97 research outputs found
Effect of Strain on Interactions of ÎŁ3{111} Silicon Grain Boundary with Oxygen Impurities from First Principles
The interaction of grain boundaries (GBs) with inherent defects and/or impurity elements in multicrystalline silicon plays a decisive role in their electrical behavior. Strain, depending on the types of GBs and defects, plays an important role in these systems. Herein, the correlation between the structural and electronic properties of ÎŁ3{111} Si-GB in the presence of interstitial oxygen impurities is studied from the first-principles framework, considering the global and local model of strain. It is observed that the distribution of strain along with the number of impurity atoms modifies the energetics of the material. However, the electronic properties of the considered Si-GBs are not particularly affected by the strain and by the oxygen impurities, unless a very high local distortion induces additional structural defects
Defects and strain enhancements of second-harmonic generation in Si/Ge superlattices
International audienceStarting 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. (C) 2014 AIP Publishing LLC
Structural and Dynamic Characterization of Li-Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach
Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li-PF6, Li-BF4, and Li-TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation-anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation-anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions
Second-harmonic generation in silicon waveguides strained by silicon nitride
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 present
owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material
engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low
optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide
by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle
calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40pm/V at
2.300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light
sources spanning the near- to mid-infrared spectrum from 1.2 to 10 micron
Screening in semiconductor nanocrystals: \textit{Ab initio} results and Thomas-Fermi theory
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 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
Role of interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2
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
Optical properties of structurally-relaxed Si/SiO superlattices: the role of bonding at interfaces
We have constructed microscopic, structurally-relaxed atomistic models of
Si/SiO superlattices. The structural distortion and oxidation-state
characteristics of the interface Si atoms are examined in detail. The role
played by the interface Si suboxides in raising the band gap and producing
dispersionless energy bands is established. The suboxide atoms are shown to
generate an abrupt interface layer about 1.60 \AA thick. Bandstructure and
optical-absorption calculations at the Fermi Golden rule level are used to
demonstrate that increasing confinement leads to (a) direct bandgaps (b) a blue
shift in the spectrum, and (c) an enhancement of the absorption intensity in
the threshold-energy region. Some aspects of this behaviour appear not only in
the symmetry direction associated with the superlattice axis, but also in the
orthogonal plane directions. We conclude that, in contrast to Si/Ge, Si/SiO
superlattices show clear optical enhancement and a shift of the optical
spectrum into the region useful for many opto-electronic applications.Comment: 11 pages, 10 figures (submitted to Phys. Rev. B
Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results
Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole
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