Search

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., S. O. acknowledges support/funding from University of Modena and Reggio Emilia under project “FAR2017INTERDISC”. S. O. and I. M. thank the Super-Computing Interuniversity Consortium CINECA for support and high-performance computing resources under the Italian Super-Computing Resource Allocation (ISCRA) initiative, PRACE for awarding us access to the resource MARCONI HPC cluster based in Italy at CINECA. I. M. acknowledges support/funding from European Union H2020-EINFRA-2015-1 and H2020-INFRAEDI-2018-1 programs under grant agreement No. 676598 and No. 824143, project MaX-MAterials at the eXascale. M. A. greatly acknowledges the Transnational Access Programme of the HPC-EUROPA3 (project HPC17PB9IZ). Some of the high-performance computing (HPC) resources for this project were granted by the Institut du development et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipment National de Calcul Intensif). This work was supported by the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. M. P. acknowledges INFN for financial support through the National project Nemesys. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) and MICIU (Ministerio de Ciencia y Universidades) under Grants FEDER-MAT2017-90024-P, FIS2015-64886-C5-4-P and PGC2018-096955-B-C44-P, the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and the Generalitat de Catalunya under Grant 2017 SGR 1506.

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.

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 a first 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., M.A. greatly acknowledges the Transnational Access Programme of the HPC-EUROPA3 (project HPC17PB9IZ). Part of the high-performance computing (HPC) resources for this project were granted by the Institut du developpement et des ressources en informatique scientifique (IDRIS) under the allocation A0040910089 via GENCI (Grand Equipement National de Calcul Intensif). This work was supported by the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grants FEDER-MAT2017-90024-P and FIS2015-64886-C5-4-P, the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496, the Generalitat de Catalunya under Grants 2017 SGR 1506. S.O. acknowledges support/funding from University of Modena and Reggio Emilia under project ”FAR2017INTERDISC”.

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.

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.

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

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 13- 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 outcorne. 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 self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects. (C) 2006 Elsevier B.V. All rights reserved.

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: interfacing Si with different materials and/or introducing strain. In this paper we theoretically investigate second-harmonic generation, described by the second-order nonlinear susceptibility ?(2) in Si/Ge heterostructures. The role of symmetry and strain will be carefully analyzed also through a comparison with the computed results for unstrained and strained bulk Si and SiC systems, the first system being initially centrosymmetric, the second from the start non centrosymmetric. Thus we are able to elucidate the type of strain and symmetry breaking necessary to induce, tune and enhance second-harmonic generations in different energy regions for Si-based systems. Copyright © 2017 American Scientific Publishers All rights reserved.

The research of new materials to complement or replace traditionalbulk semiconductors for photonic and photovoltaic applicationsis a very active field. Suitable candidates to carry this task arerepresented by silicon nanocrystals. Thesematerials present uniquechemical and physical properties which are intrinsically associatedwith their low dimensionality and with the quantum confinementeffect. Despite their great relevance, full comprehension of theseproperties is still lacking and many fundamental issues needdeep investigation. In this context the role of theoretical modelingand simulations is extraordinarily important. It is a widespreadbelief among the nanoscience research community that ab initioapproaches constitute a unique and very powerful instrument tocontrol and design the properties of novel materials and deviceswith an accuracy that complements experimental observations.Here we present density-functional and many-body perturbationtheory calculations that have been carried out in these last yearsin our group in order to study the structural, electronic, optical,and transport properties of such nanocrystals, showing that manyexperimental results can be simulated and understood throughtheoretical studies.

Being a source of clean and renewable energy, the possibility to convert solar radiation in electric current with high efficiency is one of the most important topics of modern scientific research. Currently the exploitation of interaction between nanocrystals seems to be a promising route to foster the establishment of third generation photovoltaics. Here we adopt a fully ab-initio scheme to estimate the role of nanoparticle interplay on the carrier multiplication dynamics of interacting silicon nanocrystals. Energy and charge transfer-based carrier multiplication events are studied as a function of nanocrystal separation showing 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 atomistic scale which conditions maximize the outcome., Comment: Supplementary materials are freely available online

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.

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

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 occupy nearest 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 to light 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.

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.

In this work we study the optical properties of hydrogen-passivated, free-standing silicon and germanium nanowires, oriented along the [I 00] [I I 0] [I I I I 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 [100] direction, as due to the different band structures of the corresponding bulk compounds. (C) 2007 Elsevier B.V. All rights reserved.

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.

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 (1 0 0)-oriented films the band gap becomes direct for both semiconductors, whereas the (1 1 0)- and (1 1 1)-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 Γ 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.

International audience; 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 a way 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.

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.

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 an electron 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 reliable estimations of the screening function, that well compares to recent ab-initio calculations.

International audience; 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. (C) 2014 AIP Publishing LLC.

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 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.

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 nano-structures 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 and optical 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 of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

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.

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.

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.

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.

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.

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.

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.

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.

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 AIP Publishing LLC.

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.

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 Ge x alloy nanowires with tunable Ge concentration, core–shell structures and multiple axial junctions, i.e. superlattices. In this chapter we review the most recent progresses in this field.

The theoretical analysis of two different Si wires of size 5\ifmmode\times\else\texttimes\fi{}4 and 3\ifmmode\times\else\texttimes\fi{}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 as responsible of the luminescence transition; (iv) the spatial localization of the valence- and conduction-band states participating in the luminescence transition shows that all the Si atoms of the wire are collectively involved. \textcopyright{} 1996 The American Physical Society.

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., (FP7/2007-2013), Grant Agreement No. 245977

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.

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.

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 for 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.

A systematic theoretical study of the work function behavior for Si1-xGex heterostructures 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. (C) 2016 AIP Publishing LLC.

In this work we present the ab initio study of crystal local-field effects in second-harmonic generation spectroscopy for an interface material such as Si/CaF2. Starting from an independent particle picture, we demonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimation of the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF2 interface was done by a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that the microscopic 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 absorption it is almost rigid.We also compare our theoretical study with experimental data showing unambiguously that only when crystal local fields are included, it is possible to correctly interpret experimental results.

International audience; 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 experimentalobservations 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.

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 modification of 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.

Silicon photonics is a relatively recent research field1–4 that aims to substitute electrons with photons as the carriers of information in devices such as planar lightwave circuits. The technology marries the device electronic performance of traditional semiconductor materials with the speed and bandwidth offered by light. Although in the linear regime a silicon photonic circuit can efficiently guide, modulate, and detect IR light, in the nonlinear (i.e., ultrafast, ultrahigh capacity) regime the situation is more complex. In particular, nonlinear second-order optical phenomena (e.g., the electro-optic effect) are needed to generate, convert, and modulate light at the desired speeds inside complex networks of silicon wires. Nonlinear silicon photonics has so far exploited the natural third-order optical nonlinearities— .3/—of silicon (e.g., optical-field-induced birefringence) for use in security, military, and biosensing applications. Consequently, substantial research effort has gone into enhancing these properties and to applying them to novel all-optical photonic devices.5 A few years ago, a second-order nonlinear optical susceptibility— .2/—was reported for a so-called Mach-Zehnder interferometer based on a strained silicon photonic band-gap waveguide.6 While .2/ is prohibited by the centro-symmetry of the silicon crystal itself, being able to induce it in silicon would provide access to a wide range of nonlinear optical devices at power densities intrinsically lower than those allowed by mere third-order effects. Our recent contribution7 to this field has unambiguously shown, both theoretically and experimentally, that silicon can be efficiently stressed to become a good second-order nonlinear crystal. We computed the second-order nonlinear optical response from strained bulk silicon ab initio using time-dependent Figure 1. Simulated second-order nonlinear optical susceptibility— .2/—for weakly strained silicon (Si) (black), medium-strained Si (yellow), and heavily strained Si (red). The hierarchy of the increasing strain in the Si models is indicated quantitatively by the values of pressure (HY) and shear stress (SS) as reported in the legend. w: Energy of the pump photons.