The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemic... more The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical–chemical properties of the material. Therefore, the GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different [Formula: see text]3{112} Si-GB models. These models are ([Formula: see text]) and ([Formula: see text]) [Formula: see text]3{112} Si-GBs, which lead to different structural reconstruction. Starting from these two models, we have shown that geometry optimization has an important role on the structural reconstruction of the GB interface and, therefore, on its properties. For this reason, we discussed different methodologies to define an optimal relaxation protocol. The influence of the local structures in ([Formula: see text]) and ([Formula: se...
The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 h... more 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 energy calculations. 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 comparison between 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.
ABSTRACTWe developed an ab initio formalism based on Time-Dependent Density-Functional Theory for... more ABSTRACTWe developed an ab initio formalism based on Time-Dependent Density-Functional Theory for the calculation of the second-order susceptibility Χ(2) (Luppi et al. J. Chem. Phys. 132, 241104(2010)). We apply this formalism to the calculation of second-harmonic generation spectra of hexagonal SiC polytypes, ZnGeP2 (ZGP) and GaP. Starting from the independent-particle approximation, we include manybody effects, such as quasiparticle via the scissors operator, crystal local fields and excitons. We consider two different types of kernels: the ALDA and the “long-range” kernel. We analyze the effects of the different electron-electron descriptions in the spectra, finding good agreement with experiments.
EPIOPTICS-8 - Proceedings of the 33rd Course of the International School of Solid State Physics, 2006
We review the state of the art of the theoretical approaches for ab initio studies of the electro... more We review the state of the art of the theoretical approaches for ab initio studies of the electronic and optical properties of matter. Examples within Density Functional Theory, Many-Body perturbation Theory and Time Dependent Density Functional Theory are presented and discussed. The static and the time-dependent DFT avoid dealing directly with the Many-Body equation by mapping the interacting system into
ABSTRACT CMOS circuitry dominates the current semiconductor market due to the astonishing power o... more 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.
Abstract Using first-principle density-functional theory in the GGA approximation we have studied... more 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 ...
... Nersisyan; Svetlana Serak. Light amplification in a liquid network confined in a porous matri... more ... Nersisyan; Svetlana Serak. Light amplification in a liquid network confined in a porous matrix. Author(s): Egon Gross; Dmitri Kovalev; Nicolai Kuenzner; Joachim Diener; Frederick Koch; Victor Y. Timoshenko; Minoru Fujii. Control of ...
A comprehensive understanding of the structural condition and impurity agglomerates in poly-Si st... more A comprehensive understanding of the structural condition and impurity agglomerates in poly-Si structures can be obtained through the topological analysis of ELF for the characterization of chemical changes induced by defects.
Journal of Computational Methods in Science and Engineering, 2007
The absorption and the emission spectra of undoped and doped silicon nanocrystals of different si... more 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 ...
The effects of B and P codoping on the impurity formation energies and electronic properties of S... more 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 optical properties of Si/CaF2 multiple quantum wells are studied ab initio by means of the li... more 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 ~20 Å, new transitions appear in the optical region with an evident polarization dependence. The oscillator strength
The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemic... more The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical–chemical properties of the material. Therefore, the GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different [Formula: see text]3{112} Si-GB models. These models are ([Formula: see text]) and ([Formula: see text]) [Formula: see text]3{112} Si-GBs, which lead to different structural reconstruction. Starting from these two models, we have shown that geometry optimization has an important role on the structural reconstruction of the GB interface and, therefore, on its properties. For this reason, we discussed different methodologies to define an optimal relaxation protocol. The influence of the local structures in ([Formula: see text]) and ([Formula: se...
The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 h... more 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 energy calculations. 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 comparison between 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.
ABSTRACTWe developed an ab initio formalism based on Time-Dependent Density-Functional Theory for... more ABSTRACTWe developed an ab initio formalism based on Time-Dependent Density-Functional Theory for the calculation of the second-order susceptibility Χ(2) (Luppi et al. J. Chem. Phys. 132, 241104(2010)). We apply this formalism to the calculation of second-harmonic generation spectra of hexagonal SiC polytypes, ZnGeP2 (ZGP) and GaP. Starting from the independent-particle approximation, we include manybody effects, such as quasiparticle via the scissors operator, crystal local fields and excitons. We consider two different types of kernels: the ALDA and the “long-range” kernel. We analyze the effects of the different electron-electron descriptions in the spectra, finding good agreement with experiments.
EPIOPTICS-8 - Proceedings of the 33rd Course of the International School of Solid State Physics, 2006
We review the state of the art of the theoretical approaches for ab initio studies of the electro... more We review the state of the art of the theoretical approaches for ab initio studies of the electronic and optical properties of matter. Examples within Density Functional Theory, Many-Body perturbation Theory and Time Dependent Density Functional Theory are presented and discussed. The static and the time-dependent DFT avoid dealing directly with the Many-Body equation by mapping the interacting system into
ABSTRACT CMOS circuitry dominates the current semiconductor market due to the astonishing power o... more 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.
Abstract Using first-principle density-functional theory in the GGA approximation we have studied... more 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 ...
... Nersisyan; Svetlana Serak. Light amplification in a liquid network confined in a porous matri... more ... Nersisyan; Svetlana Serak. Light amplification in a liquid network confined in a porous matrix. Author(s): Egon Gross; Dmitri Kovalev; Nicolai Kuenzner; Joachim Diener; Frederick Koch; Victor Y. Timoshenko; Minoru Fujii. Control of ...
A comprehensive understanding of the structural condition and impurity agglomerates in poly-Si st... more A comprehensive understanding of the structural condition and impurity agglomerates in poly-Si structures can be obtained through the topological analysis of ELF for the characterization of chemical changes induced by defects.
Journal of Computational Methods in Science and Engineering, 2007
The absorption and the emission spectra of undoped and doped silicon nanocrystals of different si... more 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 ...
The effects of B and P codoping on the impurity formation energies and electronic properties of S... more 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 optical properties of Si/CaF2 multiple quantum wells are studied ab initio by means of the li... more 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 ~20 Å, new transitions appear in the optical region with an evident polarization dependence. The oscillator strength
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Papers by E. Degoli