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Papers by stefano ossicini

Physical Review B, Aug 30, 2013

We study the details of electronic transport related to the atomistic structure of silicon quantu... more 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.

Physical Review B, Jun 9, 2010

In recent years many experiments have demonstrated the possibility to achieve efficient photolumi... more In recent years many experiments have demonstrated the possibility to achieve efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known that only a minor portions of the nanocrystals in the samples contribute to the observed photoluminescence, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task. Focusing on this aspect we have addressed the problem theoretically, by calculating the radiative recombination rates for different classes of Si-nanocrystals in the diameter range of 0.2-1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination rates of hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is the principal quantity in determining the system response. Interestingly, a completely different behavior emerges from the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seems intimately connected to the recombination rates, resulting the most important quantity for the characterization of the optical yield in such systems. Besides, additional conditions for the achievement of high rates are constituted by a high crystallinity of the nanocrystals and by high confinement energies (small diameters).

The Journal of Physical Chemistry C

physica status solidi c

First‐principles calculations of work function tuning induced by different chemical terminations ... more 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.

Progress in Surface Science, 2017

Si nanocrystals have been extensively studied because of their novel properties and their potenti... more 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.

Nano Letters, 2019

We studied the physics of common pand n-type dopants in hexagonal-diamond Si-a Si polymorph that ... more We studied the physics of common pand n-type 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 hexagonal-diamond phase with respect to the cubic one as

physica status solidi (a), 2017

The properties of n-and p-doped silicon nanocrystals obtained through ab-initio calculations are ... more The properties of n-and p-doped silicon nanocrystals obtained through ab-initio calculations are reviewed here. The aim is the understanding of the effect of substitutional doping on the structural, electronic and optical properties of freestanding and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through totalenergy 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 function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally we discuss the case of B and P co-doped nanocrystals 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 sizedependent red shift of the absorption spectra. Structure of a co-doped Si85BPH76 nanocrystal, diameter 1.56 nm. Yellow (gray) balls represent Si atoms, while the white (light gray) balls are the H atoms used to saturate the surface danglind bonds. B (magenta, dark gray) and P (black) dopants are here located in subsurface positions.

We present an overview of the project NASCEnT and the first results we were able to achieve after... more We present an overview of the project NASCEnT and the first results we were able to achieve after one year of project duration. The overall objective of the project is to develop new nano-materials with new production technologies and to fabricate silicon quantum dot (Si QD) materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nano-materials will enable us to design novel solar cell structures that help to overcome the efficiency limits of conventional solar cell concepts. So far we developed simple device structures and process sequences for both material systems (Si QDs in silicon oxide and silicon carbide) and tested a pin solar cell structure. Another objective of the project is to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept ...

Physical Review B, 2003

Light-emitting silicon nanocrystals embedded in SiO 2 have been investigated by x-ray absorption ... more Light-emitting silicon nanocrystals embedded in SiO 2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO 2 is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric SiO 2. This region plays an active role in the light-emission process.

Nanoscale, 2015

For B or P substitutional doping in Si/SiO2quantum dots we indicate, respectively, interfacial an... more For B or P substitutional doping in Si/SiO2quantum dots we indicate, respectively, interfacial and sub-interfacial sites as the most energetically-favored ones. B-doping enhances hole-current at a low voltage, while P-doping enhances electron-current at low and high voltage.

Physical Review B, 2002

Optical absorption and light emission of oxygen-incorporated small silicon (Si 30 H 40 O i) pyram... more Optical absorption and light emission of oxygen-incorporated small silicon (Si 30 H 40 O i) 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 3͒. In the absolute majority of the cluster configurations with low oxygen content (iϽ4) the excitations have a strong localized character and result in a significant shift from its equilibrium ground state position of one of the silicon atoms inside the cluster volume. The optical transition energies in those cases range from 2.05 to 2.35 eV. The typical Stokes shift for these structures is of the order of 100 meV. However, for some particular cluster configurations the excitations are localized at silicon sites directly adjacent to embedded oxygen atoms and this results in a considerable reduction of the emission energy down to approximately 1.40-1.60 eV and in an increase of the Stokes shift values to 600-800 meV. The same behavior was traced out for the case where the presence of a silanone (SiϭO) bond at the surface of the Si 30 H 38 O cluster is considered. For intermediate and high oxygen content (iϾ4) 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.

International Journal of Photoenergy, 2012

The combination of photonics and silicon technology is a great challenge because of the potential... more 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, ...

International Journal of Photoenergy, 2012

Surface Science Reports, 2000

The striking photoluminescence properties of porous silicon have attracted considerable research ... more The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum con®ned 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 (of the order of 500 m 2 acm 3). 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 various luminescence bands are analyzed and the underlying basic mechanisms are presented. In the quest of an ef®cient electroluminescent device, we survey the results for several porous silicon contacts, with particular attention to the interface properties, to the stability requirement and to the carrier injection mechanisms. Other device applications are discussed as well.

Superlattices and Microstructures, 2008

Doping control at the nanoscale can be used to modify optical and electronic properties thus indu... more 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 shown that 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 localized on 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 Band P-codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality, 0versus 1-dimensionality, of the systems.

Physical Review B, 2006

A first-principles calculation of the impurity screening in Si and Ge nanocrystals is presented. ... more 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 pointcharge 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 in the screening. Our first-principles calculations and the Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field effects are dominated by surface polarization, which causes a reduction of the screening in going from the bulk down to the nanoscale. Finally, the model screening function is compared with recent state-of-the-art ab initio calculations and tested with impurity activation energies.

Physical Review B, 2010

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

Physical Review B, 2007

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometr... more The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometry optimization and the many-body effects induced by the creation of an electron-hole pair have been calculated within a first-principles framework. Starting from hydrogenated silicon clusters of different size, 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 visible range that are in fair agreement with the experimental outcomes.

Physical Review B, 2005

Density-functional theory calculations have been carried out in order to study the structural, el... more 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 SiO 2. 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 Siv 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. For the silicon nanocrystals embedded in a SiO 2 matrix, the electronic and 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.

Physical Review B, 2008

The effects of the incorporation of group-III ͑B and Al͒, group-IV ͑C and Ge͒, and group-V ͑N and... more The effects of the incorporation of group-III ͑B and Al͒, group-IV ͑C and Ge͒, and group-V ͑N and P͒ 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 ab initio 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 the impurity formation energies decrease significantly when the nanocrystallites are codoped with B and P or with Al and P. Additional peaks are introduced in the absorption spectra due to codoping, giving rise to a redshift of the 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.

Physical Review B, Aug 30, 2013

We study the details of electronic transport related to the atomistic structure of silicon quantu... more 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.

Physical Review B, Jun 9, 2010

In recent years many experiments have demonstrated the possibility to achieve efficient photolumi... more In recent years many experiments have demonstrated the possibility to achieve efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known that only a minor portions of the nanocrystals in the samples contribute to the observed photoluminescence, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task. Focusing on this aspect we have addressed the problem theoretically, by calculating the radiative recombination rates for different classes of Si-nanocrystals in the diameter range of 0.2-1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination rates of hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is the principal quantity in determining the system response. Interestingly, a completely different behavior emerges from the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seems intimately connected to the recombination rates, resulting the most important quantity for the characterization of the optical yield in such systems. Besides, additional conditions for the achievement of high rates are constituted by a high crystallinity of the nanocrystals and by high confinement energies (small diameters).

The Journal of Physical Chemistry C

physica status solidi c

First‐principles calculations of work function tuning induced by different chemical terminations ... more 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.

Progress in Surface Science, 2017

Si nanocrystals have been extensively studied because of their novel properties and their potenti... more 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.

Nano Letters, 2019

We studied the physics of common pand n-type dopants in hexagonal-diamond Si-a Si polymorph that ... more We studied the physics of common pand n-type 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 hexagonal-diamond phase with respect to the cubic one as

physica status solidi (a), 2017

The properties of n-and p-doped silicon nanocrystals obtained through ab-initio calculations are ... more The properties of n-and p-doped silicon nanocrystals obtained through ab-initio calculations are reviewed here. The aim is the understanding of the effect of substitutional doping on the structural, electronic and optical properties of freestanding and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through totalenergy 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 function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally we discuss the case of B and P co-doped nanocrystals 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 sizedependent red shift of the absorption spectra. Structure of a co-doped Si85BPH76 nanocrystal, diameter 1.56 nm. Yellow (gray) balls represent Si atoms, while the white (light gray) balls are the H atoms used to saturate the surface danglind bonds. B (magenta, dark gray) and P (black) dopants are here located in subsurface positions.

We present an overview of the project NASCEnT and the first results we were able to achieve after... more We present an overview of the project NASCEnT and the first results we were able to achieve after one year of project duration. The overall objective of the project is to develop new nano-materials with new production technologies and to fabricate silicon quantum dot (Si QD) materials for all-silicon tandem solar cells to achieve increased efficiencies. The understanding of electrical transport and recombination mechanisms in these newly developed nano-materials will enable us to design novel solar cell structures that help to overcome the efficiency limits of conventional solar cell concepts. So far we developed simple device structures and process sequences for both material systems (Si QDs in silicon oxide and silicon carbide) and tested a pin solar cell structure. Another objective of the project is to ensure the compatibility of the newly developed technologies with high-throughput processing leading to further cost-reduction. Within the scope of this project the novel concept ...

Physical Review B, 2003

Light-emitting silicon nanocrystals embedded in SiO 2 have been investigated by x-ray absorption ... more Light-emitting silicon nanocrystals embedded in SiO 2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO 2 is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric SiO 2. This region plays an active role in the light-emission process.

Nanoscale, 2015

For B or P substitutional doping in Si/SiO2quantum dots we indicate, respectively, interfacial an... more For B or P substitutional doping in Si/SiO2quantum dots we indicate, respectively, interfacial and sub-interfacial sites as the most energetically-favored ones. B-doping enhances hole-current at a low voltage, while P-doping enhances electron-current at low and high voltage.

Physical Review B, 2002

Optical absorption and light emission of oxygen-incorporated small silicon (Si 30 H 40 O i) pyram... more Optical absorption and light emission of oxygen-incorporated small silicon (Si 30 H 40 O i) 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 3͒. In the absolute majority of the cluster configurations with low oxygen content (iϽ4) the excitations have a strong localized character and result in a significant shift from its equilibrium ground state position of one of the silicon atoms inside the cluster volume. The optical transition energies in those cases range from 2.05 to 2.35 eV. The typical Stokes shift for these structures is of the order of 100 meV. However, for some particular cluster configurations the excitations are localized at silicon sites directly adjacent to embedded oxygen atoms and this results in a considerable reduction of the emission energy down to approximately 1.40-1.60 eV and in an increase of the Stokes shift values to 600-800 meV. The same behavior was traced out for the case where the presence of a silanone (SiϭO) bond at the surface of the Si 30 H 38 O cluster is considered. For intermediate and high oxygen content (iϾ4) 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.

International Journal of Photoenergy, 2012

The combination of photonics and silicon technology is a great challenge because of the potential... more 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, ...

International Journal of Photoenergy, 2012

Surface Science Reports, 2000

The striking photoluminescence properties of porous silicon have attracted considerable research ... more The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum con®ned 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 (of the order of 500 m 2 acm 3). 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 various luminescence bands are analyzed and the underlying basic mechanisms are presented. In the quest of an ef®cient electroluminescent device, we survey the results for several porous silicon contacts, with particular attention to the interface properties, to the stability requirement and to the carrier injection mechanisms. Other device applications are discussed as well.

Superlattices and Microstructures, 2008

Doping control at the nanoscale can be used to modify optical and electronic properties thus indu... more 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 shown that 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 localized on 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 Band P-codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality, 0versus 1-dimensionality, of the systems.

Physical Review B, 2006

A first-principles calculation of the impurity screening in Si and Ge nanocrystals is presented. ... more 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 pointcharge 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 in the screening. Our first-principles calculations and the Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field effects are dominated by surface polarization, which causes a reduction of the screening in going from the bulk down to the nanoscale. Finally, the model screening function is compared with recent state-of-the-art ab initio calculations and tested with impurity activation energies.

Physical Review B, 2010

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

Physical Review B, 2007

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometr... more The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter including geometry optimization and the many-body effects induced by the creation of an electron-hole pair have been calculated within a first-principles framework. Starting from hydrogenated silicon clusters of different size, 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 visible range that are in fair agreement with the experimental outcomes.

Physical Review B, 2005

Density-functional theory calculations have been carried out in order to study the structural, el... more 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 SiO 2. 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 Siv 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. For the silicon nanocrystals embedded in a SiO 2 matrix, the electronic and 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.

Physical Review B, 2008

The effects of the incorporation of group-III ͑B and Al͒, group-IV ͑C and Ge͒, and group-V ͑N and... more The effects of the incorporation of group-III ͑B and Al͒, group-IV ͑C and Ge͒, and group-V ͑N and P͒ 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 ab initio 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 the impurity formation energies decrease significantly when the nanocrystallites are codoped with B and P or with Al and P. Additional peaks are introduced in the absorption spectra due to codoping, giving rise to a redshift of the 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.