Optical Properties Of Silicon Nanostructures Research Articles

Engineered near- and far-field optical response of dielectric nanostructures using focused cylindrical vector beams

Near- and far-field optical properties of silicon nanostructures under linear polarization (Gaussian beam) and azimuthally or radially focused cylindrical vector beams are investigated by finite-difference time-domain method (FDTD) in Meep open-source software. A python toolkit allowing FDTD simulations in Meep for using those excitation sources is provided. In addition to the preferential excitation of specific electric or magnetic resonance modes as a function of the excitation beam polarization, it is shown in the case of spheroids that shape anisotropy affects the resonance wavelength and the dipole orientation of the magnetic or electric dipole mode. Depending on the spheroid symmetry axis with respect to the electric field orientation, the electric dipole resonance can be split into two peaks, giving quasi-unidirectional scattering, separated by an anapole mode. The optical properties in both far-field (scattering pattern) and near-field (electric and magnetic field hot spots) can be tuned by changing the excitation polarization at a fixed wavelength and selecting properly the spheroid shape and dimensions. These numerical simulations are extended to top-down fabrication-friendly nanostructures such as nanocylinders with circular or elliptic sections.

Morphological, Structural, and Optical Properties of Silicon Nanostructures Formed in a Solution Containing Hydrogen Hexafluorosilicate H2(SiF6)

Photoluminescent boron-doped (100)-oriented porous silicon fabricated on a p-type silicon substrate by electrochemical etching in a solution containing fluorosilicic acid and ethanol is studied. The morphological, structural, and optical properties of silicon nanostructures obtained in solutions containing H2(SiF6) and ethanol are analyzed in comparison with the corresponding characteristics of samples formed in solutions of HF and ethanol. The morphological, structural, and optical properties were studied using scanning probe microscopy and spectrophotometry. It is shown that the porous silicon samples formed in solutions containing H2(SiF6) and ethanol have better optical properties, in particular, they exhibit more intense photoluminescence than the samples obtained in HF–ethanol solutions.

Морфология, структурные и оптические свойства наноструктур кремния, полученные в растворе, содержащем гексафторосиликат водорода H-=SUB=-2-=/SUB=-(SiF-=SUB=-6-=/SUB=-)

Boron doped porous silicon with the observed photoluminescence with a crystallographic orientation of (100), which was fabricated based on a p-type silicon substrate using electrochemical etching in a solution containing hexafluorosilicic acid and ethyl alcohol was studied. A comparative analysis of the morphology, structural and optical properties of silicon nanostructures obtained in a solution containing H2(SiF6) and ethanol and samples obtained in a solution containing HF and ethanol was performed. Morphology, structural and optical properties were studied using scanning probe microscopy and spectrophotometry. It was shown that samples of porous silicon obtained in a solution containing H2(SiF6) and ethanol are characterized by improved optical properties, in particular, they exhibit more intense photoluminescence compared to samples obtained in solutions with HF and ethyl alcohol.

Structural and Optical Properties of Si Nanostructures

Silicon nanostructures were realized using metal assisted chemical etching of silicon wafer (100) for two different etching times: 5 minute and 10 minute. Structural and optical properties of silicon nanostructures were investigated using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoluminescence (PL). XRD analysis probes the changes in crystallinity during etching and reveals reflection plane (220) of obtained silicon nanostructures. UV and visible spectra of photoluminescence from Si nanostructures suggest the optically active nature of Si at nanoscale and confinement of carrier. These results show that the structural and optical properties of Si nanostructures strongly depend on etching time of silicon wafer.

Anomalous Surface Doping Effect in Semiconductor Nanowires

Surface doping is being used as an effective approach to improve the mechanical, optical, electronic, and magnetic properties of various materials. For example, experimental studies have proven that rare-earth element doping can enhance the optical properties of silicon nanostructures. However, the majority of previous investigations focused on either bulk materials or nanosized spherical crystals. Here we present a comparative study on semiconducting germanium (Ge) nanowires with and without surface doping by using multiple integrated characterization probes, including high resolution scanning/transmission electron microscopy (SEM/TEM), in situ high pressure synchrotron X-ray diffraction (XRD), and Raman spectroscopy. Our results reveal that under pressure the stability of the Ge-I phase (diamond structure) in erbium (Er)-doped Ge nanowires is enhanced compared to undoped Ge nanowires. We also found an increased stability of the Ge-II phase (body centered tetragonal structure) in Er-doped Ge nanowires du...

A temperature-related boundary Cauchy–Born method for multi-scale modeling of silicon nano-structures

Abstract The surface, edge and corner effects have significant influences in the electrical and optical properties of silicon nano-structures. In this paper, a novel hierarchical temperature-related multi-scale model is presented based on the boundary Cauchy–Born method to investigate not only the surface but also the edge and corner effects in thermal properties of diamond-like structures such as silicon nano-structures at finite temperature. A combined finite element method and molecular dynamics are respectively employed in macro- and micro-scale levels. The temperature-related Cauchy–Born rule is applied using the Helmholtz free energy, as the energy density of equivalent continua relating to the Tersoff inter-atomic potential. The model employs radial quadratures at the surface, edge and corner elements as an indicator of material behavior. The capability of computational algorithm is illustrated by numerical simulation of a nano-scale cube at finite temperature and the results are compared with the atomistic model.

Size and hydrogen saturation effects on third-order polarizabilities of Si clusters

We report the frequency-dependent third-order polarizabilities (γ) of a series of hydrogen-capped silicon clusters obtained using time-dependent density functional theory. We find that the magnitude of γ increases with a decrease in silicon cluster size, while the sign of γ at low input photon energy changes from positive to negative when the ratio of hydrogen saturation increases. At higher input photon energy, the sign of γ is determined mainly by cluster size. Our results rationalize several recent experiments on the optical properties of silicon nanostructures and are helpful for designing opto-electronic devices based on them.

Porous silicon nanowires fabricated by electrochemical and laser-induced etching

Electrochemical and laser-induced etching processes were simultaneously used to synthesize the nanowires structure of porous silicon (PS). Surface morphology and structural properties of nanostructured silicon were characterized by using scanning electron microscopy (SEM) and atomic forces microscopy (AFM) images. Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical properties of silicon nanostructures were studied. Raman spectra were shifted and broadened relatively to 519.9 cm−1 of PS prepared by electrochemical etching, and shifted to 517.2 cm−1 for laser-induced etching process and to 508.9 cm−1 for electrochemical and laser etching simultaneously. Blue shift luminescence was observed at 649.6 nm for PS produced by electrochemical etching, and at 629.5 nm for laser-induced etching. PS produced a blue shift at 626.5 nm using both etching procedures simultaneously. X-Ray diffraction (XRD) was used to investigate the crystallites size of the PS as well as to provide an estimate of the degree of crystallinty of the etched sample.

Effects of simultaneous doping with boron and phosphorous on the structural, electronic and optical properties of silicon nanostructures

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.

The electronic structure and luminescence properties of porous silicon and silicon nanoclusters

We present the results of ab initio Hartree-Fock and configuration interaction calculations performed on selected small silicon clusters with a view to study their luminescent properties. We found that the covalent character of bonding is much reduced at the surface of the silicon clusters. The charge density of the clusters considered tends to be polarized towards their bulk-like core. This reflects the ‘self-passivation’ character of the chemical bonding in clusters. The optical transitions in the Si 18 cluster are found to have essentially many-electron character. The energies of three lowest optical transitions are found to be in the near-IR region and the corresponding oscillator strengths are 0.0001–0.004. Our results suggest that there is a strong dependence of the optical properties of silicon nanostructures not only on their size but also on their shape, which strongly affects the charge density and the character of correlation between the excited electrons.

Optical Properties of Silicon Nanostructures

AbstractStrong visible luminescence of porous silicon has triggered renewed general interest in the optical properties of silicon nanostructures. In addition to porous Si itself, Si‐based molecules, nanometer‐size Si particles, one‐dimensional Si chains, two‐dimensional Si layers, and many Si alloys are known to exhibit optical properties (absorption and luminescence) which strongly deviate from those of bulk crystalline Si. Here, we review and compare the main properties of the above‐mentioned systems and present a brief discussion of the experimental observations in terms of existing theories. Current prospects for Si‐based optoelectronic applications are also discussed.

Theory of the luminescence of porous silicon

The physical origin of the luminescence bands of porous silicon is discussed using results of calculations of electronic and optical properties of silicon nanostructures. Silicon crystallites, wires or molecules are investigated as possible sources of emission of light. We show that the slow visible band in porous silicon is probably due to quantum confinement in silicon crystallites with dimensions lower than ∼4.5 nm. The long radiative lifetime in this band is due to a small oscillator strength typical of an indirect bandgap semiconductor. We calculate that the presence of a dangling bond in a crystallite must kill its luminescence in the visible range but can produce an emission in the infrared region, in agreement with recent experimental results showing a correlation between the presence of dangling bonds and the infrared emission of light. We show that polysilanes can emit visible light in some cases but with radiative lifetimes in the nanosecond range. The physical origin of the fast blue band of porous silicon is examined in light of these results.