1] directions with diameters up to about 1.5 nm, using ab-initio techniques. In particular, we show how the electronic gap depends on wire’s size and orientation; such behaviour has been described in terms of quantum confinement and anisotropy effects, related to the quasi one-dimensionality of nanowires. The optical properties are analyzed taking into account different approximations: in particular, we show how the many-body effects, namely self-energy, local field and excitonic effects, strongly modify the single particle spectra. Further, we describe the differences in the optical spectra of silicon and germanium nanowires along the [1 0 0] direction, as due to the different band structures of the corresponding bulk compounds.

2007 - From Si Nanowires to Porous Silicon: The Role of Excitonic Effects [Articolo su rivista]
M., Bruno; M., Palummo; A., Marini; R., DEL SOLE; Ossicini, Stefano
abstract

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first-principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modelling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.

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 excited states calculations for semiconductor materials: From bulk to low dimensional systems [Capitolo/Saggio]
Palummo, M.; Bruno, M.; Del Sole, R.; Ossicini, S.
abstract

First-principles ground-state calculations on different kind of materials are currently carried out within density functional theory. On the other hand a correct description of the electronic excitations, which are at the origin of many experimental spectra, requires more refined theories. In this paper we summarize the main equations of the theoretical many-body approach used to describe electronic and optical properties of real materials. Some examples of excited state calculations in bulk and low dimensional semiconducting systems are given.

2005 - Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2 : Beyond the quantum confinement effect [Articolo su rivista]
M., Luppi; Ossicini, Stefano
abstract

Density-functional theory calculations have been carried out in order to study the structural, electronic, and optical properties of oxidized silicon clusters and silicon nanocrystals embedded in SiO2 . For the isolated clusters, different Si/O bonding geometries and various levels of oxidation have been investigated, checking also the dependence of the results on the structure size. 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 physical properties of these systems. In particular we show how the multiple presence of silanonelike Si=O bonds can be a reliable model for explaining the photoluminescence redshift observed in oxidized porous silicon samples and it can be used as possible explanation also for the unexpected large photoluminescence bandwidth in single oxidized Si quantum dots.

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 - Ab-initio excited states calculations of semiconductor materials: from bulk to low dimensional systems [Relazione in Atti di Convegno]
Maurizia, Palummo; Ossicini, Stefano; Mauro, Bruno; RODOLFO DEL, Sole
abstract

Ab-initio methods

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 - Excitons in germanium nanowires: Quantum confinement, orientation, and anisotropy effects within a first-principles approach [Articolo su rivista]
Bruno, M; Palummo, M; Marini, A; Del Sole, R; Olevano, V; Kholod, An; Ossicini, Stefano
abstract

Within a first-principles framework we show how many-body effects crucially modify the electronic and optical properties of free-standing Germanium nanowires. The electron-hole binding energy and probability distribution are found to depend on both wire size and orientation. Moreover, we observe an almost complete compensation of self-energy and excitonic effects for some of the analyzed quantum wires, which we explain as being due to their clusterlike atomic structure.

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 - Erratum: Orientation effects in the electronic and optical properties of germanium quantum wires (Physical Review B Condensed Matter and Materials Physics (2004) 70 (035317)) [Articolo su rivista]
Kholod, A. N.; Shaposhnikov, V. L.; Sobolev, N.; Borisenko, V. E.; Arnaud D'Avitaya, F.; Ossicini, S.
abstract

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 - Orientation effects on the electronic and optical properties of germanium quantum wires [Articolo su rivista]
A. N., Kholod; V. L., Shaposhnikov; N. V., Sobolev; V. E., Borisenko; F., ARNAUD D'AVITAYA; Ossicini, Stefano
abstract

We have performed first-principles calculations in order to analyze the influence of spatial orientation on the electronic band structure and optical properties of hydrogenated germanium quantum wires. A quantum confinement-induced direct band gap appears for the (110)-oriented wires, whereas the (100)- and (111)-oriented wires are characterized by an indirect band gap. For the same wire-effective size, an anisotropy in the fundamental band gap Eg with respect to the wire orientation is observed. For (110)-oriented wires, the imaginary part of the dielectric function shows a pronounced peak in the visible energy range with strong dimensionality and polarization dependence.

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.

2004 - What is What in the Nanoworld: A Handbook on Nanoscience and Nanotechnology [Monografia/Trattato scientifico]
V. E., Borisenko; Ossicini, Stefano
abstract

This introductory, reference handbook summarizes the terms and definitions, most important phenomena, and regulations discovered in the physics, chemistry, technology, and application of nanostructures. The short form of information taken from textbooks, special encyclopedias, recent original books and papers provides fast support in understanding "old" and new terms of nanoscience and technology widely used in scientific literature on recent developments. A representative collection of fundamental terms and definitions accompanies recommended second sources (books, reviews, websites) for an extended study of a subject. Each entry interprets the term or definition under consideration and briefly presents main features of the phenomena behind it. Additional information in the form of notes ("First described in: ?", "Recognition: ?", "More details in: ?") supplements entries and gives a historical retrospective of the subject with reference to further sources. More than 1000 entries...

2003 - Dynamics of stimulated emission in silicon nanocrystals [Articolo su rivista]
L., DAL NEGRO; M., Cazzanelli; L., Pavesi; Ossicini, Stefano; D., Pacifici; G., Franzo; F., Priolo; F., Iacona
abstract

Time-resolved luminescence measurements on silicon nanocrystal waveguides obtained by thermal annealing of plasma-enhanced chemical-vapor-deposited thin layers of silicon-rich oxide have revealed fast recombination dynamics related to population inversion which leads to net optical gain. Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission with net gain values of about 10 cm(-1). The fast emission component is strongly dependent on the pumping length for fixed excitation intensity. In addition, both the fast component intensity and its temporal decay revealed threshold behavior as a function of the incident pump intensity.

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 - Isolated and embedded silicon based nanodots: The role of surface oxygen [Relazione in Atti di Convegno]
M., Luppi; Ossicini, Stefano
abstract

We have extensively studied the effects of oxygen on the optoelectronics properties of various types of isolated silicon nanodots, through ab initio total energy calculations within the density functional theory. Varying the cluster size we have considered different Si/O bonding geometries and different levels of oxidation. We provide strong evidences that the role of the interface region surrounding the silicon nanostructures have to be carefully taken into account in order to understand the striking optical properties of these systems. Moreover the multiple presence of silanone Si=O bonds at the nanodots surface is shown to provide a consistent interpretation of the photoluminescence red-shift observed in oxidized porous silicon samples. Finally for the first time we have performed ab initio calculations on small silicon nanodots embedded in a SiO2 matrix stressing the strong interplay between the nanodot and the surrounding host environment.

2003 - Light Emitting Silicon for Microphotonics [Monografia/Trattato scientifico]
Ossicini, Stefano; Pavesi, L.; Priolo, F.
abstract

Springer Tracts on Modern Physics A fascinating insight into the state-of-the-art in silicon microphotonics and on what we can expect in the near future. The book presents an overview of the current understanding of getting light from silicon. It concentrates mainly on low dimensional silicon structures, like quantum dots, wires and wells, but covers also alternative approaches like porous silicon and the doping of silicon with rare-earths. The emphasis is on the experimental and theoretical achievements concerning the optoelectronic properties of confined silicon structures obtained during recent years. Silicon based photonic crystals are in particular considered. An in depth discussion of the route towards a silicon laser is presented.

2003 - Monte Carlo analysis of electron heating in Si/SiO2 superlattices [Articolo su rivista]
Rosini, Marcello; Jacoboni, Carlo; Ossicini, Stefano
abstract

In this work, we investigate the transport properties of Si/SiO2 superlattices with a multiband one-particle Monte Carlo simulator. Using the envelope function approximation, we solve the Kronig-Penney potential along the growth direction z using a tight-binding-like analytical form; we also consider parabolic dispersion along the xy plane. The scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shape of the bands along the growth direction, very low drift velocities are found for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly influenced by the in-plane component of the electric field: in this way higher vertical drift velocities can be obtained. The results point out that in-plane carrier heating and multiband properties are responsible for this behavior. (C) 2002 Elsevier Science B.V. All rights reserved.

2003 - Multiple Si=O bonds at silicon clusters surface [Articolo su rivista]
M., Luppi; Ossicini, Stefano
abstract

A first-principle investigation of the effects of multiple Si=O bonds at the surface of silicon-based clusters with different sizes has been carried out. Total-energy pseudopotential calculations withindensity functional theory have been applied varying systematically the number of Si=O bonds at the clusters surface. A nonlinear reduction of the energy gap with the Si=O bond number is found. A sort of saturation limit is displayed, providing a consistent interpretation of the photoluminescence redshift observed in oxidized porous silicon samples. Moreover, our results help to clarify the very recent findings on the single silicon quantum dot photoluminescence bandwidth.

2003 - Optical properties of Ge and Si nanosheets - confinement and symmetry effects [Articolo su rivista]
An, Kholod; Ossicini, Stefano; Ve, Borisenko; Fa, D'Avitaya
abstract

Comparison between silicon and germanium quantum sheets has been made regarding their electronic and optical properties. Ab initio calculations have been applied for this purpose by mean of the linearized augmented plane wave method. Quantum confinement is found to shift the band gap of both Si and Ge bulk to the blue, however this upshift depends strongly on the surface orientation. Moreover also the nature of the band gap is related to the surface symmetry: for (100)-oriented films the band gap becomes direct for both semiconductors, whereas the (110)- and (111)-oriented films show a different behavior. In the first case the band gap is direct for Si and indirect for Ge, in the second it is direct for Ge and indirect for Si. Concerning the optical properties, since in the Si films the folded bulk band energies are found to retain their original indirect character, all the Si films have an intense absorption peak only at high energies corresponding to the blueshifted direct band gap of Si bulk. In Ge films the conduction band minimum retains a strong F component. Therefore, dielectric function calculations clearly show that Ge films have a strong optical absorption in the visual energy region. Analysis of the squared optical matrix elements is also presented and the data are compared to the results for GaAs. (C) 2003 Elsevier Science B.V. All rights reserved.

2003 - Oxidized silicon nanoclusters: a theoretical study. [Capitolo/Saggio]
Luppi, M; Ossicini, Stefano
abstract

Total energy calculations have been performed to understand the role of oxidation on the structural, electronic and optical properties of Si nanoclusters. Our aim is to explain the peculiar properties of aged porous Si samples, heavily oxidized Si nanoparticles and embedded Si nanocrystals. We have studied two types of structures: isolated H-covered clusters, replacing Si-H bonds with various Si-O bonds; and Si nanoclusters embedded in a SiO2 matrix. Regarding the isolated clusters we find that the optoelectronic properties depend on the type and the number of Si-O bonds at the cluster surface. For the embedded systems our results show that a close interplay between chemical and structural effects plays a key role in the light emission processes. SINGAPORE: World Scientific (SINGAPORE).

2003 - Oxygen role on the optoelectronic properties of silicon nanodots [Articolo su rivista]
M., Luppi; Ossicini, Stefano
abstract

The optoelectronic properties of Si nanodots have been investigated using ab initio total energy calculations within the density functional theory. Structural relaxations have been considered. We have studied two types of nanodots: isolated clusters covered by H, studying the substitution of Si/H bonds with different Si/O bonds, and nanocrystals embedded in SiO2 matrix. In the first case, we find that the optoelectronic properties strongly depend on the type and the number of Si/O bonds, especially for the gap value and the arrangement of the energy levels. In the second case, the close interplay between chemical and structural effects is pointed out.

2003 - Oxygen role on the structural and optoelectronic properties of silicon nanodots [Articolo su rivista]
M., Luppi; Ossicini, Stefano
abstract

The aim of this work is to elucidate, through density functional calculations, the role of the Si–O interface bonds on the structural, electronic and optical properties of Si nanodots in order to explain the peculiar properties of aged porous Si samples and heavily oxidized Si nanoparticles. For isolated dots we show that the presence of Si=O bond results in a strong reduction of the energy gap. For Si nanodots embeddedin SiO2 we show that both the Si–O bonding and the deformation of the cage play an important role.

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 - Semiclassical and quantum transport in Si/SiO2 superlattices [Articolo su rivista]
Rosini, Marcello; Jacoboni, Carlo; Ossicini, Stefano
abstract

In the last few years, many research groups have been trying to develop electroluminescent devices based on silicon. In particular, it has been shown that low-dimensional structures, such as silicon clusters, quantum wires and quantum wells, are suitable for this purpose. In this work we investigate transport properties of a particular superlattice using two approaches. The first method is a Monte Carlo simulation of electron transport in the biased superlattice. The band structure is calculated using the envelope function approximation, and the scattering mechanisms introduced in the simulator are confined optical phonons. Owing to the particularly flat band structure, drift velocities are very low, but it will be shown that a parallel component of the electric field can significantly increase the vertical drift velocity. Moreover, a superlattice based device is proposed in order to obtain high recombination efficiency. Finally, a quantum calculation is introduced.

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 - Effect of oxygen on the optical properties of small silicon pyramidal clusters [Articolo su rivista]
A. B., Filonov; Ossicini, Stefano; F., Bassani; F., ARNAUD D'AVITAYA
abstract

Optical absorption and light emission of oxygen-incorporated small silicon (Si30H40Oi) pyramidal clusters as a function of oxygen content were theoretically studied using the self-consistent semiempirical molecular orbital method (modified neglect of diatomic overlap—parametric method). In the absolute majority of the cluster configurations with low oxygen content structures, a wide spread of the optical transition energies ranging from 1.60 to 3.00 eV is observed, due to the competition between two opposite tendencies. According to the first one, connected with the increasing of the quantum confinement effects due to oxidation, the optical transition energies tend to increase, whereas the enhanced possibility of involvement of oxygen or oxygen-adjacent silicon atoms in the process tends to decrease the energy transitions.

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 - Monte Carlo simulation of electron transport in (formula presented) superlattices: Vertical transport enhanced by a parallel field [Articolo su rivista]
Rosini, M.; Jacoboni, C.; Ossicini, S.
abstract

Considerable effort is presently devoted to develop Si quantum structures for microelectronics and nanoelectronics. In particular, well-defined (formula presented) superlattices and quantum wells are under study. We investigate here the transport properties of a (formula presented) superlattice with a multiband one-particle Monte Carlo simulator. The band structure is obtained with an analytical model and the scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shapes of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly enhanced by the in-plane component of the electric field, consequently higher vertical drift velocities can be easily obtained. © 2002 The American Physical Society.

2002 - Monte Carlo simulation of electron transport in Si/SiO2 superlattices [Relazione in Atti di Convegno]
Rosini, Marcello; Jacoboni, Carlo; Ossicini, Stefano
abstract

ABSTRACT In this work we investigate the transport properties of Si/SiO2 superlattices with a multiband one-particle Monte Carlo simulator. The band structure of the system is obtained analytically by solving the Kronig-Penney potential in a tight binding approximation along the growth direction z while we have assumed parabolic dispersion in the in-plane directions. We have introduced in the simulator confined optical phonons, both polar and non polar, as scattering mechanisms. Owing to the very flat shape of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However it turns out that for oblique fields, the in-plane component of the electric field strongly influences the transport properties along the vertical direction as effect of carrier heating. In particular higher vertical drift velocities can be obtained.

2002 - Monte Carlo simulation of electron transport in Si/SiO2 superlattices: vertical transport enhanced by a parallel field [Articolo su rivista]
Rosini, Marcello; Jacoboni, Carlo; Ossicini, Stefano
abstract

Considerable effort is presently devoted to develop Si quantum structures for microelectronics and nanoelectronics. In particular, well-defined Si/SiO2 superlattices and quantum wells are under study. We investigate here the transport properties of a Si/SiO2 superlattice with a multiband one-particle Monte Carlo simulator. The band structure is obtained with an analytical model and the scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar. Owing to the very flat shapes of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly enhanced by the in-plane component of the electric field, consequently higher vertical drift velocities can be easily obtained.

2002 - Remembering Alessandro Ossicini [Articolo su rivista]
A., Ossicini; Ossicini, Stefano; M. M., Spalevic
abstract

This paper is devoted to the memory of Professor Alessandro Ossicini.

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.

2002 - True direct gap absorption in germanium quantum films [Articolo su rivista]
Kholod, A. N.; Ossicini, Stefano; Borisenko, V. E.; D'Avitaya, F. A.
abstract

In this paper we provide strong evidence of direct gap absorption in germanium nanostructured films. By performing first-principles calculations of the optical spectra for the three main low-index orientations, we show that, due to wave-function localization both in real and reciprocal space, the Ge(100) quantum films are, in particular, good candidates for the visible photoluminescence observation.

2001 - First-principle electronic structure of rare earth arsenides [Articolo su rivista]
Said, M.; Ben Zid, F.; Bertoni, C. M; Ossicini, Stefano
abstract

The electronic properties of rare-earth arsenides have been calculated from first principles. Inthe calculations we have treated the rare-earth f electrons both as core-like and as valence-like electrons.We consider the changes in the energy bands and in the density of states near the Fermi level which arefound to be relevant, except for the case of LuAs, and discuss this in relation with the role played fromthe rare-earth 5d derived states. Moreover we show that the rare-earth 5d related bands are particularlysensitive to the variation of the lattice constant; change in the lattice constant of less than 1% leadsto a different behaviour with respect to the crossing of the rare-earth 5d derived bands and the As 4pderived bands along the-direction. This point is discussed in connection with the possibility of having asemimetal-semiconductor transition in the rare-earth arsenides.

2001 - IN PLANE ANISOTROPY OF THE OPTICAL PROPERTIES OF (In0.5Ga0.5As)n/(InP)n SUPERLATTICES. [Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract

ABSTRACT In this paper we study the electronic and optical properties of (In0.5Ga0.5As)n/(InP)n superlattices, where the Ga0.5In0.5As alloy is described both through the virtual crystal approximation (VCA) and through an appropriate ordered ternary structure. By first-principles calculations of the dielectric tensor elements we address the issue of the giant polarization anisotropy of the optical absorption experimentally observed in these superlattices. The magnitude of the anisotropy depends on the splitting between the hole states at the valence band top which is due to the lowering of the overall symmetry to the C2v point group and it is greatly influenced by strain not only at the interfaces but also in the bulk alloy.

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.

2001 - The electronic and optical properties of InGaAs/InP and InAlAs/InP superlattices [Articolo su rivista]
C., Ghidoni; Magri, Rita; Ossicini, Stefano
abstract

We study using first-principle calculations the electronic and optical properties of In0.5Ga0.5As/InP and In0.5Al0.5As/InP superlattices, where the InGaAs and InAlAs alloys are described through an appropriate ordered ternary structure. The calculated electronic properties show that the substitution of Ga with Al originate an opening of the band gap from the infrared to the near visible and a transformation of the band alignment from type I to type II. Through the analysis of the optical properties we discuss successfully the giant polarization anisotropy observed in these systems. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - The optoelectronic properties of silicon nanostructures: The role of the interfaces [Relazione in Atti di Convegno]
Ossicini, S.
abstract

The optoelectronic properties of Si nanostructures are studied ab initio to investigate their dependence on the dimensions, on the symmetry and on the bonding situations at the interfaces. We find that the dimensions are significative for the transition oscillator strengths and that the symmetry of the lattice changes the nature of the gap. The saturating species and/or the presence of dangling bonds play an important role in the formation of interface states that can occupy or leave free the band gap so making worse or improving the optical properties.

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 - Porous silicon: A quantum sponge structure for silicon based optoelectronics [Articolo su rivista]
Bisi, Olmes; Ossicini, Stefano; L., Pavesi
abstract

The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum confined in Si nanocrystals which remain after the partial electrochemical dissolution of silicon. Porous silicon is constituted by a nanocrystalline skeleton (quantum sponge) immersed in a network of pores. As a result, porous silicon is characterized by a very large internal surface area. This internal surface is passivated but remains highly chemically reactive which is one of the essential features of this new and complex material. We present an overview of the experimental characterization and theoretical modeling of porous silicon, from the preparation up to various applications. Emphasis is devoted to the optical properties of porous silicon which are closely related to the quantum nature of the Si nanostructures. The characteristics of the variousluminescence bands are analyzed and the underlying basic mechanisms are presented. In the quest of an efficientelectroluminescent device, we survey the results for several porous silicon contacts, with particular attention to the interfaceproperties, to the stability requirement and to the carrier injection mechanisms. Other device applications are discussed aswell.

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 - Ab-initio investigation of the polarization anisotropy of the optical absorption in (InGa)As-InP superlattices [Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract

In this paper we present a theory which accounts for both the intensity and the shape of the experimentally observed giant polarization anisotropy of the optical absorption in (InGa)As-InP superlattices. By first-principles self-consistent calculations of the superlattice crystal potential and of the dielectric tensor elements we derive a selection rule'' by which the radiation polarization along the 110 and -110 directions choose only one optical transition of the doublet corresponding to transitions to the conduction band from the two upper crystal field split hole states which are degenerate in common atom superlattices. However, due to the similar intensity of the oscillator strength of these transitions, it turns out that the magnitude of the anisotropy depends only on the magnitude of the energy gap between the split states.

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.

1998 - OPTICAL PROPERTIES OF CONFINED SILICON STRUCTURES [Articolo su rivista]
Ossicini, Stefano
abstract

The optical properties of bulk silicon are deeply modified if the material is manipulated at the nanometre scale. In particular the growth of Si nanostructures constitutes today a promising approach for the development of silicon-based light emitting devices. In this context I discuss theoretical results on the optoelectronic properties of low-dimensional silicon structures, e.g. Si quantum wells, quantum wires, quantum dots. The results are compared with recent experimental data.

1998 - ROLE OF SYMMETRY REDUCTION IN THE POLARIZATION DEPENDENCE OF THE OPTICAL ABSORPTION IN NON-COMMON ATOM SUPERLATTICES [Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract

Role of symmetry

1997 - Ab-initio calculation of the optical properties of silicon quantum wires [Relazione in Atti di Convegno]
Ossicini, Stefano; Biagini, M.; Bertoni, Carlo Maria; Roma, G.; Bisi, Olmes
abstract

ABSTRACT We study the effect of H. O passivation and inter-wire interaction on the optical properties of nanoscale Si wires. We find that wires with diameters as small as 10-25 AA are active in the visible range. Inter-wire interaction leads to the presence of localized states which lower the band gap energy. The presence of dangling bonds generates broad features in the infrared region. O-Si bonds reduce the optical threshold. These results are important for the discussions concerning absorption and luminescence in porous silicon.

1997 - Electronic and optical properties of silicon quantum wires at different passivation regimes [Capitolo/Saggio]
Ossicini, Stefano; Bisi, Olmes
abstract

Optoelectronic properties of porous silicon.

1997 - Optical emission from small Si particles [Articolo su rivista]
Baierle, R. J.; Caldas, M. J.; Molinari, Elisa; Ossicini, Stefano
abstract

The optical properties of hydrogen-saturated Si clusters are studied theoretically through an approach that can treat consistently both small molecules and bulk crystals and fully includes electron-electron correlation. We find that optical spectra in the ground state cannot explain the observed luminescence of porous Si for the small particle sizes consistent with structural data. However, the same clusters in their excited state relax to locally distorted equilibrium configurations, giving rise to new transitions involving localized states that lower the emission threshold. These results allow to reconcile absorption and luminescence experiments with no need to invoke extrinsic effects.

1997 - Optical properties of isolated and interacting silicon quantum wires [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; M., Biagini; A., Lugli; G., Roma; Bisi, Olmes
abstract

We have studied the effect of hydrogen passivation and inter-wire interaction on the electronic structure and optical properties of nanoscale Si wires through two first-principle techniques: linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) and norm-conserving pseudopotential. We have considered free, partially and totally H-passivated [001] Si quantum wires with various rectangular cross-sections; moreover we have investigated the inter-wire interaction, by varying the wire density. The optical properties have been computed by evaluating the imaginary part of the dielectric function and the absorption coefficient. We find that wires with diameters as small as 10-25 Angstrom are active in the visible range. Inter-wire interaction leads to the presence of localized interface states which lower the bandgap energy. These results are important for the discussion about the dimensionality of confined Si quantum particles in porous Si and for the debate on quantum confinement models.

1997 - Optically- induced defects in Si-H nanoparticles [Relazione in Atti di Convegno]
Mj, Caldas; Rj, Baierle; Molinari, Elisa; Ossicini, Stefano
abstract

We study the behavior of perfectly hydrogenated Si nanoparticles, under optical excitation. We use semiempirical selfconsistent techniques within the Hartree-Fock formalism, specially parametrized to reproduce key properties of the crystalline environment for the Si atom. We find that the optical absortion spectra of particles ranging in size from 29 to 71 Si atoms evidence both quantum confinement and relaxation effects, with optical gaps in the green-blue region. These particles show however strength for luminescence at much lower energy, related to localised surface defects created by gap-light excitation. These defects involve two surface Si atoms and a H-atom, in a Si-H-Si bridge configuration, and we propose they should be the active luminescent centers in porous Si.

1997 - Porous silicon modelled as idealised quantum wires [Capitolo/Saggio]
Ossicini, Stefano
abstract

The book can be roughly divided into three sections, covering porous silicon preparation and structure, its physical properties, and optoelectronic and other applications. Anyone who has worked on porous silicon will confirm that the large initial section on preparation and structural characterisation is essential for a critical analysis of this material. In the next section of the book, a comprehensive survey of mechanical, chemical, optical and electronic properties is given. There are many instances where conflicting experimental information is presented to demonstrate that an understanding of the structure and composition of individual samples is paramount to an understanding of the optoelectronic properties. The last section proves that the field is developing fast, with reports on miniaturisation and the first industrial applications to be taken up by giant electronics corporations.

1997 - The optical transition in a silicon wire passivated by H and O-H [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes
abstract

We report ab-initio calculations of the electronic and optical properties of Si quantum wires with different species passivating the dangling bonds, When O-H group substitutes H atoms the band gap energy lowers and new features in the low energy side of the dielectric function arise. Our results interpret succesfully the experimental data of the oxidation induced changes in the luminescence and infrared absorption spectra of porous silicon.

1996 - Auger Lineshape analysis of porous silicon: Experiment and theory [Articolo su rivista]
L., Dorigoni; L., Pavesi; Bisi, Olmes; L., Calliari; M., Anderle; Ossicini, Stefano
abstract

Different samples of porous Si have been investigated through Auger electron spectroscopy, The Si LVV lineshape of porous Si is different with respect to that of pure Si and the main peak is shifted to lower kinetic energies. Porous Si is modeled by a first-principle calculation of the electronic states of thin silicon wires (LMTO-ASA method). The main experimental finding is related to the new properties of the valence states in porous Si. The calculated Auger lineshape provides an interpretation of the characteristic shape of the measured spectra. We prove that the confinement shift is not detected by the Si L(2.3)VV Auger signal and that the Si atoms probed are bonded with H and H-2.

1996 - Confinement and passivation in isolated and coupled silicon quantum wires [Capitolo/Saggio]
Ossicini, Stefano; Bertoni, Carlo Maria; M., Biagini; G., Roma; Bisi, Olmes
abstract

Porous silicon, quantum wires, ab-initio calculation

1996 - Electron states and luminescence transition in porous silicon [Articolo su rivista]
L., Dorigoni; Bisi, Olmes; F., Bernardini; Ossicini, Stefano
abstract

The theoretical analysis of two different Si wires of size 5 × 4 and 3 × 4, simulating porous Si, has been performed through the linear-muffin-tin-orbitals method in the atomic sphere approximation. All the atomic core energies were self-consistently computed and used to directly compare the energies of the quantum wires and that of the crystalline Si, by aligning the 2p core level of a Si atom located at the center of the wire to that corresponding to crystalline Si. The optical properties of the wires have been computed by evaluating the imaginary part of the dielectric function. The main results are (i) the opening of the gap is asymmetric; 1/3 of the widening is in the valence band, while 2/3 in the conduction band; (ii) the near band-gap states originate mainly from Si atoms located at the center of the wire; (iii) the imaginary part of the dielectric function shows a low-energy structure, strongly anisotropic, that follows the blueshift for the gap and is identified ...

1996 - Electronic Structure of Rare Earth Arsenide / Gallium Arsenide superlattices [Articolo su rivista]
Said, M.; Bertoni, C. M.; Fasolino, A.; Ossicini, Stefano
abstract

LMTO Calculations

1996 - Electronic structure of the 1x1 YBa2Cu3O7/PrBa2Cu3O7 superlattice: a local spin density approximation with on-site Coulomb interaction [Articolo su rivista]
M., Biagini; Ossicini, Stefano; CALANDRA BUONAURA, Carlo
abstract

Electronic structure calculations based on the local-spin-density approximation (LSDA) fail to reproduce the antiferromagnetic ground state of PrBa2Cu3O7 (PBCO). We have performed linear muffin-tin orbital–atomic sphere approximation calculations, based on the local-spin-density approximation with on-site Coulomb correlation applied to Cu(1) and Cu(2) 3d states. We have found that inclusion of the on-site Coulomb interaction modifies qualitatively the electronic structure of PBCO with respect to the LSDA results, and gives Cu spin moments in good agreement with the experimental values. The Cu(2) upper Hubbard band lies about 1 eV above the Fermi energy, indicating a CuII oxidation state. On the other hand, the Cu(1) upper Hubbard band is located across the Fermi level, which implies an intermediate oxidation state for the Cu(1) ion, between CuI and CuII. The metallic character of the CuO chains is preserved, in agreement with optical reflectivity [K. Takenaka et al., Phys....

1996 - Luminescence in porous silicon: The role of confinement and passivation [Articolo su rivista]
Ossicini, Stefano; L., Dorigoni; Bisi, Olmes
abstract

We perform the theoretical analysis of two wires of different size, simulating porous Si, through the linear muffin tin orbitals method in the atomic sphere approximation. We consider free, partially and totally H-covered [001] Si quantum wires with rectangular cross section. We show that (a) quantum confinement originates the opening of the LDA gap; (b) this opening is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band; (c) the near band gap states originate from Si atoms located at the center of the wire; (d) the confinement is enhanced in the case of free surfaces; (e) the imaginary part of the dielectric function shows a low energy side structure strongly anisotropic, identified as responsible of the luminescence transition; (f) the presence of dangling bonds destroys the luminescent properties.

1996 - Optical absorption of small hydrogenated Si particles [Articolo su rivista]
BAIERLE R., J; CALDAS M., J; Molinari, Elisa; Ossicini, Stefano
abstract

Ab-initio calculations

1996 - Si/CaF2 superlattices: a silicon light emitting nanostructure [Relazione in Atti di Convegno]
Ossicini, Stefano; Fasolino, A.; Bernardini, F.; ARNAUD D'AVITAYA, F.; Vervoort, L.; Bassani, F.
abstract

Abstract One promising approach for the development of silicon-based-light-emitting devices is the epitaxial growth of Si nanostructures. In this context, we propose the lattice matched system CaF2/Si/CaF2 as a prototype of a well controlled and ordered Si-based system with known microscopic structure. We present here a combined theoretical and experimental investigation of ultra-thin silicon (111) layers embedded in CaF2. Our all electron calculation predicts the band gap opening and the presence of confined and interface states leading to a quasi-direct band gap. We have synthesized, by molecular beam epitaxy, Si/CaF2 multilayers which efficiently photoluminesce at room temperature. The photoluminescence spectra show a strong resemblance to those of porous silicon. There is a critical dependence of the photoluminescence efficiency on the thickness of the Si layers and a blue shift for decreasing Si layers thickness. Our results allow us to conclude unambiguously that quantum conf...

1996 - The luminescence transition in porous silicon: The nature of the electronic states [Articolo su rivista]
L., Dorigoni; Bisi, Olmes; F., Bernardini; Ossicini, Stefano
abstract

The theoretical analysis of two different wires of size 5x4 and 3x4, simulating porous Si has been performed through the linear muffin tin orbitals method in the atomic sphere approximation. We show that the opening of the local density approximation gap is asymmetric: 1/3 of the widening is in the valence band, while 2/3 is in the conduction band; the near-bandgap states originate from Si atoms located at the centre of the wire; the imaginary part of the dielectric function shows a strongly anisotropic low-energy side structure, identified as responsible for the luminescence transition; the spatial localization of the valence and conduction band states involved in the luminescence transition shows that all the Si atoms of the wire are collectively involved.

1996 - The optical transition in porous Si: The effects of quantum confinement, surface states and hydrogen passivation [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes
abstract

We present a theoretical study of two infinite wires of Si with a different lateral size. The analysis is based on the linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA). We consider free, partially and totally H-covered[001] Si quantum wires with rectangular cross-section. The results of this investigation prove the quantum wire nature of porous Si and interpret many of its physical features. In particular we show that a) as expected quantum confinement originates the opening of the LDA gap; b) the gap opening effect is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band; c) the near band gap states originate from Si atoms located at the center of the wire; d) the confinement is enhanced in the case of free surfaces; e) the imaginary part of the dielectric function shows a low-energy side structure strongly anisotropic, identified as responsible of the luminescence transition; f) the presence of dangling bonds destroys the luminescence properties; g) in spite of feature c), all Si atoms ape collectively involved in the luminescence transition; h) the shift detected by the Si L(2,3) VV Anger signal is due to H-interaction effect and is not a measure of the quantum confinement effect; i) the Si atoms probed by the Si L(2,3) VV Auger are bonded with H and H-2.

1995 - Ab-initio calculation of the electronic (valence and core) and optical properties of interfaces [Capitolo/Saggio]
Ossicini, Stefano; Bisi, Olmes
abstract

Ab-initio methods, low dimensional systems

1995 - ELECTRONIC-STRUCTURE OF PRBA2CU3O7 - A LOCAL-SPIN-DENSITY APPROXIMATION WITH ON-SITE COULOMB INTERACTION [Articolo su rivista]
Biagini, M; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

Electronic structure calculations based on the local-spin-density approximation (LSDA) fail to reproduce the antiferromagnetic ground state of PrBa2Cu3O7 (PBCO). We have performed linear muffin-tin orbital-atomic sphere approximation calculations, based on the local-spin-density approximation with on-site Coulomb correlation applied to Cu(1) and Cu(2) 3d states. We have found that inclusion of the on-site Coulomb interaction modifies qualitatively the electronic structure of PBCO with respect to the LSDA results, and gives Cu spin moments in good agreement with the experimental values. The Cu(2) upper Hubbard band lies about 1 eV above the Fermi energy, indicating a Cu-II oxidation state. On the other hand, the Cu(1) upper Hubbard band is located across the Fermi level, which implies an intermediate oxidation state for the Cu(1) ion, between Cu-I and Cu-II. The metallic character of the CuO chains is preserved, in agreement with optical reflectivity [K. Takenaka et al., Phys. Rev. B 46, 5833 (1992)] and positron annihilation experiments [L. Hoffmann et al., Phys. Rev. Lett. 71, 4047 (1993)]. These results support the view of an extrinsic origin of the insulating character of PrBa2Cu3O7.

1995 - Hole filling and interlayer coupling in YBa2 Cu3 07/PrBa2 Cu307 superlattices [Articolo su rivista]
M., Biagini; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

Charge transfer effects in YBa2Cu3O7/PrBa2Cu3O7 superlattices have been proposed by many authors as the origin of the experimentally observed strong depression of the critical temperature. We performed self-consistent LMTO-ASA calculations and found that no remarkable change in the electronic structure of the superconducting CuO2 planes occurs in the studied structures, when the PBCO layer thickness is varied. The observed depression of the critical temperature does not seem to be originated intrinsically by a severe modification of the electronic structure or by the hole-filling mechanism.

1995 - Light emission at room temperature from Si/CaF2 multilayers [Articolo su rivista]
Fa, D'Avitaya; L., Vervoort; F., Bassani; Ossicini, Stefano; A., Fasolino; F., Bernardini
abstract

We have synthesized, by Molecular Beam Epitaxy (MBE), Si/CaF2 multilayers which are optically active at room temperature. The photoluminescence spectra present a blue shift far decreasing Si layer thickness, in analogy to those obtained from porous silicon when the porosity is increased. We find a critical dependence of the photoluminescence efficiency on the thickness of the Si layers. We compare the experimental results to ab initio calculations of the band structure of Si/CaF2 multilayers which predict the band gap opening and the presence of confined and interface states leading to a quasi-direct band gap.

1995 - Si/CaF2 superlattices: a direct gap structure due to interface state coupling [Articolo su rivista]
Ossicini, Stefano; A., Fasolino; F., Bernardino
abstract

One promising approach for the development of silicon-based light-emitting devices is the epitaxial growth of Si nanostructures. In this context, the lattice matched system CaF2/Si/CaF2 as prototype of a well controlled and ordered Si-based system with known microscopic structure is proposed. For this system, a new mechanism is found leading to a direct band gap based on the coupling of interface states in very thin Si layers.

1994 - ELECTRONIC-STRUCTURE OF THIN SI LAYERS IN CAF2 - HYBRIDIZATION VERSUS CONFINEMENT [Articolo su rivista]
A., Fasolino; Ossicini, Stefano; F., Bernardini
abstract

We present first principle calculations of thin (1-7 double layers) silicon (111) layers in CaF2, a system with strong analogies to porous silicon. We show that the Si band gap opening is dominated by the valence band which follows the effective mass confinement picture, while the conduction band is found to flatten and shift very modestly due to hybridization effects between Si and Ca states which lead to bonding-antibonding interface states in the gap. The relevance of these results for Si-based low dimensional structures is discussed.

1994 - First-principle investigation of the electronic structure of Si- based layered structures [Articolo su rivista]
F., Bernardini; Ossicini, Stefano; A., Fasolino
abstract

The electronic properties of ultra-thin Si layers have been investigated. Using the linear muffin tin orbital (LMTO) method, we have studied band gap and interface states for supercells formed by Si(111) layers of variable thickness (1-7 double layers) embedded in a CaF2 host crystal, a system with interesting analogies to porous silicon. Our results show that the band gap opening is dominated by the valence band which follows the effective mass confinement picture, while the conduction band is found to flatten out and shift very modestly. The latter behaviour is due to hybridization effects between Si and Ca states which lead to bonding-antibonding interface states in the gap. For 1 and 2 double layers the bonding Si-Ca states emerge from the Si valence band and lead to an almost direct gap at finite wavevectors. This behaviour could account for efficient visible luminescence in this system. The relevance of these results for Si-based low-dimensional structures is discussed.

1994 - GAP OPENING IN ULTRATHIN SI LAYERS - ROLE OF CONFINED AND INTERFACE STATES [Articolo su rivista]
Ossicini, Stefano; A., Fasolino; F., Bernardini
abstract

We present first principle calculations of ultrathin silicon (111) layers embedded in CaF2, a lattice matched insulator. Our all electron calculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface character. The latter states are very sensitive to the sample design. We suggest that a quasidirect band gap can be achieved by stacking Si layers of different thickness.

1994 - Quasi-particle band structure of NiO: The Mott-Hubbard picture regained [Articolo su rivista]
Manghi, Franca; CALANDRA BUONAURA, Carlo; Ossicini, Stefano
abstract

We demonstrate that the Hubbard correlation among Ni 3d electrons is able to reproduce the insulating character of NiO, the correct value of the gap, the orbital character of the valence band edge, and the presence of satellite structures. We have determined the quasiparticle spectra starting from the complex single particle band structure of NiO and including the on-site Hubbard repulsion according to a recently developed three-body scattering theory which allows us to treat highly correlated and highly hybridized systems. The calculated quasiparticle band structure is in excellent agreement with photoemission data.

1994 - Surface states and surface-shifted core-levels of the Pb-Si(111)(3x3) interface [Capitolo/Saggio]
Bernardini, F.; Ossicini, Stefano
abstract

AB-initio calculation of core level shifts.

1993 - Electronic properties of low dimensional silicon structures [Relazione in Atti di Convegno]
Ossicini, Stefano; Fasolino, A.; Bernardini, F.
abstract

Ab-initio calculation of low-dimensional silicon nanosystems

1993 - Optical Properties of Low Dimensional Silicon Nanostructures [Monografia/Trattato scientifico]
BENSAHEL D., C; Canham, L. T.; Ossicini, Stefano
abstract

NATO ASI Series E: Applied Science This book describes the state of the art on the luminescent properties of porous silicon. The dramatic results of the major international laboratories and groups working in this subject are presented. The majority of the participants agree that a quantum confinement effect and a good passivation of the surface crystallites are responsible for the observed luminescence properties. New characterisation techniques are for the first time presented as a comprehensive survey of the theoretical models. Both efficient and wavelength tunable electroluminescence are demonstrated using liquid contacts and a cathodic injection system. Localised cathodoluminescence in a scanning electron miscroscope and a scanning tunneling microscope is also presented. This book demonstrates that a consensus is emerging on the origin of the visible luminescence of highly porous Si. This novel nanostructure is extending our knowledge into a new area of solid state physics.

1992 - CHEMICAL BOND AND INTERFACES STATES AT METAL-SEMICONDUCTOR AND INSULATOR-SEMICONDUCTOR INTERFACES [Articolo su rivista]
Ossicini, Stefano; Arcangeli, C.; Bisi, Olmes
abstract

LMTO Calculations

1992 - Fermi level pinning and interface states at Pb-Si(111) interface [Articolo su rivista]
Ossicini, Stefano; F., Bernardini
abstract

The first theoretical analysis of the electronic properties of the Pb-Si(111) interface at monolayer coverage is presented. We have used the self-consistent linear muffin-tin orbital method with the atomic-sphere-approximation in a supercell geometry. The results show the presence of a discrete true interface state which is responsible of the pinning of the Fermi level and of the large Schottky barrier heights found for this system.

1992 - Hydrogen covered Si(111) surfaces [Articolo su rivista]
BUONGIORNO NARDELLI, M.; Finocchi, F.; Palummo, M.; DI FELICE, R.; Bertoni, Carlo Maria; Bernardini, F.; Ossicini, Stefano
abstract

The recently discovered method for the production of an ideally H-terminated, stable and easily transferable Si(111)1 × 1 surface renews the interest for this prototypical system. Through a density functional description of the electronic structure based on pseudopotential and LMTO methods, we discuss in detail spectroscopical information, bond geometry, stretching frequency and the energetics of this surface. Further attention is devoted to the chemisorption of atomic hydrogen on the Si(111)2 × 1 surface and to the removal of the reconstruction, which leads to a less perfect 1 × 1 surface.

1992 - Interface core-level shifts for silicon interfaces [Relazione in Atti di Convegno]
Ossicini, Stefano; Bisi, Olmes
abstract

Ab-initio calculation of core-level shift

1992 - THE ELECTRONIC-PROPERTIES OF THE CAF2-SI(111) SYSTEM - FROM MONOLAYER COVERAGE TO SOLID SOLID INTERFACE [Articolo su rivista]
Arcangeli, C; Ossicini, Stefano; Bisi, Olmes
abstract

The surface linear muffin-tin method in the atomic-sphere approximation (SLMTO-ASA) is applied to study the electronic properties of the CaF2-Si(111) interface. We investigate the growth of CaF2 on Si(111) following the interface formation from low to high coverages. The early stages of the growth are simulated by an interface formed by a monolayer of Ca deposited on Si(111) followed by a F layer, i.e., a F-Ca-Si(111) system. Both valence- and core-electron states have been calculated. Their analysis gives important information about the nature of the Ca-Si and Ca-F bonds. We explore the dependence of the electronic properties on the number of deposited layers showing that the main contribution to the interface properties is established at monolayer coverage and that the transfer of charge between the two crystals is localized in their last layers.

1992 - The electronic properties of the Pb-Si(111) interface at different coverages [Relazione in Atti di Convegno]
Ossicini, Stefano; Bernardini, F.
abstract

Ab-initio calculations of electronic properties.

1992 - The electronic properties of the Pb-Si(111) interface at monolayer coverage [Capitolo/Saggio]
F., Bernardini; Ossicini, Stefano
abstract

Core level shift

1992 - Theoretical approaches to the Schottky Barrier Problem [Articolo su rivista]
Ossicini, Stefano
abstract

The description of the electronic structure of an interface between two materials is one of the main goals of solid state theory. In the case of a metal-semiconductor interface a basic problem is the calculation of Schottky barriers. A review is presented in which different theories on the formation of the Schottky barrier are discussed. The role of the interface states and their physical origin are discussed. Particular attention will be paid to the question whether the barrier heights are influenced by the details of the interface structure.

1992 - Theory of Core-level Shifts of Clean and Covered Surfaces [Relazione in Atti di Convegno]
Bisi, Olmes; Ossicini, Stefano; C., Arcangeli
abstract

The core-level binding energy of an atom in the surface region is different from that of the bulk atoms. From the measurement of this core-level shift many information on the electronic states and surface properties can be devised. The interpretation of the surface core-level shift in terms of the one-electron picture is not a simple task, because the core-level binding energy is correctly viewed as an energy difference between the ground state and a core-hole final state of the system. This total energy difference, that determines the core-level binding energy, is not simply related to the single-particle eigenvalues, due to relaxation and correlation effects. For this reason single-particle eigenvalues are only a gross approximation to core-level binding energy. On the contrary the core-level binding energy shift, on going from the surface to the bulk environment, can be successfully related to the single-particle eigenvalues shift. We will show this for various surface systems: (i) a clean surface of a metal, Ni; (ii) an interface between a metallic monolayer and a semiconductor, Na-Si(111); (iii) a semiconductor- insulator interface, F-Ca-Si(111). The investigation of these interfaces is based on the surface linear muttin-tin orbital method. Our results yield an accurate analysis of both valence and core electron states and of the nature of the interface bond.

1991 - Chemical bond and electronic states in the CaF2-Si(111) and Ca-Si(111) interfaces [Articolo su rivista]
Ossicini, Stefano; Arcangeli, C.; Bisi, Olmes
abstract

The first stage of formation of the CaF2-Si(111) and Ca-Si(111) interfaces is studied through the employment of the surface linear-muffin-tin-orbital approach in the atomic-sphere approximation. The interfaces are simulated by monolayers of F-Ca and Ca on Si(111), respectively. Both valence- and core-electron states have been calculated: their analysis gives important information about the nature of the Ca-Si and Ca-F bonds. These results are successfully compared with the available experimental data. The importance of considering the Madelung contribution in the interpretation of surface-core-level shifts is pointed out.

1991 - Electronic and structural properties of semiconductor surfaces with chemisorbed oxygen [Articolo su rivista]
Finocchi, F.; BUONGIORNO NARDELLI, M.; Bernardini, F.; Palummo, M.; DE FELICE, R.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Electronic calculations

1991 - Electronic and structural properties of semiconductor-metal and semiconductor-insulator interfaces [Capitolo/Saggio]
Bisi, Olmes; Ossicini, Stefano; Arcangeli, C.
abstract

LMTO Methods

1991 - Initial formation of the CaF2 interface: a theoretical study [Articolo su rivista]
Ossicini, Stefano; C., Arcangeli; Bisi, Olmes
abstract

The insulator-semiconductor CaF2-Si(111) interface is studied in the first stages of formation. The linear muffin-tin orbitals method in the atomic-sphere approximation is employed to investigated the F-Ca-Si(111) interface. The analysis of both valence and core electron states gives information about the nature of the bond between Ca, F and Si surface and allows one to interpret the available experimental data.

1991 - The electronic structure of the Pb-Si(111) interface [Articolo su rivista]
Bernardini, F.; Ossicini, Stefano
abstract

LMTO Calculations

1990 - Calculation of the electronic properties of simple metal surfaces through lateral averaged pseudopotential [Articolo su rivista]
Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano; Solonovich, V. K.
abstract

Metal surfaces, jellium models

1990 - Covalency on the adsorption of Na on Si(111) [Articolo su rivista]
Ossicini, Stefano; Arcangeli, C; Bisi, Olmes
abstract

ABSTRACT The linear muffin-tin orbitals method in the atomic-sphere approximation is employed to investigate the electronic structure of a semiconductor-metal interface at monolayer coverage. We studied the interface formed by one monolayer of Na adsorbed on Si(111). Analysis of both valence- and core-electron states gives information on the nature of the bond between Na and the Si surface, which is mainly covalent, and allows one to interpret the available experimental data.

1990 - Electronic and magnetic properties of interfaces [Capitolo/Saggio]
Bisi, Olmes; Ossicini, Stefano
abstract

During the last years much attention has been devoted to the investigation of the electronic properties of surfaces and interfaces from first-principle calculations. The microscopic physics of a great number of systems has been investigated in details and many mechanisms and processes have been focused and understood. First-principle calculations are mainly based on the density functional theory in its local density approximation. Recently a substantial advancement in the calculation of the electronic states has been achieved by the computation of quasi-particle energies including self-energy correction. The computational techniques used can be divided between all-electron and pseudopotential approaches. All-electron calculations are usually performed within the linear-method scheme that strongly simplifies the computational problem. After a brief introduction of the theoretical framework and methods, we will present the application of linear methods to the investigation of a) the clean surface of a semiconductor, b) the metal-semiconductor interface and c) the magnetic properties of surfaces and interfaces.

1990 - Electronic structure of Si(111)-NiSi2(111) A-type and B-type interfaces [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Bertoni, Carlo Maria
abstract

The self-consistent electronic properties of the epitaxial Si(111)-NiSi2(111) interfaces are computed for the experimentally observed A- and B-type interface structures. The densities of states projected at the different atomic sites and the two-dimensional band structure provide a detailed analysis of the electronic properties of the silicon-silicide interface. The Schottky-barrier height turns out to be dependent not only on the interface structure, but also on the interface relaxation distance. A critical analysis of existing results is also presented.

1990 - First-principle calculation for the core level shifts of clean and covered surfaces [Articolo su rivista]
Bisi, Olmes; Arcangeli, C.; Bassoli, D.; Franceschini, A.; Ossicini, Stefano
abstract

Core level calculation

1990 - SIMPLE METAL-SURFACES AND IMAGE POTENTIAL STATES [Articolo su rivista]
F., Finocchi; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

The surfaces of Al and Na are simple systems, but they show several important features like surface states and image resonances. Using the weighted density approximation within the density functional theory we are able to account self-consistently for image effects in the description of their electronic properties.

1990 - Structural and electronic properties of CaF2-Si(111) interface [Articolo su rivista]
Ossicini, Stefano; C., Arcangeli; Bisi, Olmes
abstract

semiconductor interfaces

1990 - THE ELECTRONIC-PROPERTIES OF SI-NISI2(111) EPITAXIAL INTERFACES [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Bertoni, Carlo Maria
abstract

Abstract The densities of states and the two-dimensional band structure provide a detailed investigation of the electronic structure of the A-type and B-type epitaxial Si---NiSi2(111) interfaces. We find that the Schottky barrier height is dependent not only on the type of interface growth, but also on the interface relaxation distance.

1990 - Temperature effects on the surface properties of metals [Articolo su rivista]
Solonovich, V. K.; Kukharenco, L. V.; Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Metal surfaces, theory and experiment.

1989 - Selfconsistent LMTO calculation for semiconductor clean surfaces [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes
abstract

We show that reliable selfconsistent densities of states and energy bands at the semiconductor-vacuum interface can be obtained by the all-electron linear muffin-tin orbitals (LMTO) method in the atomic sphere approximation (ASA). Our calculation is based on the empty sphere technique, i.e. interstitial spheres with zero atomic number, firstly introduced to well describe within the muffin-tin potential approximation the behaviour of solids with low coordination number, like group IV semiconductors. We show that the semiconductor-vacuum interface can be well described if the semi-infinite vacuum region is simulated by a semi-infinite empty sphere solid. The calculation is firstly applied to the Si(111)(1 × 1) surface, but its extension to more complex systems, like ultrathin overlayer chemisorbed structures, is simple. In this all-electron approach important data like surface core level energy shifts are easily obtained. Furthermore recent developments of the LMTO method, l...

1989 - Selfconsistent calculations of the electronic properties of simple metal surfaces [Articolo su rivista]
Finocchi, F.; Bertoni, Carlo Maria; Ossicini, Stefano
abstract

Jellium calculations

1988 - The electronic properties of Si(111)-NiSi2(111) interfaces [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes
abstract

LMTO Calculations

1988 - The electronic properties of Silicon - Silicide epitaxial interfaces [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes
abstract

The selfconsistent electronic properties of the epitaxial Si(111)-NiSi2 interface are computer for the experimentally observed type-A and type-B growth orientations. The densities of states projected on the various sites provide a detailed analysis of the silicon- silicide interface electronic states. The measured Schottky barrier heights may be interpreted by assuming that deviations from epitaxy must be taken into account.

1987 - Electron density profiles at charged metal surfaces in the weighted density approximation [Articolo su rivista]
Ossicini, Stefano; Finocchi, F.; Bertoni, Carlo Maria
abstract

We present a selfconsistent calculation of the electron distribution at metal surfaces in thepresence of a static electric field in the semi-infinite jellium model. The calculation is based on theHohenberg-Kohn-Sham theory with a nonlocal approximation for the exchange-correlationenergy and potential. For different metallic density r s = 2, 3, 4 we present numerical results for themean position and the spread of the induced charge density. The results for the centroid of thecharge induced by a small field are discussed and compared with the image plane position valuesobtained from the exchange-correlation potential.

1987 - First principle investigation of the electronic properties of silicon-silicides interfaces [Articolo su rivista]
Bisi, Olmes; Ossicini, Stefano
abstract

We review the theoretical studies on the reactive interfaces between silicon and transition metals. Much information on the interface compounds can be obtained through an analysis of the electron states of bulk compounds of different structure and stoichiometry. A more detailed analysis of the interface reaction and typical surface information, like the interface geometry or the Schottky barrier height B, can be obtained only through direct investigations of the interface structure. The difficulty of interface calculations can be overcome by using the linear methods devised by Andersen, like the linear augmented plane wave (LAPW) and the linear muffin tin orbitals (LMTO) methods. The LMTO method has been adapted to perform a self-consistent calculation of the electronic properties of the Si(111)-NiSi2 (111) epitaxial interface. This interface has been investigated for both the experimentally observed type-A and type-B orientations. The densities of states projected on the various sites provide, for the first time, a detailed analysis of the interface electronic states. The perturbation due to the interface is shown to be very strong and various interface features are found on the silicon and silicide layers. The computed B is different for the two orientations, with a value of 1.1 eV for the type-A orientation, and 0.8 eV for the staking-fault type-B interface.

1987 - Image force effects in the barrier height for metal-metal tunneling electrons [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

The density-functional theory is applied to calculate the interface barrier height between two metals as a function of the metal separation. Image-force effects are considered, since a nonlocal exchange-correlation energy functional is used. It is shown that the inclusion of nonlocality in the description of the exchange-correlation potential is important for a correct description of the barrier height, which turns out to be considerably lowered for metal-metal separations greater than a few angstroms.

1987 - Image potential at metal surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

The density-functional theory is applied to calculate the self-consistent electron density distribution and the surface potential at metal surfaces with different electron densities. The surface potential yields the correct long-range image potential, since a nonlocal exchange-correlation energy functional is used. It is shown that within this scheme it is possible to calculate the position of the image plane as a function of the bulk density, the influence of the image potential on the binding energy of the image potential states, and the correct behavior of the interface barrier height for metal–metal tunneling electrons.

1987 - Image potential effects on the metal-metal barrier height [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

Jellium non local calculation

1986 - Image plane for surface potential [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

A method of describing the nonlocality of exchange and correlation energy and potential, which gives the correct long-range imagelike behaviour, is adopted in a self-consistentcalculation of a solid surface in the framework of the jellium model. The exchange-correlation potential Vxc(x) whithin the approximation proposed by Gunnarson and Jones, is written for asystem with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential V seen by an electron are self-consistently calculated.We show that within this scheme it is possible to calculate the position of the image plane, as a function of the bulk density.

1986 - Image plane position for metal surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

Metal surfaces.

1986 - Interaction potentials between rare gas atoms and metal surfaces [Articolo su rivista]
Ossicini, Stefano
abstract

The interaction potential of rare-gas atoms with metal surfaces is calculated following the Zaremba-Kohn scheme. The attractive part of the potential is described by the usual van der Waals form. The change in the energy eigenvalues of the metal electron is calculated by using a pseudopotential formalism for the rare-gas-atom perturbation, the metal electrons being described by the atomic wave functions. It turns out that this model gives a qualitatively and semiquantitatively good description of the potential experienced by the metal s electrons at the surface. Calculations are performed not only for helium interactions with the noble metals, but also in the case of the other heavier rare gases.

1986 - Non-local exchange and correlation in the jellium model of surfaces [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria; Gies, P.
abstract

A method of describing the non-locality of exchange and correlation energy and potential. used by the authors in a density-functional calculation of atomic structure, is now adopted in a self-consistent calculation of a solid surface in the framework of the jellium model. The exchange-correlation potential VXC(z), within the approximation proposed by Gunnarsson and Jones, is written for a system with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential Veff(z) seen by an electron are self-consistently calculated for different electron densities (rs varying from 2 to 6). The behaviour of the different contributions to the surface potential, the intensity of the Friedel oscillations, the surface energies, and the image-plane position z0 are presented, discussed and compared with the results of the local density approximation.

1985 - AES analysis of the growth mechanism of metal layers on metal surfaces [Articolo su rivista]
Ossicini, Stefano; Ciccacci, F; Memeo, R.
abstract

A discussion on the growth of thin metal films on metal substrates on the basis of the Auger technique is presented. Within a simple model the characteristics of the different growths are related to the adsorbate and substrate Auger peak-to-peak intensity ratios. The behavior of these ratios as a function of the number of deposited atoms is studied in the case of the Frank–van der Merwe, Volmer–Weber, and Stranski–Krastanov growth mechanism. A comparison between our calculations and reported measurements is discussed in detail for the Pd/W(110), Au/Pt(100), and Pt/Au(100) systems.

1985 - Density functional calculation of atomic structure with non-local exchange and correlation [Articolo su rivista]
Ossicini, Stefano; Bertoni, Carlo Maria
abstract

A method of describing the nonlocality of the exchange and correlation energy and potential is used in a self-consistent spinless atomic calculation. The exchange and correlation potential V is analytically written and used in the calculation of the total energies and orbitals eigenvalues of the self-consistent Kohn-Sham equations for the atomic case. The results for the lighter atoms and for other examples are discussed and compared with the results obtained within the framework of the local-density approximation (LDA). We also present an analysis of the shape and asymptotic behavior of the potentials. A comparison of the outcome of the method with the results of LDA, Hartree-Fock calculations, and other approximate descriptions of nonlocality effects clearly indicates the power of the method.

1985 - Removal of orbital degeneracy of the atomic P state for matrix isolated metal atoms [Articolo su rivista]
Ruffolo, M. G.; Ossicini, Stefano; Forstmann, F.
abstract

A theoretical study is made of the matrix effects on the absorption spectra of the noble metal atoms Cu, Ag, and Au isolated in Ar, Kr, and Xe matrices. The matrix influence which is discussed in terms of level shifts and additional crystal field splittings is calculated by a nonlocal pseudopotential method. In order to get a threefold splitting of the P level, we assume an axial symmetry for the metal atom site which is caused by a distorted neighborhood in a fcc matrix. This is the first theoretical investigation of the matrix cage distortion around the impurity. The results show that only small variations of the nearest neighbor distances between isolated atoms and matrix atoms are necessary to account for the observed matrix perturbations.

1985 - Selftrapped exciton bubble size growth in solid Ne [Articolo su rivista]
Ossicini, Stefano
abstract

A pseudopotential method is suggested for the calculation of the lattice perturbation on the absorption spectra of excited Ne atoms. The calculated shifts of the transition energies are very close to the experimental values. Different models are proposed in order to explain the formation process of a microcavity around the excited Ne atom.

1985 - The He *(1s, 2s)+Ne interaction potential [Articolo su rivista]
Ossicini, Stefano
abstract

A well depth D = 0.79 meV at R = 11.56 au is calculated for the excited He(1s, 2s) atom interacting with Ne by using a non-local pseudopotential method in good agreement with the experimental values. The pseudopotential I use also contains polarization effects in an adiabatic form. The contribution of the polarization to the well depth is calculated to be 0.42 meV.

1984 - Removal orbital degeneracy for matrix isolated atoms [Articolo su rivista]
Ossicini, Stefano; Ruffolo, M. G.
abstract

Pseudopotential calculations

1983 - Matrix influence on the optical spectra of isolated Mg and Ca atoms [Articolo su rivista]
Ossicini, Stefano; Fortsmann, F.
abstract

Optical spectra calculations

1983 - On the use of Auger technique for quantitative analysis of overlayers [Articolo su rivista]
Memeo, R.; Ciccacci, F.; Mariani, C.; Ossicini, Stefano
abstract

Within a simple model we discuss here the results of a calculation which contributes to the reliability of the use of the Auger technique as a quantitative tool. We consider the adsorption of foreign species on a substrate for three different cases: the submonolayer region, uniform thin film growth and island formation. We show that in all the cases it is possible to find a relation involving the peak-to-peak height ratios of the adsorbate and substrate Auger lines that gives the following information about the system: absolute coverage, thickness of the film and characteristics of the island growth respectively for the above cases. For the submonolayer region, only one Auger spectrum is necessary to evaluate the coverage. In the other cases it is also possible to establish whether the growth proceeds layer by layer or by island formation.

1982 - Isolated noble-metal atoms in a Neon matrix [Articolo su rivista]
Ossicini, Stefano; Forstmann, F.
abstract

By a pseudopotential method, the perturbation of the outer electronic s and p levels of Cu, Ag, and Au atoms by a Ne matrix is calculated. It is concluded that for Cu and Au the trapping site consists of three vacancies. Two sites of three and four vacancies give the two different "triplets" in the absorption spectrum of Ag.

1982 - Matrix trapping site for H atoms trapped in Ne and Ar [Articolo su rivista]
Ossicini, Stefano; Forstmann, F.
abstract

The optical absorption spectra of H atoms isolated in Ne and Ar matrices are analysed. The first transition is calculated by a pseudopotential method and we can conclude that H atoms are trapped in substitutional sites. The higher excitations can be understood by the exciton formula for the host.

1982 - Stable trapping site for matrix isolated Li [Articolo su rivista]
Ossicini, Stefano; Forstmann, F.
abstract

Matrix isolated atomic optical spectra

1981 - Matrix effects on the optical spectra of alkali atoms trapped in Ar, Kr and Xe matrices: a pseudopotential calculations [Articolo su rivista]
Ossicini, Stefano; Forstmann, F.
abstract

By a pseudopotential method the perturbation of the outer electronic s and p levels of Na and K by rare gas matrices is calculated. It is concluded that the stable trapping site consists of four vacancies, except for a substitutional site for Na in Xe. A metastable site of three vacancies gives the ''red triplet'' in the absorption spectrum.

1980 - The influence of rare gas matrix on the electronic levels of isolated atoms [Articolo su rivista]
Forstmann, F.; Ossicini, Stefano
abstract

A pseudopotential model is suggested for the calculation of the matrix perturbation on the spectra of isolated atoms. The calculated shifts of the ground state and of the excited state have different signs. The energy of the outer electron of the isolated atom is lowered in the substitutional site. A blue shift for the PS absorption lines of the right magnitude is calculated for Cu, Ag, and Au atoms on substitutional sites in Ar, Kr, and Xe matrices.

1979 - Electronic structure of the (111) ideal and relaxed surfaces of silicon by the chemical pseudopotential method [Articolo su rivista]
Casula, F.; Ossicini, Stefano; Selloni, A.
abstract

The electronic energy structure for the (111)ideal and relaxed surfaces of silicon is calculated by the chemical pseudopotential method. We use a minimal basis set of localized orbitals and an atomic-like crystal potential to compute the interaction parameters and include self-consistency. Results are compared with other more involved theoretical calculations with satisfactory agreement.

1978 - Electronic vacancy states in silicon by the chemical pseudopotential method [Articolo su rivista]
Casula, F.; Ossicini, Stefano; Selloni, A.
abstract

The electronic states of a neutral vacancy in Si are studied through the chemical pseudopotential method by creating a vacancy in a large crystal unit cell containing up to 54 atoms. A localized vacancy state is found in the forbidden gap and its energy is shown to be convergent with respect to the size of the cell. The density of states of the valence band is modified by the presence of the vacancy with additional peaks which give charge localization on the vacany nearest neighbour atoms.

1977 - Chemical pseudopotential and semiconductor surface states [Articolo su rivista]
Selloni, A; Ossicini, Stefano; Tosatti, E.
abstract

Chemical pseudopotential calculation of silicon surfaces